TitlePageJava9ProgrammingByExample

Yourguidetosoftwaredevelopment

PeterVerhas

BIRMINGHAM-MUMBAI

Java9ProgrammingByExample

Copyright©2017PacktPublishingAllrightsreserved.Nopartofthisbookmaybereproduced,storedinaretrievalsystem,ortransmittedinanyformorbyanymeans,withoutthepriorwrittenpermissionofthepublisher,exceptinthecaseofbriefquotationsembeddedincriticalarticlesorreviews.

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Credits

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PeterVerhas

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AbouttheAuthorPeterVerhasisaseniorsoftwareengineerandsoftwarearchitecthavingelectricalengineeringandeconomicsbackgroundfromTUBudapest(MsC)andPTEHungary(MBA),andalsostudiedatTUDelftandTUVienna.Hecreatedhisfirstprogramsin1979,andsincethenhehasauthoredseveralopensourceprograms.HehasworkedinseveralpositionsinthetelecommunicationsandfinanceindustriesandwastheCIOoftheHungarianstart-upindex.huduringitsearlydays.

PeterworksforEPAMSystemsinSwitzerland,participatinginsoftwaredevelopmentprojectsatvariouscustomersites,andhesupportstalentacquisitionbyinterviewingcandidates,trainingprogramsfordevelopers,andinternalmentoringprograms.

YoucanfollowPeteronTwitterat@verhas,LinkedIn,andGitHub,orreadhistechnicalblog,JavaDeep,athttp://javax0.wordpress.com.

Acknowledgementisthesectionofabookthateverybodyignoresbyturningthepages.Thistime,thissectionisabitdifferent.Iwillmentionafewpeopleandtheirrolesinthemakingofthisbookbut,atthesametime,Iwillexplainwhyandhowitisimportanttorelyonpeople,beingasoftwaredeveloper.Doingprofessionalworkisnotpossiblewithouthavingalife.Itisquiteobviousifyoutakethatliterally,butitisjustastruefiguratively.Ifyoudonotfindthebalancebetweenyourpersonalandprofessionallife,youwillburnoutandwillnotoperateprofessionally.Thisistheplacetomentionmyfamily,myparentswhomIamluckytostillhavearound,mywife,andmyalreadyadultkidswhoneverstoppedbelievinginmebeingabletodothiswork,whoknowmorethanwellwhatahypocriteIam,advocatingpersonal-professionallifebalance,andwhocontinuallypushedmeclosertothisequilibriumpointinlifesothatIcouldkeepperformingprofessionally.Forprofessionalwork,coworkersarealmostasimportantasfamilysupport.Itisimportantthatyousupportyourcolleaguesasmuchasyouaskthemfortheirsupport.Youlearnalotfrombooksandfromexperience,butyoulearnthemostfromotherpeople.Payattentiontoseniordevelopers.Youcan,however,learnjustasmuchfromjuniors.Nomatterhowaceyouare,fromtimetotime,arookiemayshedlightonatopic.Duringtheyears,Ilearnedalotfromjuniorswhobroughtafreshviewtothetable,askingshockingquestionsthatwereabsolutelyvalid.Icannotnameeachandeveryjuniorwhoaidedmyworkwithfreshout-of-the-boxthinking.Ican,andshould,however,namesomepeerprofessionalswhoactivelyparticipatedinthecreationofthisbookwiththeiradvice,discussions,andsuggestions.IshouldcertainlymentionKárolyOláhwhowasveryenthusiasticaboutmyproject,andherepresented,supported,andencouragedtheideainsideEPAMsystems.Heactivelydiscussedwiththeuppermanagementthatthesupportforwritingabookwellfitstheinnovationlineanddevelopmentofthecompany,andthepeoplewhoworktogether.Withouttheofficialsupportfromthecompanyprovidingextratimeforthetask,Iwouldnothavebeenabletocreatethisbook.Goodcompanyattractsgoodpeoplewhoarecleverandalsogoodtoworkwith.Ihadmanydiscussionsaboutthebook,topics,andhowtoexplaincertainaspectswithmyfellowEPAMers:KrisztiánSallai,PeterFodor,SándorSzilágyi,MantasAleknavicius,GáborLénard,andmanyothers.IwillseparatelymentionIstvánAttilaKovácsfromourBudapestofficewithwhomIdiscussedChapter5indetail,andwhogavemeveryvaluablefeedbackaboutthetopic.Ifhedoesnotknowsomething

aboutJavaparallelcomputing,thenthatsomethingdoesnotexist.Asasummaryandtakeawayforthepatientreaderwhoreadthissectiontilltheend,technology,knowledge,skills,andexperienceareextremelyimportantforbeingaprofessionalJava9developer,butitisthepeoplewhoreallymatter.

AbouttheReviewerJeffFriesenisafreelanceauthorandsoftwaredeveloperwhohastaughtJavaintheclassroomandbywritingnumerousarticlesandbookssincethelate1990s.HeholdsaBachelorofSciencedegreeinComputerScienceandMathematics.Priortofreelancing,Jeffworkedfortelecommunications,investment,andsoftwaredevelopmentcompanies.

JefffreelancesasaJavaauthorandsoftwaredeveloper.

JeffhaswrittenJavaI/O,NIOandNIO.2andJavaThreadsandtheConcurrencyUtilitiesforApress.Fulldetailsareavailableonhiswebsite(http://javajeff.ca/cgi-bin/makepage.cgi?/books).

IthankNitinDasanfortheopportunitytotechreviewthisbook.IalsothankUlhasKambaliforassistingmewiththetechreviewprocess.

www.PacktPub.comForsupportfilesanddownloadsrelatedtoyourbook,pleasevisitwww.PacktPub.com.

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TableofContents

PrefaceWhatthisbookcovers

Whatyouneedforthisbook

Whothisbookisfor

Conventions

Readerfeedback

CustomersupportDownloadingtheexamplecode

Errata

Piracy

Questions

1. GettingStartedwithJava9GettingstartedwithJava

InstallingJavaInstallationonWindows

InstallationonMACOSX

InstallationonLinux

SettingJAVA_HOME

ExecutingjshellLookingatthebytecode

PackagingclassesintoaJARfile

ManagingtherunningJavaapplication

UsinganIDENetBeans

Eclipse

IntelliJ

IDEservicesIDEscreenstructure

Editingfiles

Managingprojects

Buildthecodeandrunit

DebuggingJava

Summary

2. TheFirstRealJavaProgram-SortingNamesGettingstartedwithsorting

Bubblesort

GettingstartedwithprojectstructureandbuildtoolsMake

AntInstallingAnt

UsingAnt

MavenInstallingMaven

UsingMaven

GradleInstallingGradle

SettinguptheprojectwithMaven

Codingthesort

UnderstandingthealgorithmandlanguageconstructsBlocks

Variables

Types

Arrays

Expressions

Loops

Conditionalexecution

Finalvariables

Classes

Inner,nested,local,andanonymousclasses

Packages

Methods

Interfaces

Argumentpassing

Fields

Modifiers

Objectinitializersandconstructors

Compilingandrunningtheprogram

Summary

3. OptimizingtheSort-MakingCodeProfessionalThegeneralsortingprogram

AbriefoverviewofvarioussortingalgorithmsQuicksort

ProjectstructureandbuildtoolsMavendependencymanagement

CodethesortCreatingtheinterfaces

CreatingBubbleSort

Amendingtheinterfaces

Architecturalconsiderations

CreatingunittestsAddingJUnitasdependency

WritingtheBubbleSortTestclass

GoodunittestsAgoodunittestisreadable

Unittestsarefast

Unittestsaredeterministic

Assertionsshouldbeassimpleaspossible

Unittestsareisolated

Unittestscoverthecode

Refactorthetest

Collectionswithwrongelements

Handlingexceptions

Generics

TestDrivenDevelopment

ImplementingQuickSortThepartitioningclass

Recursivesorting

Non-recursivesorting

ImplementingtheAPIclass

CreatingmodulesWhymodulesareneeded

WhatisaJavamodule

Summary

4. Mastermind-CreatingaGameTheGame

Themodelofthegame

JavacollectionsInterfacecollection

SetHashfunctions

Methodequals

MethodhashCode

ImplementingequalsandhashCode

HashSet

EnumSet

LinkedHashSet

SortedSet

NavigableSetTreeSet

ListLinkedList

ArrayList

Queue

Deque

MapHashMap

IdentityHashMap

Dependencyinjection

ImplementingthegameColorManager

Theclasscolor

JavaDocandcodecomments

Row

Table

GuesserUniqueGuesser

GeneralGuesser

TheGameclass

Creatinganintegrationtest

Summary

5. ExtendingtheGame-RunParallel,RunFasterHowtomakeMastermindparallel

Refactoring

Processes

Threads

Fibers

java.lang.Thread

PitfallsDeadlocks

Raceconditions

Overusedlocks

Starving

ExecutorServiceForkJoinPool

Variableaccess

TheCPUheartbeat

Volatilevariables

Synchronizedblock

Waitandnotify

LockCondition

ReentrantLock

ReentrantReadWriteLock

Atomicclasses

BlockingQueueLinkedBlockingQueue

LinkedBlockingDeque

ArrayBlockingQueue

LinkedTransferQueue

IntervalGuesser

ParallelGamePlayer

Microbenchmarking

Summary

6. MakingOurGameProfessional-DoitasaWebappWebandnetwork

IP

TCP/IP

DNS

TheHTTPprotocolHTTPmethods

Statuscodes

HTTP/2.0

Cookies

Clientserverandwebarchitecture

WritingservletsHelloworldservlet

JavaServerPages

HTML,CSS,andJavaScript

MastermindservletStoringstate

HTTPsession

Storingstateontheclient

DependencyinjectionwithGuice

TheMastermindHandlerclass

Storingstateontheserver

TheGameSessionSaverclass

RunningtheJettywebservlet

LoggingConfigurability

Performance

Logframeworks

Java9logging

Loggingpractice

Othertechnologies

Summary

7. BuildingaCommercialWebApplicationUsingRESTTheMyBusinesswebshop

Samplebusinessarchitecture

Microservices

Serviceinterfacedesign

JSON

REST

ModelViewController

SpringframeworkArchitectureofSpring

Springcore

Serviceclasses

Compilingandrunningtheapplication

TestingtheapplicationIntegrationtest

Applicationtest

Servletfilters

AuditloggingandAOP

Dynamicproxy-basedAOP

Summary

8. ExtendingOurE-CommerceApplicationTheMyBusinessordering

Settinguptheproject

OrdercontrollerandDTOs

Consistencychecker

AnnotationsAnnotationretention

Annotationtarget

Annotationparameters

Repeatableannotations

Annotationinheritance

@Documentedannotations

JDKannotations

UsingreflectionGettingannotations

Invokingmethods

Settingfields

FunctionalprogramminginJavaLambda

Streams

Functionalinterfaces

Methodreferences

ScriptinginJava9

Summary

9. BuildinganAccountingApplicationUsingReactiveProgrammingReactive...what?

Reactiveprogramminginanutshell

ReactivesystemsResponsive

Resilient

Elastic

Message-driven

Back-pressure

Reactivestreams

ReactiveprogramminginJavaImplementinginventory

Summary

10. FinalizingJavaKnowledgetoaProfessionalLevelJavadeeptechnologies

Javaagent

PolyglotprogrammingPolyglotconfiguration

Polyglotscripting

BusinessDSL

Problemswithpolyglot

Annotationprocessing

ProgrammingintheenterpriseStaticcodeanalysis

Sourcecodeversioncontrol

Softwareversioning

Codereview

Knowledgebase

Issuetracking

TestingTypesoftests

Testautomation

Blackboxversuswhitebox

SelectinglibrariesFitforthepurpose

License

Documentation

Projectalive

Maturity

Numberofusers

The"Ilikeit"factor

Continuousintegrationanddeployment

Releasemanagement

Coderepository

Walkinguptheladder

Summary

PrefaceJavadrasticallychangedwiththeintroductionofJava8,andthischangehasbeenelevatedtoawholenewlevelwiththenewversion,Java9.Javahasawell-establishedpast,beingmorethan20yearsold,butatthesametime,itisnew,functional,reactive,andsexy.Thisisalanguagethatdeveloperslove,andatthesametime,itisthenumberonechoiceofdeveloperlanguageformanyenterpriseprojects.

ItisprobablymorelucrativetolearnJavanowthaneverbefore,startingwithJava9.WeencourageyoutostartyourprofessionaldevelopercareerbylearningJava9,andwehavedoneourbestinthisbooktohelpyoualongthisroad.Weassembledthetopicsofthebooksothatitiseasytostart,andyoucanfeelthethingsworkingandmovingverysoon.Atthesametime,wehavetriedtoreachveryfar,signalingtheroadaheadforaprofessionaldeveloper.

Thesandsoftimekeptmoving,andIdiscoveredfunctionalprogramming.

Icouldverywellseewhywritingside-effect-freecodeworked!IwashookedandstartedplayingwithScala,Clojure,andErlang.Immutabilitywasthenormhere.

However,Iwonderedhowtraditionalalgorithmswouldlookinafunctionalsettingandstartedlearningaboutit.

Adatastructureisnevermutatedinplace.Instead,anewversionofthedatastructureiscreated.Thestrategyofcopyandwritewithmaximizedsharingwasanintriguingone!Allthatcarefulsynchronizationissimplynotneeded!

Thelanguagescomeequippedwithgarbagecollection.So,ifaversionisnotneededanymore,runtimewouldtakecareofreclaimingthememory.

Allingoodtime,though!Readingthisbookwillhelpyouseethatweneednotsacrificealgorithmicperformancewhileavoidingin-placemutation!

WhatthisbookcoversChapter1,GettingStartedwithJava9,givesyouajumpstartinJava,helpingyouinstallitonyourcomputerandrunyourfirstinteractiveprogramsusingthenewJshell.

Chapter2,TheFirstRealJavaProgram-SortingNames,teachesyouhowtocreateadevelopmentproject.Thistime,wewillcreateprogramfilesandcompilethecode.

Chapter3,OptimizingtheSort-MakingCodeProfessional,developsthecodefurthersothatthecodeisreusableandnotonlyatoy.

Chapter4,Mastermind-CreatingaGame,iswhensomefunstarts.Wedevelopagameapplicationthatisinterestingandnotastrivialasitfirstseems,butwewilldoit.

Chapter5,ExtendingtheGame-RunParallel,RunFaster,showsyouhowtoutilizethemulti-processorcapabilitiesofmodernarchitecture.Thisisaveryimportantchapterthatdetailstechnologies,thatonlyafewdeveloperstrulyunderstand.

Chapter6,MakingOurGameProfessional-DoitasaWebapp,transformstheuserinterfacefromcommand-linetowebbrowser-based,deliveringbetteruserexperience.

Chapter7,BuildingaCommercialWebApplicationUsingREST,takesyouthroughthedevelopmentofanapplicationthathasthecharacteristicsofmanycommercialapplications.WewillusethestandardRESTprotocolwhichhasgainedgroundinenterprisecomputing.

Chapter8,ExtendingOurE-CommerceApplication,helpsyoudeveloptheapplicationfurtherutilizingmodernlanguagefeaturessuchasscriptingandlambdaexpressions.

Chapter9,BuildinganAccountingApplicationUsingReactiveProgramming,teachesyouhowtoapproachsomeproblemsusingreactiveprogramming.

Chapter10,FinalizingJavaKnowledgetoaProfessionalLevel,givesabird's-eyeviewofdevelopertopicsthatplayanimportantroleinthelifeofaJavadeveloper,andwhichwillguideyoufurtherinworkingasaprofessionaldeveloper.

Whatyouneedforthisbook

Toimmerseintothecontentofthisbookandtosoakupmostoftheskillsandknowledge,weassumethatyoualreadyhavesomeexperiencewithprogramming.Wedonotassumetoomuchbuthopethatyoualreadyknowwhatavariableis,thatcomputershavememory,disk,networkinterfaces,andwhattheygenerallyare.Inadditiontothesebasicskills,therearesometechnicalrequirementstotryoutthecodeandtheexamplesofthebook.Youneedacomputer—somethingthatisavailabletodayandcanrunWindows,Linux,orOSX.Youneedanoperatingsystemand,probably,thatisallyouneedtopayfor.Allothertoolsandservicesthatyouwillneedareavailableasopensourceandfreeofcharge.Someofthemarealsoavailableascommercialproductswithanextendedfeatureset,butforthescopeofthisbook,startingtolearnJava9programming,thosefeaturesarenotneeded.Java,adevelopmentenvironment,buildtools,andallothersoftwarecomponentsweuseareopensource.

WhothisbookisforThisbookisforanyonewhowantstolearntheJavaprogramminglanguage.Youareexpectedtohavesomepriorprogrammingexperiencewithanotherlanguage,suchasJavaScriptorPython,butnoknowledgeofearlierversionsofJavaisassumed.

ConventionsInthisbook,youwillfindanumberoftextstylesthatdistinguishbetweendifferentkindsofinformation.Herearesomeexamplesofthesestylesandanexplanationoftheirmeaning.

Codewordsintext,databasetablenames,foldernames,filenames,fileextensions,pathnames,dummyURLs,userinput,andTwitterhandlesareshownasfollows:"Thefollowingfunctionfhasasideeffect,though."

Ablockofcodeissetasfollows:

packagepackt.java9.by.example.ch03;

publicinterfaceSort{

voidsort(SortableCollectioncollection);

}

Ifthereisaline(orlines)ofcodethatneedstobehighlighted,itissetasfollows:

id=123

title=BookJava9byExample

description=anewbooktolearnJava9

weight=300

width=20

height=2

depth=18

Newtermsandimportantwordsareshowninbold.Wordsthatyouseeonthescreen,forexample,inmenusordialogboxes,appearinthetextlikethis:"ClickingtheNextbuttonmovesyoutothenextscreen."

Warningsorimportantnotesappearinaboxlikethis.

Tipsandtricksappearlikethis.

ReaderfeedbackFeedbackfromourreadersisalwayswelcome.Letusknowwhatyouthinkaboutthisbook—whatyoulikedordisliked.Readerfeedbackisimportantforusasithelpsusdeveloptitlesthatyouwillreallygetthemostoutof.

Tosendusgeneralfeedback,simplye-mailfeedback@packtpub.com,andmentionthebook'stitleinthesubjectofyourmessage.

Ifthereisatopicthatyouhaveexpertiseinandyouareinterestedineitherwritingorcontributingtoabook,seeourauthorguideatwww.packtpub.com/authors.

CustomersupportNowthatyouaretheproudownerofaPacktbook,wehaveanumberofthingstohelpyoutogetthemostfromyourpurchase.

Downloadingtheexamplecode

Youcandownloadtheexamplecodefilesforthisbookfromyouraccountathttps://github.com/PacktPublishing/Java-9-Programming-By-Example.Ifyoupurchasedthisbookelsewhere,youcanvisithttp://www.packtpub.com/supportandregistertohavethefilese-maileddirectlytoyou.

Youcandownloadthecodefilesbyfollowingthesesteps:

1. Loginorregistertoourwebsiteusingyoure-mailaddressandpassword.2. HoverthemousepointerontheSUPPORTtabatthetop.3. ClickonCodeDownloads&Errata.4. EnterthenameofthebookintheSearchbox.5. Selectthebookforwhichyou'relookingtodownloadthecodefiles.6. Choosefromthedrop-downmenuwhereyoupurchasedthisbookfrom.7. ClickonCodeDownload.

Oncethefileisdownloaded,pleasemakesurethatyouunziporextractthefolderusingthelatestversionof:

WinRAR/7-ZipforWindowsZipeg/iZip/UnRarXforMac7-Zip/PeaZipforLinux

ThecodebundleforthebookisalsohostedonGitHubathttps://github.com/PacktPublishing/Java-9-Programming-By_Example.Wealsohaveothercodebundlesfromourrichcatalogofbooksandvideosavailableathttps://github.com/PacktPublishing/.Checkthemout!

Errata

Althoughwehavetakeneverycaretoensuretheaccuracyofourcontent,mistakesdohappen.Ifyoufindamistakeinoneofourbooks—maybeamistakeinthetextorthecode—wewouldbegratefulifyoucouldreportthistous.Bydoingso,youcansaveotherreadersfromfrustrationandhelpusimprovesubsequentversionsofthisbook.Ifyoufindanyerrata,pleasereportthembyvisitinghttp://www.packtpub.com/submit-errata,selectingyourbook,clickingontheErrataSubmissionFormlink,andenteringthedetailsofyourerrata.Onceyourerrataareverified,yoursubmissionwillbeacceptedandtheerratawillbeuploadedtoourwebsiteoraddedtoanylistofexistingerrataundertheErratasectionofthattitle.

Toviewthepreviouslysubmittederrata,gotohttps://www.packtpub.com/books/content/supportandenterthenameofthebookinthesearchfield.TherequiredinformationwillappearundertheErratasection.

PiracyPiracyofcopyrightedmaterialontheInternetisanongoingproblemacrossallmedia.AtPackt,wetaketheprotectionofourcopyrightandlicensesveryseriously.IfyoucomeacrossanyillegalcopiesofourworksinanyformontheInternet,pleaseprovideuswiththelocationaddressorwebsitenameimmediatelysothatwecanpursuearemedy.

Pleasecontactusatcopyright@packtpub.comwithalinktothesuspectedpiratedmaterial.

Weappreciateyourhelpinprotectingourauthorsandourabilitytobringyouvaluablecontent.

QuestionsIfyouhaveaproblemwithanyaspectofthisbook,youcancontactusatquestions@packtpub.com,andwewilldoourbesttoaddresstheproblem.

GettingStartedwithJava9

YouwanttolearnJavaandyouhaveagoodreasonforit.Javaisamodernandwell-establishedapplicationprogramminglanguage,whichiswidelyusedinmanyindustries,beittelecommunication,finance,orsomethingelse.Javadeveloperpositionsarethemostnumerousand,probably,thebestpaid.This,amongotherthings,makesthelanguagelucrativeforyoungprofessionalstolearn.Ontheotherhand,thisisnotwithoutreason.Javalanguage,thetools,andthewholeinfrastructurearounditiscomplexandcompound.BecomingaJavaprofessionaldoesnothappeninadayorweek;itisaworkofmanyyears.TobeaJavaexpert,youneedtoknownotonlyabouttheprogramminglanguagebutalsoaboutobject-orientedprogrammingprinciples,opensourcelibraries,applicationservers,network,databases,andmanyotherthingsthatyoucanbecomeanexpertin.Nevertheless,learningthelanguageisanabsolutemustthatallotherpracticesshouldbuildon.Throughthisbook,youwillbeabletolearnJavaversion9andabitmore.

Inthischapter,youwillbeintroducedtotheJavaenvironmentandgivenstep-by-stepinstructionsonhowtoinstallit,editsamplecode,compile,andrunJava.Youwillgetacquaintedwiththebasictoolsthathelpdevelopment,betheyareapartofJavaorareprovidedbyothervendors.Wewillcoverthefollowingtopicsinthischapter:

IntroductiontoJavaInstallingWindows,Linux,andMacOSXExecutingjshellUsingotherJavatoolsUsingintegrateddevelopmentenvironment

GettingstartedwithJavaItislikegoingthroughapathinaforest.Youcanfocusonthegraveloftheroadbutitispointless.Instead,youcanenjoytheview,thetrees,thebirds,andtheenvironmentaroundyou,whichismoreenjoyable.ThisbookissimilarasIwon'tbefocusingonlyonthelanguage.Fromtimetotime,Iwillcovertopicsthatareclosetotheroadandwillgiveyousomeoverviewanddirectionsonwhereyoucangofurtherafteryoufinishthisbook.Iwillnotonlyteachyouthelanguagebutalsotalkabitaboutalgorithms,object-orientedprogrammingprinciples,toolsthatsurroundJavadevelopment,andhowprofessionalswork.Thiswillbemixedwiththecodingexamplesthatwewillfollow.Lastly,thefinalchapterwillbefullydevotedtothetopic,whattolearnnextandhowtogofurthertobecomeaprofessionalJavadeveloper.

Bythetimethisbookgetsintoprint,Javawillhavecompleted22years.http://www.oracle.com/technetwork/java/javase/overview/javahistory-index-198355.html.Thelanguagehaschangedalotduringthisperiodandgotbetter.Therealquestiontoaskisnothowlonghasitbeenhere,butratherhowlongwillitstay?Isitstillworthlearningthislanguage?TherearenumerousnewlanguagesthathavebeendevelopedsinceJavawasborn(http://blog.takipi.com/java-vs-net-vs-python-vs-ruby-vs-node-js-who-reigns-the-job-market/).Theselanguagesaremoremodernandhavefunctionalprogrammingfeatures,which,bytheway,Javahasalsohadsinceversion8.ManysaythatJavaisthepast—thefutureisScala,Swift,Go,Kotlin,JavaScript,andsoon.Youcanaddmanyotherlanguagestothislist,andforeach,youcanfindablogarticlethatcelebratestheburialofJava.Therearetwoanswerstothisconcern-oneisapragmaticbusinessapproach,theotherismoreengineering:

ConsideringthatCOBOLisstillactivelyusedinthefinanceindustryandCOBOLdevelopersareperhapsbetterpaidthanJavadevelopers,itisnottooriskytosaythatasaJavadeveloper,youwillfindpositionsinthenext40years.Personally,Iwouldbetmorethana100years,butconsideringmyage,itwillnotbefairpredictingmorethan20to40yearsahead.Javaisnotonlyalanguage;itisalsoatechnologythatyouwilllearnabitaboutfromthisbook.ThetechnologyincludestheJavaVirtualMachine(JVM),whichisusuallyreferredtoasJVM,andgivestheruntimeenvironmentformanylanguages.KotlinandScala,forexample,cannotrunwithoutJVM.EvenifJavawillbeadumbrated,JVMwillstillbeanumberoneplayerintheenterprisescene.

TounderstandandlearnthebasicoperationofJVMisalmostasimportantasthelanguageitself.Javaisacompiledandinterpretedlanguage.Itisaspecialbeastthatforgesthebestofbothworlds.BeforeJava,therewereinterpretedandcompiledlanguages.

Interpretedlanguagesarereadfromthesourcecodebytheinterpreterandthentheinterpreterexecutesthecode.Ineachoftheselanguages,thereissomepreliminarylexicalandsyntaxanalysisstep;however,afterthat,theinterpreter,which,asaprogramitself,isexecutedbytheprocessorandtheinterpretercontinuously,interpretstheprogramcodetoknowwhattodo.Compiledlanguagesaredifferent.Insuchacase,thesourcecodeiscompiledtobinary(.exefileonWindowsplatforms),whichtheoperatingsystemloadsandtheprocessordirectlyexecutes.Compiledprogramsusuallyrunfaster,butthereisusuallyaslowercompilationphasethatmaymakethedevelopmentslower,andtheexecutionenvironmentisnotso

flexible.Javacombinedthetwoapproaches.

ToexecuteaJavaprogram,theJavasourcecodehastobecompiledtotheJVMbytecode(.classfile),whichisloadedbyJVMandisinterpretedorcompiled.Hmm...isitinterpretedorcompiled?ThethingthatcamewithJavaistheJustinTime(JIT)compiler.Thismakesthephaseofthecompilationthatiscalculation-intensiveandthecompilationforcompiledlanguagesrelativelyslow.JVMfirststartstointerprettheJavabytecodeand,whiledoingthat,itkeepstrackofexecutionstatistics.Whenitgathersenoughstatisticsaboutcodeexecutions,itcompilestonativecode(forexample,x86codeonanIntel/AMDplatform)fordirectexecutionofthepartsofcodethatareexecutedfrequentlyandkeepsinterpretingthecodefragmentsthatarerarelyused.Afterall,whywasteexpensiveCPUtimetocompilesomecodethatishardlyeverused?(Forexample,codethatreadsconfigurationduringstartupanddoesnotexecuteagainunlesstheapplicationserverisrestarted.)Compilationtothebytecodeisfastandcodegenerationisdoneonlyforthesegmentsthatpayoff.

ItisalsointerestingthatJITusesthestatisticsofthecodeexecutiontooptimizethecode.If,forexample,itseesthatsomeconditionalbranchisexecutedin99%ofthecasesandtheotherbranchisexecutedonlyin1%,thenitwillgeneratenativecodethatrunsfast,thusfavoringthefrequentbranch.Ifthebehaviorofthatpartoftheprogramchangesbytimeandthestatisticshowsthattheratioschanged,theJITautomaticallyrecompilesthebytecodefromtimetotime.Thisisallautomaticandbehindthescenes.

Inadditiontotheautomaticcompilation,thereisalsoanextremelyimportantfeatureofJVM-itmanagesthememoryfortheJavaprogram.TheexecutionenvironmentofmodernlanguagesdothatandJavawasthefirstmainstreamlanguagethathadanautomaticgarbagecollection(GC).BeforeJava,IwasprogramminginCfor20yearsanditwasagreatpaintokeeptrackofallmemoryallocationandnottoforgettoreleasethememorywhentheprogramnolongerneededit.Forgettingmemoryallocationatasinglepointinthecodeandthelongrunningprogramwaseatingupallmemoryslowly.SuchproblemspracticallyceasedtoexistinJava.Thereisapricethatwehavetopayforit—GCneedsprocessorcapacityandsomeextramemory,butthatissomethingwearenotshortofinmostoftheenterpriseapplications.Somespecialprograms,likereal-timeembeddedsystemsthatcontrolthebrakesofaheavy-dutylorrymaynothavethatluxury.ThosearestillprogrammedinassemblyorC.Fortherestofus,wehaveJava,andthoughitmayseemstrangeformanyprofessionals,evenalmost-real-timeprograms,suchashigh-frequencytradingapplications,arewritteninJava.

Theseapplicationsconnectthroughthenetworktothestockexchangeandtheysellandbuystocksrespondingtomarketchangeinmilliseconds.Javaiscapableofdoingthat.TheruntimeenvironmentofJavathatyouwillneedtoexecuteacompiledJavacode,whichalsoincludestheJVMitself,containscodethatletsJavaprogramsaccessthenetwork,filesondisks,andotherresources.Todothis,theruntimecontainshigh-levelclassesthatthecodecaninstantiate,execute,andwhichdothelow-leveljobs.Youwillalsodothis.ItmeansthattheactualJavacodedoesnotneedtohandleIPpackets,TCPconnections,orevenHTTPhandlingwhenitwantstouseorprovideaRESTserviceinsomemicroservicesarchitecture.Itisalreadyimplementedintheruntimelibraries,andalltheapplicationprogrammerhastodoistoincludetheclassesinthecodeandusetheAPIstheyprovideonanabstractionlevelthatmatchestheprogram.WhenyouprograminJava,youcanfocusontheactualproblemyouwanttosolve,whichisthebusinesscodeandnotthelow-levelsystemcode.Ifitisnotinthestandardlibrary,youwillfinditinsomeproductinsomeexternallibrary,anditisalsoveryprobablethatyouwillfindanopensourcesolutionfortheproblem.

ThisisalsoastrongpointofJava.Thereisavastnumberofopensourcelibrariesavailableforallthedifferentpurposes.Ifyoucannotfindalibraryfittingyourproblemifyoustarttocodesomelow-levelcode,thenprobablyyouaredoingsomethingwrong.Therearetopicsinthisbookthatareimportant,suchasclassloadersorreflection,notbecauseyouhavetousethemeverydaybutratherbecausetheyareusedbyframeworks,andknowingthemhelpsunderstandhowtheseframeworkswork.Ifyoucannotsolveyourproblemwithoutusingreflectionorwritingyourownclassloaderorprogrammultithreaddirectly,thenyouprobablychosethewrongframework.Thereisalmostcertainlyagoodone:Apacheproject,Google,andmanyotherimportantplayersinthesoftwareindustrypublishtheirJavalibrariesasopensource.

Thisisalsotrueformultithreadprogramming.Javaisamultithreadprogrammingenvironmentfromtheverybeginning.TheJVMandtheruntimesupportsprogramsthatexecutethecode.Theexecutionrunsparallelonmultiplethreads.Thereareruntimelanguageconstructsthatsupportparallelexecutingprogramsstartingattheverylowleveltohighabstraction.Multithreadcodeutilizesthemulticoreprocessors,whicharemoreeffective.Theseprocessorsaremoreandmorecommon.20yearsago,onlyhigh-endservershadmultipleprocessorsandonlyDigitalAlphaprocessorshad64-bitarchitectureandCPUclockabove100MHz.10yearsago,multiprocessorstructurewascommonontheserverside,andabout5yearsago,multicoreprocessorswereonsomedesktopsandonnotebooks.Today,evenmobilephoneshavethem.WhenJavawasstartedin1995,thegeniuseswhocreatedithadseenthisfuture.

TheyenvisionedJavatobeawriteonce,runanywherelanguage.Atthattime,thefirsttargetforthelanguagewasappletrunninginthebrowser.Today,manythink(andIalsosharethisopinion)thatappletswereawrongtarget,oratleastthingswerenotdoneintherightway.Asfornow,youwillmeetappletsontheInternetlessfrequentlythanFlashapplicationsordinosaurs.

However,atthesametime,theJavainterpreterwasalsoexecutingserverandclientapplicationswithoutanybrowser;furthermore,asthelanguageandtheexecutingenvironmentdeveloped,theseapplicationareasbecamemoreandmorerelevant.Today,themainuseofJavaisenterprisecomputingandmobileapplicationsmainlyfortheAndroidplatform;forthefuture,theuseoftheenvironmentisgrowinginembeddedsystemsastheInternetofthings(IoT)comesmoreandmoreintopicture.

InstallingJavaTodevelop,compile,andexecuteJavaprograms,youwillneedtheJavaexecutionenvironment.Astheoperatingsystemsthatweusuallyuseforsoftwaredevelopmentdonotcontainthelanguagepreinstalled,youwillhavetodownloadit.Although,thereismultipleimplementationofthelanguage,IrecommendthatyoudownloadtheofficialversionofthesoftwarefromOracle.Theofficialsiteforjavaishttp://java.comandthisisthesitefromwherethelatestreleaseofthelanguagecanbedownloaded.Atthetimeofwritingthisbook,the9thversionofJavaisnotyetreleased.Anearlypre-releaseversionisaccessibleviahttp://jdk9.java.net/download.Laterthereleaseversionswillalsobeavailablefromhere.

Whatyoucandownloadfromhereisasocalledearlyaccessversionofthecodethatisavailableonlytoexperimentwithit,andnoprofessionalsshoulduseitforrealprofessionalpurposes

Onthepage,youhavetoclickontheradiobuttontoacceptthelicense.Afterthat,youcanclickonthe

linkthatdirectlystartsthedownloadoftheinstallationkit.Thelicenseisaspecialearlyaccesslicenseversionthatyou,asaprofessional,shouldcarefullyread,understand,andacceptonlyifyouareagreeablewiththeterms.

ThereisaseparateinstallationkitforWindows32and64bitsystems,MacOSX,Linux32and64bitversions,LinuxforARMprocessor,SolarisforSPARCprocessorsystems,andSolarisx86versions.AsitisnotlikelythatyouwilluseSolaris,IwilldetailtheinstallationprocedureonlyforWindows,Linux,andMacOSX.Inlaterchapters,thesampleswillalwaysbeMacOSX,butsinceJavaisawriteonce,runanywherelanguage,thereisnodifferenceaftertheinstallation.Thedirectoryseparatormaybeslanteddifferently,theclasspathseparatorcharacterisasemicolononWindowsinsteadofacolon,andthelookandfeeloftheTerminalorcommandapplicationisalsodifferent.However,whereitisimportant,Iwilltrynottoforgettomentionit.

Toconfuseyou,theJavadownloadforeachoftheseoperatingsystemversionslistsalinkfortheJREandonefortheJDK.JREstandsforJavaRuntimeEnvironmentanditcontainsallthetoolsandexecutablesthatareneededtorunJavaprograms.JDKistheJavaDevelopmentKitthatcontainsallthetoolsandexecutablesthatareneededtodevelopJavaprogramsincludingtheexecutionoftheJavaprogram.Inotherwords,JDKcontainsitsownJRE.Fornow,allyouneedtodoisdownloadtheJDK.

Thereisoneimportantpointoftheinstallationthatisthesameoneachofthethreeoperatingsystemsthatyouhavetobepreparedforbeforetheinstallation:toinstallJava,youshouldhaveadministrativeprivileges.

InstallationonWindowsTheinstallationprocessonWindowsstartsbydoubleclickingonthedownloadedfile.Itwillstarttheinstallerthatwillpresentyouawelcomescreen.

PressingtheNextbuttonwegetawindowwhereyoucanselectthepartsyouwanttoinstall.Let'sleaveherethedefaultselection,whichmeansthatweinstallallthedownloadedpartsofJavaandpressthebuttonNext.Thefollowingwindowiswherewecanselectthedestinationfolderfortheinstallation.

Asfornowwedonotchangethedirectoryselectedbytheinstaller.PressNext.Later,whenyoubecomeaprofessionalJavadeveloper,youmaydecidetoinstallJavatoadifferentlocationbutthenyouwillalreadyhavetoknowwhatyouaredoing.

YoumayneedtoclicktheNextbuttonafewtimesandthentheinstallerfinishes.Providetheadministrativepasswordwhenaskedandvoila!Javaisinstalled.ThisisreallytheveryusualWindowsinstallationprocess.

ThelaststepistosettheenvironmentvariableJAVA_HOME.TodothatinWindowswehavetoopenthe

controlcenterandselecttheEditenvironmentvariablesforyouraccountmenu.

Thiswillopenanewwindowthatweshouldusetocreateanewenvironmentvariableforthecurrentuser.

ThenameofthenewvariablehastobeJAVA_HOMEandthevalueshouldpointtotheinstallationdirectoryoftheJDK.

ThisvalueonmostofthesystemsisC:ProgramFilesJavajdk-9.ThisisusedbymanyJavaprogramsandtoolstolocatetheJavaruntime.

InstallationonMACOSXInthissection,wewilltakelookathowtoinstallJavastep-by-steponanOSXplatform.Iwilldescribetheinstallationprocessforthereleasedversionavailableatthetimeofwritingthisbook.Asfornow,theJava9earlyaccessversionisabittrickytoinstall.ItisprobablethatversionJava9willhavesimilarorthesameinstallstepsasJava8update92has.

TheOSXversionofJavacomesintheformofa.dmgfile.ThisisapackagingformatofOSX.Toopenit,simplydoubleclickonthefileintheDownloadfolderwherethebrowsersavesitandtheoperatingsystemwillmountthefileasaread-onlydiskimage.

Thereisonlyonefileonthisdisk:theinstallationimage.DoubleclickonthefilenameoriconintheFinderapplicationandtheinstallationprocesswillstart.

Thefirstscreenopeningisawelcomescreen.ClickContinueandyouwillseetheSummarypagethatdisplayswhatwillbeinstalled.

ItisnotasurprisethatyouwillseeastandardJavainstallation.Thistime,thebuttoniscalledInstall.Clickonitandyouwillseethefollowing:

Thisisthetimewhenyouhavetoprovidetheloginparametersfortheadministrativeuser—ausernameandpassword.Whenprovided,installationstartsand,inafewseconds,youwillseeaSummarypage.

ClickCloseandyouareready.YouhaveJavainstalledonyourMac.Optionally,youcandismounttheinstallationdiskand,sometimelater,youcanalsodeletethe.dmgfile.Youwillnotneedthat,andincaseyoudo,youcandownloaditanytimefromOracle.

Thelastthingistocheckwhethertheinstallationwasokay.Proofofthepuddingiseatingit.StartaTerminalwindowandtypejava-versionatthepromptandJavawilltellyoutheversioninstalled.

OnthenextscreenshotyoucanseetheoutputonmyworkstationandalsotheMacOScommandsthatarehandytoswitchbetweenthedifferentversionsofJava:

Onthescreenshot,youcanseethatIhaveinstalledtheJavaJDK1.8u92versionand,atthesametime,IalsohaveaJava9earlyreleaseinstallation,whichIwillusetotestthenewfeaturesofJavaforthisbook.

InstallationonLinuxThereareseveralwaystoinstallJavaonLinux,dependingonitsflavor.Here,Iwilldescribeaninstallationmethodthatworksmoreorlessthesamewayonallflavors.TheoneIusedisDebian.

Firststepisthesameasinanyotheroperatingsystem:downloadtheinstallationkit.InthecaseofLinux,youshouldselectapackagethathasatar.gzending.Thisisacompressedarchiveformat.Youshouldalsocarefullyselectthepackagethatmatchestheprocessorinyourmachineandthe32/64bitversionoftheoperatingsystem.Afterthepackageisdownloaded,youhavetoswitchtorootmode,issuingthesucommand.Thisthefirstcommandyoucanseeonthescreenshotthatshowstheinstallationcommands.

Thetarcommanduncompressedthearchiveintoasubfolder.InDebian,thissubfolderhastobemovedto/opt/jdkandthemvcommandisusedforthispurpose.Thetwoupdate-alternativescommandisDebian-specific.ThesetelltheoperatingsystemtousethisnewlyinstalledJavaincasethereisalreadyanolderJavainstalled.TheDebianIwasusingtotestanddemonstratetheinstallationprocessonavirtualmachinecamewitha7yearoldversionofJava.

Thefinalstepoftheinstallationisthesameasanyotheroperatingsystem:checkingthattheinstallationwassuccessfulinissuingthejava-versioncommand.InthecaseofLinux,thisisevenmoreimportantbecausetheinstallationprocessdoesnotcheckthatthedownloadedversionmatchestheoperatingsystemandtheprocessorarchitecture.

SettingJAVA_HOMETheJAVA_HOMEenvironmentvariableplaysaspecialroleforJava.EventhoughtheJVMexecutable,java.exeorjava,isonthePATH(thusyoucanexecuteitbytypingthenamejavawithoutspecifyingdirectoryontheCommandPrompt)(Terminal),itisrecommendedthatyouusethecorrectJavainstallationtosetthisenvironmentvariable.ThevalueofthevariableshouldpointtotheinstalledJDK.TherearemanyJava-relatedprograms,forexample,TomcatorMaven,thatusethisvariabletolocatetheinstalledandcurrentlyusedJavaversion.InMacOSX,settingthisvariableisunavoidable.

InOSX,theprogramthatstartstoexecutewhenyoutypejavaisawrapperthatfirstlooksatJAVA_HOMEtodecidewhichJavaversiontostart.Ifthisvariableisnotset,thenOSXwilldecideonitsown,selectingfromtheavailableinstalledJDKversions.Toseetheavailableversions,youcanissuethefollowingcommand:~$/usr/libexec/java_home-VMatchingJavaVirtualMachines(10):9,x86_64:"JavaSE9-ea"/Library/Java/JavaVirtualMachines/jdk-9.jdk/Contents/Home1.8.0_92,x86_64:"JavaSE8"/Library/Java/JavaVirtualMachines/jdk1.8.0_92.jdk/Contents/Home1.7.0_60,x86_64:"JavaSE7"/Library/Java/JavaVirtualMachines/jdk1.7.0_60.jdk/Contents/Home/Library/Java/JavaVirtualMachines/jdk-9.jdk/Contents/Home

YouwillthengetthelistofinstalledJDKs.Notethatthecommandislowercase,buttheoptioniscapital.Ifyoudonotprovideanyoptionsandargumenttotheprogram,itwillsimplyreturntheJDKitthinksisthenewestandmostappropriateforthepurpose.AsIcopiedtheoutputofthecommandfrommyTerminalwindow,youcanseethatIhavequiteafewversionsofJavainstalledonmymachine.

ThelastlineoftheprogramresponseisthehomedirectoryofJDK,whichisthedefault.YoucanusethistosetyourJAVA_HOMEvariableusingsomebashprogramming:exportJAVA_HOME=$(/usr/libexec/java_home)

Youcanplacethisfileinyour.bashrcfile,whichisexecutedeachtimeyoustartTerminalapplicationandthusJAVA_HOMEwillalwaysbeset.Ifyouwanttouseadifferentversion,youcanuse-v,withthelowercaseoptionthistime,tothesameutility,asfollows:exportJAVA_HOME=$(/usr/libexec/java_home-v1.8)

TheargumentistheversionofJavayouwanttouse.Notethatthisversioningbecomes:

exportJAVA_HOME=$(/usr/libexec/java_home-v9)

IfyouwanttouseJavaJDKEarlyAccessversionandnot1.9,thereisnoexplanationforthesame—factoflife.

NotethatthereisanotherenvironmentvariablethatisimportantforJava-CLASSPATH.Wewilltalkaboutitlater.

ExecutingjshellNowthatwehavespentalotoftimeinstallingJava,itistimetogetthefingersburntabit.AsweareusingJava9,thereisanewtoolthathelpsdeveloperstoplayaroundwiththelanguage.ThisisaRead-Eval-Print-Loop(REPL)toolthatmanylanguagetoolsetscontainandtherewerealsoimplementationsfromJava,butversion9isthefirstthatcontainsthisfeatureofftheshelf.

REPLisatoolthathasinteractivepromptandlanguagecommandsthatcanbedirectlyenteredwithouteditingsomestandalonefile.Theenteredcommandsareexecuteddirectlyandthentheloopstartsagain,waitingfortheusertotypeinthenextcommand.Thisisaveryeffectivetooltotryoutsomelanguageconstructswithoutthedelayofediting,compiling,andloading.ThestepsareautomaticallyandtransparentlydonebytheREPLtool.

TheREPLtoolinJava9iscalledjshell.Tostartit,justtypeitsname.IfitisnotonthePATH,thentypethefullpathtojshellthatcomesinstalledwithJava9,asshowninthefollowingexample:

$jshell

|WelcometoJShell--Version9-ea

|Foranintroductiontype:/helpintro

jshell>

Thejshellstartsupinaninteractivewayandthepromptitdisplaysisjshell>tohelpyourecognizethatjshellisrunningandwhatyoutypeisreadbytheprogramandnottheoperatingsystemshell.Asthisisthefirsttimeyouwillstartjshell,ittellsyoutotype/helpintro.Let'sdoit.Itwillprintoutashorttextaboutwhatjshellis,asshowninthefollowingcode:

jshell>/helpintro

|

|intro

|

|ThejshelltoolallowsyoutoexecuteJavacode,gettingimmediateresults.

|YoucanenteraJavadefinition(variable,method,class,etc),like:intx=8

|oraJavaexpression,like:x+x

|oraJavastatementorimport.

|TheselittlechunksofJavacodearecalled'snippets'.

|

|Therearealsojshellcommandsthatallowyoutounderstandand

|controlwhatyouaredoing,like:/list

|

|Foralistofcommands:/help

Okay,sowecantypeJavasnippetsand/list,butthatisonlyoneexampleoftheavailablecommands.Wecanhopeformoreinformationbytyping/help,asshowninthefollowingcode:

jshell>/help

|TypeaJavalanguageexpression,statement,ordeclaration.

|Ortypeoneofthefollowingcommands:

|/list[<nameorid>|-all|-start]--listthesourceyouhavetyped

|/edit<nameorid>--editasourceentryreferencedbynameorid

|/drop<nameorid>--deleteasourceentryreferencedbynameorid

|/save[-all|-history|-start]<file>--Savesnippetsourcetoafile.

...

Whatyougetisalonglistofcommands.Mostofitisnotpresentedheretosavepaperandyourattention.Wewillusemanyofthesecommandsonourjourneythroughthenextfewpages.Let'sstartwithasmallJavasnippetthatistheagelessHelloWorldexample:

jshell>System.out.println("HelloWorld!")

HelloWorld!

ThisistheshortesteverHelloWorldprograminJava.TillJava9,ifyouwantedtodonothingmorethanprintoutHelloWorld!,youhadtocreateaprogramfile.Ithadtocontainthesourcecodeofaclassincludingthepublicstaticmainmethod,whichcontainedtheonelinewehadtotypeinwithJava9jshell.Itwascumbersomejustforasimpleprintoutofsamplecode.Nowitismucheasierandjshellisalsolenient,forgivingusthemissingsemicolonattheendoftheline.

Thenextthingweshouldtryisdeclaringavariable,asfollows:

jshell>inta=13

a==>13

jshell>

Wedeclaredavariable,nameda,andassignedthevaluetoit-13.Thetypeofthevariableisint,whichisanabbreviationforintegertypesinJava.Nowwehavethisvariablealreadyinoursnippet,sowecanprintitoutifwewanttoasshown:

jshell>System.out.println(a)

13

Itistimetowritesomethingmorecomplexintojshellthanaoneliner.

jshell>voidmain(String[]args){

...>System.out.println("HelloWorld")

...>}

|Error:

|';'expected

|System.out.println("HelloWorld")

|

Thejshellrecognizesthatthisisnotaone-linerandthatitcannotprocesswhatwetypedsofar,whenwepressEnterattheendofthefirstline,anditsignalsthatitexpectsmorecharactersfromus,soitdisplays...>asacontinuationprompt.Wetypeinthecommandsthatmakeupthewholehelloworldmainmethod,butthistimejshelldoesnotletusmissthesemicolon.Thatisallowedonlyinthecaseofone-linesnippets.Asjshellisinteractive,itiseasytocorrectthemistake;presstheuparrowkeyafewtimestogetbackthepreviouslinesand,thistime,addthesemicolonattheendofthesecondline:

jshell>voidmain(String[]args){

...>System.out.println("HelloWorld");

...>}

|createdmethodmain(String[])

Thismethodwascreatedforusasasnippetandnowwecancallit:

jshell>main(null)

HelloWorld

Anditworks.Youcanlistallthesnippetsthatwerecreated,asfollows:

jshell>/list

1:System.out.println("HelloWorld!")

2:inta=13;

3:System.out.println(a)

4:voidmain(String[]args){

System.out.println("HelloWorld");

}

And,aswewanttogoonwritingafullJavaversionofhelloworld,wecansaveourworkfromjshelltoafile,asfollows:

jshell>/saveHelloWorld.java

Finally,weexitedfromjshellbytyping/exit.Asyougetbacktothesystemprompt,typecatHelloWorld.java(ortypeHelloWorld.javaonWindows)toseethecontentofthefile.Itisasfollows:

$catHelloWorld.java

System.out.println("HelloWorld!")

inta=13;

System.out.println(a)

voidmain(String[]args){

System.out.println("HelloWorld");

}

Thefilecontainsallthesnippetsthatwetypedinoneaftertheother.Ifyouthinkthatyouhavemesseduptheshellwithlotsofvariablesandcodesnippetsthatyoudonotneedanymore,youcanissuethe/resetcommand:

jshell>/reset

|Resettingstate.

Afterthiscommand,thejshellisascleanaswhenitwasstartedearlier

jshell>/list

jshell>

Listingjustdoesnotproduceanythingaswedeleteditall.Fortunately,wesavedthestateofjshelltoafileandwecanalsoloadthecontentofthefileissuingthe/opencommand:

jshell>/openHelloWorld.java

HelloWorld!

13

ItloadsthelinefromthefileandexecutesitjustasthecharactersweretypedintotheCommandPrompt.

Youmayrecallthatthe/listcommandprintedanumberinfrontofeachsnippet.Wecanuseittoeditthesnippetsindividually.Todoso,issuethe/editcommandfollowedbythenumberofthesnippet:

jshell>/edit1

YoumayrecallthatthefirstcommandweenteredwastheSystem.out.printlnsystemcallthatprintsouttheargumenttotheconsole.WhenyoupressEnterafterthe/edit1command,youdonotgetthepromptback.Instead,jshellopensaseparategraphicaleditorthatcontainsthesnippettoeditasshowninthefollowingimage:

Editthetextintheboxsothatitwilllooklikethis:

printf("HelloWorld!")

ClickonAcceptandthenExit.WhenyouclickonAccept,theTerminalwillexecutethesnippetanddisplaythefollowingresult:

HelloWorld!

Themethodthatweused,printf,standsforformattedprinting.Thismaybewellknownfrommanyotherlanguages.ItwasfirstintroducedbytheClanguageandthoughcryptic,thenamesurvived.ThisisalsopartofthestandardJavaclass,PrintStream,justlikeprintln.Incaseofprintln,wehadtowriteSystem.outinfrontofthemethodname.Incaseofprintf,wedidnot.Why?

Thereasonisthatjshelldefinesafewsnippetsthatareautomaticallyloadedwhenjshellstartsorresets.Youcanseetheseifyouissuethe/listcommandwiththe-startoption,asfollows:

jshell>/list-start

s1:importjava.util.*;

s2:importjava.io.*;

s3:importjava.math.*;

s4:importjava.net.*;

s5:importjava.util.concurrent.*;

s6:importjava.util.prefs.*;

s7:importjava.util.regex.*;

s8:voidprintf(Stringformat,Object...args){System.out.printf(format,args);}

Thesepredefinedsnippetshelptheuseofjshell.Mostoftheuserswillimporttheseclasses,andtoeasetheprinttoscreen,itdefinesamethodsnippetthathappenstohavethename,printf,whichisalsothenameofamethodinthePrintStreamclass.

Ifyouwanttolistallthesnippetsyouenteredaswellasthepredefinedsnippetsandalsothosethatcontainedsomeerrorandthuswerenotexecuted,youcanusethe-alloptiontothe/listcommand,asfollows:

jshell>/list-all

...

s7:importjava.util.regex.*;

...

1:System.out.println("HelloWorld!")

...

e1:System.out.println("HelloWorld!")

inta=14;

5:System.out.println("HelloWorld!");

...

Someofthelinesweredeletedfromtheactualoutputforbrevity.Thelinesthatarepreloadedarenumberedwiththesprefix.Thesnippetsthatcontainanerrorhaveanumberprefixedwithe.

Ifyouwanttoexecutesomeofthesnippetsagain,youonlyhavetotype/nwherenisthenumberofthesnippet,asfollows:

jshell>/1

System.out.println("HelloWorld!")

HelloWorld!

Youcannotre-executethepreloadedsnippetsorsnippetsthatcontainederrors.Thereisnoneedforanyofthoseanyway.Preloadedsnippetsdeclaresomeimportsanddefineasnippetmethod;erroneoussnippetsdonotexecutebecausetheyare,well...erroneous.

Youneednotrelyonthenumberofjshellwhenyouwanttore-executeasnippet.Whenyoualreadyhavealotofsnippetsinyourjshellsession,listingthemallwouldbetoocumbersome;thereisashortcuttore-executethelastn-thsnippet.Youhavetowrite/-n.Here,nisthenumberofthesnippetcountingfromthelastone.So,ifyouwanttoexecutetheverylastsnippet,thenyouhavetowrite/-1.Ifyouwanttoexecutetheonebeforethelastone,youhavetowrite/-2.Notethatifyoualreadytyped/-1,thenthelastoneisthere-executionofthelastsnippetandsnippetnumber-2willbecomenumber-3.

Listingallthesnippetscanalsobeavoidedinotherways.Whenyouareinterestedonlyincertaintypesofsnippets,youcanhavespecialcommands.

Ifwewanttoseeonlythevariablesthatwedefinedinthesnippets,thenwecanissuethe/varscommand,asfollows:

jshell>/vars

|inta=13

Ifwewanttoseeonlytheclasses,thecommand/typeswilldothat:

jshell>classs{}

|createdclasss

jshell>/types

|classs

Here,wejustcreatedanemptyclassandthenwelistedit.

Tolistthemethodsthatweredefinedinthesnippets,the/methodscommandcanbeissued:

jshell>/methods

|printf(String,Object...)void

|main(String[])void

Youcanseeintheoutputthatthereareonlytwomethods,whichareasfollows:

printf:Thisisdefinedinapreloadedsnippetmain:This,wedefined

Ifyouwanttoseeeverythingyoutyped,youhavetoissuethe/historycommandforallthesnippetsandcommandsthatyoutyped.(Iwillnotcopytheoutputhere;Idonotwanttoshamemyself.Youshouldtryyourselfandseeyourownhistory.)

Recallthatwecandeleteallthesnippetsissuingthe/resetcommand.Youcanalsodeletesnippetsindividually.Todoso,youshouldissuethe/dropncommand,wherenisthesnippednumber:

jshell>/drop1

|Thiscommanddoesnotacceptthesnippet'1':System.out.println("HelloWorld!")

|See/types,/methods,/vars,or/list

Oops!Somethingwentwrong.Thereisnothingdefinedwhensnippetnumber1wasexecutedandthe/drop

commandactuallydropsthedefinedvariable,type,ormethod.Thereisnothingtobedroppedinthefirstsnippet.But,ifwereissuethe/listcommand,wewillgetthefollowingresults:

jshell>/list

1:System.out.println("HelloWorld!")

2:inta=13;

3:System.out.println(a)

4:voidmain(String[]args){

System.out.println("HelloWorld");

}

Wecanseethatwecandropthesecondorthefourthsnippet,too:

jshell>/drop2

|droppedvariablea

jshell>/drop4

|droppedmethodmain(String[])

Thejshellerrormessagesaystoseetheoutputofthe/types,/methods,/vars,or/listcommands.Theproblemwiththisisthat/types,/methods,and/varsdonotdisplaythenumberofthesnippet.ThisismostprobablyasmallbuginthejshellprereleaseversionandmaybefixedbythetimetheJDKisreleased.

Whenwewereeditingthesnippets,jshellopenedaseparategraphicaleditor.Itmayhappenthatyouarerunningjshellusingsshonaremoteserverandwhereitisnotpossibletoopenaseparatewindow.Youcansettheeditorusingthe/setcommand.Thiscommandcansetquiteafewconfigurationoptionsofthejshell.Tosettheeditortousetheubiquitousvi,issuethefollowingcommand:

jshell>/seteditor"vi"

|Editorsetto:vi

Afterthis,jshellwillopenthesnipped-inviinthesameTerminalwindowwhereyouissuethe/editcommand.

Itisnotonlytheeditorthatyoucanset.Youcansetthestartupfile,andthewayjshellprintsthefeedbacktotheconsoleafteracommandwasexecuted.

Ifyousetthestartupfile,thenthecommandslistedinthestartupfilewillbeexecutedinsteadofthebuilt-incommandsofjshellafterthe/resetcommand.Thisalsomeansthatyouwillnotbeabletousetheclassesdirectlythatareimportedbydefaultandyouwillnothavetheprintfmethodsnippet,unlessyourownstartupfilecontainstheimportsandthedefinitionofthesnippet.

Createthesample.startupfilewiththefollowingcontent:

voidprintln(Stringmessage){System.out.println(message);}

Startingupanewjshellandexecutingitisdoneasfollows:

jshell>/setstartsample.startup

jshell>/reset

|Resettingstate.

jshell>println("wuff")

wuff

jshell>printf("Thiswon'twork...")

|Error:

|cannotfindsymbol

|symbol:methodprintf(java.lang.String)

|printf("Thiswon'twork...")

|^----^

Theprintlnmethodisdefinedbuttheprintfmethod,whichwasdefinedinthedefaultstartup,isnot.

Thefeedbackdefinesthepromptjshellprintsandthenwaitsfortheinput,thepromptforthecontinuationlines,andthemessagedetailsaftereachcommand.Therearepredefinedmodes,whichareasfollows:

NormalSilentConciseVerbose

Normalisselectedbydefault.Ifyouissue/setfeedbacksilent,thenpromptbecomes->andjshellwillnotprintdetailsaboutthecommands.The/setfeedbackconcisecodeprintsabitmoreinformationand/setfeedbackverboseprintsverboseinformationaboutthecommandsexecuted:

jshell>/setfeedbackverbose

|Feedbackmode:verbose

jshell>intz=13

z==>13

|modifiedvariablez:int

|updateoverwrotevariablez:int

Youcanalsodefineyourownmodes,givinganametothenewmodeusingthe/setmodexyzcommandwherexyzisthenameofthenewmode.Afterthis,youcansetprompt,truncation,andformatforthemode.Whentheformatisdefined,youcanuseitthesamewayasthebuilt-inmodes.

Last,butnotleast,themostimportantcommandofjshellis/exit.Thiswilljustterminatetheprogramandyouwillreturntotheoperatingsystemshellprompt.

Now,let'sedittheHelloWorld.javafiletocreateourfirstJavaprogram.Todoso,youcanusevi,notepad,Emacs,orwhateverisavailableonyourmachineandfitsyou.Lateron,wewillusesomeintegrateddevelopmentenvironment(IDE),NetBeans,Eclipse,orIntelliJ;however,fornow,asimpletexteditorisenough.

Editthefilesothatthecontentwillbeasfollows:

publicclassHelloWorld{

publicstaticvoidmain(String[]args){

System.out.println("HelloWorld");

}

}

Tocompilethesourcecodetobytecode,whichisexecutablebyJVM,wehavetousetheJavacompilernamedjavac:

javacHelloWorld.java

Thisgeneratesthejava.classfileinthecurrentdirectory.Thisisacompiledcodethatcanbeexecutedasfollows:

$javaHelloWorld

HelloWorld

Withthisone,youhavecreatedandexecutedyourfirstfullJavaprogram.Youmaystillwonderwhatweweredoing.Howandwhy,Iwillexplainit;butfirst,Iwantedyoutohaveafeelingthatitworks.

Thefileweeditedcontainedonlythesnippetandwedeletedmostofthelines,exceptthedeclarationofthemainmethodandweinsertedthedeclarationoftheclassaroundit.

InJava,youcannothavestandalonemethodsorfunctions,likeinmanyotherlanguages.Everymethodbelongstosomeclassandeveryclassshouldbedeclaredinaseparatefile(well,almost,butfornow,let'sskiptheexceptions).Thenameofthefilehastobethesameasthenameoftheclass.Thecompilerrequiresthisforpublicclasses.Evenfornon-publicclassesweusuallyfollowthisconvention.IfyourenamedthefilefromHelloWorld.javatoHello.java,thecompilerwilldisplayanerrorwhenyoutrytocompilethefilewiththenewname.

$mvHelloWorld.javaHello.java

~/Dropbox/java_9-by_Example$javacHello.java

Hello.java:2:error:classHelloWorldispublic,shouldbedeclaredinafilenamedHelloWorld.java

publicclassHelloWorld{

^

1error

So,let'smoveitbacktotheoriginalname:mvHello.javaHelloWorld.java.

Thedeclarationoftheclassstartswiththekeywordclass,thenthenameoftheclass,anopeningcurlybrace,andlastsuntilthematchingclosingbrace.Everythinginbetweenbelongstotheclass.

Fornow,let'sskipwhyIwrotepublicinfrontoftheclassandfocusonthemainmethodinit.Themethoddoesnotreturnanyvalue,therefore;thereturnvalueofitisvoid.Theargument,namedargs,isastringarray.WhenJVMstartsthemainmethod,itpassesthecommand-lineargumentstotheprograminthisarray.However,thistimewedonotuseit.ThemainmethodcontainsthelinethatprintsoutHelloWorld.Now,let'sexaminethislineabitmore.

Inotherlanguages,printingsomethingtotheconsolerequiresonlyaprintstatementoraverysimilarcommand.IrememberthatsomeBASICinterpretersevenallowedustotype?insteadofprintbecauseprintingtothescreenwassocommon.Thishaschangedalotduringthelast40years.Weusegraphicalscreens,Internet,andmanyotherinputandoutputchannels.Thesedays,itisnotverycommontowritetotheconsole.

Usually,inprofessionallarge-scaleenterpriseapplications,thereisnotevenasinglelinethatdoesthat.Instead,wewilldirectthetexttologfiles,sendmessagestomessagequeues,andsendrequestsandreplywithresponsesoverTCP/IPprotocol.Asthisissoinfrequentlyused,thereisnoreasontocreateashortcutforthepurposeinthelanguage.Afterthefirstfewprograms,whenyougetacquaintedwiththedebuggerandloggingpossibilities,youwillnotprintanythingdirectlytotheconsoleyourself.

Still,Javahasfeaturesthatletyousendtextdirectlytothestandardoutputofaprocessthegoodoldway,

asitwasinventedoriginallyforUNIX.ThisisimplementedinaJavawaywhereeverythinghastobeanobjectorclass.Togetaccesstothesystemoutput,thereisaclassnamedSystemandit,amongotherthings,hasthefollowingthreevariables:

in:Thisisthestandardinputstreamout:Thisisthestandardoutputstreamerr:Thisisthestandarderrorstream

Torefertotheoutputstreamvariable,becauseitisnotinourclassbutinSystem,wewillhavetospecifytheclassnamesowewillrefertoitasSystem.outinourprogram.ThetypeofthisvariableisPrintStream,whichisalsoaclass.ClassandtypearesynonymsinJava.EveryobjectthatisoftypePrintStreamhasamethodnamedprintlnthatacceptsaString.Iftheactualprintstreamisthestandardoutput,andweareexecutingourJavacodefromthecommandline,thenthestringissenttotheconsole.

ThemethodisnamedmainandthisisaspecialnameinJavaprograms.WhenwestartaJavaprogramfromthecommandline,JVMinvokesthemethodnamedmainfromtheclassthatwespecifyonthecommandline.Itcandothatbecausewedeclaredthismethodpublicsothatanyonecanseeandinvokeit.Ifitwasprivate,itwouldbeseenandcallableonlyfromwithinthesameclass,orclasses,thataredefinedinthesamesourcefile.

Themethodisalsodeclaredasstatic,whichmeansthatitcanbeinvokedwithoutanactualinstanceoftheclassthatcontainsthemethods.Usingstaticmethodsisusuallyseenasnotagoodpracticethesedays,unlesstheyareimplementingfunctionsthatcannotreallyeverberelatedtoaninstance,orhavedifferentimplementationssuchasthefunctionsinthejava.lang.Mathclass;but,somewhere,thecodeexecutionhastostartandtheJavaruntimewillnotusuallycreateinstancesofclassesforusautomatically.

Tostartthecode,thecommandlineshouldbeasfollows:

java-cp.HelloWorld

The-cpoptionstandsforclasspath.Theclasspathisafairlycomplexideaforjavabut,fornow,let'smakeitsimpleandsaythatitisalistofdirectoriesandJARfilesthatcontainourclasses.Thelistseparatorfortheclasspathis:(colon)onUNIX-likesystemsand;(semicolon)onWindows.Inourcase,theclasspathistheactualdirectory,asthatistheplacewheretheJavacompilercreatedHelloWorld.class.Ifwedonotspecifyclasspathonthecommandline,Javawillusethecurrentdirectoryasadefault.Thatisthereasonourprogramwasworkingwithoutthe-cpoptioninthefirstplace.

Bothjavaandjavachandlemanyoptions.Togetalistoftheoptionstypejavac-helporjava-help.WeusetheIDEtoeditthecodeand,manytimes,tocompile,build,andrunitduringdevelopment.Theenvironmentinthiscasesetsthereasonableparameters.Forproductionweusebuildtoolsthatalsosupporttheconfigurationoftheenvironment.Becauseofthis,werarelymeetthesecommandlineoptions.Nevertheless,professionalshavetounderstandtheirmeaningsatleastandknowwheretolearntheiractualuseincaseitisneeded.

LookingatthebytecodeTheclassfileisabinaryfile.ThemainroleofthisformatistobeexecutedbytheJVMandtoprovidesymbolicinformationfortheJavacompilerwhenacodeusessomeoftheclassesfromalibrary.WhenwecompileourprogramthatcontainsSystem.out.println,thecompilerlooksatthecompiled.classfilesandnotatthesourcecode.IthastofindtheclassnamedSystem,thefieldnamedout,andthemethodprintln.Whenwedebugapieceofcodeortrytofindoutwhyaprogramdoesnotfindaclassormethod,wewillneedawaytolookintothebinaryofthe.classfiles.Thisisnotaneverydaytaskandittakessomeadvancedknowledge

Todoso,thereisadecompilerthatcandisplaythecontentofa.classfileinamoreorlessreadableformat.Thiscommandiscalledjavap.Toexecuteit,youcanissuethefollowingcommand:

$javapHelloWorld.class

Compiledfrom"HelloWorld.java"

publicclassHelloWorld{

publicHelloWorld();

publicstaticvoidmain(java.lang.String[]);

}

TheoutputoftheprogramshowsthattheclassfilecontainsJavaclassthathassomethingcalledHelloWorld();itseemstobeamethodhavingthesamenameastheclassanditalsocontainsthemethodwehavewritten.

Themethodthathasthesamenameastheclassistheconstructoroftheclass.Aseveryclassinjavacanbeinstantiated,thereisaneedforaconstructor.Ifwedonotgiveone,thentheJavacompilerwillcreateoneforus.Thisisthedefaultconstructor.Thedefaultconstructordoesnothingspecialbutreturnsanewinstanceoftheclass.Ifweprovideaconstructoronourown,thentheJavacompilerwillnothavebotheredcreatingone.

ThejavapdecompilerdoesnotshowwhatisinsidethemethodsorwhatJavacodeitcontainsunlessweprovidethe-coption:

$javap-cHelloWorld.class

Compiledfrom"HelloWorld.java"

publicclassHelloWorld{

publicHelloWorld();

Code:

0:aload_0

1:invokespecial#1//Methodjava/lang/Object."<init>":()V

4:return

publicstaticvoidmain(java.lang.String[]);

Code:

0:getstatic#2//Fieldjava/lang/System.out:Ljava/io/PrintStream;

3:ldc#3//Stringhali

5:invokevirtual#4//Methodjava/io/PrintStream.println:(Ljava/lang/String;)V

8:return

}

Itisverycrypticandisnotforordinaryhumans.Onlyafewexperts,whodealwiththeJavacodegeneration,canfluentlyreadthat.But,tohavealookatithelpsyougetaglimpseofwhatbytecodemeans.Itissomethinglikeagoodoldassembly.Althoughthisisbinarycode,thereisnothingsecretinit:Javaisopensource,theclassfileformatiswelldocumentedanddebuggablefortheexperts.

PackagingclassesintoaJARfileWhenyoudeliveraJavaapplication,usuallythecodeispackagedintoJAR,WAR,EAR,orsomeotherpackagedformat.Welearnsomethingagainthatseemstobeobscureatfirstsight,butinreality,thisisnotthatcomplex.TheyareallZIPfiles.YoucanopenanyofthesefilesusingWinZiporsomeotherzipmanagerthatyouhavealicensefor.Theextrarequirementisthat,forexample,inthecaseofaJARfile,thearchiveshouldcontainadirectorynamedMETA-INFandinsideitafilenamedMANIFEST.MF.Thisfileisatextfileandcontainsmetainformationintheformat,whichisasfollows:Manifest-Version:1.0Created-By:9-ea(OracleCorporation)

Therecanbealotofotherinformationinthefile,butthisistheminimumthattheJavaprovidedtooljarputsthereifwepackageourclassfileintoajarissuingthefollowingcommand:

jar-cfhello.jarHelloWorld.class

The-coptiontellstheJARarchivertocreateanewJARfileandtheoptionfisusedtospecifythenameofthenewarchive.Theonewespecifiedhereishello.jarandthefileaddedtoitistheclassfile.

ThepackagedJARfilecanalsobeusedtostarttheJavaapplication.JavacanreaddirectlyfromJARarchivesandloadclassesfromthere.Theonlyrequirementisthattheyareontheclasspath.

Notethatyoucannotputindividualclassesontheclasspath,onlydirectories.AsJARfilesarearchiveswithaninternaldirectorystructureinthem,theybehavelikeadirectory.

CheckthattheJARfilewascreatedusinglshello.jarandremovethermHelloWorld.classclassfilejusttoensurethatwhenweissuethecommandline,thecodeisexecutedfromtheJARfileandnottheclass.

$java-cphello.jarHelloWorld

HelloWorld

ToseethecontentoftheJARfile,however,itisrecommendedthatyouusetheJARtoolandnotWinZipeventhoughthatmaybecozier.RealprofessionalsusetheJavatoolstohandleJavafiles.

$jar-tfhello.jar

META-INF/

META-INF/MANIFEST.MF

HelloWorld.class

ManagingtherunningJavaapplicationTheJavatoolsetthatcomeswiththeJDKsupportstheexecutionandmanagementofrunningJavaapplicationsaswell.Tohavesomeprogramthatwecanmanagewhileexecuting,wewillneedacodethatrunsnotonlyforafewmillisecondsbut,whileitruns,italsoprintssomethingtotheconsole.Let'screateanewprogramcalledHelloWorldLoop.javawiththefollowingcontent:

publicclassHelloWorldLoop{

publicstaticvoidmain(String[]args){

for(;;){

System.out.println("HelloWorld");

}

}

}

Theprogramcontainsaforloop.Loopsallowrepeatedexecutionofacodeblock,andwewilldiscusstheminChapter2,TheFirstRealJavaProgram-SortingNames.Theloopwecreatedhereisaspecialonethatneverterminatesbutrepeatstheprintingmethodcall,printingHelloWorlduntilwekilltheprogrambypressingCtrl+corissuingakillcommandonLinuxoronOSX,orterminatetheprograminthetaskmanagerunderWindows.

CompileandstartitinonewindowandopenanotherTerminalwindowtomanagetheapplication.

Thefirstcommandthatweshouldgetfamiliarwithisjps.http://docs.oracle.com/javase/7/docs/technotes/tools/share/jps.htmlItliststheJavaprocessesthatrunonthemachine,whichareasfollows:

$jps

21873sun.tools.jps.Jps

21871HelloWorldLoop

Youcanseethattherearetwoprocesses—oneistheprogramweexecuteandtheotheristhejpsprogramitself.Notsurprisingly,thejpstoolisalsowritteninJava.Youcanalsopassoptionstojps,whicharedocumentedontheweb.

Therearemanyothertoolsandwewillexamineoneofthem,whichisaverypowerfulandeasy-to-usetool—JavaVisualVM.

VisualVMisacommand-linegraphicaltoolthatconnectstotherunningJavaprocessanddisplaysthedifferentperformanceparameters.TostarttheVisualVMtool,youwillissuethejvisualvmcommandwithoutanyparameters.Soon,awindowwillappearwithanexploringtreeontheleft-handsideandawelcomepaneontheright.TheleftsideshowsalltherunningJavaprocessesunderthebranchnamedLocal.IfyoudoubleclickonHelloWorldLoop,itwillopenthedetailsoftheprocessontherightpane.Ontheheadertabs,youcanselectOverview,Monitor,Threads,Sampler,andProfiler.ThefirstthreetabsarethemostimportantandgiveyouagoodviewofwhatishappeninginJVMregardingthenumberofthreads,CPUusage,memoryconsumption,andsoon.

UsinganIDEIntegrateddevelopmentenvironmentsareoutstandingtoolsthathelpthedevelopmentbyoffloadingthemechanicaltasksfromthedeveloper'sshoulders.Theyrecognizemanyoftheprogrammingerrorsaswetypethecode,helpusfindtheneededlibrarymethods,displaythedocumentationofthelibraries,andprovideextratoolsforstylechecking,debugging,andsoon.

Inthissection,wewilllookatsomeIDEsandhowtoleveragethefunctionstheyprovide.

TogetanIDE,youwillhavetodownloadandinstallit.ItdoesnotcomewiththeJavadevelopmenttoolsbecausetheyarenotpartofthelanguageenvironment.But,don'tworry.Theycanbedownloadedfreeofchargeandareeasytoinstall.Theymaybemorecomplextostartupthananotepadeditor,butevenafterafewhoursofwork,theywillpaybackthetimeyoudevotetolearningthem.Afterall,itisnotwithoutreasonthatnodeveloperiscodingJavainnotepadorvi.

ThethreetopmostIDEsareNetBeans,Eclipse,andIntelliJ.Allareavailableincommunityversions,whichmeansthatyouneednotpayforthem.IntelliJhasafullversionthatyoucanalsobuy.Thecommunityeditionwillbeusableforlearningthelanguage.IncaseyoudonotlikeIntelliJ,youcanuseEclipseorNetBeans.Theseareallfreeofcharge.Personally,IusetheIntelliJcommunityeditionformostofmyprojectsandthescreensamplesthatshowanIDEinthisbookwillfeaturethisIDE.But,itdoesnotnecessarilymeanthatyouhavetosticktothisIDE.

Inthedevelopercommunity,therearetopicsthatcanbeheavilydebated.Thesetopicsareaboutopinions.Weretheyaboutfactsthedebatewouldeasilybesoonover.Onesuchtopicis:"WhichisthebestIDE?"Itisamatteroftaste.Thereisnodefiniteanswer.Ifyoulearnhowtouseone,youwilllikethatandyouwillbereluctanttolearnanotherone,unlessyouseethattheotheroneissomuchbetter.ThatisthereasondeveloperslovetheIDEtheyuse(orjusthate,dependingontheirpersonality),buttheykeepusingthesameIDEusuallyforalongtime.ThereisnobestIDE.

TodownloadtheIDEofyourchoice,youcanvisiteitheroneofthefollowingwebsites:

https://netbeans.org/forNetBeanshttp://www.eclipse.org/forEclipsehttps://www.jetbrains.com/idea/forIntelliJ

NetBeans

NetBeansissupportedbyOracleandiscontinuouslydeveloped.Itcontainscomponents,suchastheNetBeansprofiler,thatbecamepartoftheOracleJavadistribution.YoumaynoticethatwhenyoustartVisualVMandstarttheprofiling,theJavaprocessstartedhasnetbeansinitsname.

Generally,NetBeansisaframeworktodeveloprichclientapplicationsandtheIDEisonlyoneapplicationofthemanythatarebuiltontopoftheframework.Itsupportsmanylanguages,notonlyJava.YoucandevelopPHP,C,orJavaScriptcodeusingNetBeansandhavesimilarservicesforJava.Forthesupportofdifferentlanguages,youcandownloadpluginsoraspecialversionofNetBeans.ThesespecialversionsareavailablefromthedownloadpageoftheIDEandtheyarenothingmorethanthebasicIDEwithsomepreconfiguredplugins.IntheCpackage,thedevelopersconfigurethepluginsthatareneededwhenyouwanttodevelopC;inthePHPversion,theypluginforPHP.

Eclipse

EclipseissupportedbyIBM.SimilartoNetBeans,itisalsoaplatformforrichclientapplicationanditisbuiltaroundtheOSGicontainerarchitecture,whichitselfisatopicthatcanfillabooklikethis.MostofthedevelopersuseEclipseand,almostexclusively,itisthechoicewhendeveloperscreatecodefortheIBMWebSphereapplicationserver.TheEclipsespecialversioncontainsadeveloperversionofWebSphere.

EclipsealsohaspluginstosupportdifferentprogramminglanguagesandalsohasdifferentvariationssimilartoNetBeans.ThevariationsarepluginsprepackagedwiththebasicIDE.

IntelliJ

ThelastoneintheprecedingenumerationisIntelliJ.ThisIDEistheonlyonethatdoesnotwanttobeaframework.IntelliJisanIDE.Italsohasplugins,butmostofthepluginsthatyouwillneedtodownloadtouseinNetBeansorEclipsearepreconfigured.Whenyouwanttousesomemoreadvancedplugin,itmayhoweverbesomethingyouhavetopayfor,whichshouldnotbeaproblemwhenyouaredoingprofessional,paidwork,shouldit?Thesethingsarenotthatexpensive.Forlearningthesubjectsinthisbook,youwillnotneedanypluginthatisnotinthecommunityedition.Asinthisbook,IwilldevelopthesamplesusingIntelliJandIrecommendthatyoufollowmeduringyourlearningexperience.

IwanttoemphasizethattheexamplesinthisbookareindependentoftheactualIDEtobeused.YoucanfollowthebookusingNetBeans,Eclipse,orevenEmacs,notepad,orvi.

IDEservices

Integrateddevelopmentenvironmentsprovideuswithservices.Themostbasicserviceisthatyoucaneditfileswiththem,buttheyalsohelpbuildthecode,findbugs,runthecode,deploytotheapplicationserverindevelopmentmode,debug,andsoon.Inthefollowingsections,wewilllookatthesefeatures.IwillnotgiveanexactandpreciseintroductiononhowtouseoneortheotherIDE.Abooklikethisisnotagoodmediumforsuchatutorial.

IDEsdifferonmenuplacement,keyboardshortcuts,andtheymayevenchangeasnewerversionsarereleased.ItisbesttolookattheactualIDEtutorialvideooronlinehelp.Theirfeatures,ontheotherhand,areverysimilar.IntelliJhasthevideodocumentationathttps://www.jetbrains.com/idea/documentation/.

IDEscreenstructureThedifferentIDEslooksimilar,andhavethesamescreenstructuremoreorless.Inthefollowingscreenshot,youcanseeanIntelliJIDE:

Ontheleftside,youcanseethefilestructureofaJavaproject.AJavaprojecttypicallycontainsmanyfilesindifferentdirectorieswhichwewilldiscussinthenextchapter.ThesimpleHelloWorldapplicationcontainsapom.xmlprojectdescriptionfile.ThisfileisneededfortheMavenbuildtool,whichisalsoatopicforthenextchapter.Fornow,youshouldonlyknowthatitisafilethatdescribestheprojectstructureformaven.TheIDEalsokeepstrackofsomeadministrativedataforitself.ItisstoredinHelloWorld.iml.Themainprogramfileisstoredinthesrc/main/javadirectoryandnamedHelloWorld.java.

Ontherightside,youcanseethefiles.Inthescreenshot,wehaveonlyonefileopened.Incasethereismorethanonefileopened,thentherearetabs-oneforeachfile.Now,theactivefileisHelloWorld.javathatcanbeeditedinthesourcecodeeditor.

EditingfilesWhenediting,youcantypeincharactersordeletecharacters,words,andlines,butthisissomethingthatalleditorscando.IDEsofferextra.IDEsanalyzethesourcecodeandformatit,which,inturn,automaticallyindentsthelines.Italsocontinuouslycompilesthecodeinthebackgroundwhileyouedititandifthereissomesyntaxerror,thenitunderlinesitwitharedwaivingline.Whenyoufixtheerror,theredunderliningdisappears.

Theeditoralsoautomaticallygivessuggestionsforfurthercharactersasyoutype.Youcanignorethewindowthatpopsupandcontinuetyping.However,manytimes,itiseasiertostopafteracharacterandusetheupanddownarrowstoselectthewordthatneedsfinishingbeforepressingEnter:thewordwillbeinsertedintothesourcecodeautomatically.

Inthescreenshot,youcanseethatIwroteSystem.oandtheeditorimmediatelysuggestedthatIwantedtowriteout.TheotheralternativesaretheotherstaticfieldsandmethodsthatareintheclassSystemandwhichcontainthelettero.

TheIDEeditorgivesyouhintsnotonlywhenitcantypeforyou,butalsowhenitcannottypeinsteadofyou.Inthescreenshot,theIDEtellsyoutotypesomeexpressionasargumenttotheprintln()methodthatisboolean,char,int,andsoon.TheIDEhasabsolutelynoideawhattotypethere.Youhavetoconstructtheexpression.Still,itcantellyouthatitneedstobeofacertaintype.

Itisnotonlythebuilt-intypesthattheeditorknows.TheeditorintegratedwiththeJDKcontinuouslyscansthesourcefilesandknowswhatclasses,methods,andfieldsarethereinthesourcecodewhichare

usableattheplaceofediting.

Thisknowledgeisalsoheavilyusedwhenyouwanttorenameamethodorvariable.Theoldmethodwastorenamethefieldormethodinthesourcefileandthendoanexhaustivesearchforallreferencestothevariable.UsingtheIDE,themechanicalworkisdonebyit.Itknowsalltheusesofafieldormethodandautomaticallyreplacestheoldidentifierwiththenewone.Italsorecognizeswhetheralocalvariablehappenstohavethesamenameastheonethatwerename,andtheIDEonlyrenamesthoseoccurrencesthatarereallyreferringtotheonewearerenaming.

Youcanusuallydomorethanjustrenaming.Therearemoreorlessmechanicaltasksthatprogrammerscallrefactoring.ThesearesupportedbytheIDEsusingsomekeyboardshortcutandcontextsensitivemenuintheeditor—rightclickonthemouseandclickMenu.

TheIDEalsohelpsyoutoreadthedocumentationofthelibrariesandsourcecodeasshowninthefollowingimage:

LibrariesprovideJavadocdocumentationforthepublicmethodsandyoushouldalsowriteJavadocforyourownmethod.JavadocdocumentationisextractedfromspecialcommentsinthesourcecodeandwewilllearnhowtocreatethoseinChapter4,Mastermind-CreatingaGame.Thesearelocatedincommentsinfrontoftheactualmethodhead.Ascreatingcompileddocumentationispartofthecompilationflow,theIDEalsoknowsthedocumentationanditdisplaysasahoveringboxoverthemethodnames,classnames,orwhateverelementyouwanttouseinthesourcefilewhenyoupositionthecursorontheelement.

ManagingprojectsOntheleftsideoftheIDEwindow,youcanseethedirectorystructureoftheproject.TheIDEknowsthedifferenttypesoffilesandshowstheminawaythatismeaningfulfromtheprogrammingpointofview.Forexample,itdoesnotdisplayMain.javaasafilename.Instead,itdisplaysMainandaniconthatsignalsthatMainisaclass.ItcanalsobeaninterfacestillinafilenamedMain.javabut,inthatcase,theiconwillshowthatthisisaninterface.ThisisagaindonebytheIDEcontinuouslyscanningandcompilingthecode.

ThefilesarestructuredintosubdirectorieswhenwedevelopaJavacode.Thesesubdirectoriesfollowthepackagingstructureofthecode.Manytimes,inJava,weusecompoundandlongpackagenames,anddisplayingitasadeepandnesteddirectorystructurewillnotbesoeasytohandle.

Packagesareusedtogroupthesourcefiles.Thesourcefilesforclassesthatarerelatedinsomewayshouldgointoonepackage.Wewilldiscussthenotionofpackagesandhowtousetheminthenextchapter

TheIDEiscapableofshowingthepackagestructureinsteadofthenesteddirectoriesforthosedirectoriesoftheprojectthatcontainsourcefiles.

Whenyoumoveaclassoraninterfacefromonepackagetoanother,ithappensinasimilarwayasrenamingorotherrefactoring.Allreferencestotheclassorinterfaceinthesourcefilesgetrenamedtothenewpackage.Ifafilecontainsanimportstatementreferringtotheclass,thenameoftheclassinthestatementiscorrected.Tomoveaclass,youcanopenthepackageandusethegoodolddraganddrop.

PackagehierarchyisnottheonlyhierarchydisplayedintheIDE.Theclassesareinpackagesbut,atthesametime,thereisaninheritancehierarchy.Classesmayimplementinterfacesandcanextendotherclasses.TheJavaIDEshelpusbyshowingtypehierarchieswhereyoucannavigateacrossagraphicalinterfacealongtheinheritancerelations.

ThereisanotherhierarchythatIDEscanshowtohelpuswithdevelopment:methodcallhierarchy.Afteranalyzingthecode,theIDEcanshowusthegraphdisplayingtherelationsbetweenthemethods:whichmethodcallswhichothermethods.Sometimes,thiscallgraphisalsoimportantinshowingthe

dependenciesofmethodsoneachother.

BuildthecodeandrunitTheIDEsusuallycompilethecodeforanalysistohelpusspotsyntaxerrorsorundefinedclassesandmethodsonthefly.Thiscompilationisusuallypartial,coveringapartofthecode,andasitrunsallthetime,thesourcecodechangesandisneveractuallycomplete.Tocreatethedeployablefile,thatis,thefinaldeliverablecodeoftheproject,aseparatebuildprocesshastobestarted.MostoftheIDEshavesomebuilt-intoolforthat,butit'snotrecommendedtousetheseexceptforthesmallestprojects.ProfessionaldevelopmentprojectsuseAnt,Maven,orGradleinstead.HereisanexampleofMaven.

TheIDEsarepreparedtousesuchanexternaltool,andtheycanhelpusinstartingthem.Thisway,thebuildprocesscanrunonthedevelopermachinewithoutstartinganewshellwindow.IDEscanalsoimportthesettingsfromtheconfigurationfileoftheseexternalbuildtoolstorecognizetheprojectstructure,wheresourcefilesare,andwhattocompiletosupporttheerrorcheckingwhileediting.

Thebuildingprocessusuallycontainstheexecutionofcertainchecksonthecode.AbunchoftheJavasourcefilemaycompilesmoothlyandthecodemaystillcontainalotofbugsandmaybewritteninbadstyle,whichwillmaketheprojectbecomesunmaintainableinthelongrun.Toavoidsuchproblems,wewilluseunittestsandstaticcodeanalysistools.Thesedonotguaranteeerrorfreecodebutthechancesaremuchbetter.

IDEshavepluginstorunthestaticcodeanalysistoolsaswellasunittests.BeingintegratedintotheIDEhasahugeadvantage.Whenthereisanyproblemidentifiedbytheanalysistool,orbysomeunittests,theIDEprovidesanerrormessagethatalsofunctionslikealinkonawebpage.Ifyouclickonthemessage,usuallyblueandunderlined,exactlylikeonawebpage,theeditoropenstheproblematicfileandplacesthecursorwheretheissueis.

DebuggingJavaDevelopingcodeneedsdebugging.Javahasverygoodfacilitiestodebugcodeduringdevelopment.JVMsupportsdebuggersviatheJavaPlatformDebuggerArchitecture.ThisletsyouexecutecodeindebugmodeandJVMwillacceptexternaldebuggertoolstoattachtoitviaanetwork,oritwilltrytoattachtoadebuggerdependingoncommand-lineoptions.JDKcontainsaclient,thejdbtool,whichcontainsadebugger;however,itissocumbersometousewhencomparedtothegraphicalclientbuiltintotheIDEsthatIhaveneverheardofanyoneusingitforrealwork.

TostartaJavaprogramindebugmodesothatJVMwillacceptadebuggerclienttoattachtheoptionstoit,executethefollowingcommand:-Xagentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=7896

TheXagentliboptioninstructstheJavaruntimetoloadthejdwpagent.Thepartoftheoptionthatfollows-Xagentlib:jdwp=isinterpretedbythedebuggeragent.Theseoptionsareasfollows:

transport:Thisshouldspecifywhichtransporttouse.Itcanbeasharedmemory(dt_shmem)socketoraTCP/IPsockettransportbut,inpractice,youwillalwaysusethelatter.Thisisspecifiedintheprecedingdt_socketsample.server:ThisspecifiesifthedebuggedJVMstartsinservermodeorclientmode.WhenyoustarttheJVMinservermode,itstartstolistenonasocketandacceptsthedebuggertoconnecttoit.Ifitisstartedinclientmode,thenittriestoconnectadebuggerthatissupposedtobestartedinservermode,listeningonaport.Thevalueoftheoptionisymeaningservermodeornmeaningnonserver,a.k.a.clientmode.suspend:Thiscanalsobeyorn.IfJVMisstartedinsuspendmode,itwillnotstarttheJavacodeuntiladebuggerisattachedtoit.Ifitisstartedwithsuspend=n,thentheJVMstartsandwhenadebuggerattaches,itstopsassoonasabreakpointisreached.IfyoustartastandaloneJavaapplication,youwillusuallystartthedebuggingwithsuspend=y,whichisthedefault.Ifyouwanttodebuganapplicationinanapplicationserverorservlet-containerenvironment,thenitisbettertostartwithsuspend=n;otherwise,theserverdoesnotstartuntilthedebuggerattachestoit.StartingtheJavaprocessinsuspend=ymodeincaseservletapplicationisonlyusefulwhenyouwanttodebugtheservletstaticinitializercode,whichisexecutedwhentheserverisstartingup.Withoutsuspendmode,youwillberequiredtoattachthedebuggerveryfast.ItisbetterthatJVMjustwaitsforyouinthatsituation.address:ThisshouldspecifytheaddressthatJVMcommunicateswith.IftheJVMstartedinclientmode,thenitwillstarttoconnecttothisaddress.IftheJVMrunsinservermode,thenitwillacceptconnectionsfromthedebuggeronthataddress.Theaddressmayspecifyonlytheport.Inthiscase,theIPaddressisthatofthelocalmachine.

Theotheroptionsthedebuggeragentmayhandleareforspecialcases.Forthetopicscoveredinthisbook,theprecedingoptionsareenough.

ThefollowingscreenshotshowsatypicaldebuggingsessionwherewedebugthesimplestprograminIntelliJIDE:

WhenyoustartaprogramfromtheIDEindebugmode,alltheseoptionsareautomaticallysetforyou.Youcansetbreakpointjustbyclickingonthesourcecodeintheeditor.Youcanhaveaseparateformtoadd,remove,andeditbreakpoints.Breakpointscanbeattachedtospecificlinesorspecificevents,likewhenanexceptionisthrown.Breakpointsattachedtoaspecificlinecanalsohaveconditionsthattellthedebuggertostoptheexecutionofthecodeonlywhentheconditionistrue;forexample,ifavariablehassomepredefinedvalue.

Summary

InthischapterwewereintroducedtoeachotherwithJava.Wedonotknowtoomuchfromeachotherbutwegotacquainted.WehaveinstalledtheJavaenvironment:Java,JDKandintegrateddevelopmentenvironment.Wehavewrittenasmallprogramandhadabrieflookatwhatcanbedoneusingthedevelopmenttools.Thisisfarfrommasterybuteventhelongestjourneystartswithafirststep,whichissometimesthehardesttomake.WehavedoneitinourJavajourney.Westartedrollingandfortheenthusiaststhatweare,nothingcanstopuswalkingallthewaylong.

TheFirstRealJavaProgram-SortingNames

Inthepreviouschapter,wegotacquaintedwithJava,andespeciallywithusingtheREPLtoolandinteractivelyexecutingsomesimplecode.Thatisagoodstart,butweneedmore.Inthischapter,wewilldevelopasimplesortprogram.Usingthiscodeasanexample,wewilllookatdifferentbuildtools,whicharefrequentlyusedforJavaprojects,andlearnthebasicfeaturesoftheJavalanguage.Thischapterwillcoverthefollowingtopics:

ThesortingproblemTheprojectstructureandbuildtoolsTheMake,Ant,Maven,andGradlebuildtoolsJavalanguagefeaturesrelatedtothecodeexample

GettingstartedwithsortingThesortingproblemisoneoftheoldestprogrammingtasksthatanengineerdealswith.Wehaveasetofrecordsandweknowthatwewanttofindaspecificonesometimelater,andwewanttofindthatonefast.Tofindit,wesorttherecordsinaspecificorderthathelpsusfindtherecordwewantquickly.

Asanexample,wehavethenamesofstudentswiththeirmarksonsomecards.Whenstudentscometotheofficeaskingfortheirresults,welookthroughallofthecardsoneaftertheothertofindthenameoftheenquiringstudent.However,itisbetterifwesortthecardsbythenamesofthestudentsalphabetically.Whenastudentmakesanenquiry,wecansearchthemarkattachedtothenamemuchfaster.

Wecanlookatthemiddlecard;ifitshowsthenameofthestudent,thenwearehappytohavefoundthenameandthemark.Ifthecardprecedesthenameofthestudentalphabetically,thenwewillcontinuesearchinginthesecondhalf;otherwise,wewillcheckthefirsthalf.

Followingthatapproach,wecanfindthenameofthestudentinafewsteps.Thenumberofstepscannotbemorethanthenumberasmanytimesthepackofcardscanbehalved.Ifwehavetwocards,thenitistwostepsatmost.Ifitisfour,thenwewillneedthreestepsatmost.Ifthereareeightcards,thenwemayneedfoursteps,butnotmore.Ifthereare1,000cards,thenwemayneedatmost11steps,whiletheoriginal,non-sortedsetwillneed1,000steps,worstcase.Thatis,approximately,itspeedsupthesearch100times,sothisisworthsortingthecards,unlessthesortingitselftakestoomuchtime.Thealgorithmfindinganelementinthealreadysortedsetwejustdescribediscalledbinarysearch(https://en.wikipedia.org/wiki/Binary_search_algorithm).

Inmanycases,itisworthsortingthedataset,andtherearemanysortingalgorithmstodothat.Therearesimplerandmorecomplexalgorithms,and,asinmanycases,morecomplexalgorithmsaretheonesthatrunfaster.

AswearefocusingontheJavaprogrammingpartandnotthealgorithmforging,inthischapter,wewilldevelopaJavacodethatimplementsasimpleandnot-that-fastalgorithm.

Bubblesort

Thealgorithmthatwewillimplementinthischapteriswell-knownasbubblesort.Theapproachisverysimple.Beginatthestartofthecardsandcomparethefirstandthesecondcard.Ifthefirstcardislaterinlexicographicorderthanthesecondone,thenswapthetwocards.Thenrepeatthisforthecardthatisatthesecondplacenow,thenthethird,andsoon.Thereisacardthatislexicographicallythelatest,sayWilson.Whenwegetthiscardandstarttocompareitwiththenextone,wewillalwaysswapthem;thisway,Wilson'scardwilltraveltothelastplacewhereithastobeafterthesort.Allwehavetodoisrepeatthistravellingfromthestartanddotheoccasionalswappingofcardsagain,butthistimeonlytothelastbutoneelement.Thistime,thesecondlatestelementwillgettoitsplace—say,WilkinsonwillberightbeforeWilson.Ifwehavencards,andwerepeatthisn-1times,allcardswillgettotheirplace.

Inthefollowingsections,wewillcreateaJavaprojectthatimplementsthisalgorithm.

GettingstartedwithprojectstructureandbuildtoolsWhenaprojectismorecomplexthanasingleclass,anditusuallyis,thenitiswisetodefineaprojectstructure.Wewillhavetodecidewherewestorethesourcefiles,wheretheresourcefiles(thosethatcontainsomeresourcefortheprogram,butarenotJavasource)are,wherethe.classfilesshouldbewrittenbythecompiler,andsoon.Generally,thestructureismainlythedirectorysetupandtheconfigurationofthetoolsthatperformthebuild.

Thecompilationofcomplexprogramscannotbefeasiblydoneusingthecommandlineissuingjavaccommands.Ifwehave100Javasourcefiles,thecompilationwillrequirethatmanyjavaccommandstobeissued.Itcanbeshortenedusingwildcards,suchasjavac*.java,orwecanwriteasimplebashscriptoraBATcommandfilethatdoesthat.First,itwillbejust100lines,eachcompilingonesourceJavafiletoclassfile.Then,wewillrealizethatitisonlytime,CPU,andpowerconsumingtocompilethefilesthatarenotchangedsincethelastcompilationssowecanaddsomebashprogrammingthatchecksthetimestamponthesourceandgeneratedfiles.Then,wewillprobablyrealizethat...whatever.Attheend,wewillendupwithatoolthatisessentiallyabuildtool.Buildtoolsareavailablereadymade;itisnotworthreinventingthewheel.

Insteadofcreatingone,wewilluseabuildtoolthatisready.Thereareafewofthemthatcanbefoundathttps://en.wikipedia.org/wiki/List_of_build_automation_software.Inthischapter,wewilluseonecalledMaven;however,beforejumpingintothedetailsofthistool,wewilllookatsomeothertoolsthatyouarelikelytomeetasaJavaprofessionalinenterpriseprojects.

Inthefollowingsections,wewilldiscussabitofthefourbuildtools:

MakeAntMavenGradle

WewillmentionMakeonlybrieflybecauseitisnotusedinJavaenvironmentsthesedays.However,Makewasthefirstbuildtool,andmanyideasthatmodernJavabuildtoolsarebasedoncomefromthegoodoldmake.You,asaprofessionalJavadeveloper,shouldalsobefamiliarwithMakesothatyouwillnotfreakoutifyouhappentoseetheuseofitinaprojectforsomepurpose,andcanknowwhatitisandwhereitsdetaileddocumentationcanbefound.

AntwasthefirstbuildtoolwidelyusedforJavamanyyearsago,anditisstillusedinmanyprojects.

MavenisnewerthanAnt,anditusesadifferentapproach.Wewilllookatitindetail.MavenisalsotheofficialbuildtooloftheApachesoftwarefoundationfortheJavaproject.WewillalsouseMavenasabuildtoolinthischapter.

Gradleisevennewer,andithasstartedtocatchuptoMaventhesedays.Wewillusethistoolinlater

chaptersinmoredetail.

MakeThemakeprogramwasoriginallycreatedinApril1976,sothisisnotanewtool.ItisincludedintheUnixsystem,sothistoolisavailablewithoutanyextrainstallationonLinux,MacOSX,oranyotherUnix-basedsystem.Additionally,therearenumerousportsofthetoolonWindows,andsomeversionis/wasincludedintheVisualStudiocompilertoolset.

TheMakeisnottiedtoJava.ItwascreatedwhenthemajorprogramminglanguagewasC,butitisnottiedtoCoranyotherlanguage.Themakeisadependencydescriptionlanguagethathasaverysimplesyntax.Themake,justlikeanyotherbuildtool,iscontrolledbyaprojectdescriptionfile.Inthecaseofmake,thisfilecontainsaruleset.ThedescriptionfileisusuallynamedMakefile,butincasethenameofthedescriptionfileisdifferent,itcanbespecifiedasacommand-lineoptiontothemakecommand.

RulesinMakefilefolloweachotherandconsistofoneormorelines.Thefirstlinestartsatthefirstposition(thereisnotaborspaceatthestartoftheline)andthefollowinglinesstartwithatabcharacter.Thus,Makefilemaylooksomethinglikethefollowingcode:

run:hello.jar

java-cphello.jarHelloWorld

hello.jar:HelloWorld.class

jar-cfhello.jarHelloWorld.class

HelloWorld.class:HelloWorld.java

javacHelloWorld.java

Thisfiledefinesthreeso-calledtargets:run,hello.jar,andHelloWorld.class.TocreateHelloWorld.class,typethefollowinglineatthecommandprompt:

makeHelloWorld.class

MakewilllookattheruleandseethatitdependsonHelloWorld.java.IftheHelloWorld.classfiledoesnotexist,orHelloWorld.javaisnewerthantheJavaclassfile,makewillexecutethecommandthatiswrittenonthenextlineanditwillcompiletheJavasourcefile.IftheclassfilewascreatedfollowingthelastmodificationofHelloWorld.java,thenmakeknowsthatthereisnoneedtorunthecommand.

InthecaseofcreatingHelloWorld.class,themakeprogramhasaneasytask.Thesourcefilewasalreadythere.Ifyouissuethemakehello.jarcommand,theprocedureismorecomplex.Themakecommandseesthatinordertocreatehello.jar,itneedsHelloWorld.class,whichitselfisalsoatargetonanotherrule.Thus,itmayneedtobecreated.

First,itstartstheproblemthesamewayasbefore.IfHelloWorld.classisthere,andisolderthanhello.jar,thereisnothingtodo.Ifitisnotthere,orisnewerthanhello.jar,thenthejar-cfhello.jarHelloWorld.classcommandneedstobeexecuted,althoughnotnecessarilyatthemomentwhenitrealizesthatithastobeperformed.ThemakeprogramremembersthatthiscommandhastobeexecutedsometimeinthefuturewhenallthecommandsthatareneededtocreateHelloWorld.classarealreadyexecutedsuccessfully.Thus,itcontinuestocreatetheclassfileexactlythesamewayasIdescribedearlier.

Ingeneral,arulecanhavethefollowingformat:

target:dependencies

command

Themakecommandcancreateanytargetusingthemaketargetcommandbyfirstcalculatingwhichcommandstoexecuteandthenexecutingthemonebyone.ThecommandsareshellcommandsexecutinginadifferentprocessandmayposeproblemsunderWindows,whichmayrendertheMakefilefiles'operatingsystemdependent.

Notethattheruntargetisnotanactualfilethatmakecreates.Atargetcanbeafilenameorjustanameforthetarget.Inthelattercase,makewillneverconsiderthetargettobereadilyavailable.

AswedonotusemakeforaJavaproject,thereisnoreasontogetintomoredetails.Additionally,Icheatedabitbymakingthedescriptionofarulesimplerthanitshouldbe.Themaketoolhasmanypowerfulfeaturesoutofthescopeofthisbook.Therearealsoseveralimplementationsthatdifferalittlefromeachother.YouwillmostprobablymeettheonemadebytheFreeSoftwareFoundation—theGNUmake.And,ofcourse,justincaseofanyUnixcommand-linetool,manisyourfriend.Themanmakecommandwilldisplaythedocumentationofthetoolonthescreen.

Themainpointsthatyoushouldrememberaboutmakeareasfollows:

Itdefinesthedependenciesoftheindividualartifacts(targets)inadeclarativewayItdefinestheactionstocreatethemissingartifactsinanimperativeway

Thisstructurewasinventeddecadesagoandhassurvivedupuntilnowformostofthebuildtools,asyouwillseeinthenextfewchapters.

AntTheantbuildtoolwasbuiltespeciallyforJavaprojectsaroundtheyear2000.TheaimofJavatobeawrite-once-run-anywherelanguageneededatoolthatcanalsobeusedindifferentenvironments.AlthoughmakeisavailableonUnixmachines,andWindowsaswell,Makefileswerenotalwayscompatible.Therewasasmallproblemwiththeuseofthetabcharacterthatsomeeditorsreplacedwithspace,renderingMakefileunusable,butthiswasnotthemajorreason.ThemainproblemwithmakethatignitedthedevelopmentofAntisthatthecommandsareshellcommands.Eveniftheimplementationofthemakeprogramwasmadetobecompatibleondifferentoperatingsystems,theusedcommandsweremanytimesincompatible,andthatwassomethingmakeitselfcouldnotchange.Becausemakeissuesexternalcommandstobuildthetargets,developersarefreetouseanyexternaltoolthatisavailableforthemonthedevelopmentmachine.Anothermachineusingthesameoperatingsystemjustmaynothavethesamesetoftoolsinvokedbymake.Thisunderminestheportabilityofthemakebuiltprojects.

Atthesametime,Antisfollowingthemajorprinciplesofmake.Therearetargetsthatmaydependoneachotherandtherearecommandsthatneedtobeexecutedinanappropriatesequencetocreatethetargetsoneaftertheother,followingthedependencyorder.ThedescriptionofthedependenciesandthecommandsisXML(tabissuesolved)andthecommandsareimplementedinJava(systemdependencyissolved,well...moreorless).

AsAntisneitherpartoftheoperatingsystemnortheJDK,youwillhavetodownloadandinstallitseparatelyifyouwanttouseit.

InstallingAntAntcanbedownloadedfromitsofficialwebsite(http://ant.apache.org).Youcandownloadthesourceortheprecompiledversion.Theeasiestwayistodownloadthebinaryinatar.gzformat.

WheneveryoudownloadsoftwarefromtheInternet,itishighlyrecommendedthatyouchecktheintegrityofthedownloadedfile.TheHTTPprotocoldoesnotcontainerrorchecking,anditmayhappenthatanetworkerrorremainshiddenoramalevolentinternalproxymodifiesthedownloadedfile.Downloadsitesusuallyprovidechecksumsforthedownloadablefiles.TheseareusuallyMD5,SHA1,SHA512,orsomeotherchecksums.

WhenIdownloadedtheApacheAnt1.9.7versionintar.gzformat,IalsoopenedthepagethatledtotheMD5checksum.Thechecksumvalueisbc1d9e5fe73eee5c50b26ed411fb0119.

Thedownloadedfilecanbecheckedusingthefollowingcommandline:$md5apache-ant-1.9.7-bin.tar.gz

MD5(apache-ant-1.9.7-bin.tar.gz)=bc1d9e5fe73eee5c50b26ed411fb0119

ThecalculatedMD5checksumisthesameastheoneonthewebsite,whichsaysthatthefileintegrityisnotharmed.OntheWindowsoperatingsystem,notooltocalculateMD5digestisincluded.ThereisatoolthatMicrosoftprovides,calledFileIntegrityChecksumVerifierUtility,whichisavailableviathepagehttps://support.microsoft.com/en-us/help/841290/availability-and-description-of-the-file-checksum-integrity-verifier-utility.IfyouuseLinux,itmayhappenthatthemd5ormd5sumutilityisnotinstalled.Inthatcase,youcaninstallitusingthecommandapt-getorwhateverinstallationtoolyourLinuxdistributionsupports.

Afterthefileisdownloaded,youcanexplodeittoasubdirectoryusingthefollowingcommand:

tarxfzapache-ant-1.9.7-bin.tar.gz

ThecreatedsubdirectoryistheusablebinarydistributionofAnt.Usually,Imoveitunder~/bin,makingitavailableonlyformyuseronOSX.Afterthat,youshouldsettheenvironmentvariableasANT_HOMEtopointtothisdirectoryandalsoaddthebindirectoryoftheinstallationtothePATH.Todothat,youshouldeditthe~/.bashrcfileandaddthefollowinglinestoit:

exportANT_HOME=~/bin/apache-ant-1.9.7/

exportPATH=${ANT_HOME}bin:$PATH

Then,restarttheterminalapplication,orjusttype.~/.bashrcandtesttheinstallationofAntbytypingthefollowingcommand:

$ant

Buildfile:build.xmldoesnotexist!

Buildfailed

Iftheinstallationwascorrect,youshouldseetheprecedingerrormessage.

UsingAntWhenyouseeaprojecttobebuiltbyAnt,youwillseeabuild.xmlfile.Thisistheprojectbuildfile,theonethatAntwasmissingwhenyoucheckedthattheinstallationwascorrect.Itcanhaveanyothername,andyoucanspecifythenameofthefileasacommand-lineoptionforAnt,butthisisthedefaultfilename,asMakefilewasformake.Abuild.xmlsamplelookslikethefollowing:

<projectname="HelloWorld"default="jar"basedir=".">

<description>

ThisisasampleHelloWorldprojectbuildfile.

</description>

<propertyname="buildDir"value="build"/>

<propertyname="srcDir"value="src"/>

<propertyname="classesDir"value="${buildDir}/classes"/>

<propertyname="jarDir"value="${buildDir}/jar"/>

<targetname="dirs">

<mkdirdir="${classesDir}"/>

<mkdirdir="${jarDir}"/>

</target>

<targetname="compile"depends="dirs">

<javacsrcdir="${srcDir}"destdir="${classesDir}"/>

</target>

<targetname="jar"depends="dirs,compile">

<jardestfile="${jarDir}/HelloWorld.jar"basedir="${classesDir}"/>

</target>

</project>

Thetop-levelXMLtagisproject.Eachbuildfiledescribesoneproject,hencethename.Therearethreepossibleattributestothetag,whichareasfollows:

name:ThisdefinesthenameoftheprojectandisusedbysomeIDEstodisplayitintheleftpanelidentifyingtheprojectdefault:ThisnamesthetargettousewhennotargetisdefinedonthecommandlinestartingAntbasedir:Thisdefinestheinitialdirectoryusedforanyotherdirectorynamecalculationinthebuildfile

Thebuildfilecancontainadescriptionfortheproject,aswellaspropertiesinpropertytags.Thesepropertiescanbeusedasvariablesintheattributesofthetasksbetweenthe${and}characters,andplayanimportantroleinthebuildprocess.

ThetargetsaredefinedintargetXMLtags.Eachtagshouldhaveanamethatuniquelyidentifiesthetargetinthebuildfileandmayhaveadependstagthatspecifiesoneormoreothertargetsthatthistargetdependson.Incasethereismorethanonetarget,thetargetsarecommaseparatedintheattribute.Thetasksbelongingtothetargetsareexecutedinthesameorderasthetargetsdependencychainrequires,inaverysimilarwayaswesawinthecaseofmake.

YoucanalsoaddadescriptionattributetoatargetthatisprintedbyAntwhenthecommand-lineoption,-projecthelp,isused.Thishelpstheusersofthebuildfiletoknowwhattargetsarethereandwhichdoeswhat.Buildfilestendtogrowlargewithmanytargets,andwhenyouhavetenormoretargets,itishardtoremembereachandeverytarget.

ThesampleprojectwithHelloWorld.javaisnowarrangedinthefollowingdirectories:

build.xmlintherootfolderoftheprojectHelloWorld.javainthesrcfolderoftheprojectThebuild/folderdoesnotexist;itwillbecreatedduringthebuildprocessThebuild/classesandbuild/jaralsodonotexistyet,andwillbecreatedduringthebuildprocess

WhenyoustartthebuildfortheHelloWorldprojectthefirsttime,youwillseethefollowingoutput:

$ant

Buildfile:/Users/verhasp/Dropbox/java_9-by_Example/sources/ch02/build.xml

dirs:

[mkdir]Createddir:/Users/verhasp/Dropbox/java_9-by_Example/sources/ch02/build/classes

[mkdir]Createddir:/Users/verhasp/Dropbox/java_9-by_Example/sources/ch02/build/jar

compile:

...

[javac]Compiling1sourcefileto/Users/verhasp/Dropbox/java_9-by_Example/sources/ch02/build/classes

jar:

[jar]Buildingjar:/Users/verhasp/Dropbox/java_9-by_Example/sources/ch02/build/jar/HelloWorld.jar

BUILDSUCCESSFUL

Totaltime:0seconds

Someunimportantlinesaredeletedfromtheactualoutput.

Antrealizesthatfirstithastocreatethedirectories,thenithastocompilethesourcecode,andfinallyitcanpackthe.classfilesintoa.jarfile.NowitisuptoyoutorememberthecommandtoexecutetheHelloWorldapplication.Itwaslistedalreadyinthefirstchapter.Notethatthistime,theJARfileisnamedHelloWorld.jar,anditisnotinthecurrentdirectory.YoucanalsotrytoreadtheonlinedocumentationofAntandcreateatargetrunthatexecutesthecompiledandpackedprogram.

Anthasabuilt-intasknamedjavathatexecutesaJavaclassinalmostthesamewayasyoutypedthejavacommandintheterminal.

MavenAsAntwascreatedtoovercometheshortagesofmake,Mavenwascreatedwithasimilarintention—toovercometheshortagesofAnt.Youmayrecallthatmakecouldnotguaranteebuildportabilitybecausethecommandsmakeexecutesarearbitraryshellcommandsthatmaybesystemspecific.AnAntbuild,ifallthetasksareavailableontheclasspath,isportableaslongasJavarunsthesamewayonthedifferentplatforms.

TheproblemwithAntisabitdifferent.Whenyoudownloadthesourcecodeofaprojectandyouwanttobuild,whatwillthecommandbe?YoushouldaskAnttolistallthetargetsandselecttheonethatseemstobethemostsuitable.Thenameofthetaskdependsontheengineerwhocraftedthebuild.xmlfile.Therearesomeconventions,buttheyarenotstrictrules.

WherewillyoufindtheJavasourcefiles?Aretheyinthesrcdirectoryornot?WilltherealsobesomeGroovyorotherprogramminglanguagefilesincasetheprojectispolyglot?Thatdepends.Again,theremaybesomeconventionsthatsomegroupsorcompanyculturessuggest,butthereisnogeneralbestindustrypractice.

WhenyoustartanewprojectwithAnt,youwillhavetocreatethetargetsforcompilation,testexecution,andpackaging.Itissomethingthatyouwillhavealreadydoneforotherprojects.Afterthesecondorthirdproject,youwilljustcopyandpasteyourpreviousbuild.xmltoyournewproject.Isthataproblem?Yes,itis.Itiscopy/pasteprogramming,evenifitisonlysomebuildfiles.

DevelopersrealizedthatasignificanteffortoftheprojectsutilizingAntisdevotedtoprojectbuildtoolconfiguration,includingrepetitivetasks.Whenanewjoinercomestotheteam,theywillfirsthavetolearnhowthebuildisconfigured.Ifanewprojectisstarted,thebuildconfigurationhastobecreated.Ifitisarepetitivetask,thenbetterletthecomputersdoit.Thatisgenerallywhatprogrammingisallabout,isn'tit?

Mavenapproachesthebuildissueabitdifferently.WewanttobuildJavaprojects.Sometimes,someGroovyorJythonthings,buttheyarealsoJVMlanguages;thus,sayingthatwewanttobuildJavaprojectsisnotreallyahugerestriction.JavaprojectscontainJavafiles,sometimessomeotherprogramminglanguage'ssourcefiles,resourcefiles,andgenerally,thatisit.Antcandoanything,butwedonotwanttodojustanythingwithabuildtool.Wewanttobuildprojects.

Okay,afterwerestrictedourselvesandacceptedthatwedonotneedabuildtoolthatcanbeusedforanything,wecangoon.Wecanrequirethatthesourcefilesbeunderthesrcdirectory.Therearefilesthatareneededfortheoperationalcodeandtherearefilesthatcontainsometestcodeanddata.Therefore,wewillhavetwodirectories,src/testandsrc/main.Javafilesareinsrc/main/javaaswellassrc/test/java.Resourcefilesareundersrc/main/resourcesandsrc/test/resources.

Ifyouwanttoputyoursourcefilessomewhereelse,thendon't.Imeanit.Itispossible,butIwillnoteventellyouhow.Nobodydoesit.IdonotevenhaveanyideawhyMavenmakesitpossible.WheneveryouseeaprojectthatisusingMavenasabuildtool,thesourcesareorganizedlikethat.Thereisnoneedtounderstandthedirectorystructureenvisionedbytheproject'sbuildengineer.Itisalwaysthesame.

Howaboutthetargetsandthetasks?TheyarealsothesameforallMaven-basedprojects.WhatelsewouldyouliketodowithaJavaprojectotherthancompile,test,package,ordeployit?Mavendefinestheseprojectlifecyclesforus.WhenyouwanttocompileaprojectusingMavenasabuildtool,youwillhavetotype$mvncompiletocompiletheproject.Youcandothatevenbeforeunderstandingwhattheprojectactuallyis.

Aswehavethesamedirectorystructureandthesamegoals,theactualtasksleadingtothegoalsarealsoallthesame.WhenwecreateaMavenproject,wedonothavetodescribewhatthebuildprocesshastodoandhowithastodoit.Wewillhavetodescribetheproject,andonlythepartsthatareprojectspecific.

ThebuildconfigurationofaMavenprojectisgiveninanXMLfile.Thenameofthisfileisusuallypom.xml,anditshouldbeintherootdirectoryoftheproject,whichshouldbethecurrentworkingdirectorywhenfiringupMaven.ThewordPOMstandsforProjectObjectModel,anditdescribestheprojectsinahierarchicalway.Thesourcedirectories,thepackaging,andotherthingsaredefinedinaso-calledsuperPOM.ThisPOMispartoftheMavenprogram.AnythingthatthePOMdefines,overridesthedefaultsdefinedinthesuperPOM.Whenthereisaprojectwithmultiplemodules,thePOMsarearrangedintoahierarchy,andtheyinherittheconfigurationvaluesfromtheparentdowntothemodules.AswewilluseMaventodevelopoursortingcode,wewillseesomemoredetailslater.

InstallingMavenMavenisneitherapartoftheoperatingsystemnortheJDK.IthastobedownloadedandinstalledinaverysimilarwaytoAnt.YoucandownloadMavenfromitsofficialwebsite(https://maven.apache.org/)underthedownloadsection.Currently,thelateststableversionis3.3.9.Whenyoudownloadit,theactualreleasemaybedifferent;instead,usethelateststableversion.Youcandownloadthesourceortheprecompiledversion.Theeasiestwayistodownloadthebinaryintar.gzformat.

Icannotskipdrawingyourattentiontotheimportanceofcheckingthedownloadintegrityusingchecksums.IhavedetailedthewaytodoitinthesectionaboutAntinstallation.

Afterthefileisdownloaded,youcanexplodeittoasubdirectoryusingthefollowingcommand:

tarxfzapache-maven-3.3.9-bin.tar.gz

ThecreatedsubdirectoryistheusablebinarydistributionofMaven.Usually,Imoveitunder~/bin,makingitavailableonlyformyuseronOSX.Afterthat,youshouldaddthebindirectoryoftheinstallationtothePATH.Todothat,youshouldeditthe~/.bashrcfileandaddthefollowinglinestoit:

exportM2_HOME=~/bin/apache-maven-3.3.9/

exportPATH=${M2_HOME}bin:$PATH

Then,restarttheterminalapplication,orjusttype.~/.bashrcandtesttheinstallationofMaventyping,asfollows:

$mvn-v

ApacheMaven3.3.9(bb52d8502b132ec0a5a3f4c09453c07478323dc5;2015-11-10T17:41:47+01:00)

Mavenhome:/Users/verhasp/bin/apache-maven-3.3.9

Javaversion:9-ea,vendor:OracleCorporation

Javahome:/Library/Java/JavaVirtualMachines/jdk-9.jdk/Contents/Home

Defaultlocale:en_US,platformencoding:UTF-8

OSname:"macosx",version:"10.11.6",arch:"x86_64",family:"mac"

YoushouldseeasimilarmessageonthescreenthatdisplaystheinstalledMavenversionandotherinformation.

UsingMavenUnlikeAnt,Mavenhelpsyoucreatetheskeletonofanewproject.Todothat,youwillhavetotypethefollowingcommand:

$mvnarchetype:generate

Mavenwillfirstdownloadtheactuallyavailableprojecttypesfromthenetworkandpromptyoutoselecttheoneyouwanttouse.ThisapproachseemedtobeagoodideawhileMavenwasnew.WhenIfirststartedMaven,thenumberoflistedprojectswassomewherebetween10and20.Today,asIwritethisbook,itlists1,635differentarchetypes.Thisnumberseemsmorelikeahistoricaldate(theconstitutionoftheFrenchAcademyofScience)thanausablesizelistofdifferentarchetypes.However,donotfreakout.Mavenoffersadefaultvaluewhenitasksforyourchoice,anditisgoodfortheHelloWorldwegofor.

Chooseanumber:817:

Theactualnumbermaybedifferentonyourinstallation.Whateveritis,acceptthesuggestionandpressEnter.Afterthat,Mavenwillaskyoufortheversionoftheproject:

Chooseversion:

1:1.0-alpha-1

2:1.0-alpha-2

3:1.0-alpha-3

4:1.0-alpha-4

5:1.0

6:1.1

Chooseanumber:6:5

Selectthe1.0versionthatislistedasnumber5.ThenextthingMavenasksforisthegroupIDandtheartifactIDoftheproject.Thedependencymanagementthatwewilldiscusslaterusesthese.IselectedagroupIDbasedonthebookandthepublisher.TheartifactoftheprojectisSortTutorialaswewillstartourchapterexampleinthisproject.

Definevalueforproperty'groupId'::packt.java9.by.example

Definevalueforproperty'artifactId'::SortTutorial

Thenextquestionisthecurrentversionoftheproject.Wehavealreadyselected1.0andMavenoffers1.0-SNAPSHOT.Here,Iselected1.0.0-SNAPSHOTbecauseIprefersemanticversioning.

Definevalueforproperty'version':1.0-SNAPSHOT::1.0.0-SNAPSHOT

Semanticversioning,definedonhttp://semver.org/,isaversioningschemethatsuggeststhreedigitversionnumbersasM.m.p.forMajor,minor,andpatchversionnumbers.Thisisveryusefulforlibraries.Youwillincrementthelastversionnumberifthereisonlyabugfixsincethepreviousrelease.Youwillincrementtheminornumberwhenthenewreleasealsocontainsnewfeatures,butthelibraryiscompatiblewiththepreviousversion;inotherwords,anyprogramthatisusingtheolderversioncanstillusethenewerversion.Themajorreleasenumberisincreasedwhenthenewversionissignificantlydifferentfromthepreviousone.Inthecaseofapplicationprograms,thereisnocodethatusestheapplicationAPI;thus,theminorversionnumberisnotthatimportant.Itdoesnothurt,though,anditoften

provestobeusefultosignalsmallerchangesintheapplication.Wewilldiscusshowtoversionsoftwareinthelastchapter.

Mavenhandlestheversionsthathavethe-SNAPSHOTpostfixasnon-releaseversions.Whilewedevelopthecode,wewillhavemanyversionsofourcode,allhavingthesamesnapshotversionnumber.Ontheotherhand,non-snapshotversionnumberscanonlybeusedonlyforasingleversion.

Definevalueforproperty'package':packt.java9.by.example::

ThelastquestionfromtheprogramskeletongenerationisthenameoftheJavapackage.ThedefaultisthevaluewegaveforgroupId,andwewillusethis.Itisarareexceptiontousesomethingelse.

Whenwehavespecifiedalltheparametersthatareneeded,thefinalrequestistoconfirmthesetting:

Confirmpropertiesconfiguration:

groupId:packt.java9.by.example

artifactId:SortTutorial

version:1.0.0-SNAPSHOT

package:packt.java9.by.example

Y::Y

AfterenteringY,Mavenwillgeneratethefilesthatareneededfortheprojectanddisplaythereportaboutthis:

[INFO]-----------------------------------------------------------

[INFO]UsingfollowingparametersforcreatingprojectfromOld(1.x)Archetype:maven-archetype-quickstart:1.0

[INFO]-----------------------------------------------------------

[INFO]Parameter:basedir,Value:.../mavenHelloWorld

[INFO]Parameter:package,Value:packt.java9.by.example

[INFO]Parameter:groupId,Value:packt.java9.by.example

[INFO]Parameter:artifactId,Value:SortTutorial

[INFO]Parameter:packageName,Value:packt.java9.by.example

[INFO]Parameter:version,Value:1.0.0-SNAPSHOT

[INFO]***EndofdebuginfofromresourcesfromgeneratedPOM***

[INFO]projectcreatedfromOld(1.x)Archetypeindir:.../mavenHelloWorld/SortTutorial

[INFO]-----------------------------------------------------------

[INFO]BUILDSUCCESS

[INFO]-----------------------------------------------------------

[INFO]Totaltime:01:27min

[INFO]Finishedat:2016-07-24T14:22:36+02:00

[INFO]FinalMemory:11M/153M

[INFO]-----------------------------------------------------------

Youcantakelookatthefollowinggenerateddirectorystructure:

Youcanalsoseethatitgeneratedthefollowingthreefiles:

SortTutorial/pom.xmlthatcontainstheProjectObjectModel

SortTutorial/src/main/java/packt/java9/by/example/App.javathatcontainsaHelloWorldsampleapplicationSortTutorial/src/test/java/packt/java9/by/example/AppTest.javathatcontainsaunittestskeletonutilizingthejunit4library

Wewilldiscussunittestsinthenextchapter.Fornow,wewillfocusonthesortingapplication.AsMavenwassokindandgeneratedasampleclassfortheapp,wecancompileandrunitwithoutactualcoding,justtoseehowwecanbuildtheprojectusingMaven.ChangethedefaultdirectorytoSortTutorialissuingcdSortTutorialandissuethefollowingcommand:

$mvnpackage

Wewillgetthefollowingoutput:

Mavenfiresup,compiles,andpackagestheprojectautomatically.Ifnot,pleasereadthenextinfobox.

WhenyoufirststartMaven,itdownloadsalotofdependenciesfromthecentralrepository.Thesedownloadstaketime,andarereportedonthescreen,sotheactualoutputmaybedifferentfromwhatyousawintheprecedingcode.MavencompilescodewiththedefaultsettingsforJavaversion1.5.Itmeansthatthe

generatedclassfileiscompatiblewithJavaversion1.5,andalsothatthecompileronlyacceptslanguageconstructsthatwereavailablealreadyinJava1.5.Ifwewanttousenewerlanguagefeatures,andinthisbookweusealot,thepom.xmlfileshouldbeeditedtocontainthefollowinglines:

<build>

<plugins>

<plugin>

<groupId>org.apache.maven.plugins</groupId>

<artifactId>maven-compiler-plugin</artifactId>

<configuration>

<source>1.9</source>

<target>1.9</target>

</configuration>

</plugin>

</plugins>

</build>

WhenusingJava9'sdefaultsettingsforMaven,itbecomesevenmorecomplex,becauseJava9doesnotgenerateclassformatnorrestrictsourcecompatibilityearlierthanJava1.6.Atthisverymoment,asIwritetheselines,thelatestMavenreleaseis3.3.9.WhenItrytocompiletheprecedingcodewithoutthemodifications,theJavacompilerstopswithanerrordisplayingthefollowing:[ERROR]Sourceoption1.5isnolongersupported.Use1.6orlater.

[ERROR]Targetoption1.5isnolongersupported.Use1.6orlater.Later,Mavenreleasesmaybehavedifferentlyinthefuture.

Now,youcanstartthecodeusingthefollowingcommand:

$java-cptarget/SortTutorial-1.0.0-SNAPSHOT.jarpackt.java9.by.example.App

Youcanseetheresultofasampleruninthefollowingpicture:

GradleAntandMavenaretwoworlds,andusingoneortheothermayleadtoheateddebatesonInternetforums.Antgivesfreedomtodeveloperstocreateabuildprocessthatfitstheirtaste.Mavenrestrictstheteamtouseabuildprocessthatismorestandard.Somespecialprocessesthatdonotmatchanystandardbuild,butwhicharesometimesneededinsomeenvironments,arehardtoimplementusingMaven.InAnt,youcanscriptalmostanythingusingthebuilt-intasks,almostthesamewayasyoucanprogrambash.UtilizingMavenisnotthatsimple,and,itoftenrequireswritingaplugin.Eventhoughwritingapluginisnotrocketscience,developersusuallyliketohavethepossibilityofmakingthingsinasimplerway:Scripting.Wehavetwoapproaches,twomindsetsandstyles,andnotasingletooltofulfillalltheneeds.NosurprisethatbytheJavatechnologiesweredeveloped,anewbuildtoolwasemerging.

Gradletriestousethebestofbothworlds,utilizingtechniquesthatwerenotavailablebythetimeMavenandAntwerefirstdeveloped.

Gradlehasbuilt-intargetsandlifecycle,butatthesametime,youcanalsowriteyourowntargets.Youcanconfigureaproject,justlikeusingMaven,withoutscriptingthetaskstodoso,butatthesametime,youcanalsoscriptyourowntargetjustlikeinAnt.Whatismore,GradleintegratedAnt,soanytaskimplementedforAntisavailableforGradleaswell.

MavenandAntuseXMLfilestodescribethebuild.Today,XMLisatechnologyofthepast.Westilluseit,andadevelopershouldbefluentinhandling,reading,andwritingXMLfiles,butamoderntooldoesnotuseXMLforconfiguration.New,fancyformatssuchasJSONaremorepopular.Gradleisnoexception.TheconfigurationfileofGradleusesadomain-specificlanguage(DSL)basedonGroovy.Thislanguageismorereadableforprogrammersandgivesmorefreedomtoprogrambuildprocesses.And,thisisalsothedangerofGradle.

HavingthepowerfulJVMlanguageGroovyinthehandsofdeveloperstocreatebuildtoolsgivesafreedomandtemptationtocreatecomplexbuildprocessesthatseemtobeagoodideaatthestart,butlatermayprovetobejusttoocomplexandhard,and,therefore,expensivetomaintain.ThisisexactlywhyMavenwasimplementedinthefirstplace.

Ihavetostopbeforegettingintoanotherareathatisthegroundforheatedandpointlessdebates.Gradleisanextremelypowerfulbuildtool.Youshoulduseitcarefully,justlikeyouwoulduseaweapon—don'tshootyourlegs.

InstallingGradleToinstallGradle,youwillhavetodownloadthecompiledbinariesfromthehttps://gradle.org/gradle-download/website.

Again,I'dliketoemphasizetheimportanceofcheckingthedownloadintegrityusingchecksums.IhavegivenadetailedwaytodoitinthesectionaboutAntinstallation.Unfortunately,theGradlewebsitedoesnotprovidethechecksumvaluesforthedownloadablefiles.

GradleisdownloadableintheZIPformat.Tounpackthefile,youwillhavetousetheunzipcommand:

$unzipgradle-3.3-bin.zip

ThecreatedsubdirectoryistheusablebinarydistributionofGradle.Usually,Imoveitunder~/bin,makingitavailableonlyformyuseronOSX.Afterthat,youshouldaddthebindirectoryoftheinstallationtothePATH.Todothat,youshouldeditthe~/.bashrcfileandaddthefollowinglines:

exportGRADLE_HOME=~/bin/gradle-3.3/

exportPATH=${GRADLE_HOME}bin:$PATH

Then,restarttheterminalapplication,orjusttype.~/.bashrcandtesttheinstallationofGradle,typingthefollowing:

$gradle-version

Wegettothefollowingoutput,ascanbeseeninthisscreenshot:

SettinguptheprojectwithMavenTostarttheproject,wewillusethedirectorystructureandpom.xmlthatwascreatedbyMavenitselfwhenwestartedwiththefollowingcommandline:$mvnarchetype:generate

Itcreatedthedirectories,thepom.xmlfile,andanApp.javafile.Now,wewillextendthisprojectbycreatingnewfiles.Wewillcodethesortingalgorithmfirstinthepackt.java9.by.example.stringsortpackage:

WhenwecreatethenewpackageintheIDE,theeditorwillautomaticallycreatethestringsortsubdirectoryunderthealreadyexistingsrc/main/java/packt/java9/by/exampledirectory:

CreatingthenewSortclassusingtheIDEwillalsoautomaticallycreateanewfilenamedSort.javainthisdirectory,anditwillfillintheskeletonoftheclass:packagepackt.java9.by.example.stringsort;

publicclassSort{}

WewillnowhaveApp.javacontainingthefollowingcode:

packagepackt.java9.by.example;

publicclassApp

{

publicstaticvoidmain(String[]args)

{

System.out.println("HelloWorld!");

}

}

Mavencreateditasastartingversion.Wewilleditthisfiletoprovideasamplelistthatthesortingalgorithmcansort.IrecommendthatyouusetheIDEtoeditthefileandalsotocompileandrunthecode.TheIDEprovidesashortcutmenutostartthecodeandthisisabiteasierthantypingthecommandinTerminal.Generally,itisrecommendedthatyougetacquaintedwiththeIDEfeaturestosavetimeavoidingrepetitivetasks,suchastypingterminalcommands.Professionaldevelopersusethecommandlinealmostexclusivelytotestcommand-linefeaturesandusetheIDEwheneveritispossible.

CodingthesortMavenandtheIDEcreatedthefilesforthesortprogram.Theyformtheskeletonforourcode,andnowitistimetogrowsomemusclesonthemtoletitmove.Wespentquitesometimetosetuptheprojectbyvisitingthedifferentbuildtools,onlytolearnhowtocompilethecode.Ihopethatthisdidnotdistractyoumuch,butanyhow,wedeservetoseesomerealcode.

First,wewillcreatethecodeforthesortingcode,andafterthat,thecodethatinvokesthesorting.Thecodethatinvokesthesortingisakindoftestingcode.Forsimplicity,wewillnowsimplyuseapublicstaticvoidmainmethodtostartthecode.Wewillusethetestframeworkinlaterchapters.

Asfornow,thecodeforthesortingwilllooklikethis:

packagepackt.java9.by.example.stringsort;

publicclassSort{

publicvoidsort(String[]names){

intn=names.length;

while(n>1){

for(intj=0;j<n-1;j++){

if(names[j].compareTo(names[j+1])>0){

finalStringtmp=names[j+1];

names[j+1]=names[j];

names[j]=tmp;

}

}

n--;

}

}

}

Thisistheclassthatdoesthesorting.Thereisonlyonemethodinthisclassthatdoesthesorting.Theargumenttothemethodisanarraycontainingthestrings,andthemethodsortsthisarray.Themethodhasnoreturnvalue.Thisisdenotedinthedeclarationusingthepseudotypevoid.Methodsusetheirargumentstoperformsometasks,andmayreturnonevalue.Theargumentstothemethodarepassedbyvalue,whichmeansthatthemethodcannotmodifythevariablepassedasargument.However,itcanmodifytheobjectstheargumentscontain.Inthiscase,thearrayismodifiedandwewillsortit.Ontheotherhand,theactualNamesvariablewillpointtothesamearrayandthesortmethodcannotdoanythingtomakethisvariablepointtoadifferentarray.

Thereisnomainmethodinthisclass,whichmeansthatitcannotbestartedfromthecommandlineonitsown.Thisclasscanonlybeusedfromsomeotherclass,aseveryJavaprogramshouldhaveaclassthathasapublicstaticvoidmainmethodthatwecreatedseparately.

Icouldalsoputamainmethodintotheclasstomakeitexecutable,butthatisnotagoodpractice.Realprogramsarecomposedofmanyclasses,andoneclassshouldnotdomanythings.Rather,it'stheopposite.Thesingleresponsibilityprinciplesaysthatasingleclassshouldberesponsibleforonesinglething;therefore,classsortdoesthesorting.Executingtheapplicationisadifferenttask,andthusithastobeimplementedinadifferentclass.

Often,wedonotimplementtheclasscontainingthemainmethod.Often,aframeworkprovidesit.For

example,writingaservletthatrunsinaservletcontainerrequirescontainingaclassthatimplementsthejavax.servlet.Servletinterface.Inthiscase,theprogramseeminglydoesnothaveamainmethod.Theactualimplementationoftheservletcontainerdoes.TheJavacommandlinestartsthecontainerandthecontainerloadstheservletswhentheyareneeded.

Inthefollowingexamplecode,weimplementedtheAppclasscontainingthemainmethod:

packagepackt.java9.by.example;

importpackt.java9.by.example.stringsort.Sort;

publicclassApp{

publicstaticvoidmain(String[]args){

String[]actualNames=newString[]{

"Johnson","Wilson",

"Wilkinson","Abraham","Dagobert"

};

finalSortsorter=newSort();

sorter.sort(actualNames);

for(finalStringname:actualNames){

System.out.println(name);

}

}

}

Thiscodecontainsastringarrayinitializedtocontainconstantvalues,createsanewinstanceoftheSortclass,invokesthesortmethod,andthenprintsoutthecodetothestandardoutput.

Inrealprograms,wealmostneverhavesuchconstantsinprogramcodes;weputthemintoresourcefilesandhavesomecodetoreadtheactualvalues.Thisseparatesthecodefromdataandeasesmaintenance,eliminatingtheriskofaccidentalmodificationofcodestructurewhenonlythedataistobechanged.Similarly,wewillalmostneverwriteanythingtostandardoutputusingSystem.out.Usually,wewilluseloggingpossibilitiesthatareavailablefromdifferentsources.TherearedifferentlibrariesthatprovideloggingfunctionalitiesandloggingisalsoavailablefromtheJDKitself.

Asfornow,wewillfocusonsimplesolutionssoastonotdistractyourfocusfromJavabytheplethoraofdifferentlibrariesandtools.Inthefollowingsection,wewilllookattheJavalanguageconstructsthatweusedtocodethealgorithm.First,wewilllookatthemgenerally,andthen,inabitmoredetail.Theselanguagefeaturesarenotindependentofeachother:onebuildsupontheother,andtherefore,theexplanationwillfirstbegeneral,andwewillgointodetailsinthesubsections.

intn=names.length;<br/>while(n>1){<br/>for(intj=0;j<n-1;j++){<br/>if(names[j].compareTo(names[j+1])>0){<br/>finalStringtmp=names[j+1];<br/>names[j+1]=names[j];<br/>names[j]=tmp;<br/>}<br/>}<br/>n--;<br/>}

Thenvariableholdsthelengthofthearrayatthestartofthesorting.ArraysinJavaalwayshaveapropertythatgivesthelengthanditiscalledlength.Whenwestartthesorting,wewillgofromthestartofthearraytotheendofitand,asyoumayrecall,thelastelement,Wilson,willwalkuptothelastpositionduringthisfirstiteration.Subsequentiterationswillbeshorterand,therefore,thevariablenwillbedecreased.

Blocks

ThecodeinJavaiscreatedincodeblocks.Anythingthatisbetweenthe{and}charactersisablock.Intheprecedingexample,thecodeofthemethodisablock.Itcontainscommands,andsomeofthem,likethewhileloop,alsocontainablock.Insidethatblock,therearetwocommands.Oneofthemisaforloop,againwithablock.Althoughwecanhavesingleexpressionstoformthebodyofaloop,weusuallyuseblocks.Wewilldiscussloopsindetailinjustafewpages.

Aswecouldseeintheprecedingexample,theloopscanbenested,andthusthe{and}charactersformpairs.Ablockcanbeinsideanotherblock,buttwoblockscannotoverlap.Whenthecodecontainsa}character,itisclosingtheblockthatwasopenedlast.

Variables

InJava,justlikeinalmostanyprogramminglanguage,weusevariables.ThevariablesinJavaaretyped.Itmeansthatavariablecanholdavalueofasingletype.ItisnotpossibleforavariabletoholdaninttypeatsomepointintheprogramandlateraStringtype.Whenvariablesaredeclared,theirtypeiswritteninfrontofthevariablename.

Variablesalsohavevisibilityscope.Localvariablesinmethodscanonlybeusedinsidetheblockinwhichtheyaredefined.Avariablecanbeusedinsidemethodsortheycanbelongtoaclassoranobject.Todifferentiatethetwo,weusuallycallthesevariablesfields.

TypesEachvariablehasonetype.InJava,therearetwomajorgroupsoftype:primitiveandreferencetypes.Theprimitivetypesarepredefined,andyoucannotdefineorcreateanewprimitivetype.Thereareeightprimitivetypes:byte,short,int,long,float,double,boolean,andchar.

Thefirstfourtypes,byte,short,int,andlong,aresignednumericintegertypes,capableofstoringpositiveandnegativenumberson8,16,32,and64bits.

Thefloatanddoubletypesstorefloatingpointnumberson32and64bitsintheIEEE754floating-pointformat.

Thebooleantypeisaprimitivetypethatcanonlybetrueorfalse.

Thechartypeisacharacterdatatypethatstoresasingle16-bitUnicodecharacter.

Foreachprimitivetype,thereisaclassthatcanstorethesametypeofvalue.Whenaprimitivetypehastobeconvertedtothematchingclasstypeitisdoneautomatically.Itiscalledautoboxing.ThesetypesareByte,Short,Integer,Long,Float,Double,Boolean,andCharacter.Take,forexample,thefollowingvariabledeclaration:

Integera=113;

Thisconvertsthevalue113,whichisanintnumber,toanIntegerobject.

Thesetypesarepartoftheruntime,andalsopartofthelanguage.Althoughthereisnoprimitivecounterpartofit,thereisaveryimportantandubiquitousclassthatwehavealreadyused:String.Astringcontainscharacters.

Themajordifferencesbetweenprimitivetypesandobjectsarethatprimitivetypescannotbeusedtoinvokemethods,buttheyconsumelessmemory.Thedifferencebetweenthememoryconsumptionanditsconsequencesforspeedisimportantinthecaseofarrays.

ArraysVariablescanbeaprimitivetypeaccordingtotheirdeclaration,ortheymayholdareferencetoanobject.Aspecialobjecttypeisanarray.Whenavariableholdsareferencetoanarray,thenitcanbeindexedwiththe[and]characters,alongwithanintegralvalueconsistingof0orapositivevaluerangingtoonelessthanthearray'slength,toaccessacertainelementofthearray.Multi-dimensionalarraysarealsosupportedbyJavawhenanarrayhaselementsthatarealsoarrays.ArraysareindexedfromzeroinJava.Underoroverindexingischeckedatruntime,andtheresultisanexception.

Anexceptionisspecialconditionthatinterruptsthenormalexecutionflowandstopstheexecutionofthecodeorjumpstotheclosestenclosingcatchstatement.Wewilldiscussexceptionsandhowtohandletheminthenextchapter.

Whenacodehasanarrayofaprimitivetype,thearraycontainsmanymemoryslots,eachholdingthevalueofthetype.Whenthearrayhasareferencetype,inotherwords,whenitisanarrayofobjects,thenthearrayelementsarereferencestoobjects,eachcontainingthetype.Inthecaseofintforexample,eachelementofthearrayis32-bit,whichis4bytes.IfthearrayisatypeofInteger,thentheelementsarereferencestoobjects,pointers,sotosay,whichisusually64-bitusing64-bitJVMand32-biton32-bitJVM.Inadditiontothat,thereisanIntegerobjectsomewhereinmemorythatcontainsthe4-bytevalueandalsoanobjectheaderthatmaybeasmuchas24bytes.

Theactualsizeoftheextrainformationneededtoadministereachobjectisnotdefinedinthestandard.ItmaybedifferentondifferentimplementationsoftheJVM.Theactualcoding,oreventheoptimizationofthecodeinanenvironment,shouldnotdependontheactualsize.However,thedevelopersshouldbeawarethatthisoverheadexistsandisintherangeofaround20orsobytesforeveryobject.Objectsareexpensiveintermsofmemoryconsumption.

Memoryconsumptionisoneissue,butthereissomethingelse.Whentheprogramworkswithalargeamountofdataandtheworkneedstheconsecutiveelementsofthearray,thentheCPUloadsachunkofmemoryintotheprocessorcache.ItmeansthattheCPUcanaccesselementsofthearraythatareconsecutivelyfaster.Ifthearrayisofaprimitivetype,itisfast.Ifthearrayisofsomeclasstype,thentheCPUhastoaccessmemorytogettheactualvalue,whichmaybeasmuchas50timesslower.

ExpressionsExpressionsinJavaareverymuchlikeinotherprogramminglanguages.YoucanusetheoperatorsthatmaybesimilarfromlanguagessuchasCorC++.Theyareasfollows:

Unaryprefixandpostfixincrementoperators(--and++beforeandafteravariable)Unarysign(+and-)operatorsLogical(!)andbitwise(~)negationMultiplication(*),division(/),andmodulo(%)Additionandsubtraction(+and-again,butthistimeasbinaryoperators)Shiftoperatorsmovethevaluesbitwise,andthereisleft(<<)andright(>>)shiftandunsignedrightshift(>>>)Thecomparingoperatorsare<,>,<=,>=,==,!=andinstanceofthatresultinbooleanvalueTherearebitwiseor(|),and(&),exclusiveor(^)operators,andsimilarlylogicalor(||),and(&&)operators

Whenlogicaloperatorsareevaluated,theyareshortcutevaluated.Itmeanstheright-handoperandisevaluatedonlyiftheresultcannotbeidentifiedfromtheresultoftheleftoperand.

Theternaryoperatorisalsosimilartotheone,likeitisonC,selectingfromoneoftheexpressionsbasedonsomecondition:condition?expression1:expression2.Usually,thereisnoproblemwiththeternaryoperator,butsometimesyouhavetobecarefulasthereisacomplexrulecontrollingthetypeconversionsincasethetwoexpressionsarenotofthesametype.It'salwaysbettertohavethetwoexpressionsbeofthesametype.

Finally,thereisanassignmentoperator(=)thatassignsthevalueofanexpressiontoavariable.Foreachbinaryoperator,thereisanassignmentversionthatcombines=withabinaryoperatortoperformanoperationinvolvingtherightoperandandassigntheresulttotheleftoperand,whichmustbeavariable.Theseare+=,-=,*=,/=,%=,&=,^=,|=,<<=,>>=,and>>>=.

Theoperatorshaveprecedenceandcanbeoverriddenbyparentheses,asusual.

Animportantpartofexpressionsisinvokingmethods.Staticmethodscanbeinvokedbythenameoftheclassandthenameofthemethod.Forexample,tocalculatethesineof1.22,wecanwritethefollowingline:

doublez=Math.sin(1.22);

Here,Mathistheclassfromthepackagejava.lang.ThemethodsinisinvokedwithoutusinganyinstanceofMath.Thismethodisstatic,anditisnotlikelythatwewilleverneedanyotherimplementationofitthantheoneprovidedintheclassMath.

Non-staticmethodscanbeinvokedusinganinstanceandthenameofthemethodwithadotseparatingthetwo.Forexample,takethefollowingcodelineasanexample:

System.out.println("HelloWorld");

TheprecedingcodeusesaninstanceoftheclassPrintStreamthatisreadilyavailablethroughastaticfieldintheclassSystem.Thisvariableiscalledout,andwhenwewriteourcode,wehavetoreferenceitasSystem.out.ThemethodprintlnisdefinedintheclassPrintStreamandweinvokeitontheobjectreferencedbythevariableout.Thisexamplealsoshowsthatstaticfieldscanalsobereferencedthroughthenameoftheclassandthefieldseparatedbyadot.Similarly,whenweneedtoreferenceanon-staticfield,wecandoitthroughaninstanceoftheclass.

Staticmethodsdefinedinthesameclassfromwhereitisinvokedorinheritedcanbeinvokedwithouttheclassname.Invokinganon-staticmethoddefinedinthesameclassorbeinginheritedcanbeinvokedwithoutaninstance.Inthiscase,theinstanceisthecurrentobjecttheexecutionisin.Thisobjectisalsoavailablethroughthethiskeyword.Similarly,whenweuseafieldofthesameclasswhereourcodeis,wesimplyusethename.Incaseofastaticfield,theclassweareinbydefault.Inthecaseofanon-staticfield,theinstanceistheobjectreferencedbythethiskeyword.

Youcanalsoimportastaticmethodintoyourcodeusingtheimportstaticlanguagefeature,inwhichcaseyoucaninvokethemethodwithoutthenameoftheclass.

Theargumentsofthemethodcallsareseparatedusingcommas.Methodsandmethodargumentpassingisanimportanttopicthatwewillmentionindetailinaseparatesubsection.

LoopsTheforloopinsidethewhileloopwillgothroughalltheelementsfromthefirst(indexedwithzeroinJava)uptillthelast(indexedwithn-1).Generally,theforloophasthesamesyntaxasinC:

for(initialexpression;condition;incrementexpression)

block

First,theinitialexpressionisevaluated.Itmaycontainvariabledeclaration,asinourexample.Thevariablejintheprecedingexampleisvisibleonlyinsidetheblockoftheloop.Afterthis,theconditionisevaluated,andaftereachexecutionoftheblock,theincrementexpressionisexecuted.Thelooprepeatssolongastheconditionistrue.Iftheconditionisfalserightaftertheexecutionoftheinitialexpression,theloopdoesnotexecuteatall.Theblockisalistofcommandsseparatedbysemicolonsandenclosedbetweenthe{and}characters.

Insteadof{and},enclosedblockJavaletsyouuseasinglecommandfollowingtheheadoftheforloop.Thesameistrueinthecaseofthewhileloop,andalsofortheif...elseconstructs.Practiceshowsthatthisisnotsomethingaprofessionalshoulduse.Professionalcodealwaysusescurlybraces,evenwhenthereisonlyasinglecommandwheretheblockisinplace.Thispreventsthedanglingelseproblemandgenerallymakesthecodemorereadable.ThisissimilartomanyC-likelanguages.Mostofthemallowasinglecommandattheseplaces,andprofessionalprogrammersavoidusingasinglecommandintheselanguagesforreadabilitypurposes.Itisironicthattheonlylanguagethatstrictlyrequirestheuseofthe{and}bracesattheseplacesisPerl—theonelanguageinfamousforunreadablecode.

Theloopinthefor(intj=0;j<n-1;j++){samplestartsfromzeroandgoeston-2.Writingj<n-1isthesame,inthiscase,asj<=n-2.Wewilllimitjtostopintheloopbeforetheendofthesectionofthearray,becausewereachbeyondtheindexjbyonecomparingandconditionallyswappingtheelementsindexedbyjandj+1.Ifwewentoneelementfurther,wewouldtrytoaccessanelementofthearraythatdoesnotexist,anditwouldcausearuntimeexception.Tryandmodifytheloopconditiontoj<norj<=n-1andyouwillgetthefollowingerrormessage:

ItisanimportantfeatureofJavathattheruntimechecksmemoryaccessandthrowsanexceptioninthecaseofbadarrayindexing.Inthegoodolddays,whilecodinginC,often,wefacedunexplainableerrorsthatstoppedourcodemuchlaterandattotallydifferentcodelocationsfromwheretherealerrorwas.ArrayindexinCsilentlycorruptedthememory.Javastopsyouassoonasyoumakeamistake.Itfollowsthefail-fastapproachthatyoualsoshoulduseinyourcode.Ifsomethingiswrong,theprogramshouldfail.Nocodeshouldtrytolivewithorovercomeanerrorthatcomesfromacodingerror.Codingerrorsshouldbefixedbeforetheycauseevenmoredamage.

TherearealsotwomoreloopconstructsinJava:thewhileloopandthedoloop.Theexamplecontainsawhileloop:itistheouterloopthatrunssolongasthereareatleasttwoelementsthatmayneedswappinginthearray:

while(n>1){

Thegeneralsyntaxandsemanticsofthewhileloopisverysimple,asseenhere:

while(condition)block

Repeattheexecutionoftheblocksolongastheconditionistrue.Iftheconditionisnottrueattheverystartoftheloop,thendonotexecutetheblockatall.Thedoloopisalsosimilar,butitcheckstheconditionaftereachexecutionoftheblock:

doblockwhile(condition);

Forsomereason,programmersrarelyusedoloops.

ConditionalexecutionTheheartofthesortistheconditionandthevalueswappinginsidetheloop.

if(names[j].compareTo(names[j+1])>0){

finalStringtmp=names[j+1];

names[j+1]=names[j];

names[j]=tmp;

}

ThereisonlyoneconditionalcommandinJava,theifcommand.Ithasthefollowingformat:

if(condition)blockelseblock

Themeaningofthecodestructureisquitestraightforward.Iftheconditionistrue,thenthefirstblockisexecuted,otherwise,thesecondblockisexecuted.Theelsekeyword,alongwiththesecondblock,isoptional.Ifthereisnothingtobeexecutedincasethattheconditionisfalse,thenthereisnoneedfortheelsebranch,justlikeintheexample.Ifthearrayelementindexedwithjislaterinthesortorderthantheelementj+1,thenweswapthem,butiftheyarealreadyinorder,thereisnothingtodowiththem.

Toswapthetwoarrayelements,wewilluseatemporaryvariablenamedtmp.ThetypeofthisvariableisString,andthisvariableisdeclaredtobefinal.ThefinalkeywordhasdifferentmeaningsdependingonwhereitisusedinJava.Thismaybeconfusingforbeginnersunlessyouarewarnedaboutit,justlikenow.Afinalclassormethodisatotallydifferentthingthanafinalfield,whichisagaindifferentthanafinallocalvariable.

FinalvariablesInourcase,tmpisafinallocalvariable.Thescopeofthisvariableislimitedtotheblockfollowingtheifstatement,andinsidethisblock,thisvariablegetsavalueonlyonce.Theblockisexecutedmanytimesduringthecodeexecution,andeachtimethevariablegetsintoscope,itgetsavalue.However,thisvaluecannotbechangedintheblock.Thismaybeabitconfusing.Youcanthinkaboutitashavinganewtmpeachtimetheblockexecutes.Thevariablegetsdeclaredandhasanundefinedvalueandcangetavalueonlyonce.

Finallocalvariablesdonotneedtogetthevaluewheretheyaredeclared.Youcanassignavaluetoafinalvariablesometimelater.Itisimportantthatthereshouldnotbeacodeexecutionthatassignsavaluetoafinalvariablethatwasalreadyassignedavaluebefore.Thecompilerchecksitanddoesnotcompilethecodeifthereisapossibilityofthereassignmentofafinalvariable.

Todeclareavariabletobefinalisgenerallytoeasereadabilityofthecode.Whenyouseeavariableinacodedeclaredtobefinal,youcanassumethatthevalueofthevariablewillnotchangeandthemeaningofthevariablewillalwaysbethesamewhereveritwasusedinthemethod.ItwillalsohelpyouavoidsomebugswhenyoutrytomodifysomefinalvariablesandtheIDEwillimmediatelycomplainaboutit.Insuchsituations,itislikelytobeaprogrammingmistakethatisdiscoveredextremelyearly.

Inprinciple,itispossibletowriteaprogramwhereallvariablesarefinal.Itisgenerallyagoodpracticetodeclareallfinalvariablesthatcanbedeclaredtobefinaland,incasesomevariablemaynotbedeclaredfinal,thentrytofindsomewayofcodingthemethodabitdifferently.

Ifyouneedtointroduceanewvariabletodothat,itprobablymeansyouwereusingonevariabletostoretwodifferentthings.Thesethingsareofthesametypeandstoredinthesamevariableatdifferenttimesbut,logically,theystillaredifferentthings.Donottrytooptimizetheuseofvariables.Neveruseavariablebecauseyoualreadyhaveavariableofthetypeinyourcodethatisavailable.Ifitislogicallyadifferentthing,thendeclareanewvariable.Whilecoding,alwaysprefersourcecodeclarityandreadability.InJava,especially,theJustInTimecompilerwilloptimizeallthisforyou.

Althoughwedonotexplicitlytendtousethefinalkeywordontheargumentlistofamethod,itisgoodpracticetomakesurethatyourmethodscompileandworkiftheargumentsaredeclaredfinal.Someexperts,includingme,believethatthemethodparametersshouldhavebeenmadefinalbydefaultinthelanguage.ThisissomethingthatwillnothappeninanyversionofJava,solongasJavafollowsthebackwardcompatibilityphilosophy.

ClassesNowthatwehavelookedattheactualcodelinesandhaveunderstoodhowthealgorithmworks,let'slookatthemoreglobalstructuresofthecodethatbringsittogether:classesandpackagesenclosingthemethods.

EveryfileinaJavaprogramdefinesaclass.AnycodeinaJavaprogramisinsideaclass.ThereisnothinglikeglobalvariablesorglobalfunctionsasinC,Python,Go,orotherlanguages.Javaistotallyobjectoriented.

Therecanbemorethanoneclassinasinglefile,butusuallyonefileisoneclass.Later,wewillseethatthereareinnerclasseswhenaclassisinsideanotherclass,but,fornow,wewillputoneclassintoonefile.

TherearesomefeaturesintheJavalanguagethatwedonotuse.Whenthelanguagewascreated,thesefeaturesseemedtobeagoodidea.CPU,memory,andotherresources,includingmediocredevelopers,werealsomorelimitedthantoday.Someofthefeatures,perhaps,mademoresensebecauseoftheseenvironmentalconstraints.Sometimes,Iwillmentionthese.Inthecaseofclasses,youcanputmorethanoneclassintoasinglefilesolongasonlyoneispublic.Thatisbadpractice,andwewillneverdothat.Javaneverobsoletesthesefeatures.ItisaphilosophyofJavatoremaincompatiblewithallpreviousversions.Thisphilosophyisgoodforthealreadywritten,hugeamountoflegacycode.Javacodewrittenandtestedwithanoldversionwillworkinanewerenvironment.Atthesametime,thosefeatureslurebeginnerstoawrongstyle.Forthisreason,sometimes,Iwillnotevenmentionthesefeatures.Forexample,here,Icouldsay:Thereisoneclassinafile.Thiswouldnotbeabsolutelycorrect.Atthesametime,itismoreorlesspointlesstoexplainingreatdetailafeaturethatIrecommendnottobeused.Later,Imaysimplyskipthemand"lie".Therearenottoomanyofthosefeatures.

Aclassisdefinedusingtheclasskeywordandeachclasshastohaveaname.Thenameshouldbeuniquewithinthepackage(seethenextsection)andhastobethesameasthenameofthefile.Aclasscanimplementaninterfaceorextendanotherclass,forwhichwewillseeanexamplelater.Aclasscanalsobeabstract,final,andpublic.Thesearedefinedwiththeappropriatekeywords,asyouwillseeinexamples.

Ourprogramhastwoclasses.Bothofthemarepublic.Thepublicclassesareaccessiblefromanywhere.Classesthatarenotpublicarevisibleonlyinsidethepackage.Innerandnestedclassescanalsobeprivatevisibleonlyinsidethetop-levelclassdefinedonthefilelevel.

ClassesthatcontainamainmethodtobeinvokedbytheJavaenvironmentshouldbepublic.ThatisbecausetheyareinvokedbytheJVM.

Theclassstartsatthebeginningofthefilerightafterthepackagedeclarationandeverythingbetweenthe{and}charactersbelongtotheclass.Themethods,fields,innerornestedclasses,andsoonarepartoftheclass.Generally,curlybracesdenotesomeblockinJava.ThiswasinventedintheClanguage,andmany

languagesfollowthisnotation.Classdeclarationissomeblock,methodsaredefinedusingsomeblock,loops,andconditionalcommandsuseblocks.

Whenweusetheclasses,wewillhavetocreateinstancesofclasses.Theseinstancesareobjects.Inotherwords,objectsarecreatedinstantiatingaclass.Todothat,thenewkeywordisusedinJava.WhenthelinefinalSortsorter=newSort();isexecutedintheAppclass,itcreatesanewobjectinstantiatingtheSortclass.WewillalsosaythatwecreatedanewSortobjectorthatthetypeoftheobjectisSort.Whenanewobjectiscreated,aconstructoroftheobjectisinvoked.Abitsloppy,Imaysay,thattheconstructorisaspecialmethodintheclassthathasthesamenameastheclassitselfandhasnoreturnvalue.Thatisbecauseitreturnsthecreatedobject.Tobeprecise,constructorsarenotmethods.Theyareinitializersandtheydonotreturnthenewobject.Theyworkonthenot-ready-yetobject.Whenaconstructorexecutingtheobjectisnotfullyinitialized,someofthefinalfieldsmaynotbeinitializedandtheoverallinitializationstillcanfailiftheconstructorthrowsanexception.Inourexample,wedonothaveanyconstructorinthecode.Insuchacase,Javacreatesadefaultconstructorthatacceptsnoargumentanddoesnotmodifythealreadyallocatedbutuninitializedobject.IftheJavacodedefinesaninitializer,thentheJavacompilerdoesnotcreateadefaultone.

Aclasscanhavemanyconstructors,eachhavingdifferentparameterlist.

InadditiontoconstructorsJavaclassescancontaininitializerblocks.Theyareblocksontheclasslevel,thesamelevelastheconstructorandmethods.Thecodeintheseblocksiscompiledintotheconstructorsandisexecutedwhentheconstructorisexecuting.

Itisalsopossibletoinitializestaticfieldsinstaticinitializerblocks.Thesearetheblocksonthetoplevelinsidetheclasswiththestatickeywordinfrontofthem.Theyareexecutedonlyoncewhentheclassisloaded.

WenamedtheclassesinourexampleAppandSort.ThisisaconventioninJavatonamealmosteverythinginCamelCase.

CamelCaseiswhenthewordsarewrittenwithoutspacesbetweenthem.Thefirstwordmaystartwithlowercaseoruppercase,and,todenotethestartofthesecondandsubsequentwords,theystartwithuppercase.ForExampleThisIsALongCamelCasename.

Classnamesstartwithanuppercaseletter.Thisisnotarequirementofthelanguageformally,butthisisaconventionthateveryprogrammershouldfollow.Thesecodingconventionshelpyoucreatecodethatiseasiertounderstandbyotherprogrammers,andleadtoeasiermaintenance.Staticcodeanalyzertools,suchasCheckstyle(http://checkstyle.sourceforge.net/),alsocheckthattheprogrammersfollowtheconventions.

Inner,nested,local,andanonymousclassesIhavealreadymentionedinnerandnestedclassesintheprevioussection.Nowwelookattheminbitmoredetail.

Thedetailsofinnerandnestedclassesatthispointmaybedifficult.Don'tfeelashamedifyoudonotunderstandthissectionfully.Ifitistoodifficult,skiptothenextsectionandreadaboutpackagesandreturnherelater.Nested,inner,andlocalclassesarerarelyused,thoughtheyhavetheirrolesanduseinJava.AnonymousclasseswereverypopularinGUIprogrammingwiththeSwinguserinterfacethatalloweddeveloperstocreateJavaGUIapplications.WithJava8andthelambdafeature,anonymousclassesarenotsoimportantthesedays,andwiththeemergingJavaScriptandbrowsertechnology,theJavaGUIbecamelesspopular.

Whenaclassisdefinedinafileonitsown,itiscalledatop-levelclass.Classesthatareinsideanotherclassare,obviously,nottop-levelclasses.Iftheyaredefinedinsideaclassonthesamelevelasfields(variablesthatarenotlocaltosomemethodorotherblock),theyareinnerornestedclasses.Therearetwodifferencesbetweenthem.Oneisthatnestedclasseshavethestatickeywordbeforetheclasskeywordattheirdefinition,andinnerclassesdon't.

Theotherdifferenceisthatinstancesofnestedclassescanexistwithoutaninstanceofthesurroundingclass.Innerclassinstancesalwayshaveareferencetoaninstanceofthesurroundingclass.

Becauseinnerclassinstancescannotexistwithoutaninstanceofthesurroundingclass,theirinstancecanonlybecreatedbyprovidinganinstanceoftheouterclass.Wewillseenodifferenceifthesurroundingclassinstanceistheactualthisvariable,butifwewanttocreateaninstanceofaninnerclassfromoutsidethesurroundingclass,thenwehavetoprovideaninstancevariablebeforethenewkeywordseparatedbyadot,justlikeifnewwereamethod.Forexample,wecouldhaveaclassnamedTopLevelthathasaclassnamedInnerClass,likeinthefollowingcodesnippet:

publicclassTopLevel{

classInnerClass{}

}

ThenwecancreateaninstanceoftheInnerClassfromoutsidewithonlyaTopLevelobject,likeinthissnippet:

TopLeveltl=newTopLevel();

InnerClassic=tl.newInnerClass();

Asinnerclasseshaveanimplicitreferencetoaninstanceoftheenclosingclass,thecodeinsidetheinnerclasscanaccessthefieldsandthemethodsoftheenclosingclass.

Nestedclassesdonothaveanimplicitreferencetoanyinstanceoftheenclosingclass,andtheymaybeinstantiatedwiththenewkeywordwithoutanyreferencetoanyinstanceofanyotherclass.Becauseofthat,theycannotaccessthefieldsoftheenclosingclassunlesstheyarestaticfields.

Localclassesareclassesthataredefinedinsideamethod,constructor,oraninitializerblock.Wewillsoontalkaboutinitializerblocksandconstructors.Localclassescanbeusedinsidetheblockwheretheyaredefined.

Anonymousclassesaredefinedandinstantiatedinasinglecommand.Theyareashortformofanested,inner,orlocalclass,andtheinstantiationoftheclass.Anonymousclassesalwaysimplementaninterfaceorextendanamedclass.Thenewkeywordisfollowedbythenameoftheinterfaceortheclasswiththeargumentlisttotheconstructorbetweenparentheses.Theblockthatdefinesthebodyoftheanonymousclassstandsimmediatelyaftertheconstructorcall.Inthecaseofextendinganinterface,theconstructorcanonlybetheonewithoutargument.Theanonymousclasswithnonamecannothaveitsownconstructors.InmodernJavaweusuallyuselambdainsteadofanonymousclasses.

Lastbutnotleast—well,actually,leastIshouldmentionthatnestedandinnerclassescanalsobenestedindeeperstructures.Innerclassescannotcontainnestedclasses,butnestedclassescancontaininnerclasses.Why?Ihavenevermetanyonewhocouldreliablytellmetherealreason.Thereisnoarchitecturalreason.Itcouldbelikethat.Javadoesnotpermitthat.However,itisnotreallyinteresting.Ifyouhappentowritecodethathasmorethanonelevelofclassnestingthenjuststopdoingit.Mostprobablyyouaredoingsomethingwrong.

PackagesClassesareorganizedintopackagesandthefirstcodelineinafileshouldspecifythepackagethattheclassisin.

packagepackt.java9.by.example.stringsort;

Ifyoudonotspecifythepackage,thentheclasswillbeinthedefaultpackage.Thisshouldnotbeused,exceptinthesimplestcasewhenyouwanttotrysomecode.WithJava9,youcanusejshellforthispurpose,so,asopposedtopreviousversionsofJava,nowthesuggestionbecomesverysimple—neverputanyclassinthedefaultpackage.

Thenameofthepackagesishierarchical.Thepartsofthenamesareseparatedbydots.Usingpackagenameshelpsyouavoidnamecollisions.Namesoftheclassesareusuallykeptshortandputtingthemintopackageshelpstheorganizationoftheprogram.Thefullnameofaclassincludesthenameofthepackagetheclassisin.Usually,wewillputthoseclassesintoapackagethatareinsomewayrelated,andaddsomethingtoasimilaraspectofaprogram.Forexample,controllersinanMVCpatternprogramarekeptinasinglepackage.Packagesalsohelpyouavoidnamecollisionofclasses.However,thisonlypushestheproblemfromclassnamecollisiontopackagenamecollision.Wehavetomakesurethatthenameofthepackageisuniqueanddoesnotcauseanyproblemwhenourcodeisusedtogetherwithanyotherlibrary.Whenanapplicationisdeveloped,wejustcannotknowwhatotherlibrarieswillbeusedinlaterversions.Tobepreparedfortheunexpected,theconventionistonamethepackagesaccordingtosomeInternetdomainnames.Whenadevelopmentcompanyhasthedomainnameacmecompany.com,thentheirsoftwareisusuallyunderthecom.acmecompany...packages.Itisnotastrictlanguagerequirement.Itisonlyaconventiontowritethedomainnamefromrighttoleft,anduseitaspackagename,butthisprovestobefairlygoodinpractice.Sometimes,likeIdointhisbook,onecandeviatefromthispracticesoyoucanseethatthisruleisnotcarvedinstone.

Whentherubberhitstheroad,andthecodeiscompiledintobytecode,thepackagebecomesthenameoftheclass.Thus,thefullnameoftheSortclassispackt.java9.by.example.stringsort.Sort.Whenyouuseaclassfromanotherpackage,youcanusethisfullnameorimporttheclassintoyourclass.Again,thisisonthelanguagelevel.UsingthefullyqualifiednameorimportingmakesnodifferencewhenJavabecomesbytecode.

MethodsWehavealreadydiscussedmethods,butnotindetail,andtherearestillsomeaspectsthatweshouldmeetbeforewegoon.

Therearetwomethodsinthesampleclasses.Therecanbemanymethodsinaclass.Methodnamesarealsocamelcasedbyconvention,andthenamestartswithalowercaseletter,asopposedtoclasses.Methodsmayreturnavalue.Ifamethodreturnsavalue,themethodhastodeclarethetypeofthevalueitreturnsand,inthatcase,anyexecutionofthecodehastofinishwithareturnstatement.Thereturnstatementhasanexpressionafterthekeyword,whichisevaluatedwhenthemethodisexecutedandisreturnedbythemethod.Itisgoodpracticetohaveonlyonesinglereturnfromamethodbut,insomesimplecases,breakingthatcodingconventionmaybeforgiven.Thecompilerchecksthepossiblemethodexecutionpaths,anditisacompile-timeerrorifsomeofthepathsdonotreturnavalue.

Whenamethoddoesnotreturnanyvalue,ithastobedeclaredtobevoid.Thisisaspecialtypethatmeansnovalue.Methodsthatarevoid,suchasthepublicstaticvoidmainmethod,maysimplymissthereturnstatementandjustend.Ifthereisareturnstatement,thereisnoplaceforanyexpressiondefiningareturnvalueafterthereturnkeyword.Again,thisisacodingconventiontonotusethereturnstatementincaseofamethodthatdoesnotreturnanyvalue,butinsomecodingpatterns,thismaynotbefollowed.

Methodscanbeprivate,protected,public,andstatic,andwewilldiscusstheirmeaninglater.

Wehaveseenthatthemainmethodthatwasinvokedwhentheprogramstartedisastaticmethod.Suchamethodbelongstotheclassandcanbeinvokedwithouthavinganyinstanceoftheclass.Staticmethodsaredeclaredwiththestaticmodifier,andtheycannotaccessanyfieldormethodthatisnotstatic.

Inourexample,thesortmethodisnotstatic,butasitdoesnotaccessanyfieldanddoesnotcallanynon-staticmethod(asamatteroffact,itdoesnotcallanymethodatall),itcouldjustaswellbestatic.Ifwechangethedeclarationofthemethodtopublicstaticvoidsort(String[]names){(notethewordstatic),theprogramstillworks,buttheIDEwillgiveawarningwhileediting,forexample:

Staticmember'packt.java9.by.example.stringsort.Sort.sort(java.lang.String[])'accessedviainstancereference

ThatisbecauseyoucanaccessthemethodwithoutaninstancedirectlythroughthenameoftheSort.sort(actualNames);classwithouttheneedofthesortervariable.CallingastaticmethodviaaninstancevariableispossibleinJava(againsomethingthatseemedtobeagoodideaatthegenesisofJava,butisprobablynot),butitmaymisleadthereaderofthecodeintothinkingthatthemethodisaninstancemethod.

Makingthesortmethodstatic,themainmethodcanbeasfollows:

publicstaticvoidmain(String[]args){

String[]actualNames=newString[]{

"Johnson","Wilson",

"Wilkinson","Abraham","Dagobert"

};

Sort.sort(actualNames);

for(finalStringname:actualNames){

System.out.println(name);

}

}

Itseemstobemuchsimpler(itis),and,incasethemethoddoesnotuseanyfield,youmaythinkthatthereisnoreasontomakeamethodnon-static.DuringthefirsttenyearsofJava,staticmethodswereinheavyuse.Thereisevenaterm,utilityclass,whichmeansaclassthathasonlystaticmethodsandshouldnotbeinstantiated.WiththeadventofInversionofControlcontainers,wetendtouselessstaticmethods.Whenstaticmethodsareused,itishardertousedependencyinjection,anditisalsomoredifficulttocreatetests.Wewilldiscusstheseadvancedtopicsinthenextfewchapters.Fornow,youareinformedastowhatstaticmethodsareandthattheycanbeused;however,usually,unlessthereisaveryspecialneedforthem,wewillavoidthem.

Later,wewilllookathowclassesareimplementedinthehierarchy,andhowclassesmayimplementinterfacesandextendotherclasses.Whenthesefeaturesarelookedat,wewillseethatthereareso-calledabstractclassesthatmaycontainabstractmethods.Thesemethodshavetheabstractmodifier,andtheyarenotdefined—onlythename,argumenttypes(andnames),andreturntypearespecified.Aconcrete(non-abstract)classextendingtheabstractclassshoulddefinethem.

Theoppositeofabstractmethodisthefinalmethoddeclaredwiththefinalmodifier.Afinalmethodcannotbeoverriddeninsubclasses.

InterfacesMethodsarealsodeclaredininterfaces.Amethoddeclaredinaninterfacedoesnotdefinetheactualbehaviorofthemethod;theydonotcontainthecode.Theyhaveonlytheheadofthemethod;inotherwords,theyareabstractimplicitly.Althoughnobodydoes,youmayevenusetheabstractkeywordinaninterfacewhenyoudefineamethod.

Interfaceslookverysimilartoclasses,butinsteadofusingtheclasskeyword,weusetheinterfacekeyword.Becauseinterfacesaremainlyusedtodefinemethods,themethodsarepublicifnomodifierisused.

Interfacescanalsodefinefields,butsinceinterfacescannothaveinstances(onlyimplementingclassescanhaveinstances),thesefieldsareallstaticandtheyalsohavetobefinal.Thisisthedefaultforfieldsininterfaces,thuswedonotneedtowritetheseifwedefinedfieldsininterfaces.

Itwasacommonpracticetodefineonlyconstantsinsomeinterfacesandthenusetheseinclasses.Todothat,theeasiestwaywastoimplementtheinterface.Sincetheseinterfacesdonotdefineanymethod,theimplementationisnothingmorethanwritingtheimplementskeywordandthenameoftheinterfaceintotheheaderoftheclassdeclaration.Thisisbadpracticebecausethiswaytheinterfacebecomespartofthepublicdeclarationoftheclass,althoughtheseconstantsareneededinsidetheclass.Ifyouneedtodefineconstantsthatarenotlocaltoaclassbutareusedinmanyclasses,thendefinetheminaclassandimportthefieldsusingimportstaticorjustusethenameoftheclassandthefield.

Interfacescanalsohavenestedclasses,buttheycannothaveinnerclasses.Theobviousreasonforthatisthatinnerclassinstanceshaveareferencetoaninstanceoftheenclosingclass.Inthecaseofaninterface,therearenoinstances,soaninnerclasscouldnothaveareferencetoaninstanceofanenclosinginterface,becausethatjustdoesnotexist.Thejoyfulpartofitisthatwedonotneedtousethestatickeywordinthecaseofnestedclassesbecausethatisthedefault,justasinthecaseoffields.

WiththeadventofJava8,youcanalsohavedefaultmethodsininterfacesthatprovidedefaultimplementationofthemethodfortheclassesthatimplementtheinterface.TherecanalsobestaticandprivatemethodsininterfacessinceJava9.

Methodsareidentifiedbytheirnameandtheargumentlist.Youcanreuseanameforamethodandhavedifferentargumenttypes;Javawillidentifywhichmethodtousebasedonthetypesoftheactualarguments.Thisiscalledmethodoverloading.Usually,itiseasytotellwhichmethodyoucall,butwhentherearetypesthatextendeachother,thesituationbecomesmorecomplex.Thestandarddefinesverypreciserulesfortheactualselectionofthemethodthatthecompilerfollows,sothereisnoambiguity.However,fellowprogrammerswhoreadthecodemaymisinterpretoverloadedmethodsor,atleast,willhavehardtimeidentifyingwhichmethodisactuallycalled.Methodoverloadingmayalsohinderbackwardcompatibilitywhenyouwanttoextendyourclass.Thegeneraladviceistothinktwicebeforecreatingoverloadedmethods.Theyarelucrative,butmaysometimesbecostly.

ArgumentpassingInJava,argumentsarepassedbyvalue.Whenthemethodmodifiesanargumentvariable,thenonlythecopyoftheoriginalvalueismodified.Anyprimitivevalueiscopiedduringthemethodcall.Whenanobjectispassedasanargument,thenthecopyofthereferencetotheobjectispassed.

Thatway,theobjectisavailabletobemodifiedforthemethod.Inthecaseofclassesthathavetheirprimitivecounterpart,andalsointhecaseofStringandsomeotherclasstypes,theobjectssimplydonotprovidemethodsorfieldstomodifythestate.Thisisimportantfortheintegrityofthelanguage,andtonotgetintotroublewhenobjectsandprimitivevaluesautomaticallygetconverted.

Inothercases,whentheobjectismodifiable,themethodcaneffectivelyworkontheveryobjectitwaspassedto.Thisisalsothewaythesortmethodinourexampleworksonthearray.Thesamearray,whichisalsoanobjectitself,ismodified.

Thisargumentpassingismuchsimplerthanitisinotherlanguages.Otherlanguagesletthedevelopermixthepassbyreferenceandthepassbyvalueargumentpassing.InJava,whenyouuseavariablebyitselfasanexpressiontopassaparametertoamethod,youcanbesurethatthevariableitselfisnevermodified.Theobjectitrefersto,however,incaseitismutable,maybemodified.

Anobjectismutableifitcanbemodified,alteringthevalueofsomeofitsfielddirectlyorviasomemethodcall.Whenaclassisdesignedinawaythatthereisnonormalwaytomodifythestateoftheobjectafterthecreationoftheobject,theobjectisimmutable.TheclassesByte,Short,Integer,Long,Float,Double,Boolean,Character,aswellasString,aredesignedintheJDKsothattheobjectsareimmutable.Itispossibletoovercomethelimitationofimmutabilityimplementationofcertainclassesusingreflection,butdoingthatishackingandnotprofessionalcoding.Doingthatcanbedoneforonesinglepurpose—gettingabetterknowledgeandunderstandingoftheinnerworkingsofsomeJavaclasses,butnothingelse.

FieldsFieldsarevariablesontheclasslevel.Theyrepresentthestateofanobject.Theyarevariables,withdefinedtypeandpossibleinitialvalue.Fieldscanbestatic,final,transient,andvolatile,andtheaccessmaybemodifiedwiththepublic,protected,andprivatekeywords.

Staticfieldsbelongtotheclass.Itmeansthatthereisoneofthemsharedbyalltheinstancesoftheclass.Normal,non-staticfieldsbelongtotheobjects.Ifyouhaveafieldnamedf,theneachinstanceoftheclasshasitsownf.Iffisdeclaredstatic,thentheinstanceswillsharetheverysameffield.

Thefinalfieldscannotbemodifiedaftertheyareinitialized.Initializationcanbedoneonthelinewheretheyaredeclared,inaninitializerblockorintheconstructorcode.Thestrictrequirementisthattheinitializationhastohappenbeforetheconstructorreturns.Thisway,themeaningofthefinalkeywordisverydifferent,inthiscase,fromwhatitmeansinthecaseofaclassoramethod.Afinalclasscannotbeextendedandafinalmethodcannotbeoverriddeninanextendingclass,aswewillseeinthenextchapter.Thefinalfieldsareeitheruninitializedorgetavalueduringinstancecreation.Thecompileralsochecksthatthecodedoesinitializeallfinalfieldsduringtheobject-instancecreationorduringtheclassloading,incasethefinalfieldisstatic,andthatthecodeisnotaccessing/readinganyfinalfieldthatwasnotyetinitialized.

Itisacommonmisconceptionthatthefinalfieldshavetobeinitializedatthedeclaration.Itcanbedoneinaninitializercodeorinaconstructor.Therestrictionisthat,nomatterwhichconstructoriscalledincasetherearemore,thefinalfieldshavetobeinitializedexactlyonce.

Thetransientfieldsarenotpartoftheserializedstateoftheobject.Serializationisanactofconvertingtheactualvalueofanobjecttophysicalbytes.Deserializationistheoppositewhentheobjectiscreatedfromthebytes.Itisusedtosavethestateinsomeframeworks.Thecodethatdoestheserialization,java.lang.io.ObjectOutputStream,worksonlywithclassesthatimplementtheSerializableinterface,andusesonlythefieldsfromthoseobjectsthatarenottransient.Veryobviously,transientfieldsarealsonotrestoredfromthebytesthatrepresenttheserializedformoftheobjectbecausetheirvalueisnotthere.

Serializationisusuallyusedindistributedprograms.Agoodexampleisthesessionobjectofaservlet.Whentheservletcontainerrunsonaclusterednode,somefieldsofobjectsstoredintothesessionobjectmaymagicallydisappearbetweenHTTPhits.Thatisbecauseserializationsavesandreloadsthesessiontomovethesessionbetweenthenodes.Serialization,insuchasituation,mayalsobeaperformanceissueifadeveloperdoesnotknowthesideeffectsofthestoredlargeobjectsinthesession.

Thevolatilekeywordisakeywordthattellsthecompilerthatthefieldmaybeusedbydifferentthreads.Whenavolatilefieldisaccessedbyanycode,theJITcompilergeneratescodewhichensuresthatthevalueofthefieldaccessedisuptodate.Whenafieldisnotvolatile,thecompiler-generatedcodemaystorethevalueofthefieldinaprocessorcacheorregistryforfasteraccesswhenitseesthatthevaluewillbeneededsoonbysomesubsequentcodefragment.Inthecaseofvolatilefields,thisoptimizationcannotbedone.Additionally,notethatsavingthevaluetomemoryandloadingfromthereallthetimemaybe50ormoretimesslowerthanaccessingavaluefromaregistryorcache.

ModifiersMethods,constructors,fields,interfaces,andclassescanhaveaccessmodifiers.Thegeneralruleisthatincasethereisnomodifier,thescopeofthemethod,constructor,andsoon,isthepackage.Anycodeinthesamepackagecanaccessit.

Whentheprivatemodifierisused,thescopeisrestrictedtotheso-calledcompilationunit.Thismeanstheclassthatisinonefile.Whatisinsideonefilecanseeanduseanythingdeclaredtobeprivate.Thisway,innerandnestedclassescanhaveaccesstoeachother'sprivatevariables,whichmaynotreallybeagoodprogrammingstyle,butJavapermitsthat.

Theoppositeofprivateispublic.ItextendsthevisibilitytothewholeJavaprogram,oratleasttothewholemodule,iftheprojectisaJava9module.

Thereisamiddleway:protected.Anythingwiththismodifierisaccessibleinsidethepackageandalsoinclassesthatextendtheclass(regardlessofpackage)thattheprotectedmethod,field,andsoon,isin.

ObjectinitializersandconstructorsWhenanobjectisinstantiated,theappropriateconstructoriscalled.Theconstructordeclarationlookslikeamethodwiththefollowingdeviation:theconstructordoesnothaveareturnvalue.Thatisbecausetheconstructorsworkonthenot-fully-readyinstancewhenthenewcommandoperatorisinvokedanddoesnotreturnanything.Constructors,havingthesamenameastheclass,cannotbedistinguishedfromeachother.Ifthereisaneedformorethanoneconstructor,theyhavetobeoverloaded.Constructors,thus,cancalleachother,almostasiftheywerevoidmethodswithdifferentarguments.However,thereisarestriction—whenaconstructorcallsanother,ithastobetheveryfirstinstructionintheconstructor.Youusethis()syntaxwithanappropriateargumentlist,whichmaybeempty,toinvokeaconstructorfromanotherconstructor.

Theinitializationoftheobjectinstancealsoexecutesinitializerblocks.Theseareblockscontainingexecutablecodeinsidethe{and}charactersoutsidethemethodsandconstructors.Theyareexecutedbeforetheconstructorintheordertheyappearinthecode,togetherwiththeinitializationofthefieldsincasetheirdeclarationscontainvalueinitialization.

Ifyouseethestatickeywordinfrontofaninitializerblock,theblockbelongstotheclassandisexecutedwhentheclassisloadedalongwiththestaticfieldinitializers.

CompilingandrunningtheprogramFinally,wewillcompileandexecuteourprogramfromthecommandline.Thereisnothingnewinthisone;wewillonlyapplywhatwehavelearnedinthischapterusingthefollowingtwocommands:$mvnpackage

Thiscompilestheprogram,packagestheresultintoaJARfile,andfinallyexecutesthefollowingcommand:

$java-cptarget/SortTutorial-1.0.0-SNAPSHOT.jarpackt.java9.by.example.App

Thiswillprintthefollowingresultonthecommandline:

Summary

Inthischapter,wehavedevelopedaverybasicsortalgorithm.ItwasmadepurposefullysimplesothatwecouldreiteratethebasicandmostimportantJavalanguageelements,classes,packages,variables,methods,andsoon.Wealsolookedatbuildtools,sowearenotemptyhandedinthenextchapterswhenprojectswillcontainmorethanjusttwofiles.WewilluseMavenandGradleinthefollowingchapters.

Intheverynextchapter,wewillmakethesortprogrammorecomplex,implementingmoreeffectivealgorithmsandalsomakingourcodeflexible,givingustheopportunitytolearnmoreadvancedJavalanguagefeatures.

OptimizingtheSort-MakingCodeProfessionalInthischapter,wewilldevelopthesortingcodeandmakeitmoregeneral.WewanttosortnotonlyanarrayofStrings.Essentially,wewillwriteaprogramthatcansortanythingthatissortable.Thatway,wewillbringthecodingtoitsfullextenttowardoneofthemajorstrengthsofJava:abstraction.

Abstraction,however,doesnotcomewithoutapricetag.Whenyouhaveaclassthatsortsstringsandyouaccidentallymixanintegerorsomethingelse,whichisnotastring,intothesortabledata,thenthecompilerwillcomplainaboutit:JavadoesnotallowyoutoputanintintoaStringarray.Whenthecodeismoreabstract,suchprogrammingerrorsmayslipin.WewilllookathowtohandlesuchexceptionalcasescatchingandthrowingExceptions.

Toidentifythebugs,wewilluseunittesting,applyingtheindustrystandardJUnitversion4.AsJUnitheavilyusesannotation,andbecauseannotationsareimportant,youwilllearnaboutitabit.

Afterthat,wewillmodifythecodetousethegenericsfeatureofJavathatwasintroducedintothelanguageinversion5.Usingthatpossibility,wewillcatchthecodingerrorduringcompilationtime,whichisbetterthanduringruntime.Theearlierabugisidentified,thecheaperitistofix.

Forthebuild,wewillstilluseMaven,butthistime,wewillsplitthecodeintosmallmodules.Thus,wewillhaveamulti-moduleproject.Wewillhaveseparatemodulesforthedefinitionofasortingmoduleandforthedifferentimplementations.Thatway,wewilllookathowclassescanextendeachotherandimplementinterfaces,and,generally,wewillreallystarttoprogramintheobject-orientedway.

WewillalsodiscussTestDrivenDevelopment(TDD),andattheendofthesection,wewillstartusingthebrandnewfeatureofJava9:modulesupport.

Inthischapter,wewillcoverthefollowingtopics:

Object-orientedprogrammingprinciplesUnittestingpracticesAlgorithmiccomplexityandquicksortExceptionhandlingRecursivemethodsModulesupport

ThegeneralsortingprogramInthepreviouschapter,weimplementedasimplesortalgorithm.ThecodecansortelementsofaStringarray.Wedidthistolearn.Forpracticaluse,thereisareadycookedsortsolutionintheJDKthatcansortmembersofcollections,whicharecomparable.

TheJDKcontainsautilityclasscalledCollections.ThisclasscontainsastaticCollections.sortmethodthatiscapableofsortinganyListthathasmembersthatareComparable.ListandComparableareinterfacesdefinedintheJDK.Thus,ifwewanttosortalistofStrings,thesimplestsolutionisasfollows:

publicclassSimplestStringListSortTest{

@Test

publicvoidcanSortStrings(){

ArrayListactualNames=newArrayList(Arrays.asList(

"Johnson","Wilson",

"Wilkinson","Abraham","Dagobert"

));

Collections.sort(actualNames);

Assert.assertEquals(newArrayList<String>(Arrays.<String>asList(

"Abraham","Dagobert","Johnson","Wilkinson","Wilson")),actualNames);

}

}

ThiscodefragmentisfromasampleJUnittest,whichisthereasonwehavethe@Testannotationinfrontofthemethod.Wewilldiscussthatindetaillater.Toexecutethattest,youcanissuethefollowingcommand:

$mvn-Dtest=SimplestStringListSortTesttest

Thissortimplementation,however,doesnotfitourneeds.Firstofall,becauseitisthereready(noneedtocode)andusingitdoesnotneedanythingnewthatyouhavenotlearnedinthepreviouschapters.Exceptfortheannotationinfrontofthemethod,thereisnothingnewinthecodethatyoucannotunderstand.YoumayrefreshBYturningsomepagesback,orelseconsulttheoracleonlinedocumentationoftheJDK(https://docs.oracle.com/javase/8/docs/api/),butthatisall.Youalreadyknowthesethings.

YoumaywonderwhyIwrotetheURLfortheJavaversion8APItothelink.Well,thenthisisthemomentofhonestyandtruth-whenIwrotethisbook,theJava9JDKwasnotavailableinitsfinalform.IcreatedmostoftheexamplesonmyMacBookusingJava8andIonlytestedthefeaturesthatareJava9specific.SupportatthemomentforJava9intheIDEsisnotperfect.Whenyoureadthisbook,Java9willbeavailable,soyoucantryandchangethatonesingledigitfrom8to9intheURLandgettothedocumentationoftheversion9.Atthemoment,IgetHTTPERROR404.Sometimes,youmayneedthedocumentationofolderversions.Youcanuse3,4,5,6,or7insteadof8intheURL.Documentationfor3and4isnotavailabletoreadonline,butitcanbedownloaded.Hopefully,youwillneverneedthatanymore.Version5,perhaps.Version6isstillwidelyusedatlargecorporations.

Althoughyoucanlearnalotfromreadingcodethatwaswrittenbyotherprogrammers,IdonotrecommendtryingtolearnfromtheJDKsourcecodeatthisearlystageofyourstudies.Theseblocksofcodeareheavilyoptimized,notmeanttobetutorialcodes,andold.Theydonotgetrustedduringtheyears,buttheywerenotrefactoredtofollowthecodingstylesofJavaasitmatured.Atsomeplaces,you

canfindreallyuglycodeintheJDK.

Okay,sayingthatweneedtodevelopanewsortcodebecausewecanlearnfromit,isabitcontrived.TherealreasonwhyweneedasortimplementationisthatwewantsomethingthatcansortnotonlyListdatatypesandaListofsomethingthatimplementstheComparableinterface.Wewanttosortabunchofobjects.Allwerequireisthatthebunchcontainingtheobjectsprovidessimplemethodsthatarejustenoughtosortthemandhaveasortedbunch.

OriginallyIwantedtousethewordcollectioninsteadofbunch,butthereisaCollectioninterfaceinJavaandIwantedtoemphasizethatwearenottalkingaboutjava.util.Collectionofobjects.

WealsodonotwanttheobjectstoimplementtheComparableinterface.IfwerequiretheobjecttoimplementtheComparableinterface,itmayviolatetheSingleResponsibilityPrinciple(SRP).

Whenwedesignaclass,itshouldmodelsomeobjectclassoftherealworld.Wewillmodeltheproblemspacewithclasses.Theclassshouldimplementthefeaturesthatrepresentthebehavioroftheobjectsthatitmodels.Ifwelookattheexampleofstudentsfromthesecondchapter,thenaStudentclassshouldrepresentthefeaturesthatallstudentsshare,andisimportantfromthemodelingpointofview.AStudentobjectshouldbeabletotellthenameofthestudent,theage,theaveragescoresofthelastyear,andsoon.Allstudentshavefeet,andcertainlyeachofthosefeethavesizesowemaythinkthataStudentclassshouldalsoimplementamethodthatreturnsthesizeofthestudent'sfoot(onefortheleftandonefortherightjusttobeprecise),butwedonot.Wedonot,becausethesizeofthefootisirrelevantfromthemodelpointofview.IfwewanttosortalistcontainingStudentobjects,theStudentclasshastoimplementtheComparableinterface.Butwait!Howdoyoucomparetwostudents?Bynames,byage.orbytheaveragescoreofthem?

ComparingastudenttoanotherisnotafeatureoftheStudent.Everyclass,orforthatmatter,package,library,orprogrammingunitshouldhaveoneresponsibilityanditshouldimplementonlythatandnothingelse.Itisnotexact.Thisisnotmathematics.Sometimes,itishardtotellifafeaturefitsintotheresponsibilityornot.Therearesimpletechniques.Forexample,incaseofastudent,youcanasktherealpersonabouthisnameandage,andprobablytheycanalsotellyoutheiraveragescore.IfyouaskoneofthemtocompareTo(anotherstudent),astheComparableinterfacerequiresthismethod,theywillprobablyaskback,butbywhatattribute?Orhow?Orjust,what?Insuchacase,youcansuspectthatimplementingthefeatureisprobablynotintheareaofthatclassandthisconcern;thecomparisonshouldbesegregatedfromtheimplementationoftheoriginalclass.ThisisalsocalledSegregationofConcerns,whichiscloselyrelatedtoSRP.

JDKdeveloperswereawareofthis.Collections.sortthatsortsaListofComparableelementsisnottheonlysortingmethodinthisclass.ThereisanotherthatjustsortsanyListifyoupassasecondargumentandobjectthatimplementstheComparatorinterfaceandiscapableofcomparingtwoelementsofList.Thisisacleanpatterntoseparatetheconcerns.Insomecases,separatingthecomparisonisnotneeded.Inothercases,itisdesirable.TheComparatorinterfacedeclaresonesinglemethodthattheimplementingclasseshavetoprovide:compare.Ifthetwoargumentsareequal,thenthismethodreturns0.Iftheyaredifferent,itshouldreturnanegativeorapositiveintdependingonwhichargumentprecedeswhichone.

TherearealsosortmethodsintheJDKclass,java.util.Arrays.Theysortarrays,oronlyasliceofanarray.Themethodisagoodexampleofmethodoverloading.Therearemethodswiththesamename,butwithdifferentargumentstosortawholearrayforeachprimitivetype,forasliceofeach,andalsotwoforobjectarrayimplementingtheComparableinterface,andalsoforobjectarraytobesortedusingComparator.Asyousee,thereisawholerangeofsortimplementationsavailableintheJDK,andin99percentofthecases,youwillnotneedtoimplementasortyourself.Thesortsusethesamealgorithm,astablemergesortwithsomeoptimization.

Whatwewillimplementisageneralapproachthatcanbeusedtosortlists,arrays,orjustanythingthathaselementsanditispossibletoswapanytwoelementsofit;thesolutionwillbeabletousethebubblesortthatwehavealreadydevelopedandalsootheralgorithms.

AbriefoverviewofvarioussortingalgorithmsTherearemanydifferentsortingalgorithms.AsIsaid,therearesimplerandmorecomplexalgorithmsand,inmanycases,morecomplexalgorithmsaretheonesthatrunfaster.Inthischapter,wewillimplementthebubblesortandquicksort.Wehavealreadyimplementedthebubblesortforstringsinthepreviouschapter,sointhiscase,theimplementationwillmainlyfocusontherecodingforgeneralsortableobjectsorting.Implementingquicksortwillinvolveabitofalgorithmicinterest.

Bewarnedthatthissectionisheretogiveyouonlyatasteofalgorithmiccomplexity.ItisfarfrompreciseandIaminthevainhopethatnomathematicianreadsthisandputsacurseonme.Someoftheexplanationsarevague.Ifyouwanttolearncomputerscienceindepth,thenafterreadingthisbook,findsomeotherbooksorvisitonlinecourses.

Whenwetalkaboutthegeneralsortingproblem,wewillthinkaboutsomegeneralsetofobjectsthatcanbecomparedandanytwoofthemcanbeswappedwhilewesort.Wewillalsoassumethatthisisanin-placesort;thus,wedonotcreateanotherlistorarraytocollecttheoriginalobjectsinsortedorder.Whenwetalkaboutthespeedofanalgorithm,wearetalkingaboutsomeabstractthingandnotmilliseconds.Whenwewanttotalkaboutmilliseconds,actualreal-worldduration,weshouldalreadyhavesomeimplementationinsomeprogramminglanguagerunningonarealcomputer.

Algorithms,intheirabstractform,don'tdothatwithoutimplementation.Still,itisworthtalkingaboutthetimeandmemoryneedofanalgorithm.Whenwedothat,wewillusuallyinvestigatehowthealgorithmbehavesforalargesetofdata.Forasmallsetofdata,mostalgorithmsarejustfast.Sortingtwonumbersisusuallynotanissue,isit?

Incaseofsorting,wewillusuallyexaminehowmanycomparisonsareneededtosortacollectionofnelements.Bubblesortneedsapproximatelyn2(ntimesn)comparisons.Wecannotsaythatthisisexactlyn2becauseincaseofn=2,theresultis1,forn=3itis3,forn=4itis6,andsoon.However,asnstartstogetlarger,theactualnumberofcomparisonsneededandn2willasymptoticallybeofthesamevalue.WesaythatthealgorithmiccomplexityofthebubblesortisO(n2).Thisisalsocalledthebig-Onotation.IfyouhaveanalgorithmthatisO(n2)anditworksjustfinefor1,000elementsfinishinginasecond,thenyoushouldexpectthesamealgorithmfinishingfor1millionelementsinaroundtendaysorinamonth.Ifthealgorithmislinear,sayO(n),thenfinishing1,000elementinonesecondshouldmakeyouexpect1milliontobefinishedin1,000seconds.Thatisabitlongerthanacoffeebreakandtooshortforlunch.

Thismakesitfeasiblethatifwewantsomeseriousbusinesssortingobjects,wewillneedsomethingbetterthanbubblesort.Thatmanyunnecessarycomparisonsarenotonlywastingourtime,butalsoCPUpower,consumingenergy,andpollutingtheenvironment.Thequestion,however,is:howfastcanasortbe?Isthereaprovableminimumthatwecannotovercome?

Theanswerisyes.

Whenweimplementanysortingalgorithm,theimplementationwillexecutecomparisonsandelementswaps.Thatistheonlywaytosortacollectionofobjects.Theoutcomeofacomparisoncanhavetwovalues.Say,thesevaluesare0or1.Thisisonebitofinformation.Iftheresultofthecomparisonis1,

thenweswap,iftheresultis0,thenwedonotswap.

Wecanhavetheobjectsindifferentordersbeforewestartthecomparisonandthenumberofdifferentordersisn!(nfactorial).Thatis,thenumbersmultipliedfrom1ton,inotherwordsn!=1*2*3*...*(n-1)*n.

Let'sassumethatwestoredtheresultoftheindividualcomparisonsinanumberasaseriesofbitsforeachpossibleinputforthesort.Now,ifwereversetheexecutionofthesortandrunthealgorithmstartingfromthesortedcollection,controltheswappingusingthebitsthatdescribedtheresultsofthecomparison,andweusethebitstheotherwayarounddoingthelastswapfirstandtheonethatwasdonefirstduringthesortingfirst,weshouldgetbacktheoriginalorderoftheobjects.Thisway,eachoriginalorderisuniquelytiedtoanumberexpressedasanarrayofbits.

Now,wecanexpresstheoriginalquestionthisway:howmanybitsareneededtodescribenfactorialdifferentnumbers?Thatisexactlythenumberofcomparisonswewillneedtosortnelements.Thenumberofbitsislog2(n!).Usingsomemathematics,wewillknowthatlog2(n!)isthesameaslog2(1)+log2(2)+...+log2(n).Ifwelookatthisexpression'sasymptoticvalue,thenwecansaythatthisisthesameO(n*logn).Weshouldnotexpectanygeneralsortingalgorithmtobefaster.

Forspecialcases,therearefasteralgorithms.Forexample,ifwewanttosort1millionnumbersthatareeachbetweenoneand10,thenweonlyneedtocountthenumberofthedifferentnumbersandthencreateacollectionthatcontainsthatmanyones,twos,andsoon.ThisisanO(n)algorithm,butthisisnotapplicableforthegeneralcase.

Again,thiswasnotaformalmathematicalproof.

QuicksortSirCharlesAntonyRichardHoaredevelopedthequicksortalgorithmin1959.Itisatypicaldivideandconqueralgorithm.Tosortalongarray,pickanelementfromthearraythatwillbethepivotelement.Then,partitionthearraysothattheleftsidewillcontainalltheelementsthataresmallerthanthepivotandtherightsidewillcontainalltheelementsthatarelargerthan,orequaltothepivot.Whenthisisdone,theleftsideandtherightsideofthearraycanbesortedbycallingthesortrecursively.Tostoptherecursion,whenwehaveonesingleelementinthearray,wewilldeclareitsorted.

Wetalkaboutarecursivealgorithmwhenthealgorithmisdefinedpartiallyusingitself.ThemostfamousrecursivedefinitionistheFibonacciseriesthatis0and1forthefirsttwoelementsandanylaterelementthenthelementisthesumofthe(n-1)thandthe(n-2)thelement.Recursivealgorithmsaremanytimesimplementedinmodernprogramminglanguagesimplementingamethodthatdoessomecalculationbutsometimescallsitself.Whendesigningrecursivealgorithms,itisofutmostimportancetohavesomethingthatstopstherecursivecalls;otherwise,recursiveimplementationwillallocateallmemoryavailablefortheprogramstackandstoptheprogramwitherror.

Thepartitioningalgorithmgoesthefollowingway:wewillstarttoreadthearrayusingtwoindicesfromthestartandend.Wewillfirststartwiththeindexthatissmallandincreasetheindexuntilitissmallerthanthelargeindex,oruntilwefindanelementthatisgreaterthanorequaltothepivot.Afterthis,wewillstarttodecreasethelargerindexsolongasitisgreaterthanthesmallindexandtheelementindexedisgreaterthanorequaltothepivot.Whenwestop,weswapthetwoelementspointedbythetwoindices,iftheindicesarenotthesame,andwewillstartincreasinganddecreasingthesmallandlargeindices,respectively.Iftheindicesarethesame,thenwearefinishedwiththepartitioning.Theleftsideofthearrayisfromthestarttotheindexwheretheindicesmetminusone;therightsidestartswiththeindexandlastsuntiltheendoftheto-be-sortedarray.

ThisalgorithmisusuallyO(nlogn),butinsomecasesitcandegradetobeO(n2),dependingonhowthepivotischosen.Therearedifferentapproachesfortheselectionofthepivot.Inthisbook,wewillusethesimplest:wewillselectthefirstelementofthesortablecollectionasapivot.

<project><br/>...<br/><modules><br/><module>SortInterface</module><br/><module>bubble</module><br/><module>quick</module><br/></modules><br/></project>

$tree<br/>|-SortInterface<br/>|---src/main/java/packt/java9/by/example/ch03<br/>|-bubble<br/>|---src<br/>|-----main/java/packt/java9/by/example/ch03/bubble<br/>|-----test/java/packt/java9/by/example/ch03/bubble<br/>|-quick/src/<br/>|-----main/java<br/>|-----test/java

MavendependencymanagementDependenciesarealsoimportantinthePOMfile.Thepreviousprojectdidnothaveanydependency,butthistimewewilluseJUnit.Dependenciesaredefinedinpom.xmlusingthedependenciestag.Forexample,thebubblesortmodulecontainsthefollowingpieceofcode:

<dependencies>

<dependency>

<groupId>packt.java9.by.example</groupId>

<artifactId>SortInterface</artifactId>

</dependency>

<dependency>

<groupId>junit</groupId>

<artifactId>junit</artifactId>

</dependency>

</dependencies>

Theactualpom.xmlinthecodesetyoucandownloadwillcontainmorecodethanthis.Inprint,weoftenpresentaversionoronlyafractionthathelpstheunderstandingofthetopicthatwearediscussingatthatpoint.

IttellsMaventhatthemodulecodeusesclasses,interfaces,andenumtypesthataredefinedinthesemodulesthatareavailablefromsomerepository.

WhenyouuseMaventocompilethecode,thelibrariesthatareusedbyyourcodeareavailablefromrepositories.WhenAntwasdeveloped,thenotionofrepositorieswasnotinvented.Atthattime,thedeveloperscopiedtheusedversionofthelibraryintoafolderinthesourcecodestructure.Usually,thedirectorylibwasusedforthepurpose.Thereweretwoproblemswiththisapproach.Oneisthesizeofthesourcecoderepository.If,forexample,100differentprojectsusedJUnit,thentheJARfileoftheJUnitlibrarywascopiedthere100times.Theotherproblemwastogatherallthelibraries.Whenalibraryusedanotherlibrary,thedevelopershadtoreadthedocumentationofthelibrarythatdescribed(manytimesoutdatedandnotprecise)whatotherlibrariesareneededtousethislibrary.Thoselibrarieshadtobedownloadedandinstalledthesameway.Thiswastimeconsuminganderrorprone.Whenalibrarywasmissingandthedevelopersjustdidnotnoticeit,theerrorwasmanifestedduringcompiletimewhenthecompilercouldnotfindtheclassorevenonlyatruntimewhentheJVMwasnotabletoloadtheclass.

Tosolvethisissue,Mavencomeswithabuilt-inrepositorymanagerclient.Therepositoryisastoragethatcontainsthelibraries.Astherecanbeothertypesoffilesinarepository,notonlylibraries,Maventerminologyisartifact.ThegroupId,theartifactId,andtheversionnumberidentifyanartifact.Thereisaverystrictrequirementthatanartifactcanonlybeputintoarepositoryonce.Evenifthereisanerrorduringthereleaseprocessthatisidentifiedaftertheerroneousreleasewasuploaded,theartifactcannotbeoverwritten.ForthesamegroupId,artifactId,andversion,therecanonlybeonesinglefilethatwillneverchange.Iftherewasanerror,thenanewartifactistobecreatedwithnewversionnumberandtheerroneousartifactmaybedeletedbutnotreplaced.

Iftheversionnumberendswith-SNAPSHOT,thenthisuniquenessisnotguaranteedorrequired.Snapshotsareusuallystoredinseparaterepositoryandarenotpublishedfortheworld.

Repositoriescontaintheartifactsindirectoriesthatareorganizedinadefinedway.WhenMavenruns,itcanaccessdifferentrepositoriesusinghttpsprotocol.

Formerly,thehttpprotocolwasalsoused,andfornon-payingcustomers,thecentralrepositorywasavailableviahttponly.However,itwasdiscoveredthatmodulesdownloadedfromtherepositorycouldbetargetsformen-in-the-middlesecurityattacksandSonatype(http://www.sonatype.com)changedthepolicyandusedhttpsprotocolonly.Neverconfigureorusearepositorywiththehttpprotocol.NevertrustafilethatyoudownloadedfromHTTP.

Thereisalocalrepositoryonthedevelopermachine,usuallyunderthe~/.m2/repositorydirectory.Whenyouissuethemvninstallcommand,Mavenstoresthecreatedartifacthere.MavenalsostoresanartifactherewhenitisdownloadedfromarepositoryviaHTTPS.Thisway,subsequentcompilationsdonotneedtogoouttothenetworkfortheartifacts.

Companiesusuallysetuptheirownrepositorymanager(theonethatSonatype,thecompanybackingMaven,isprovidingNexus).Theseapplicationscanbeconfiguredtocommunicatewithseveralotherrepositoriesandcollecttheartifactsfromthereondemand,essentiallyimplementingproxyfunctionality.Artifactstraveltothebuildfromthefarendrepositoriestothecloseronesinahierarchicalstructuretothelocalrepoandessentiallytothefinalartifactifthepackagingtypeoftheprojectiswar,ear,orsomeotherformatthatenclosesthedependentartifacts.Thisisessentiallyfilecachingwithoutrevalidationandcacheeviction.Thiscanbedonebecauseofthestrictrulesofartifactuniqueness.Thisisthereasonforsuchastrictrule.

Iftheprojectbubblewereastandaloneproject,andnotpartofamulti-moduleone,thenthedependencywouldlooklikethis:

<dependencies>

<dependency>

<groupId>packt.java9.by.example</groupId>

<artifactId>SortInterface</artifactId>

<version>1.0.0-SNAPSHOT</version>

</dependency>

<dependency>

<groupId>junit</groupId>

<artifactId>junit</artifactId>

<version>4.12</version>

</dependency>

</dependencies>

Ifversionisnotdefinedforadependency,Mavenwillnotbeabletoidentifywhichartifacttouse.Inthecaseofamulti-moduleproject,versioncanbedefinedintheparentandthemodulescaninherittheversion.Astheparentisnotdependentontheactualdependency,itonlydefinestheversionattachedtothegroupIdandartifactId;theXMLtagisnotdependencies,butdependencyManagement/dependenciesunderthetop-levelprojecttagasinthefollowingexample:

<dependencyManagement>

<dependencies>

<dependency>

<groupId>packt.java9.by.example</groupId>

<artifactId>SortInterface</artifactId>

<version>${project.version}</version>

</dependency>

<dependency>

<groupId>junit</groupId>

<artifactId>junit</artifactId>

<version>4.12</version>

<scope>test</scope>

</dependency>

</dependencies>

</dependencyManagement>

IftheparentPOMusesthedependenciestagdirectly,Mavenisnotabletodecideiftheparentdependsonthatartifactorsomemodules.Whenthemoduleswanttousejunit,theyneednotspecifytheversion.Theywillgetitfromtheparentprojectdefinedas4.12,whichisthelatestfromJUnit4.Ifevertherewillbeanewversion4.12.1,withsomeseriousbugsfixed,thentheonlyplacetomodifytheversionnumberistheparentPOM,andthemoduleswillusethenewversionstartingwiththenextexecutionoftheMavencompilation.

Whenthenewversion,JUnit5,comesout,however,themoduleswillallhavetobemodifiedbecauseJUnitisnotjustanewversion.Version5ofJUnitissplitintoseveralmodulesand,thisway,groupIdandartifactIdwillalsochange.

ItisalsoworthnotingthatthemodulesthatimplementtheinterfacesfromtheSortInterfacemoduleareeventuallydependentonthismodule.Inthiscase,theversionisdefinedasfollows:

<version>${project.version}</version>

Thatseemstobeabittautological(itis,actually).The${project.version}propertyistheversionoftheprojectanditisinheritedbytheSortInterfacemodule.Thisistheversionoftheartifactthattheothermodulesdependon.Inotherwords,themodulesalwaysdependontheversionthatwearecurrentlydeveloping.

Codethesort

Toimplementthesort,first,wewilldefinetheinterfacesthatasortlibraryshouldimplement.Definingtheinterfacebeforetheactualcodingisagoodpractice.Whentherearemanyimplementations,itissometimesrecommendedtofirstcreateasimpleoneandstartusingitsothattheinterfacemayevolveduringthephase,andwhenthemorecompleximplementationsaredue,thentheinterfacetobeimplementedisalreadyfixed,moreorless.

packagepackt.java9.by.example.ch03;<br/><br/>publicinterfaceSort{<br/>voidsort(SortableCollectioncollection);<br/>}

packagepackt.java9.by.example.ch03;<br/><br/>publicinterfaceSortableCollection{<br/>}

CreatingBubbleSortNow,wecanstartcreatingthebubblesortthatimplementstheSortinterface:

packagepackt.java9.by.example.ch03.bubble;

importpackt.java9.by.example.ch03.*;

importjava.util.Comparator;

publicclassBubbleSortimplementsSort{

@Override

publicvoidsort(SortableCollectioncollection){

intn=collection.size();

while(n>1){

for(intj=0;j<n-1;j++){

if(comparator.compare(collection.get(j),

collection.get(j+1))>0){

swapper.swap(j,j+1);

}

}

n--;

}

}

Normally,thealgorithmtoexecuteneedstwooperationsthatweimplementedinthecodelasttimespecifictoaStringarray:comparingtwoelementsandswappingtwoelements.Asthistimethesortimplementationitselfdoesnotknowwhattypetheelementsareusedandalsodoesnotknowifthesomethingitsortsisanarray,alistsorsomethingelse,itneedssomethingthatdoesitforthesortwhenneeded.Moreprecisely,itneedsacomparatorobjectcapableofcomparingtwoelementsanditneedsaswapperobjectthatiscapableofswappingtwoelementsinthecollection.

Togetthose,wecanimplementtwosettermethodsthatcansettheobjectsforthepurposebeforesortisinvoked.Asthisisnotspecifictothebubblesortalgorithmbutisrathergeneral,thesetwomethodsshouldalsobemadeapartoftheinterface,sotheimplementationisoverridingit.

privateComparatorcomparator=null;

@Override

publicvoidsetComparator(Comparatorcomparator){

this.comparator=comparator;

}

privateSwapperswapper=null;

@Override

publicvoidsetSwapper(Swapperswapper){

this.swapper=swapper;

}

}

The@OverrideannotationsignalsfortheJavacompilerthatthemethodisoverridingamethodoftheparentclass,or,asinthiscase,oftheinterface.Amethodcanoverrideaparentmethodwithoutthisannotation;however,ifweusetheannotation,thecompilationfailsifthemethoddoesactuallynotoverridesomething.Thishelpsyoudiscoverduringcompiletimethatsomethingwaschangedintheparentclassorintheinterfaceandwedidnotfollowthatchangeintheimplementation,orthatwejustmadesomemistakethinkingthatwewilloverrideamethodwhenweactuallydonot.Asannotationsareheavilyusedinunittests,wewilltalkaboutannotationsinabitmoredetaillater.

AmendingtheinterfacesThemodifiedSortinterfacewilllooklikethis:

publicinterfaceSort{

voidsort(SortableCollectioncollection);

voidsetSwapper(Swapperswap);

voidsetComparator(Comparatorcompare);

}

Thisalsomeansthatwewillneedtwonewinterfaces:SwapperandComparator.WeareluckythattheJavaruntimealreadydefinesaComparatorinterfacethatjustfitsthepurpose.Youmayhaveguessedthatfromthefollowingimportstatement:

importjava.util.Comparator;

Whenyouneedsomethingverybasic,likeacomparatorinterface,itismostprobablydefinedintheruntime.Itisadvisabletoconsulttheruntimebeforewritingyourownversion.TheSwapperinterface,however,wewillhavetocreate.

packagepackt.java9.by.example.ch03;

publicinterfaceSwapper{

voidswap(inti,intj);

}

AsitisusedtoswaptwoelementsspecifiedbytheindicesinSortableCollection,thereisamethod,quitetriviallynamedswapforthepurpose.But,wearenotreadyyet.Ifyoutrytocompiletheprecedingcode,thecompilerwillcomplainaboutthegetandsizemethods.Theyareneededbythealgorithmtoimplementthesort,buttheyarenotinherentlypartofthesortingitself.Thisisaresponsibilitythatshouldnotbeimplementedinthesort.Aswedonotknowwhattypeofcollectionswewillsort,itisnotonlyunadvisablebutalsoimpossibletoimplementthesefunctionalitiesinsidethesort.Itseemsthatwejustcannotsortanything.Therearesomerestrictionswewillhavetoset.Thesortingalgorithmmustknowthesizeofthecollectionwesortandalsoshouldhaveaccesstoanelementbyindexsothatitcanpassitontothecomparator.

TheserestrictionsareexpressedintheSortableCollectioninterfacethatwejustleftemptynotknowingbeforethefirstsortimplementationwhatisrequiredtobethere.

packagepackt.java9.by.example.ch03;

publicinterfaceSortableCollection{

Objectget(inti);

intsize();

}

Now,wearereadywiththeinterfacesandtheimplementationandwecangoontestingthecode.But,beforethat,wewillbrieflyreiteratewhatwedidandwhywedidthat.

ArchitecturalconsiderationsWecreatedaninterfaceandasimpleimplementationofit.Duringtheimplementation,wediscoveredthattheinterfaceneedsotherinterfacesandmethodsthatareneededtosupportthealgorithm.Thisusuallyhappensduringthearchitecturaldesignofthecode,beforeimplementation.Fordidacticalreasons,Ifollowedthebuild-upoftheinterfaceswhilewedevelopedthecode.Inreallife,whenIcreatedtheinterfaces,IcreatedthemallinonestepasIhaveenoughexperience.Iwrotemyfirstquicksortcodearound1983inFortran.However,itdoesnotmeanthatIhitthebull'seyewithjustanyproblemandcomeoutwiththefinalsolution.Itjusthappensthatsortisatoowellknownproblem.Ifyouneedtomodifytheinterfacesorotheraspectsofyourdesignduringdevelopment,donotfeelembarrassed.Itisanaturalconsequenceandaproofthatyouunderstandthingsbetterandbetterastimegoesby.Ifthearchitectureneedschange,itisbettertobedonethannot,andthesooneritis,thebetter.Inreallifeenterpriseenvironments,wewilldesigninterfacesjusttolearnduringdevelopmentthatthereweresomeaspectsthatweforgot.Theyareverytrueandbitmorecomplexoperationsthansortingacollection.

Inthecaseofthesortingproblem,weabstractedthesomethingwewanttosorttothemostpossibleextreme.TheJavabuildinsortcansortarraysorlists.Ifyouwanttosortsomethingthatisnotalistoranarray,youhavetocreateaclassthatimplementsthejava.util.Listinterfacewithmorethan24methodsitrequirestowrapyoursortableobjecttomakeitsortablebytheJDKsort.Tobehonest,thatisnottoomany,andinareal-worldproject,Iwouldconsiderthatasanoption.

However,wedonot,andcannotknow,whatmethodsoftheinterfacethebuilt-insortuses.Thosethatareusedshouldbefunctionallyimplementedandthosethatarenot,cancontainasimplereturnstatementbecausetheyarejustneverinvoked.AdevelopercanconsultthesourcecodeoftheJDKandseewhatmethodsareactuallyused,butthatisnotthecontractofthesearchimplementation.Itisnotguaranteedthatanewversionwillstilluseonlythosemethods.Ifanewversionstartstouseamethodthatweimplementedwithasinglereturnstatement,thesortwillmagicallyfail.

ItisalsoaninterestingperformancequestionhowtheswappingoftwoelementsisimplementedbythesearchusingonlytheListinterface.Thereisnoput(int,Object)methodintheListinterface.Thereisadd(intObject),butthatinsertsanewelementanditmaybeextremelycostly(burningCPU,disk,energy)topushallelementsofthelistupiftheobjectsarestored,forexample,ondisk.Furthermore,thenextstepmayberemovingtheelementaftertheonewejustinserted,doingthecostlyprocessofmovingthetailofthelistagain.Thatis,thetrivialimplementationofput(int,Object)thatthesortmayormaynotfollow.Again,thisissomethingthatshouldnotbeassumed.

Whendevelopersuselibraries,classes,andmethodsfromtheJDK,opensource,orcommerciallibraries,thedevelopersmayconsultthesourcecodebuttheyshouldnotrelyontheimplementation.YoushouldrelyonlyonthecontractandthedefinitionoftheAPIthatthelibrarycomeswith.Whenyouimplementaninterfacefromsomeexternallibrary,andyoudonotneedtoimplementsomepartofit,andcreatesomedummymethods,feelthedangerintheair.Itisanambush.Itislikelythateitherthelibraryispoorqualityoryoudidnotunderstandhowtouseit.

Inourcase,weseparatedtheswappingandthecomparisonfromthesort.Thecollectionshouldimplementtheseoperationsandprovidethemforthesort.Thecontractistheinterface,andtousethesort,

youhavetoimplementallmethodsoftheinterfaceswedefined.

TheinterfaceofSortdefinessettersthatsetSwapperandComparator.HavingdependenciessetthatwaymayleadtoacodethatcreatesanewinstanceofaclassimplementingtheSortinterface,butdoesnotsetSwapperandComparatorbeforeinvokingSort.ThiswillleadtoNullPointerExceptionthefirsttimetheComparatorisinvoked(orwhentheSwapperisinvokedincasetheimplementationinvokesthatfirst,whichisnotlikely,butpossible).Thecallingmethodshouldinjectthedependenciesbeforeusingtheclass.Whenitisdonethroughsetters,itiscalledsetterinjection.ThisterminologyisheavilyusedwhenweuseframeworkssuchasSpring,Guice,orsomeothercontainer.Creatingtheseserviceclassesandinjectingtheinstanceintoourclassesisfairlysimilarallthetime.

Containerimplementationscontainthefunctionalityinageneralwayandprovideconfigurationoptionstoconfigurewhatinstancesaretobeinjectedintowhatotherobjects.Usually,thisleadstoshorter,moreflexible,andmorereadablecode.However,dependencyinjectionisnotexclusivetocontainers.Whenwewritethetestingcodeinthenextsection,andinvokethesetters,weactuallydodependencyinjection.

Thereisanotherwayofdependencyinjectionthatavoidstheproblemofdependenciesnotbeingset.Thisiscalledconstructorinjection.Thedependenciesarefinalprivatefieldswithnovalues.Rememberthatthesefieldsshouldgettheirfinalvaluesbythetimetheconstructorfinishes.Constructorinjectionpassestheinjectedvaluestotheconstructorasargumentsandtheconstructorsetsthefields.Thisway,thefieldsareguaranteedtobesetbythetimetheobjectwasconstructed.Thisinjection,however,cannotbedefinedinaninterface.

Now,wealreadyhavethecode,andweknowtheconsiderationsofhowtheinterfaceswerecreated.Thisisthetimetodosometesting.

CreatingunittestsWhenwewritecode,weshouldtestit.Nocodehasevergoneintoproductionbeforeatleastdoingsometestruns.Therearedifferentlevelsoftestshavingdifferentaims,technologies,industrypractices,andnames.

Unittests,asthenamesuggests,testaunitofcode.Integrationteststesthowtheunitsintegratetogether.Smoketeststestalimitedsetofthefeaturesjusttoseethatthecodeisnottotallybroken.Thereareothertests,untilthefinaltest,whichistheproofofthework:useracceptancetest.Proofofthepuddingiseatingit.Acodeisgoodiftheuseracceptsit.

Manytimes,Itelljuniorsthatthenameuseracceptancetestisabitmisleading,becauseitisnottheuserwhoacceptstheresultofaproject,butthecustomer.Bydefinition,thecustomeristhepersonwhopaysthebill.Professionaldevelopmentispaid;otherwise,itisnotprofessional.Theterminologyis,however,useracceptancetest.Itjusthappensthatcustomersaccepttheprojectonlyiftheuserscanusetheprogram.

WhenwedevelopinJava,unittestistestingstandaloneclasses.Inotherwords,inJavadevelopment,aunitisaclasswhenwetalkaboutunittests.Tofurnishunittests,weusuallyusetheJUnitlibrary.Thereareotherlibraries,suchasTestNG,butJUnitisthemostwidelyused,sowewilluseJUnit.Touseitasalibrary,first,wewillhavetoaddittotheMavenPOMasadependency.

AddingJUnitasdependencyRecallthatwehaveamulti-moduleproject,andthedependencyversionsaremaintainedintheparentPOMunderthedependencyManagementtag.

<dependencyManagement>

<dependencies>

...

<dependency>

<groupId>junit</groupId>

<artifactId>junit</artifactId>

<version>4.12</version>

<scope>test</scope>

</dependency>

</dependencies>

</dependencyManagement>

Thescopeofthedependencyistest,whichmeansthatthislibraryisneededonlytocompilethetestcodeandduringtheexecutionofthetest.TheJUnitlibrarywillnotmakeitswaytothefinalreleasedproduct;thereisnoneedforit.IfyoufindtheJUnitlibraryinsomedeployedproductionWebArchive(WAR)file,suspectthatsomebodywasnotproperlymanagingthescopesofthelibraries.

MavensupportsthecompilationandtheexecutionofJUnittestsinthelifecycleoftheproject.Ifwewanttoexecutethetests,onlywecanissuethemvntestcommand.TheIDEsalsosupporttheexecutionoftheunittests.Usually,thesamemenuitemthatcanbeusedtoexecuteaclassthathasapublicstaticmainmethodcanbeused.IftheclassisaunittestutilizingJUnit,theIDEwillrecognizeitandexecutethetestsandusuallygiveagraphicalfeedbackonwhattestwasexecutingfineandwhichonesfailed,andhow.

WritingtheBubbleSortTestclassThetestclassesareseparatedfromtheproductionclasses.Theygointothesrc/test/javadirectory.Whenwehaveaclassnamed,forexample,BubbleSort,thenthetestwillbenamedBubbleSortTest.Thisconventionhelpstheexecutingenvironmenttoseparatethetestsfromthoseclassesthatdonotcontaintestsbutareneededtoexecutethetests.Totestthesortimplementationwehavejustcreated,wecanfurnishaclassthatcontains,fornow,asinglecanSortStringsmethod.

Unittestmethodnamesareusedtodocumentthefunctionalitybeingtested.AstheJUnitframeworkinvokeseachandeverymethodthathasthe@Testannotation,thenameofthetestisnotreferencedanywhereinourcode.Wecanbravelyusearbitrarylongmethodnames;itwillnothinderreadabilityattheplacewherethemethodisinvoked.

packagepackt.java9.by.example.ch03.bubble;

//importsdeletedfromprint

publicclassBubbleSortTest{

@Test

publicvoidcanSortStrings(){

ArrayListactualNames=newArrayList(Arrays.asList(

"Johnson","Wilson",

"Wilkinson","Abraham","Dagobert"

));

ThemethodcontainsArrayListwiththeactualnamesthatwehavealreadygottenfamiliarwith.AswehaveasortimplementationandinterfacethatneedsSortableCollection,wewillcreateonebackedupbyArrayList.

SortableCollectionnamesCollection=newSortableCollection(){

@Override

publicObjectget(inti){

returnactualNames.get(i);

}

@Override

publicintsize(){

returnactualNames.size();

}

};

WedeclaredanewobjectthathastheSortableCollectiontype,whichisaninterface.ToinstantiatesomethingthatimplementsSortableCollection,wewillneedaclass.Wecannotinstantiateaninterface.Inthiscase,definetheclassintheplaceoftheinstantiation.ThisiscalledananonymousclassinJava.Thenamecomesfromthefactthatthenameofthenewclassisnotdefinedinthesourcecode.TheJavacompilerwillautomaticallycreateanameforthenewclass,butthatisnotinterestingfortheprogrammers.WewillsimplywritenewSortableCollection()andprovidetheneededimplementationimmediatelyfollowingbetween{and}.Itisveryconvenienttodefinethisanonymousclassinsidethemethodas,thisway,itcanaccessArrayListwithoutpassingareferencetoArrayListintheclass.

Asamatteroffact,thereferenceisneeded,buttheJavacompilerautomaticallydoesthis.TheJavacompiler,inthiscase,alsotakescarethatautomaticreferencepassingthiswaycanonlybedoneusingvariablesthatwereinitializedandwillnotchangeduringtheexecutionofthecodeaftertheinstantiationoftheanonymousclass.ThevariableactualNameswassetanditshouldnotbechangedinthemethodlater.

Asamatteroffact,wecanevendefineactualNamestobefinalandthiswouldhavebeenarequirementifweusedJava1.7orearlier.Startingwith1.8,therequirementisthatthevariableiseffectivelyfinal,butyouneednotdeclareittobefinal.

ThenextthingthatweneedisaSwapperimplementationforArrayList.Inthiscase,wewilldefineawholeclassinsidethemethod.Itcanalsobeananonymousclass,butthistimeIdecidedtouseanamedclasstodemonstratethataclasscanbedefinedinsideamethod.Usually,wedonotdothatinproductionprojects.

classSwapActualNamesArrayElementsimplementsSwapper{

@Override

publicvoidswap(inti,intj){

finalObjecttmp=actualNames.get(i);

actualNames.set(i,actualNames.get(j));

actualNames.set(j,tmp);

}

}

Last,butnotleast,wewillneedacomparatorbeforewecaninvokethesort.AswehaveStringstocompare,thisiseasyandstraightforward.

ComparatorstringCompare=newComparator(){

@Override

publicintcompare(Objectfirst,Objectsecond){

finalStringf=(String)first;

finalStrings=(String)second;

returnf.compareTo(s);

}

};

Havingeverythingpreparedforthesorting,wewillfinallyneedaninstanceoftheSortimplementation,setthecomparatorandtheswapper,andinvokethesort.

Sortsort=newBubbleSort();

sort.setComparator(stringCompare);

sort.setSwapper(newSwapActualNamesArrayElements());

sort.sort(namesCollection);

Thelast,butmostimportantpartofthetestistoassertthattheresultistheonethatweexpect.JUnithelpsusdothatwiththeaidoftheAssertclass.

Assert.assertEquals(Arrays.asList("Abraham","Dagobert","Johnson","Wilkinson","Wilson"),actualNames);

}

}

ThecalltoassertEqualschecksthatthefirstargument,theexpectedresult,equalsthesecondargument,thesortedactualNames.Iftheydiffer,thenAssertionErroristhrown;otherwise,thetestjustfinishesfine.

GoodunittestsIsthisagoodunittest?Ifyoureaditinatutorialbooklikethis,ithastobe.Actually,itisnot.ItisagoodcodetodemonstratesomeofthetoolsthatJUnitprovidesandsomeJavalanguagefeatures,butasarealJUnittest,Iwillnotuseitinareallifeproject.

Whatmakesaunittestgood?Toanswerthisquestion,wewillhavetofindwhattheunittestisgoodforandwhatitisthatweuseitfor.

Wewillcreateunitteststovalidatetheoperationoftheunitsandtodocument.

Unittestsarenottofindbugs.Developerseventuallyuseunittestsduringdebuggingsessionsbut,manytimes,thetestingcodecreatedforthedebuggingisatemporaryone.Whenthebugisfixed,thecodeusedtofinditwillnotgetintothesourcecode.Foreverynewbug,thereshouldbeanewtestcreatedthatcoversthefunctionalitythatwasnotproperlyworking,butitishardlythetestcodethatisusedtofindthebug.Thisisbecauseunittestsaremainlyfordocumentation.YoucandocumentaclassusingJavaDoc,buttheexperienceshowsthatthedocumentationoftenbecomesoutdated.Thedevelopersmodifythecode,buttheydonotmodifythedocumentation,andthedocumentationbecomesobsoleteandmisleading.Unittests,however,areexecutedbythebuildsystemandifContinuousIntegration(CI)isinuse(anditshouldbe,inaprofessionalenvironment),thenthebuildwillbebrokenifatestfails,alldeveloperswillgetmailnotificationaboutit,anditwilldrivethedeveloperbreakingthebuildtofixthecodeorthetest.Thisway,thetestsverifythatcontinuousdevelopmentdidnotbreakanythinginthecodeor,atleast,notsomethingthatcanbediscoveredusingunittests.

AgoodunittestisreadableOurtestisfarfrombeingreadable.Atestcaseisreadableifyoulookatitandin15secondsyoucantellwhatitdoes.Itassumes,ofcourse,someexperienceinJavaonbehalfofthereader,butyougetthepoint.Ourtestisclutteredwithsupportclassesthatarenotcoretothetest.

Ourtestalsohardlyvalidatesthatthecodeisworkingproperly.Itactuallydoesnot.TherearesomebugsinitthatIputtheredeliberately,whichwewilllocateandzapinthefollowingsections.OnesingletestthatsortsasingleStringarrayisfarfromvalidatingasortimplementation.IfIweretoextendthistesttoareal-worldtest,wewouldneedmethodsthatwouldhavethenamecanSortEmptyCollection,canSortOneElementCollection,canSortTwoElements,canSortReverseOrder,orcanSortAlreadySorted.Ifyoulookatthenames,youwillseewhattestsweneed.Comingfromthenatureofthesortproblem,animplementationmaybereasonablysensitivetoerrorsinthesespecialcases.

Whatarethegoodpointsinourunittest,inadditiontoitbeinganacceptabledemonstrationtool?

Unittestsarefast

Ourunittestrunsfast.Asweexecuteunittestseachtime,theCIfiresupabuildandtheexecutionofthetestsshouldnotlastlong.Youshouldnotcreateaunittestsortingbillionsofelements.Thatisakindofstabilityorloadtestandtheyshouldruninseparatetestperiodsandnoteverytimethebuildisrunning.Ourunittestsortsfiveelementsthatarereasonable.

Unittestsaredeterministic

Ourunittestisdeterministic.Non-deterministicunittestsarethenightmareofthedevelopers.IfyouareinagroupwheresomebuildsbreakontheCIserver,andwhenabuildbreaks,yourfellowdevelopersaysthatyoujusthavetotryitagain;noway!Ifaunittestruns,itshouldrunalltimes.Ifitfails,itshouldfailnomatterhowmanytimesyoustartit.Anon-deterministicunittest,inourcase,willbetorenderrandomnumbersandhavethemsorted.Wewillendupwithdifferentarraysineachtestrunand,incasethereissomebuginthecodethatmanifestsforsomearray,wewillnotbeabletoreproduceit.Nottomentionthattheassertionthatthecodewasrunningfineisalsodifficult.

Ifwesortedarandomarrayinaunittest(somethingwedonot),wecould,hypothetically,assertthatthearrayissorted,comparingtheelementsoneaftertheothercheckingthattheyareinascendingorder.Itwouldalsobeatotallywrongpractice.

Assertionsshouldbeassimpleaspossible

Iftheassertioniscomplex,theriskofintroducingbugsintheassertionishigher.Themorecomplextheassertion,thehighertherisk.Wewillwritetheunitteststoeaseourlivesandnottohavemorecodetodebug.

Additionally,onetestshouldassertonlyonething.ThisoneassertionmaybecodedwithmultipleAssertclassmethods,oneaftertheother.Still,theaimoftheseistoassertthecorrectnessofonesinglefeatureoftheunit.RemembertheSRP:onetest,onefeature.Agoodtestislikeagoodsniper:oneshot,onekill.

UnittestsareisolatedWhenwetestaunitA,anychangeinanotherunitB,orabuginadifferentunitshouldnotaffectourunittestthatisfortheunitA.Inourcase,itwaseasybecausewehaveonlyoneunit.Later,whenwedevelopthetestforthequicksort,wewillseethatthisseparationisnotthatsimple.

Iftheunittestsareproperlyseparated,afailingunittestclearlypointsoutthelocationoftheproblem.Itisintheunitwheretheunittestfailed.Iftestsdonotseparatetheunits,thenafailureinonetestmaybecausedbyabuginadifferentunitthanweexpect.Inthiscase,thesetestsarenotreallyunittests.

Inpractice,youshouldmakeabalance.Iftheisolationoftheunitswillbetoocostly,youcandecidetocreateintegrationtests;and,iftheystillrunfast,havethemexecutedbytheCIsystem.Atthesametime,youshouldalsotrytofindoutwhytheisolationishard.Ifyoucannoteasilyisolatetheunitsinthetests,itmeansthattheunitsaretoostronglycoupled,whichmaynotbeagooddesign.

UnittestscoverthecodeUnittestsshouldtestallusualandalsoallspecialcasesofthefunctionality.Ifthereisaspecialcaseofcodethatisnotcoveredbytheunittest,thecodeisindanger.Incaseofasortimplementation,thegeneralcaseissorting,sayfiveelements.Thespecialcasesaremuchmorenumeroususually.Howdoesourcodebehaveifthereisonlyoneelementoriftherearenoelements?Whatiftherearetwo?Whatiftheelementsareinreverseorder?Whatiftheyarealreadysorted?

Usually,thespecialcasesarenotdefinedinthespecification.Theprogrammerhastothinkaboutitbeforecoding,andsomespecialcasesarediscoveredduringcoding.Thehardthingisthatyoujustcannottellifyoucoveredallspecialcasesandthefunctionalityofthecode.

Whatyoucantellisifallthelinesofcodewereexecutedduringthetestingornot.If90%ofthecodelinesareexecutedduringthetests,thenthecodecoverageis90%,whichisfairlygoodinreallife,butyoushouldneverbecontentwithanythinglessthan100%.

Codecoverageisnotthesameasfunctionalcoverage,butthereisacorrelation.Ifthecodecoverageislessthan100%,thenatleastoneofthefollowingtwostatementsistrue:

Thefunctionalcoverageisnot100%Thereisunusedcodeinthetestedunit,whichcanjustbedeleted

Thecodecoveragecanbemeasured,thefunctionalcoveragecannot.ThetoolsandIDEssupportcodecoveragemeasurement.Thesemeasurementsareintegratedintotheeditorsoyouwillnotonlygetthepercentageofthecoverage,buttheeditorwillshowyouexactlywhichlinesarenotcoveredbythecoveragecoloringthelines(inEclipse,forexample)orthegutterontheleftsideoftheeditorwindow(IntelliJ).ThepictureshowsthatinIntelliJ,thetestscoverthelinesindicatedbyagreencoloronthegutter.(Intheprintversionthisisjustagreyrectangle).

RefactorthetestNowthatwehavediscussedwhatagoodunittestis,let'simproveourtest.Thefirstthingistomovethesupportingclassestoseparatefiles.WewillcreateArrayListSortableCollection:

packagepackt.java9.by.example.ch03.bubble;

importpackt.java9.by.example.ch03.SortableCollection;

importjava.util.ArrayList;

publicclassArrayListSortableCollectionimplementsSortableCollection{

finalprivateArrayListactualNames;

ArrayListSortableCollection(ArrayListactualNames){

this.actualNames=actualNames;

}

@Override

publicObjectget(inti){

returnactualNames.get(i);

}

@Override

publicintsize(){

returnactualNames.size();

}

}

ThisclassencapsulatesArrayListandthenimplementsthegetandsizemethodstoArrayListaccess.ArrayListitselfisdeclaredasfinal.Recallthatafinalfieldhastobedefinedbythetimetheconstructorfinishes.Thisguaranteesthatthefieldistherewhenwestarttousetheobjectandthatitdoesnotchangeduringtheobjectlifetime.Note,however,thatthecontentoftheobject,inthiscase,theelementsofArrayList,maychange.Ifitwerenotthecase,wewouldnotbeabletosortit.

ThenextclassisStringComparator.ThisissosimplethatIwillnotlistithere;Iwillleaveittoyoutoimplementthejava.util.ComparatorinterfacethatcancomparetwoStrings.Itshouldnotbedifficult,especiallyasthisclasswasalreadyapartofthepreviousversionoftheBubbleSortTestclass(hint:itwasananonymousclassthatwestoredinthevariablenamedstringCompare).

WealsohavetoimplementArrayListSwapper,whichalsoshouldnotbeabigsurprise.

packagepackt.java9.by.example.ch03.bubble;

importpackt.java9.by.example.ch03.Swapper;

importjava.util.ArrayList;

publicclassArrayListSwapperimplementsSwapper{

finalprivateArrayListactualNames;

ArrayListSwapper(ArrayListactualNames){

this.actualNames=actualNames;

}

@Override

publicvoidswap(inti,intj){

Objecttmp=actualNames.get(i);

actualNames.set(i,actualNames.get(j));

actualNames.set(j,tmp);

}

}

Finally,ourtestwilllookthis:

packagepackt.java9.by.example.ch03.bubble;

//...importsdeletedfromprint...

publicclassBubbleSortTest{

@Test

publicvoidcanSortStrings(){

ArrayListactualNames=newArrayList(Arrays.asList(

"Johnson","Wilson",

"Wilkinson","Abraham","Dagobert"

));

ArrayListexpectedResult=newArrayList(Arrays.asList(

"Abraham","Dagobert",

"Johnson","Wilkinson","Wilson"

));

SortableCollectionnames=

newArrayListSortableCollection(actualNames);

Sortsort=newBubbleSort();

sort.setComparator(

newStringComparator());

sort.setSwapper(

newArrayListSwapper(actualNames));

sort.sort(names);

Assert.assertEquals(expectedResult,actualNames);

}

}

Nowthisisalreadyatestthatcanbeunderstoodin15seconds.Itdocumentswellhowtouseasortimplementationthatwedefined.Itstillworksanddoesnotrevealanybug,asIpromised.

CollectionswithwrongelementsThebugisnottrivial,andasusual,thisisnotintheimplementationofthealgorithm,butratherinthedefinition,orthelackofit.Whatshouldtheprogramdoiftherearenotonlystringsinthecollectionthatwesort?

IfIcreateanewtestthatstartswiththefollowinglines,itwillthrowClassCastException:

@Test

publicvoidcanNotSortMixedElements(){

ArrayListactualNames=newArrayList(Arrays.asList(

42,"Wilson",

"Wilkinson","Abraham","Dagobert"

));

...therestofthecodeisthesameastheprevioustest

TheproblemhereisthatJavacollectionscancontainanytypeofelements.Youcannoteverbesurethatacollection,suchasArrayList,containsonlythetypesthatyouexpect.Evenifyouusegenerics(wehavenotlearnedthat,butwewillinthischapter),thechancesofabugsomehowconjuringupsomeobjectofaninappropriatetypeintoacollection,aresmallerbutarestillthere.Don'taskmehow;Icannottellyou.Thisisthenatureofthebugs—youcannottellhowtheyworkuntilyouzapthem.Thethingisthatyouhavetobepreparedforsuchanexceptionalcase.

HandlingexceptionsExceptionalcasesshouldbehandledinJavausingexceptions.TheClassCastExceptionisthereandithappenswhenthesorttriestocompareStringtoIntegerusingStringComparator,andtodothat,ittriestocastanIntegertoString.

Whenanexceptionisthrownbytheprogramusingthethrowcommand,orbytheJavaruntime,theexecutionoftheprogramstopsatthatpoint,andinsteadofexecutingthenextcommand,itcontinueswheretheexceptioniscaught.Itcanbeinthesamemethod,orinsomecallingmethodupinthecallchain.Tocatchanexception,thecodethrowingtheexceptionshouldbeinsideatryblock,andthecatchstatementfollowingthetryblockshouldspecifyanexceptionthatiscompatiblewiththeexceptionthrown.

Iftheexceptionisnotcaught,thentheJavaruntimewillprintoutthemessageoftheexceptionalongwithastacktracethatwillcontainalltheclasses,methods,andlinenumbersonthecallstackatthetimeoftheexception.Inourcase,themvntestcommandwillproducethefollowingtraceintheoutput:

java.lang.ClassCastException:java.lang.Integercannotbecasttojava.lang.String

atpackt.java9.by.example.ch03.bubble.StringComparator.compare(StringComparator.java:9)

atpackt.java9.by.example.ch03.bubble.BubbleSort.sort(BubbleSort.java:13)

atpackt.java9.by.example.ch03.bubble.BubbleSortTest.canNotSortMixedElements(BubbleSortTest.java:49)

atsun.reflect.NativeMethodAccessorImpl.invoke0(NativeMethod)

atsun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)

atsun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)

atjava.lang.reflect.Method.invoke(Method.java:498)

...somelinesdeletedfromtheprint

atorg.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:141)

atorg.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:112)

atsun.reflect.NativeMethodAccessorImpl.invoke0(NativeMethod)

atsun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)

...somelinesdeletedfromtheprint

atorg.apache.maven.surefire.booter.ProviderFactory.invokeProvider(ProviderFactory.java:85)

atorg.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:115)

atorg.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:75)

Thisstacktraceisnotreallylong.Intheproductionenvironmentinanapplicationthatrunsonanapplicationserver,thestacktracemaycontainafewhundredelements.Inthistrace,youcanseethatMavenwasstartingthetestexecution,involvedMavensurefireplugin,andthentheJUnitexecutor,untilwegetthroughthetesttothecomparator,wheretheactualexceptionwasthrown.

ThisexceptionwasnotprintedbytheJavaruntimetotheconsole.ThisexceptioniscaughtbytheJUnitlibrarycodeandthestacktraceisloggedouttotheconsoleusingMavenloggingfacility.

Theproblemwiththisapproachisthattherealissueisnottheclasscastingfailure.Therealissueisthatthecollectioncontainsmixedelements.ItisonlyrealizedbytheJavaruntimewhenittriestocasttwoincompatibleclasses.Ourcodecanbesmarter.Wecanamendthecomparator.

packagepackt.java9.by.example.ch03.bubble;

importjava.util.Comparator;

publicclassStringComparatorimplementsComparator{

@Override

publicintcompare(Objectfirst,Objectsecond){

try{

finalStringf=(String)first;

finalStrings=(String)second;

returnf.compareTo(s);

}catch(ClassCastExceptioncce){

thrownewNonStringElementInCollectionException(

"Therearemixedelementsinthecollection.",cce);

}

}

}

ThiscodecatchestheClassCastExceptionandthrowsanewone.Theadvantageofthrowinganewexceptionisthatyoucanbesurethatthisexceptionisthrownfromthecomparatorandthattheproblemreallyisthattherearemixedelementsinthecollection.Classcastingproblemsmayhappenatotherplacesofthecodeaswell,insidesomeofthesortimplementations.Someapplicationcodemaywanttocatchtheexceptionandwanttohandlethecase;forexample,sendinganapplication-specificerrormessageandnotdumpingonlyastacktracetotheuser.ThiscodecancatchClassCastExceptionaswell,butitcannotbesurewhattherealcauseoftheexceptionis.Ontheotherhand,NonStringElementInCollectionExceptionisdefinite.

TheNonStringElementInCollectionExceptionisanexceptionthatdoesnotexistintheJDK.Wewillhavetocreateit.ExceptionsareJavaclassesandourexceptionlooksasfollows:

packagepackt.java9.by.example.ch03.bubble;

publicclassNonStringElementInCollectionExceptionextendsRuntimeException{

publicNonStringElementInCollectionException(Stringmessage,Throwablecause){

super(message,cause);

}

}

Javahasthenotionofcheckedexceptions.ItmeansthatanyexceptionthatisnotextendingRuntimeExceptionshouldbedeclaredinthemethoddefinition.Supposeourexceptionwasdeclaredasfollows:

publicclassNonStringElementInCollectionExceptionextendsException

Then,wewillhavetodeclarethecomparemethodasfollows:

publicintcompare(Objectfirst,Objectsecond)throwsNonStringElementInCollectionException

Theproblemisthattheexceptionamethodthrowsispartofthemethodsignature,andthiswaycomparewillnotoverridethecomparemethodoftheinterface,and,thatway,theclasswillnotimplementtheComparatorinterface.Thus,ourexceptionhastobearuntimeexception.

Therecanbeahierarchyofexceptionsinanapplication,andoften,noviceprogrammerscreatehugehierarchiesofthem.Ifthereissomethingyoucando,itdoesnotmeanthatyoushoulddoit.Hierarchiesshouldbekeptasflataspossible,andthisisespeciallytrueforexceptions.IfthereisanexceptionintheJDKthatdescribestheexceptionalcase,thenusethereadymadeexception.Justaswellasforanyotherclass:ifitisready,donotimplementitagain.

Itisalsoimportanttonotethatthrowinganexceptionshouldonlybedoneinexceptionalcases.Itisnottosignalsomenormaloperationalcondition.DoingthathindersreadabilityofthecodeandalsoeatsCPU.ThrowinganexceptionisnotaneasytaskfortheJVM.

Itisnotonlytheexceptionthatcanbethrown.Thethrowcommandcanthrow,andthecatchcommandcancatchanythingthatextendstheThrowableclass.TherearetwosubclassesofThrowable:Error,andException.TheErrorexceptionisthrownifsomeerrorhappenedduringtheexecutionoftheJavacode.ThetwomostinfamouserrorsareOutOfMemoryErrorandStackOverflowError.Ifanyofthesehappens,youcannotdoanythingreliablytocatchtheerror.

ThereisalsoInternalErrorandUnknownErrorintheJVM,butsinceJVMisfairlystable,youwillhardlyevermeettheseerrors.

Whenanyofthoseerrorshappen,trytodebugthecodeandtrytofindoutwhyyouusethatmuchmemoryorsuchdeepmethodcallsandtrytooptimizeyoursolution.WhatIhavejustsaidaboutcreatingexceptionhierarchiesistrueagaintocatcherrors.Thefactthatyoucancatcherrorsdoesnotmeanthatyoushould.Onthecontrary,youshouldnevercatchanerrorand,especially,neverevercatchaThrowable.

Thisway,wehandledthisspecialcasewhensomeprogrammeraccidentallywrites42amongthenames,butwillitbeniceriftheerrorwasidentifiedduringcompiletime?Todothat,wewillintroducegenerics.

Justalastthoughtbeforewegothere.WhatclassbehaviordowetestwiththecanNotSortMixedElementsunittest?ThetestisinsidetheBubbleSortTesttestclass,butthefunctionalityisinthecomparatorimplementation,StringComparator.Thistestcheckssomethingthatisoutofthescopeoftheunittestclass.Icanuseitfordemonstrationpurposes,butthisisnotaunittest.Therealfunctionalityofthesortimplementationcanbeformulizedthisway:whateverexceptionthecomparatorthrowsisthrownbythesortimplementation.Youcantrytowritethisunittest,orreadon;wewillhaveitinthenextsection.

TheStringComparatorclassdoesnothaveatestclassbecauseStringComparatorispartofthetestandwewillneverwriteatestforatest.Otherwise,wewillsinkintoanendlessrabbithole.

GenericsThegenericsfeaturewasintroducedintoJavainversion5.Tostartwithanexample,ourSortableinterfaceuntilnowwasthis:

packagepackt.java9.by.example.ch03;

publicinterfaceSortableCollection{

Objectget(inti);

intsize();

}

Afterintroducinggenerics,itwillbeasfollows:

packagepackt.java9.by.example.ch03;

publicinterfaceSortableCollection<E>{

Eget(inti);

intsize();

}

TheEidentifierdenotesatype.Itcanbeanytype.Itsaysthataclassisasortablecollectionifitimplementstheinterface,namelythetwomethods—sizeandget.ThegetmethodshouldreturnsomethingthatisoftypeE,whateverEis.Thismaynotmaketoomuchsenseupuntilnow,butyouwillsoongetthepoint.Afterall,genericsisadifficulttopic.

TheSortinterfacewillbecomethefollowing:

packagepackt.java9.by.example.ch03;

importjava.util.Comparator;

publicinterfaceSort<E>{

voidsort(SortableCollection<E>collection);

voidsetSwapper(Swapperswap);

voidsetComparator(Comparator<E>compare);

}

Thisstilldoesnotprovidemuchmorevaluethanthepreviousversionwithoutgenerics,but,atleast,itdoessomething.IntheactualclassimplementingtheSortinterface,ComparatorshouldacceptthesametypethatSortableCollectionuses.ItisnotpossiblethatSortableCollectionworksonstringsandweinjectacomparatorforintegers.

TheimplementationofBubbleSortisasfollows:

packagepackt.java9.by.example.ch03.bubble;

importpackt.java9.by.example.ch03.*;

importjava.util.Comparator;

publicclassBubbleSort<E>implementsSort<E>{

@Override

publicvoidsort(SortableCollection<E>collection){

...sortcodesameasbefore

}

privateComparator<E>comparator=null;

@Override

publicvoidsetComparator(Comparator<E>comparator){

this.comparator=comparator;

}

...methodswappersameasbefore

}

Therealpowerofgenericswillcomewhenwewillwritethetests.Thefirsttestdoesnotchangemuch,

althoughwiththegenerics,itismoredefinite.

@Test

publicvoidcanSortStrings(){

ArrayList<String>actualNames=newArrayList<>(Arrays.asList(

"Johnson","Wilson",

"Wilkinson","Abraham","Dagobert"

));

ArrayList<String>expectedResult=newArrayList<>(Arrays.asList(

"Abraham","Dagobert",

"Johnson","Wilkinson","Wilson"

));

SortableCollection<String>names=

newArrayListSortableCollection<>(actualNames);

Sort<String>sort=newBubbleSort<>();

sort.setComparator(String::compareTo);

sort.setSwapper(newArrayListSwapper<>(actualNames));

sort.sort(names);

Assert.assertEquals(expectedResult,actualNames);

}

WhenwedefineArrayList,wewillalsodeclarethattheelementsofthelistwillbestrings.WhenweallocatethenewArrayList,thereisnoneedtospecifyagainthattheelementsarestringsbecauseitcomesfromtheactualelementsthere.Eachofthemisastring;therefore,thecompilerknowsthattheonlythingthatcancomebetweenthe<and>characterisString.

Thetwocharacters<and>,withoutthetypedefinitioninbetween,iscalleddiamondoperator.Thetypeisinferred.Ifyougetusedtogenerics,thiscodebringsyoumoreinformationonthetypesthatthecollectionsworkonandthecodebecomesmorereadable.Thereadabilityandtheextrainformationisnottheonlypoint.

AsweknowthattheComparatorargumentisComparator<String>now,wecanuseadvancedfeaturesofJavaavailablesinceJava8andcanpasstheString::compareTomethodreferencetothecomparatorsetter.

Thesecondtestistheimportantoneforusnow.ThisisthetestwhichensuresthatSortdoesnotinterferewiththeexceptionthatthecomparatorthrows.

@Test(expected=RuntimeException.class)

publicvoidthrowsWhateverComparatorDoes(){

ArrayList<String>actualNames=newArrayList<>(Arrays.asList(

42,"Wilson",

"Wilkinson","Abraham","Dagobert"

));

SortableCollection<String>names=

newArrayListSortableCollection<>(actualNames);

Sort<String>sort=newBubbleSort<>();

sort.setComparator((Stringa,Stringb)->{

thrownewRuntimeException();

});

finalSwapperneverInvoked=null;

sort.setSwapper(neverInvoked);

sort.sort(names);

}

Thethingis,thatitdoesnotevencompile.ThecompilersaysthatitcannotinferthetypeofArrayList<>onthethirdline.WhenalltheargumentsoftheasListmethodwerestrings,themethodreturnedalistofStringelementsandthereforethenewoperatorwasknowntogenerateArrayList<String>.Thistime,thereisaninteger,andthus,thecompilercannotinferthatArrayList<>isforStringelements.

TochangethetypedefinitionfromArrayList<>toArrayList<String>isnotacure.Inthatcase,thecompiler

willcomplainaboutthevalue42.Thisisthepowerofgenerics.Whenyouuseclassesthathavetypeparameters,thecompilercandetectwhenyouprovideavalueofthewrongtype.TogetthevalueintoArrayListtocheckthattheimplementationreallythrowsanexception,wewillhavetoconjurethevalueintoit.Wecantrytoreplacethevalue42withanemptyStringandthenaddthefollowinglinewhichwillstillnotcompile:

actualNames.set(0,42);

ThecompilerwillstillknowthatthevalueyouwanttosetinArrayListissupposedtobeString.TogetthearraywiththeIntegerelement,youwillhavetoexplicitlyunlockthesafetyhandleandpullthetrigger,shootingyourself:

((ArrayList)actualNames).set(0,42);

Now,thetestlookslikethis:

@Test(expected=RuntimeException.class)

publicvoidthrowsWhateverComparatorDoes(){

ArrayList<String>actualNames=newArrayList<>(Arrays.asList(

"","Wilson",

"Wilkinson","Abraham","Dagobert"

));

((ArrayList)actualNames).set(0,42);

SortableCollection<String>names=

newArrayListSortableCollection<>(actualNames);

Sort<String>sort=newBubbleSort<>();

sort.setComparator((a,b)->{

thrownewRuntimeException();

});

finalSwapperneverInvoked=null;

sort.setSwapper(neverInvoked);

sort.sort(names);

}

WewillsettheSwappertobenullbecauseitisneverinvoked.WhenIfirstwrotethiscode,itwasevidenttome.Afewdayslater,IreadthecodeandIstopped.Whyisswappernull?ThenIrememberedinasecondortwo.Butanytime,whenreadingandunderstandingthecodehicksup,Itendtothinkaboutrefactoring.Icanaddacommenttothelinesaying//neverinvoked,butcommentstendtoremainthereevenwhenfunctionalitychanges.Ilearneditthehardwayin2006,whenawrongcommentpreventedmefromseeinghowthecodewasexecuting.Iwasreadingthecommentwhiledebugging,insteadofthecode,andbugfixingtooktwodayswhilethesystemwasdown.Insteadofacomment,Itendtouseconstructsthatmakethecodeexpresswhathappens.Theextravariablemaymaketheclassfileafewbytesbigger,butitisoptimizedoutbytheJITcompilersothefinalcodedoesnotrunslower.

Thecomparatorthatthrowsanexceptionwasprovidedasalambdaexpression.Lambdaexpressionscanbeusedincaseswhereananonymousclassornamedclasswillbeusedhavingonlyonesimplemethod.Lambdaexpressionsareanonymousmethodsstoredinvariablesorpassedinargumentforlaterinvocation.WewilldiscussthedetailsoflambdaexpressionsinChapter8,ExtendingourE-CommerceApplication.

Fornow,wewillgoonimplementingQuickSort,andtodothat,wewillusetheTDDmethodology.

TestDrivenDevelopmentTestDrivenDevelopment(TDD)isacodewritingapproachwhenthedevelopersfirstwriteatestbasedonthespecificationandthenwritethecode.Thisisjusttheoppositethatthedevelopercommunitygotusedto.Theconventionalapproachthatwefollowedwastowritethecodeandthenwritetestsforit.Tobehonest,therealpracticemanytimeswastowritethecodeandtestitwithad-hoctestsandnounittestsatall.Beingaprofessional,youwillneverdothat,bytheway.Youalwayswritetests.(Andnow,writeitdownahundredtimes:Iwillalwayswritetests.)

OneoftheadvantagesofTDDisthatthetestsdonotdependonthecode.Asthecodedoesnotexistatthecreationofthetest,developerscannotrelyontheimplementationoftheunitand,thus,itcannotinfluencethetestcreationprocess.Thisisgenerallygood.Unittestsshouldbeblackboxtestsasmuchaspossible.

Blackboxtestisatestthatdoesnottakeintoaccounttheimplementationofthetestedsystem.Ifasystemisrefactored,implementedinadifferentway,buttheinterfaceitprovidestowardtheexternalworldisthesame,thentheblackboxtestsshouldrunjustfine.Awhiteboxtestdependsontheinternalworkingofthesystemtested.Whenthecodechangesthewhiteboxtest,thecodemayalsoneedtuningtofollowthechange.Theadvantageofawhiteboxtestcanbethesimplertestcode.Notalways.Grayboxtestisamixtureofthetwo.

Unittestsshouldbeblackboxtests,but,manytimes,itisnotsimpletowriteablackboxtest.Developerswillwriteatestthattheythinkisblackbox,butmanytimes,thisbeliefprovestobefalse.Whentheimplementationchanges,somethingisrefactoredandthetestdoesnotworkanymoreanditneedstobecorrected.Itjusthappensthatknowingtheimplementation,thedevelopers,especiallythosewhowrotetheunit,willwriteatestthatdependsontheinternalworkingofthecode.Writingthetestbeforethecodeisatooltopreventthis.Ifthereisnocode,youcannotdependonit.

TDDalsosaysthatthedevelopmentshouldbeaniterativeapproach.Youwriteonlyonetestatthestart.Ifyourun,itfails.Ofcourseitfails!Asthereisnocodeyet,ithastofail.Then,youwillwritethecodethatfulfillsthistest.Nothingmore,onlythecodethatmakesthistestpass.Then,youwillgoonwritinganewtestforanotherpartofthespecification.Youwillrunitanditfails.Thisprovesthatthenewtestdoestestsomethingthatwasnotdevelopedyet.Then,youwilldevelopthecodetosatisfythenewtestand,possibly,youwillalsomodifyablockofcodethatyouhavealreadywritteninthepreviousiterations.Whenthecodeisready,thetestswillpass.

Manytimes,developersarereluctanttomodifythecode.Thisisbecausetheyareafraidofbreakingsomethingthatwasalreadyworking.WhenyoufollowTDD,youshouldnot,andatthesametime,youneednotbeafraidofthis.Therearetestsforallfeaturesthatwerealreadydeveloped.Ifsomeofthecodemodificationbreakssomefunctionality,thetestswillimmediatelysignaltheerror.Thekeyisthatyourunthetestsasoftenaspossiblewhenthecodeismodified.

ImplementingQuickSort

Quicksort,aswehavealreadydiscussed,ismadeoftwomajorparts.Oneispartitioningandtheotheroneisdoingthepartitioningrecursivelyuntilthewholearrayissorted.TomakeourcodemodularandreadytodemonstratetheJava9module-handlingfeature,wewilldevelopthepartitioningandtherecursivesortingintoseparateclassesandinaseparatepackage.Thecomplexityofthecodewillnotjustifythisseparation.

ThepartitioningclassThepartitioningclassshouldprovideamethodthatmovestheelementsofthecollectionbasedonapivotelement,andwewillneedtoknowthepositionofthepivotelementafterthemethodfinishes.Thesignatureofthemethodshouldlooksomethinglikethis:

publicintpartition(SortableCollection<E>sortable,intstart,intend,Epivot);

TheclassshouldalsohaveaccesstoSwapperandComparator.Inthiscase,wedefinedaclassandnotaninterface;therefore,wewilluseconstructorinjection.

Theseconstructs,likesettersandconstructorinjectors,aresocommonandhappensofrequentlythatIDEssupportthegenerationofthese.Youwillneedtocreatethefinalfieldsinthecodeandusethecodegenerationmenutocreatetheconstructor.

Thepartitioningclasswilllooklikethefollowing:

packagepackt.java9.by.example.ch03.qsort;

importpackt.java9.by.example.ch03.SortableCollection;

importpackt.java9.by.example.ch03.Swapper;

importjava.util.Comparator;

publicclassPartitioner<E>{

privatefinalComparator<E>comparator;

privatefinalSwapperswapper;

publicPartitioner(Comparator<E>comparator,Swapperswapper){

this.comparator=comparator;

this.swapper=swapper;

}

publicintpartition(SortableCollection<E>sortable,intstart,intend,Epivot){

return0;

}

}

Thiscodedoesnothing,butthatishowTDDstarts.Wewillcreatethedefinitionofarequirementprovidingtheskeletonofthecodeandthetestthatwillcallit.Todothat,wewillneedsomethingthatwecanpartition.ThesimplestchoiceisanIntegerarray.ThepartitionmethodneedsaobjectoftypeSortableCollection<E>,andwewillneedsomethingthatwrapsthearrayandimplementsthisinterface.WenamethatclassArrayWrapper.Thisclassservesageneralpurposeanditisnotonlyforthetest.Becauseofthat,wecreateitasproductioncodeandassuchweputitinthedirectorymainandnotinthedirectorytest.AsthiswrapperisindependentfromtheimplementationofSort,theproperpositionofthisclassisinanewSortSupportClassesmodule.Wewillcreatethenewmoduleasitisnotpartoftheinterface.Implementationsdependontheinterface,butnotonthesupportclasses.Therecanalsobesomeapplicationthatusesourlibrariesandmayneedtheinterfacemoduleandsomeoftheimplementationbutstilldoesnotneedthesupportclasseswhentheydeliverthewrappingfunctionalitythemselves.Afterall,wecannotimplementallpossiblewrappingfunctionality.TheSRPalsoholdsforthemodules.

Javalibrariestendtocontainunrelatedfunctionalities.Fortheshortrun,itmakestheuseofthelibrarysimpler.YouwillonlyneedtospecifyonedependencyinyourPOMfileandyouwillhavealltheclassesandAPIsthatyouneed.Inthelongrun,theapplicationgetsbigger,carryingalotofclassesthatarepart

ofsomeofthelibrariesbuttheapplicationneverusesthem.

Toaddthenewmodule,themoduledirectoryhastobecreatedalongwiththesourcedirectoriesandthePOMfile.ThemodulehastobeaddedtotheparentPOManditalsohastobeaddedtothedependencyManagementsectionsothatthetestcodeoftheQuickSortmodulecanuseitwithoutspecifyingtheversion.Thenewmoduledependsontheinterfacemodule,sothisdependencyhastobeaddedtothePOMofthesupportclasses.

TheArrayWrapperclassissimpleandgeneral.

packagepackt.java9.by.example.ch03.support;

importpackt.java9.by.example.ch03.SortableCollection;

publicclassArrayWrapper<E>implementsSortableCollection<E>{

privatefinalE[]array;

publicArrayWrapper(E[]array){

this.array=array;

}

publicE[]getArray(){

returnarray;

}

@Override

publicEget(inti){

returnarray[i];

}

@Override

publicintsize(){

returnarray.length;

}

}

TheArraySwapperclass,whichwealsoneed,comesintothesamemodule.Itisjustassimpleasthewrapper.

packagepackt.java9.by.example.ch03.support;

importpackt.java9.by.example.ch03.Swapper;

publicclassArraySwapper<E>implementsSwapper{

privatefinalE[]array;

publicArraySwapper(E[]array){

this.array=array;

}

@Override

publicvoidswap(intk,intr){

finalEtmp=array[k];

array[k]=array[r];

array[r]=tmp;

}

}

Havingtheseclasses,wecancreateourfirsttest.

packagepackt.java9.by.example.ch03.qsort;

//importsdeletedfromprint

publicclassPartitionerTest{

Beforecreatingthe@Testmethod,wewillneedtwohelpermethodsthatmakeassertions.Assertionsarenotalwayssimple,andinsomecases,theymayinvolvesomecoding.Thegeneralruleisthatthetestandtheassertionsinitshouldbeassimpleaspossible;otherwise,theyarejustpossiblesourceofprogrammingerrors.Additionally,wecreatedthemtoavoidprogrammingerrors,nottocreatenewones.

TheassertSmallElementsmethodassertsthatallelementsbeforecutIndexaresmallerthanpivot.

privatevoidassertSmallElements(Integer[]array,intcutIndex,Integerpivot){

for(inti=0;i<cutIndex;i++){

Assert.assertTrue(array[i]<pivot);

}

}

TheassertLargeElementsmethodmakessurethatallelementsfollowingcutIndexareatleastaslargeaspivot.

privatevoidassertLargeElemenents(Integer[]array,intcutIndex,Integerpivot){

for(inti=cutIndex;i<array.length;i++){

Assert.assertTrue(pivot<=array[i]);

}

}

ThetestusesaconstantarrayofIntegersandwrapsitintoanArrayWrapperclass.

@Test

publicvoidpartitionsIntArray(){

Integer[]partitionThis=newInteger[]{0,7,6};

Swapperswapper=newArraySwapper<>(partitionThis);

Partitioner<Integer>partitioner=

newPartitioner<>((a,b)->a<b?-1:a>b?+1:0,swapper);

finalIntegerpivot=6;

finalintcutIndex=partitioner.partition(newArrayWrapper<>(partitionThis),0,2,pivot);

Assert.assertEquals(1,cutIndex);

assertSmallElements(partitionThis,cutIndex,pivot);

assertLargeElemenents(partitionThis,cutIndex,pivot);

}

}

ThereisnoComparatorforIntegertypeintheJDK,butitiseasytodefineoneasalambdafunction.Nowwecanwritethepartitionmethod,asfollows:

publicintpartition(SortableCollection<E>sortable,intstart,intend,Epivot){

intsmall=start;

intlarge=end;

while(large>small){

while(comparator.compare(sortable.get(small),pivot)<0&&small<large){

small++;

}

while(comparator.compare(sortable.get(large),pivot)>=0&&small<large){

large--;

}

if(small<large){

swapper.swap(small,large);

}

}

returnlarge;

}

Ifwerunthetest,itrunsfine.However,ifwerunthetestwithcoverage,thentheIDEtellsusthatthecoverageisonly92%.Thetestcoveredonly13ofthe14linesofthepartitionmethod.

Thereisaredrectangleonthegutteratline28.Thisisbecausethetestarrayisalreadypartitioned.Thereisnoneedtoswapanyelementinitwhenthepivotvalueis6.Itmeansthatourtestisgood,butnotgood

enough.Whatifthereisanerroronthatline?

Toamendthisproblem,wewillextendthetest,changingthetestarrayfrom{0,7,6}to{0,7,6,2}.Runthetestanditfails.Why?Aftersomedebugging,wewillrealizethatweinvokethemethodpartitionwiththefixedparameter2asthelastindexofthearray.But,wemadethearraylonger.Whydidwewriteaconstantthereinthefirstplace?Itisabadpractice.Let'sreplaceitwithpartitionThis.length-1.Now,itsaysthatcutIndexis2,butweexpected1.Weforgottoadjusttheassertiontothenewarray.Let'sfixit.Nowitworks.

Thelastthingistorethinktheassertions.Thelesscodethebetter.Theassertionmethodsarequitegeneral,andwewilluseitforonesingletestarray.Theassertionmethodsaresocomplexthattheydeservetheirowntest.But,wedonotwritecodetotest.Insteadofthat,wecansimplydeletethemethodsandhavethefinalversionofthetest.

@Test

publicvoidpartitionsIntArray(){

Integer[]partitionThis=newInteger[]{0,7,6,2};

Swapperswapper=newArraySwapper<>(partitionThis);

Partitioner<Integer>partitioner=

newPartitioner<>((a,b)->a<b?-1:a>b?+1:0,swapper);

finalIntegerpivot=6;

finalintcutIndex=partitioner.partition(newArrayWrapper<>(partitionThis),0,partitionThis.length-1,pivot);

Assert.assertEquals(2,cutIndex);

finalInteger[]expected=newInteger[]{0,2,6,7};

Assert.assertArrayEquals(expected,partitionThis);

}

Andthenagain,isthisablack-boxtest?Whatifthepartitioningreturns{2,1,7,6}?Itfitsthedefinition.Wecancreatemorecomplexteststocoversuchcases.Butamorecomplextestmayalsohaveabuginthetestitself.Asadifferentapproach,wecancreateteststhatmaybesimplerbutrelyontheinternalstructureoftheimplementation.Thesearenotblack-boxtestsandthusnotidealunittests.Iwillgoforthesecondone,butIwillnotargueifsomeonechoosestheother.

RecursivesortingWewillimplementthequicksortwithanextraclassthatisintheqsortpackagealongwiththepartitioningclass,whichisasfollows:

packagepackt.java9.by.example.ch03.qsort;

//importsdeletedfromtheprint

publicclassQsort<E>{

//constructorinjectedfinalfieldsdeletedfromtheprint

publicvoidqsort(SortableCollection<E>sortable,intstart,intend){

if(start<end){

finalEpivot=sortable.get(start);

finalPartitioner<E>partitioner=newPartitioner<>(comparator,swapper);

intcutIndex=partitioner.partition(sortable,start,end,pivot);

if(cutIndex==start){

cutIndex++;

}

qsort(sortable,start,cutIndex-1);

qsort(sortable,cutIndex,end);

}

}

}

ThemethodgetsSortableCollection<E>andtwoindexparameters.Itdoesnotsortthewholecollection;itsortsonlytheelementsbetweenthestartandtheendindex.

Itisalwaysimportanttobeextremelyprecisewiththeindexing.Usually,thereisnoproblemwiththestartindexinJava,butalotofbugssourcefromhowtheendindexisinterpreted.Inthismethod,thevalueofendcanmeanthattheindexisalreadynotpartoftheto-be-sortedinterval.Inthatcase,thepartitionmethodshouldbeinvokedwithend-1andthefirstrecursivecallwithcutIndexaslastparameter.Itisamatteroftaste.Theimportantthingistobepreciseanddefinetheinterpretationofindexparameters.

Ifthereisonlyoneelement(start==end),thenthereisnothingtobesortedandthemethodreturns.Thisistheendcriterionoftherecursion.Themethodalsoassumesthattheendindexisneversmallerthanthestartindex.Asthismethodisusedonlyinsidethelibrarythatwearedevelopingatthemoment,suchanassumptionisnottooriskytomake.

Ifthereissomethingtobesorted,thenthemethodtakesthefirstelementoftheto-be-sortedintervalandusesitaspivotandcallsthepartitionmethod.Whenthepartitionisdone,themethodrecursivelycallsitselfforthetwohalves.

Thisalgorithmisrecursive.Thismeansthatthemethodcallsitself.Whenamethodcallisexecuted,theprocessorallocatessomememoryinanareacalledstackanditstoresthelocalvariablesthere.Thisareathatbelongstothemethodinthestackiscalledstackframe.Whenthemethodreturns,thisareaisreleasedandthestackisrestored,simplymovingthestackpointerwhereitwastothepreviousstate.Thiswayamethodcancontinueitsexecutionaftercallinganothermethod;thelocalvariablesarethere.

Whenamethodcallsitself,itisnotdifferent.Thelocalvariablesarelocaltotheactualcallofthemethod.Whenthemethodcallsitself,itallocatesspaceforthelocalvariablesagainonthestack.Inother

words,thesearenewinstancesofthelocalvariables.

WewilluserecursivemethodsinJava,andinotherprogramminglanguages,whenthedefinitionofthealgorithmisrecursive.Itisextremelyimportanttounderstandthatwhentheprocessorcoderuns,itisnotrecursiveanymore.Onthatlevel,thereareinstructions,registerstores,andmemoryloadsandjumps.Thereisnothinglikefunctionormethodandtherefore,onthatlevel,thereisnothinglikerecursion.

Ifyougetthat,itiseasytounderstandthatanyrecursioncanbecodedasaloop.

Asamatteroffact,itisalsotruetheotherwayaround—everyloopcanbecodedasrecursionbutthatisnotreallyinterestinguntilyoustartfunctionalprogramming.

TheproblemwiththerecursioninJava,andinmanyotherprogramminglanguages,isthatitmayrunoutofstackspace.Inthecaseofquicksort,thisisnotthecase.YoucansafelyassumethatthestackformethodcallinginJavaisafewhundredsoflevels.Quicksortneedsastackthatisapproximatelylog2ndeep,wherenisthenumberofelementstobesorted.Inthecaseofonebillionelements,thisis30thatshouldjustfit.

Whyisthestacknotmovedorresized?Thatisbecausethecodethatrunsoutofthestackspaceisusuallybadstyle.Theycanbeexpressedmorereadableinformofsomeloop.Amorerobuststackimplementationwouldonlylurethenoviceprogrammertodosomelessreadablerecursivecoding.

Thereisaspecialcaseofrecursionnamedtailrecursion.Atailrecursivemethodcallsitselfasthelastinstructionofthemethod.Whentherecursivecallreturnsthecode,executingthemethoddoesnothingelsebutreleasethestackframethatwasusedforthismethodinvocation.Inotherwords,wewillkeepthestackframeduringtherecursivecalljusttothrowitawayafterwards.Whynotthrowitawaybeforethecall?Inthatcase,theactualframe,whichhasthesamesizeandcall,willallocatebecausethisisjustthesamemethodthatiskeptandtherecursivecallistransformedintoajumpinstruction.ThisisanoptimizationthatJavadoesnotdo.Functionallanguagesaredoingit,butJavaisnotreallyafunctionallanguageandthereforetail-recursivefunctionsshouldratherbeavoidedandtransformedtoaloopintheJavasourcelevel.

Non-recursivesortingTodemonstratethatevennon-tailrecursivemethodscanbeexpressedinanon-recursiveway,hereisthequicksortthatway:

publicclassNonRecursiveQuickSort<E>{

//injectedfinalfieldsandconstructordeletedfromprint

privatestaticclassStack{

finalintbegin;

finalintfin;

publicStack(intbegin,intfin){

this.begin=begin;

this.fin=fin;

}

}

publicvoidqsort(SortableCollection<E>sortable,intstart,intend){

finalList<Stack>stack=newLinkedList<>();

finalPartitioner<E>partitioner=newPartitioner<>(comparator,swapper);

stack.add(newStack(start,end));

inti=1;

while(!stack.isEmpty()){

Stackiter=stack.remove(0);

if(iter.begin<iter.fin){

finalEpivot=sortable.get(iter.begin);

intcutIndex=partitioner.partition(sortable,iter.begin,iter.fin,pivot);

if(cutIndex==iter.begin){

cutIndex++;

}

stack.add(newStack(iter.begin,cutIndex-1));

stack.add(newStack(cutIndex,iter.fin));

}

}

}

}

ThiscodeimplementsastackontheJavalevel.Whileitseesthatthereisstillsomethingscheduledtobesortedinstack,itfetcheditfromthestackanddoesthesortpartitioning,andschedulesthetwopartsforbeingsorted.

ThiscodeismorecomplexthanthepreviousoneandyouhavetounderstandtheroleoftheStackclassandhowitworks.Ontheotherhand,theprogramusesonlyoneinstanceofthePartitionerclassanditisalsopossibletouseathreadpooltoschedulethesubsequentsortsinsteadofhandlingthetasksinasingleprocess.Thismayspeedupthesortwhenitisexecutedonamulti-CPUmachine.However,thisisabitmorecomplextaskandthischaptercontainsalotofnewthingswithoutmultitasking;therefore,wewilllookatmultithreadcodeintwochapterslateronly.

Intheveryfirstversionofthesort,IwascodingitwithoutthethreelinesthatcomparecutIndexagainsttheintervalstartandincrementsitintheifbranch.Itisneededverymuch.But,theunittestswecreatedinthisbookdonotdiscoverthebugifwemissthoselines.Irecommendthatyoujustdeletethoselinesandtrytowritesomeunitteststhatfail.Thentrytounderstandwhatthespecialcaseiswhenthoselinesarevitalandtrytomodifyyourunittestsothatitisthesimplestpossiblethatstilldiscoversthatbug.(Finally,putthefourlinesbackandseeifthecodeworks.)Additionally,findsomearchitecturalreasonwhynottoputthismodificationintothemethodpartition.Thatmethodcouldjustreturnlarge+1incaselarge==start.

ImplementingtheAPIclassHavingdoneallthis,thelastthingwewillneedistohaveQuickSortasasimpleclass(alltherealworkwasalreadydoneindifferentclasses).

publicclassQuickSort<E>implementsSort<E>{

publicvoidsort(SortableCollection<E>sortable){

intn=sortable.size();

Qsort<E>qsort=newQsort<>(comparator,swapper);

qsort.qsort(sortable,0,n-1);

}

//...setterinjectorsweredeletedfromtheprint

}

Donotforgetthatwealsoneedatest!But,inthiscase,thatisnotmuchdifferentthanthatofBubbleSort.

@Test

publicvoidcanSortStrings(){

finalString[]actualNames=newString[]{

"Johnson","Wilson",

"Wilkinson","Abraham","Dagobert"

};

finalString[]expected=newString[]{"Abraham","Dagobert","Johnson","Wilkinson","Wilson"};

Sort<String>sort=newQuickSort<>();

sort.setComparator(String::compareTo);

sort.setSwapper(newArraySwapper<String>(actualNames));

sort.sort(newArrayWrapper<>(actualNames));

Assert.assertArrayEquals(expected,actualNames);

}

Thistime,weusedStringarrayinsteadofArrayList.Thismakesthistestsimplerand,thistime,wealreadyhavethesupportclasses.

Youmayrecognizethatthisisnotaunittest.InthecaseofBubbleSort,thealgorithmwasimplementedinasingleclass.Testingthatsingleclassisaunittest.InthecaseofQuickSort,weseparatedthefunctionalityintoseparateclasses,andevenintoseparatepackages.ArealunittestoftheQuickSortclasswilldisclosethedependencyofthatclassonotherclasses.Whenthistestruns,itinvolvestheexecutionofPartitionerandalsoQsort;therefore,itisnotreallyaunittest.

Shouldwebotheraboutthat?Notreally.Wewanttocreateunitteststhatinvolveasingleunittoknowwheretheproblemiswhenaunittestfails.Iftherewereonlyintegrationtests,afailingtestcasewouldnothelpalotinpointingoutwheretheproblemis.Allitsaysisthatthereissomeproblemintheclassesthatareinvolvedinthetest.Inthiscase,thereareonlyalimitednumberofclasses(three)thatareinvolvedinthistestandtheyaretiedtogether.Theyareactuallytiedtogetherandrelatedtoeachothersocloselythatintherealproductioncode,Iwouldhaveimplementedtheminasingleclass.IseparatedthemheretodemonstratehowtotestasingleunitandalsotodemonstrateJava9modulesupportthatneedsabitmorethanasingleclassinaJARfile.

Creatingmodules

Modulehandling,alsoknownasprojectJigsaw,isafeaturethatwasmadeavailableonlyinJava9.Itwasalongplannedfeaturethatthedeveloperswerewaitingfor.FirstitwasplannedforJava7,butitwassocomplexthatitgotpostponedtoJava8andthentoJava9.Ayearago,itseemedthatitwouldgetpostponedagain,butfinally,theprojectcodegotintotheearlyreleasesandnownothingcanstopfrombeingpartoftherelease.

WhymodulesareneededWehavealreadyseenthattherearefourlevelsofaccessinJava.Amethodorfieldcanbeprivate,protected,public,ordefault(alsoknownaspackageprivate)whennomodifierissupplied.Whenyoudevelopacomplexlibrarytobeusedinseveralprojects,thelibraryitselfwillcontainmanyclassesinmanypackages.Therewillcertainlybeclassesandmethods,fieldsinthosethatareusedinsidethelibrarybyotherclassesfromdifferentpackages,butclassesthatarenottobeusedbythecodeoutsidethelibrary.Makingthemanythinglessvisiblethanpublicwillrenderthemunusableinsidethelibrary.Makingthempublicwillmakethemvisiblefromoutside.

Inourcode,theMavenmodulequickcompiledtoaJARcanonlybeusedifthemethodsortcaninvokeqsort.But,wedonotwantqsorttobeuseddirectlyfromoutside.Inthenextversion,wemaywanttodevelopaversionofthesortthatusesqsortfromtheNonRecursiveQuickSortclassandwedonotwantcomplainingcustomerswhosecodedoesnotcompileorworkbecauseofaminorlibraryupgrade.Wecandocumentthattheinternalmethodsandclassesarestillpublicbutnotforuse,butinvain.Developersusingourlibrarydonotreaddocumentation.Thisisalsowhywedonotwriteexcessivecomments.Nobodywillreadit,noteventheprocessorexecutingthecode.

Themostwell-knownandinfamousexampleofthisproblemisthesun.misc.UnsafeclassintheJDK.Thereissomereallyunsafecodeinit,asthenameimplies.Youcanaccessmemoryoutofheap,createobjectswithoutinitialization,andsoon.Youshouldnot.Whybother?Youareawell-behavingdeveloperandyoujuststicktotherulesandyoudonotusethatpackage.WheneveritchangesinanewversionoftheJDK,yourprogramissafeusingonlypublicandwell-documentedJDKAPI.Right?

Wrong!Withoutbeingawareofthis,youmayusesomelibrariesthatdependonotherlibrariesthatusethepackage.MockitoandSpringFrameworkareonlytwoofthenumerousindanger.Inaddition,Java9willdefinitelycomewithanewversionofthispackage.However,itwillalsocomewithmodulehandling.WhileJava9willprovidesomeusefulAPIforthelibrariesthatwereusingtheUnsafepackagebecausetherewasnoprovidedAPIforthefunctionalitytheyneeded,itwilldelivermodulesnottorecreatethesameproblemagain.

WhatisaJavamoduleAJavamoduleisacollectionofclassesinaJARorinadirectorythatalsocontainaspecialclassnamedmodule-info.IfthereisthisfileinaJARordirectorythenitisamodule,otherwiseitisjustacollectionofclassesthatareontheclasspath(ornot).Java8,andtheearlierversions,willjustignorethatclassasitisneverusedascode.Thisway,usingolderJava,causesnoharmandbackwardcompatibilityismaintained.

Themoduleinformationdefineswhatthemoduleexportsandwhatitrequires.Ithasaspecialformat.Forexample,wecanplacemodule-info.javainourSortInterfaceMavenmodule.

modulepackt.java9.by.example.ch03{

exportspackt.java9.by.example.ch03;

}

Thismeansthatanyclass,whichispublicandinsidethepackt.java9.by.example.ch03package,canbeusedfromoutside.Thispackageisexportedfromthemodule,butotherclassesfromotherpackagesarenotvisiblefromoutsideofthemoduleeveniftheyarepublic.Thenameofthemoduleissameasthepackage,butthisismereconventionincasethereisonlyonepackageexported.Therequirementisthesameasinthecaseofpackages:thereshouldbeanamethatisnotlikelytocollidewithothermodulenames.Thereverseddomainnameisagoodchoicebutitisnotamustasyoucanseeinthisbook.Thereisnotop-leveldomainpackt,yet.

WeshouldalsoconfiguretheparentPOMtoensurethatthecompilerweuseisJava9,

<build>...

<plugins>...

<plugin>

<groupId>org.apache.maven.plugins</groupId>

<artifactId>maven-compiler-plugin</artifactId>

<version>3.5.1</version>

<configuration>

<source>1.9</source>

<target>1.9</target>

</configuration>

</plugin>

...

Olderversionswouldbeconfusedwiththemodule-info.javafile.(Bytheway,eventheearlyaccessversionofJava9Iuseforthisbooksometimesgivesahardtime.)

Wealsocreateamodule-info.javafileintheMavenmodule,quick,whichisasfollows:

modulepackt.java9.by.example.ch03.quick{

exportspackt.java9.by.example.ch03.quick;

requirespackt.java9.by.example.ch03;

}

Thismoduleexportsanotherpackageandrequiresthepackt.java9.by.example.ch03modulethatwehavejustcreated.Now,wecancompilethemodulesandthecreatedJARsinthe./quick/targetand./SortInterface/targetdirectoriesarenowJava9modules.

AsMavendoesnotfullysupportthemodulesyet,whenIissuethemvninstallcommand,I

getthefollowingerrormessage:[ERROR].../genericsort/quick/src/main/java/module-info.java:[3,40]modulenotfound:

packt.java9.by.example.ch03

Mavenputsthecompiledmodulesonclasspath,butJava9seeksmodulepathformodules.Mavendoesnothandlemodulepathyet.Tohackmodulepathtothecompiler,wewillhavetoaddthefollowingconfigurationlinestotheparentPOMtotheconfigurationofthecompilerplugin:<compilerArgs><arg>-modulepath</arg>

<arg>${project.parent.basedir}/SortInterface/target/SortInterface-1.0.0-SNAPSHOT.jar:...</arg>

</compilerArgs>

TheactualfileshouldlistallthecolonseparatedJARfilesthatMavengenerates,andonwhichsomeofthemodulesdepend.ThesearetheSortInterface,quick,andSortSupportClasses.

Totestthefunctionalityofmodulesupport,wewillcreateanotherMavenmodulecalledMain.Ithasonlyoneclass,calledMain,withapublicstaticvoidmainmethod:

packagepackt.java9.by.example.ch03.main;

//...importsdeletedfromtheprint

publicclassMain{

publicstaticvoidmain(String[]args)throwsIOException{

StringfileName=args[0];

BufferedReaderbr=newBufferedReader(newInputStreamReader(newFileInputStream(newFile(fileName))));

List<String>lines=newLinkedList<>();

Stringline;

while((line=br.readLine())!=null){

lines.add(line);

}

br.close();

String[]lineArray=lines.toArray(newString[0]);

Sort<String>sort=newQuickSort<>();

Qsort<String>qsort=newQsort<>(String::compareTo,newArraySwapper<>(lineArray));

sort.setComparator(String::compareTo);

sort.setSwapper(newArraySwapper<>(lineArray));

sort.sort(newArrayWrapper<>(lineArray));

for(finalStringoutLine:lineArray){

System.out.println(outLine);

}

}

}

Ittakesthefirstargument(withoutcheckingthatthereisone,whichweshouldnotuseinaproductioncode)andusesthatasafilename.Then,itreadsthelinesofthefileintoaStringarray,sortsit,andprintsittothestandardoutput.

Asthemodulesupportonlyworksformodules,thisMavenmodulealsohastobeaJavamoduleandhaveamodule-info.javafile.

modulepackt.java9.by.example.ch03.main{

requirespackt.java9.by.example.ch03.quick;

requirespackt.java9.by.example.ch03;

requirespackt.java9.by.example.ch03.support;

}

Additionally,wewillhavetocreateamodule-info.javafileforthesupportmodule;otherwise,wewillnotbeabletouseitfromourmodule.

Aftercompilingthemodulesusingmvninstall,wecanrunittoprintouttheparentPOM.

java-cpMain/target/Main-1.0.0-SNAPSHOT.jar:SortInterface/target/SortInterface-1.0.0-SNAPSHOT.jar:quick/target/quick-1.0.0-SNAPSHOT.jar:SortSupportClasses/target/SortSupportClasses-1.0.0-SNAPSHOT.jarpackt.java9.by.example.ch03.main.Mainpom.xml

Notethatthisisonelineofcommandthatprintbreaksintoseverallines.

Now,ifwetrytoaccessQsortdirectlyinsertingthefollowinglineQsort<String>qsort=newQsort<>(String::compareTo,newArraySwapper<>(lineArray));intothemainmethod,MavenwillcomplainbecausethemodulesystemhidesitfromourMainclass:

[ERROR]Failedtoexecutegoalorg.apache.maven.plugins:maven-compiler-plugin:3.5.1:compile(default-compile)onprojectMain:Compilationfailure:Compilationfailure:

[ERROR].../Main/src/main/java/packt/java9/by/example/ch03/main/Main.java:[4,41]packagepackt.java9.by.example.ch03.qsortdoesnotexist

[ERROR].../Main/src/main/java/packt/java9/by/example/ch03/main/Main.java:[25,9]cannotfindsymbol

Themodulesystemalsosupportsthejava.util.ServiceLoaderbasedclass-loadingmechanism,whichwewillnotdiscussinthisbook.ThisisanoldtechnologythatisrarelyusedinanenterpriseenvironmentwhenSpring,Guice,orsomeotherdependencyinjectionframeworkisused.Ifyouseeamodule-info.javafilethatcontainstheusesandprovideskeywords,thenfirstconsultwiththeJavadocumentationabouttheServiceLoaderclassathttp://docs.oracle.com/javase/8/docs/api/java/util/ServiceLoader.html,andthentheJava9languagedocumentationonmodulesupport(http://openjdk.java.net/projects/jigsaw/quick-start).

Summary

Inthischapter,wedevelopedageneralsortingalgorithmimplementingquicksort.Wemodifiedourprojecttobeamulti-moduleMavenprojectandalsotouseJavamoduledefinitions.WewereusingJUnittodevelopunittests,andwedevelopedthecodeusingTDD.WeconvertedthecodefromoldstyleJavatonewusinggenerics,andweusedexceptionhandling.Thesearethebasictoolsthatareneededforthecomingchapters,wherewewilldevelopaguessinggame.Firstwewilldevelopasimplerversionandinthesubsequentchapterwewilldevelopaversionthatusesparallelcomputing,andmultipleproccessors.

Mastermind-CreatingaGameInthischapter,wewillstarttodevelopasimplegame.ThegameistheMastermindgamefortwoplayers.Playeroneselectsfourdifferentlycoloredpinsoutofsixpossiblecolorsandarrangesthemonaboardinarowhiddenfromtheotherplayer.Theotherplayertriestoguessthecolorsofthepinsanditspositions.Aftereachtry,playeronetellsthenumberofmatchingcolorsandthepinsmatchingbothcolorandposition.Theprogramwillactasbothplayeroneandplayertwo.Ourcodewillplayalone.However,whatremainsforustoplaywithisthemostimportant:thecode.

ThisexampleiscomplexenoughtodeepentheOOprinciplesandhowwedesignclassesandmodeltherealworld.WehavealreadyusedclassesprovidedintheJavaruntime.Thistime,wewillusecollectionsanddiscussthisimportantarea.TheseclassesandinterfacesarewidelyusedandavailableintheJDKandasimportantforaprofessionalJavadeveloperasthelanguageitself.

ThebuildtoolthistimeisGradle.

Inthischapterwewillcover:

JavacollectionsDependencyinjectionHowtocommentourcodeandtocreateJavaDocdocumentationHowtocreateintegrationtests

TheGameMastermind(https://en.wikipedia.org/wiki/Mastermind_(board_game))isanoldgame.Theplasticversionthatwasubiquitousineveryhousewithchildrenwasinventedin1970.Igotaboardaround1980asaChristmasgiftandsolvingthegamepuzzleinBASIClanguagewasoneofthefirstprogramsthatIcreatedaround1984.

Thegameboardcontainsholesinseveralrowsinfourcolumns.Thereareplasticpinsofsixdifferentcolorsthatcanbeinsertedintotheholes.Eachpinhasonecolor.Theyareusuallyred,green,blue,yellow,black,andwhite.Thereisaspecialrowthatishiddenfromoneoftheplayers(theguesser).

Toplaythegame,oneoftheplayers(hider)hastoselectfourpinsfromasetofpins.Theselectedpinsshouldhavedifferentcolors.Thepinsareplacedinthehiddenrowonebyone,eachintoaposition.

Theguessertriestofindoutwhatcolorsareinwhichpositionguessing.Eachguesstakesplaceselectingfourpinsandplacingtheminarow.Thehidertellstheguesserhowmanypinsareincorrectpositionandhowmanyhaveacolorthatisonthetable,butarenotinthepositionwherethatcolorishidden.

Asampleplaymaygolikethis:

Thehiderhidesfourpinswithcolorblue,yellow,white,andblack.Guesserguessesyellow,blue,green,andred.Thehidertellstheguesserthattherearetwocolorsmatching,butnoneofthemisinthepositioninthehiddenrow.Thehidersaysthisbecauseyellowandblueareinthehiddenrowbutnotinthepositionsastheguesserguessed.Theyareactuallyswapped,butthisinformationthehiderkeepsasecret.Allshesaysisthattherearetwocolorsmatching,noneinthecorrectposition.Thenextguessis...

Thegamefinisheswhentheguesserfindsthecorrectcolorsinthecorrectorder.Thesamegame,asonthefigure,canalsobedescribedwithtextualnotation,Bforblue,Yforyellow,Gforgreen,Wforwhite,Rforred,andbforblack(luckywehaveupperandlowercaselettersonthecomputer).

RGBY0/0

GRWb0/2

YBbW0/2

BYGR0/4

RGYB2/2

RGBY4/0

Guesswhat!Thisistheactualoutputoftheprogramthatwedevelopinthischapter.

ThemodelofthegameWhenwedevelopapieceofcodewithanobject-orientedmindset,wetrytomodeltherealworldandmapreal-worldobjectstoobjectsintheprogram.Youcertainlyhaveheardofobjectorientationexplainedwiththeverytypicalexamplesofgeometricobjects,orthecarandthemotorthingtoexplaincomposition.Personally,Ibelievethattheseexamplesaretoosimpletogetagoodunderstanding.Theymaybegoodforstarters,butwearealreadyinthefourthchapterofthebook.TheMastermindgameismuchbetter.Itisabitmorecomplexthanjustrectanglesandtriangles,butnotascomplexasatelecombillingapplicationoranatomicpowerplantcontrol.

Whatarethereal-worldobjectsthatwehaveinthatgame?Wehaveatableandwehavepinsofdifferentcolors.TherearetwoJavaclassesthatwecertainlywillneed.Whatisinatable?Therearerowseachhavingfourpositions.Perhapswewillneedaclassforarow.Atablewillhaverows.Wewillalsoneedsomethingthathidesthesecret.Thisalsomaybearowandeachrowmayalsoholdtheinformationabouthowmanypositionsandhowmanycolorsarematching.Incaseofthesecretrow,thisinformationisobvious:4and0.

Whatisapin?Eachpinhasacolorandgenerally,thatisit.Therearenootherfeaturesofapin,exceptthatitcanbeinsertedintoaholeonthetable,butthisisareallifefeaturewewillnotmodel.Essentially,apinisacolorandnothingelse.Thisway,wecaneliminatethepinclassfromourmodelearlyon,evenbeforewecreateditinJava.Instead,wehavecolors.

Whatisacolor?Thisissomethingthatmaybehardtoimmerseintothefirsttime.Weallknowwellwhatacoloris.Itisamixtureofdifferentfrequencyoflights,asoureyesperceiveit.Wecanhavepaintsandprintsindifferentcolors,andsoon.Thereareverymanythingsthatwedonotmodelinthisprogram.Itisreallyhardtotellwhatwemodelaboutcolorinourcodebecausethesefeaturesaresoobviousthatwetakeitforgrantedinreallife;wecantellabouttwocolorsthattheyaredifferent.Thisistheonlyfeatureweneed.Todothis,thesimplestclassofJavacanbeused:

packagepackt.java9.by.example.mastermind;

publicclassColor{}

IfyouhavetwovariablesofthetypeColor,youcantelliftheyarethesameornot.Youcanuseobjectidentitycomparingaandbusingtheexpressiona==boryoucanusetheequalsmethodinheritedfromtheObjectclass,a.equals(b).Itistemptingtoencodethecolorswithletters,oruseStringconstantstodenotethem.Itmaybeeasierfirst,butthereareseriousdrawbackslater.Whenthecodebecomescomplex,itleadstobugs;itwillbeeasytopasssomethingalsoencodedasStringinsteadofacolorandonlyunittestsmaysavetheday.Better,thecompileralreadycomplainsintheIDEwhenyoutypethewrongargument.

Whenweplaythegame,thepinsareinsmallboxes.Wepullpinsoutoftheboxes.Howdowegetthecolorsintheprogram?Weneedsomethingfromwherewecanfetchcolorsorlookingattheotherwaysomethingthatcangiveuscolors.WewillcallitColorManager.ColorManagerknowshowmanydifferentcolorswehaveandanytimeweneedacolor,wecanaskforit.

Again,thereisatemptationtodesigntheColorManagerthatitcanserveacolorbyitsserialnumber.Ifwe

havefourcolors,wecouldaskforcolornumber0,1,2,or3.Butthenagain,itwouldjustimplicitlyencodethecolorsasintegernumbers,whichweagreedwewillnot.Weshouldfindtheminimumfeaturethatwewillneedtomodelthegame.

Todescribethestructureoftheclasses,professionaldevelopersusuallyuseUMLclassdiagrams.UMLisadiagramnotationthatisstandardizedandisalmostexclusivelyusedtovisualizesoftwarearchitecture.TherearemanydiagramtypesinUMLtodescribethestaticstructureandthedynamicbehaviorofaprogram.Thistime,wewilllookataverysimplifiedclassdiagram.

WehavenoroomtogetintothedetailsofUMLclassdiagrams.Rectanglesdenotetheclasses,normalarrowsdenotetherelationswhenaclasshasfieldoftheotherclasstype,andtriangleheadedarrowmeansthataclassextendsanother.Thearrowpointstothedirectionoftheclassbeingextended.

AGamecontainsasecretRowandaTable.TheTablehasaColorManagerandaList<>ofRow.TheColorManagerhasafirstcolorandhasaMap<>ofColor.Wehavenotdiscussedwhythatisthedesign,wewillgetthereandthediagramhelpsuswalkingthatroad.ARowisessentiallyanarrayofColor.

Theonewhoplaysthegamehasonefunction:ithastoguessmanytimesuntilitfindsthehiddensecret.TogettothemodeloftheColorManager,wewillhavetodesignthealgorithmoftheGuesser.

Whentheplayermakesthefirstguess,anycombinationofcolorsisjustasgoodasanyother.Later,theguessesshouldconsidertheresponsesthatweregivenforpreviousguesses.Itisareasonableapproachtotryonlycolorvariationsthatcanbetheactualsecret.Theplayerselectsavariationandlooksatallpreviousguessesassumingthattheselectedvariationisthesecret.Iftheresponsestotherowshehasalreadymadearethesameforthisvariationasfortheunknownsecretinthegame,thenitisreasonabletotrythisvariation.Ifthereisanydifferenceintheresponses,thenthisvariationiscertainlynotthe

variationthatwashidden.

Tofollowthisapproach,theguesserhastogenerateallpossiblecolorvariationsoneaftertheotherandcompareitagainstthetable.Theguessercodewillnotcreateandstoreallthepossiblevariationsahead,butithastoknowwhereitwasandhastobeabletocalculatethenextvariationthatcomes.Thisassumesanorderofthevariations.Forashortwhile,let'sforgetthatnocolormayappeartwiceinavariation.Asimpleorderingcanbemadethesamewayaswesortdecimalnumbers.Ifwehaveathree-digitnumber,thenthefirstoneis000,thenextoneis001,andsoonuntil009,alwaysfetchingthenextdigitforthelastposition.Afterthat,010comes.Weincreasedadigitnexttothelastoneandwesetthelastoneto0again.Now,wehave011,012,andsoon.Youknow,howwecountnumbers.Now,replacethedigitswithcolorsandwehaveonlysixandnotten.Or,wehaveasmanyaswewantwhenweinstantiateaColorManagerobject.

ThisleadstothefunctionalityoftheColorManager.Ithastodothefollowingtwothings:

GivethefirstcolortothecallerGivethenextcolorthatfollowsagivencolor(wewillnamethemethodnextColor)

Thelatterfunctionalityshouldalsosignalsomewaywhenthereisnonextcolor.Thiswillbeimplementedusinganothermethod,namedthereIsNextColor.

ItisaconventiontostartthemethodnamesthatreturnaBooleanvaluewithis.ThatwouldleadtothenamefollowingthisconventionisThereNextColor,orisNextColor.Eitherofthesenamesexplainsthefunctionalityofthemethod.IfIaskthequestionisThereNextColor,themethodwillanswermetrueorfalse.But,thisisnothowwewillusethemethod.Wewilltalkinsimplesentences.Wewilluseshortsentences.Wewillavoidunnecessary,gibberishexpressions.Wewillalsoprogramthatway.Mostprobably,thecallerwillusethismethodinanifstatement.Theywillwritethefollowing:If(thereIsNextColor(currentColor)){...}

andnotif(isThereNextColor(currentColor)){...}

Ithinkthefirstversionismorereadableandreadabilitycomesfirst.Last,butnotleast,nobodywillblameyouifyoufollowtheoldconvention,andincasethatisthecompanystandard,youhavetoanyway.

Todothese,theColorManageralsohastocreatethecolorobjectsandshouldstoretheminastructurethathelpstheoperationsbeingperformed.

packagepackt.java9.by.example.mastermind;

importjava.util.HashMap;

importjava.util.Map;

publicclassColorManager{

finalprotectedintnrColors;

finalprotectedMap<Color,Color>successor=newHashMap<>();

finalprivateColorfirst;

publicColorManager(intnrColors){

this.nrColors=nrColors;

first=newColor();

ColorpreviousColor=first;

for(inti=1;i<nrColors;i++){

finalColorthisColor=newColor();

successor.put(previousColor,thisColor);

previousColor=thisColor;

}

successor.put(previousColor,Color.none);

}

publicColorfirstColor(){

returnfirst;

}

booleanthereIsNextColor(Colorcolor){

returnsuccessor.get(color)!=Color.none;

}

publicColornextColor(Colorcolor){

returnsuccessor.get(color);

}

}

ThestructureweuseisaMap.MapisaninterfacedefinedintheJavaruntimeandisavailablesincetheveryearlyreleasesofJava.AMaphaskeysandvalue,andforanykey,youcaneasilyretrievethevalueassignedtothekey.

Youcanseeontheline,wherethevariablesuccessorisdefinedthatwedefinethetypeofthevariableasaninterface,butthevalueisaninstanceofaclass.Obviously,thevaluecannotbeaninstanceofaninterfacebecausesuchbeastsdonotexist.But,whydowedefinethevariabletobeaninterface?Thereasonisabstractionandcodingpractice.Ifweneedtochangetheimplementationweuseforsomereason,thevariabletypestillmayremainthesameandthereisnoneedtochangethecodeelsewhere.ItisalsoagoodpracticetodeclarethevariabletobeaninterfacesothatwewillnothavethetemptationtousesomespecialAPIoftheimplementationthatisnotavailableintheinterfacejustbyconvenience.Whenitisreallyneeded,wecanchangethetypeofthevariableandusethespecialAPI.Afterall,thereisareasonthatAPIisthere,butthemeretemptationtousesomespecialthingjustbecauseitisthereishindered.Thishelpstowritesimplerandcleanerprogram.

MapisonlyoneoftheinterfacesdefinedintheJavaruntimebelongingtotheJavacollections.Therearemanyotherinterfacesandclasses.Although,theJDKandalltheclassesareavastamountandalmostnobodyknowsalltheclassesthatarethere,collectionsisaspecialareathataprofessionaldevelopershouldbeknowledgeableabout.BeforegettingintodetailsonwhyHashMapisusedinthiscode,wewillhaveanoverviewofthecollectionclassesandinterfaces.Thiswillhelpusalsounderstandtheothercollectionsusedinthisprogram.

JavacollectionsCollectionsareinterfacesandclassesthathelpusstoremorethanoneobject.Wehavealreadyseenarraysthatcandothat,andalsoArrayListinthepreviouschapters,butwedidnotdiscussindetailwhatotherpossibilitiesthereareintheJDK.Here,wewillgointomoredetail,butleavethestreamsandthefunctionalmethodsforlaterchapters,andwewillalsorefraintogointodetailsthatisratherthetaskofareferencebook.

Usingimplementationofthecollectionclassesandinterfacesreducestheprogrammingeffort.Firstofall,youdonotneedtoprogramsomethingthatisalreadythere.Secondly,theseclassesarehighlyoptimized,bothinimplementationandintheirfeatures.TheyhaveverywelldesignedAPIaswellasthecodeisfastandusessmallmemoryfootprint.Sorrytosaythattheircodewaswrittenlongtimeagoandmanytimesitisnotagoodstyle,hardtoread,andunderstand.

WhenyouuseacollectionfromtheJDK,itismorelikelythatyoucaninteroperatewithsomelibrary.Ifyoucookyourownversionoflinkedlists,itisnotlikelythatyouwillfindareadymadesolutionthatwillsortyourlist.IfyouusetheLinkedListclassintheJDK'sstandardclasslibrary,youwillgetareadymadesolutionfromtheCollectionsclass,rightfromtheJDK.ItisalsoworthmentioningthattheJavalanguageitselfsupportstheseclasses,forexample,youcaneasilyiteratethroughtheelementsofaCollectionwithashortenedspecialsyntax.

ThecollectionsinJDKcontaininterfacesthatdefinethebehaviorofthedifferentcollectiontypes,implementationclasses,andalgorithmsthatperformcertainactionssuchassorting.Manytimes,thesealgorithmsworkondifferentimplementationversions,gettingthesameresult,butoptimizedfortheimplementationspecificclass.

YoucanusetheAPIgivenbytheinterface,andifyouchangetheimplementationinyourcode,youwillgetanoptimizedversionfittingtheimplementation.

Thecollectioninterfacescanbecategorizedintwobags.OnebagcontainstheinterfacesthatextendtheCollectioninterface,andtheotheronecontainsMap,andaSortedMapextendingMap.Thisway,Mapisnotreallyacollection,asitdoesnotsimplycontainotherobjectsbutalsopairvaluestokeys.

InterfacecollectionCollectionisthetopoftheinterfacehierarchy.Thisinterfacedefinesthemethodsthatallimplementationsshouldprovide,nomatteriftheyimplementtheSet,SortedSet,List,Queue,orDequeinterfacedirectly.AsCollectionsimplysaysthatanobjectthatimplementstheCollectioninterfaceisonlyanobjectthatcollectsotherobjectstogether,themethodsitdefinesarelikeaddinganewobjecttothecollection,clearingallelementsfromthere,checkingthatanobjectisalreadyamemberofthecollection,anditeratingthroughtheelements.

Foranup-to-datedefinitionoftheinterface,consulttheJavapidocumentation(http://download.java.net/java/jdk9/docs/api/overview-summary.html).YoucanconsulttheonlineAPIanytime,anditisrecommendedtodoso.

TheJavalanguageitselfdirectlysupportstheinterface.YoucaniteratethroughtheelementsoftheCollectionwiththeenhancedforloopsyntax,thesamewayasyoucaniterateovertheelementsofanarraywherethecollectionshouldbeanexpressionthatresultsanobjectthatimplementstheCollectioninterface:for(Eelement:collection){...}

Intheprecedingcode,EiseitherObjectorthegenerictypeoftheelementsoftheCollection.

TheinterfaceCollectionisnotdirectlyimplementedintheJDK.ClassesimplementoneofthesubinterfacesofCollection.

SetTheSetisaspecialcollectionthatcannotcontainduplicateelements.Whenyouwanttoaddanobjectintoasetthatalreadyhasanobjectthatisthesameorequaltotheactualone,thentheaddmethodwillnotaddtheactualobject.Theaddmethodwillreturnfalseindicatingthefailure.

YoucanuseSetinyourprogramwhenyouneedacollectionofuniqueelementswhereyousimplywanttocheckthatanelementisamemberofasetornot,whetheranobjectbelongstoacertaingroupornot.

Aswewillreturntoourprogramcode,wewillseethattheUniqueGuesserclasshastoimplementanalgorithmthatchecksthatacolorinaguessispresentonlyonce.ThisalgorithmistheidealcandidateforaSettobeused:

privatebooleanisNotUniqueWithSet(Color[]guess){

finalSet<Color>alreadyPresent=newHashSet<>();

for(Colorcolor:guess){

if(alreadyPresent.contains(color)){

returntrue;

}

alreadyPresent.add(color);

}

returnfalse;

}

Thecodecreatesaset,whichisemptywhenthemethodstarts.Afterthat,itchecksforeachcolor(noticetheenhancedforloopoverthearrayelements)ifitwasalreadypresentbefore.Todothat,thecodechecksifthecolorisalreadyintheset.Ifitisthere,theguessisnotuniqueaswehavefoundacolorthatispresentatleasttwice.Ifthecolorwasnotintheset,thentheguesscanstillbeuniqueincolors.Tobeabletodetectthatlater,thecodeputsthecolorintotheset.

TheactualimplementationofSetthatwewilluseisHashSet.IntheJDK,therearemanyclassesimplementingtheSetinterface.ThemostwidelyusedisHashSet,anditisalsoworthmentioningEnumSet,LinkedHashSet,andTreeSet.ThelastonealsoimplementstheSortedSetinterface,sowewilldetailitthere.

TounderstandwhatHashSet(andlaterHashMap)areandhowtheywork,wewillhavetodiscusswhathashesare.Theyplayveryimportantandcentralroleinmanyapplications.TheydotheirjobunderthehoodintheJDKbuttherearesomeveryimportantconstraintsthatprogrammershavetofolloworelsereallyweirdandextremelyhardtofindbugswillmaketheirlifemiserable.IdaretosaythatviolationofthehashcontractinHashSetandHashMaparethecauseofthesecondmostdifficulttofindbugsnexttomultithreadissues.

Thus,beforegoingonwiththedifferentcollectionimplementations,wewillvisitthistopic.Wearealreadyoneleveldeepfromourexampleinthisdetourdiscussingcollectionsandnowwewillgooneleveldeeper.Ipromisethisisthelastin-depthlevelofdetours.

HashfunctionsAhashisamathematicalfunctionthatassignsanumbertoanelement.SayyouworkatauniversityadministrationandyouhavetotellifWilkinsonisastudentatyourclass.Youcanstorethenamesonsmallpapersinenvelopesoneforeachstartingletter.Insteadofsearchingthroughthe10thousandstudents,youcanlookatthepapersintheenvelopetitledW.Thisverysimplehashfunctionassignsthefirstletterofthenametothename(ortheordinalnumberoftheletter,aswesaidthatahashfunctionresultsanumber).Thisisnotreallyagoodhashfunctionbecauseitputsonlyafewelements,ifany,intotheenvelopedenotedXandmanytoAforexample.

Agoodhashfunctionresultseachpossibleordinalnumberwithsimilarprobability.Inhashtables,weusuallyhavemorebuckets(envelopesinthepreviousexample)thanthenumberofelementstobestored.Therefore,whenanelementissearchedfor,itislikelythatthereisonlyoneelementthere.Atleastthatiswhatwewouldliketohave.Iftherearemultipleelementsinasinglebucket,itiscalledcollision.Agoodhashfunctionhasaslittlecollisionsaspossible.

Forbackwardcompatibility,thereisaHashtableclassintheJDK.ThiswasoneofthefirsthashtableimplementationsinJavarightintheveryfirstversion,andasJavaisbackwardcompatible,itwasnotthrownaway.TheMapinterfacewasintroducedinversion1.2only.Hashtablehasmanydrawbacksanditsuseisnotrecommended.(EventhenameisviolatingtheJavanamingconventions.)Wedonotdiscussthisclassinthisbook.Wheneverwetalkabouthashtables,itisreferringtotheactualarraythatisinsidetheimplementationofHashSet,HashMap,oranyothercollectionthatusessomehashindexedtable.

Hashtablesarearraysthatusetheresultofthehashfunctiontoindexthearray.Usually,linkedlistsmanagecollisions.Hashtableimplementationsalsoimplementastrategytoresizethearraywhenthenumberofelementstobestoredbecomestoohighandthelikelihoodofcollisionsincrease.Thisoperationmaytakeconsiderabletimeand,duringthis,theindividualelementsaremovedbetweenthebuckets.

Duringthisoperation,thehashtablecannotreliablybeusedandthismaybesomesourceofissuesinamultithreadenvironment.Insinglethreadcode,youdonotmeetthisproblem.Whenyoucalltheaddmethod,thehashtable(setormap)decidesthatthetablehastoberesized.Theaddmethodcallstheresizingmethodanddoesnotreturnuntilitisfinished.Singlethreadcodehasnopossibilitytousethehashtableduringthisperiod:theoneandsinglethreadisexecutingtheresizingitself.Inamultithreadenvironment,however...

HashSetandHashMapusethehashfunctionprovidedbytheObjectthatisstoredinthecollection.TheObjectclassimplementsthehashCodeandequalsmethods.Youcanoverridethemandifyoudo,youshouldoverridebothinaconsistentmanner.First,wewillseewhattheyareandthenhowtooverridethemconsistently.

Methodequals

Thedocumentationofsetsays"setscontainnopairofelementse1ande2suchthate1.equals(e2)".Theequalsmethodreturnstrueifthee1ande2areinsomewayequal.Itmaybedifferentfromtwoobjectsbeingidentical.Therecanbetwodistinctobjectsthatareequal.Forexample,wecouldhaveacolorimplementationthathasthenameofthecolorsasanattributeandtwocolorobjectsmayreturntruecallingtheequalsmethodononeofthemandpassingtheargumentastheotherwhenthetwostringsareequal.ThedefaultimplementationoftheequalsmethodisinthecodeoftheObjectclassandthisreturnstrueifandonlyife1ande2areexactlythesameandsingleobject.

Itseemstobeobvious,butmyexperienceshowsthatitcannotbestressedenoughthattheimplementationofequalsinanobjecthastobeasfollows:

Reflexive:ThismeansthatanobjectthatalwaysequalsitselfSymmetric(commutative):Thismeansife1.equals(e2)istrue,thene2.equals(e1)shouldalsobetrueTransitive:Thismeansife1.equals(e2)ande2.equals(e3),thene1.equals(e3)Consistent:Thismeansthatthereturnvalueshouldnotchangeiftheobjectswerenotchangedbetweentheinvocations

MethodhashCodeThehashCodemethodreturnsanint.Thedocumentationsaysthatanyclassredefiningthismethodshouldprovidethefollowingimplementation:

ConsistentlyreturnthesamevalueiftheobjectwasnotmodifiedResultthesameintvaluefortwoobjectsthatareequal(theequalsmethodreturnstrue)

Thedocumentationalsomentionsthatthisisnotarequirementtoresultdifferentintvaluesforobjectsthatarenotequal,butitisdesirabletosupporttheperformanceofthehashimplementingcollections.

IfyouviolateanyoftheserulesintheimplementationofequalsandhashCode,thentheJDKclassesusingthemwillfail.AsyoucanbesurethatHashSet,HashMap,andsimilarclasseswerefullydebugged,seeingthatyouaddedanobjecttoasetandthenthesetreportingthatitisnottherewillbeabewilderingexperience.However,onlyuntilyoufindoutthatthetwoobjectsbeingequalandstoredinthesethavedifferenthashCodevalues,HashSetandHashMapwilllookfortheobjectonlyinthebucketthatisindexedbythehashCodevalue.

ItisalsoacommonmistaketostoreanobjectinaHashSetorHashMapandthenmodifyit.TheobjectisinthecollectionbutyoucannotfinditbecausethehashCodereturnsadifferentvalue.Objectsstoredinacollectionshouldnotbemodifiedunlessyouknowwhatyouaredoing.

Manytimes,objectscontainfieldsthatarenotinterestingfromtheequalitypointofview.ThehashCodeandequalsmethodsshouldbeidempotenttothosefieldsandyoucanalterthosefieldsevenafterstoringtheobjectinaHashSetorinHashMap.

Asanexample,youmayadministertrianglesinobjectsmaintainingthecoordinatesoftheverticesandthecolorofthetriangle.However,youdonotcareaboutthecolorforequality,onlythatthetwotrianglesareattheexactsamelocationinthespace.Inthatcase,theequalsandhashCodemethodshouldnottakethefieldcolorintoaccount.Thisway,wecanpaintourtriangles;theywillstillbefoundinHashSetorHashMapnomatterwhatthecolorfieldis.

ImplementingequalsandhashCodeImplementingthesemethodsisfairlysimple.Asthisisaverycommontask,theIDEssupportthegenerationofthesemethods.ThesemethodsaretiedtogethersomuchthatthemenuitemsintheIDEsarenotseparate;theyofferyoutogeneratethesemethodsatonce.

AskingtheIDEtogeneratetheequalsmethodwillresultinsomethinglikethefollowingcode:

@Override

publicbooleanequals(Objecto){

if(this==o)returntrue;

if(o==null||getClass()!=o.getClass())returnfalse;

MyObjectJava7that=(MyObjectJava7)o;

returnObjects.equals(field1,that.field1)&&

Objects.equals(field2,that.field2)&&

Objects.equals(field3,that.field3);

}

Forthissample,wehavethreeObjectfieldsnamedfield1,field2,andfield3.Thecodewithanyothertypesandfieldswilllookverysimilar.

First,themethodchecksforobjectidentity.OneObjectalwaysequalsitself.Ifthereferencepassedasargumentisnullandnotanobject,ortheyareofdifferentclass,thenthisgeneratedmethodwillreturnfalse.Inothercases,thestaticmethodoftheclassObjects(notetheplural)willbeusedtocompareeachofthefields.

TheutilityclassObjectswasintroducedinJava7,hencethenameofthesampleclass.Thestaticmethods,equalsandhash,supporttheoverrideoftheObjectequalsandhashCodemethods.ThehashCodecreationbeforeJava7wasfairlycomplexandrequiredtheimplementationofmoduloarithmeticwithsomemagicnumbersthatishardtoexplainjustlookingatthecodewithoutknowingthemathematicsbehindit.

ThiscomplexityisnowhiddenbehindthefollowingObjects.hashmethod.

@Override

publicinthashCode(){

returnObjects.hash(field1,field2,field3);

}

ThegeneratedmethodsimplycallstheObjects.hashmethodpassingtheimportantfieldsasarguments.

HashSet

Now,weknowessentiallyalotofthingsabouthashessowecanbravelydiscusstheHashSetclass.HashSetisanimplementationoftheSetinterfacethatinternallyuseshashtable.Generally,thatisit.Youstoreobjectsthereandyoucanseeifanobjectisalreadythereornot.WhenthereisaneedforaSetimplementation,almostalwaysHashSetisthechoice.Almost...

EnumSet

EnumSetcancontainelementsfromacertainenumeration.Recallthatenumerationsareclassesthathavefixedanumberofinstancesdeclaredinsidetheenumitself.Asthislimitsthenumberofthedifferentobjectinstances,andthisnumberisknownduringcompilationtime,theimplementationoftheEnumSetcodeisfairlyoptimized.Internally,EnumSetisimplementedasabitfieldandisagoodchoicewherebitfieldmanipulationscanbeused.

LinkedHashSet

LinkedHashSetisaHashSetthatalsomaintainsadoublylinkedlistoftheelementsitholds.WhenweiteratethoughaHashSet,thereisnoguaranteedorderoftheelement.WhentheHashSetismodified,thenewelementsareinsertedintooneofthebucketsand,possibly,thehashtablegetsresized.Thismeansthattheelementsgetrearrangedandgetintototallydifferentbuckets.IterationovertheelementsinHashSetjusttakesthebucketsandtheelementsinitinsomeorderthatisarbitraryfromthecallerpointofview.

LinkedHashSet,however,iteratesovertheelementsusingthelinkedlistitmaintainsandtheiterationisguaranteedtohappenintheordertheelementswereinserted.

SortedSet

TheSortedSetisaninterfacethatguaranteesthattheclassesimplementingitwilliterateoverthesetinasortedorder.TheordermaybethenaturalorderingoftheobjectsiftheobjectsimplementtheComparableinterfaceoraComparatorobjectmaydriveit.ThisobjectshouldbeavailablewhentheinstanceoftheclassimplementingtheSortedSetiscreated;inotherwords,ithastobeaconstructorparameter.

NavigableSet

NavigableSetextendstheSortedSetinterfacewithmethodsthatletyoudoproximitysearchintheset.Thisessentiallyletsyousearchforanelementthatisinthesearchandislessthanthesearchedobject,lessorequaltothesearchedelement,greaterorequal,orgreaterthanthesearchedobject.

TreeSet

TreeSetisanimplementationofNavigableSetand,thiswaythisisalsoaSortedSetand,asamatteroffact,isalsoaSet.AsaSortableSetdocumentationimpliestherearetwotypesoftheconstructors,eachhavingmultipleversionsthough.OnerequiressomeComparator,theotheronereliesonthenaturalorderingoftheelements.

List

Listisaninterfacethatrequiresimplementingclasstokeeptrackoftheorderoftheelements.TherearealsomethodsthataccessanelementbyindexanditerationdefinedbytheCollectioninterfacethatguaranteestheorderoftheelements.TheinterfacealsodefinesthelistIteratormethodthatreturnsanIteratoralsoimplementingtheListIteratorinterface.Thisinterfaceprovidesmethodsthatletthecallerinsertelementstothelistwhileiteratingthroughitandalsogoingbackandforthintheiteration.ItisalsopossibletosearchforacertainelementintheListbutmostimplementationsoftheinterfaceprovidepoorperformancewhilethesearchingissimplygoingthroughallelementsuntiltheelementsearchedforisfound.TherearemanyclassesimplementingthisinterfaceintheJDK.Here,wewillmentiontwo.

LinkedList

Thisisadoubly-linkedlistimplementationoftheListinterfacethathasareferencetotheprevious,andalsotothenextelementinthelistforeachelement.TheclassalsoimplementstheDequeinterface.Itisfairlycheaptoinsertordeleteanelementfromthelistbecauseitneedsonlytheadjustmentoffewreferences.Ontheotherhand,theaccesstoanelementbyindexwillneediterationfromthestartofthelist,orfromtheendofthelist,whicheverisclosertothespecifiedindexedelement.

ArrayListThisclassisanimplementationoftheListinterfacethatkeepsthereferencestotheelementsinanarray.Thatway,thisisfairlyfasttoaccessanelementbyindex.Ontheotherhand,insertinganelementtoArrayListcanbecostly.Itneedsmovingallreferencesabovetheinsertedelementoneindexhigher,anditmayalsorequireresizingthebackingarrayincasethereisnoroomintheoriginalonetostorethenewelement.Essentially,thismeansallocatinganewarrayandcopyingallreferencestoit.

ThereallocationofthearraymaybeoptimizedifweknowhowlargethearraywillgrowandcalltheensureCapacitymethod.Thiswillresizethearraytothesizeprovidedasargument,evenifthecurrentlyusedslotsarelessnumbered.

MyexperienceisthatnoviceprogrammersuseArrayListwhentheyneedalistwithoutconsideringthealgorithmicperformanceofthedifferentimplementations.IdonotactuallyknowwhythereisthispopularityofArrayList.Theactualimplementationusedinaprogramshouldbebasedonproperdecisionandnothabit.

QueueQueueisacollectionthatusuallystoreselementforlateruse.Youcanputelementsintoaqueueandyoucanpullthemout.Animplementationmayspecifythegivenorder,thatmaybefirstinfirstout(FIFO)orlastinfirstout(LIFO)orsomeprioritybasedordering.

Onaqueue,youcaninvoketheaddmethodtoaddanelement,removetoremovetheheadelement,andtheelementmethodtoaccesstheheadelementwithoutremovingitfromthequeue.Theaddmethodwillthrowanexceptionwhenthereisacapacityproblemandtheelementcannotbeaddedtothequeue.Whenthequeueisempty,andthereisnoheadelement,theelementandremovemethodsthrowexception.

Asexceptionscanonlybeusedinexceptionalcases,andthecallingprogrammayhandlethesesituationsinthenormalcourseofthecode,thusallthesemethodshaveaversionthatjustreturnsomespecialvaluesignalingthesituation.Insteadofadd,acallermaycalloffertoofferanelementforstorage.Ifthequeuecannotstoretheelement,itwillreturnfalse.Similarly,peekwilltrytogetaccesstotheheadelementorreturnnullifthereisnone,andpollwillremoveandreturntheheadelementorjustreturnnullifthereisnone.

Notethatthesemethodsreturningnulljustmakethesituationambiguouswhentheimplementation,suchasLinkedList,allowsnullelements.Neverstoreanullelementinaqueue.

DequeDequeisaninterfacewhichisadouble-endedqueue.ItextendstheQueueinterfacewiththemethodsthatallowaccesstobothendsofthequeuetoadd,lookat,andremoveelementsfrombothends.

FortheQueueinterfaceweneededsixmethods.Dequeuehavingtwomanageableendsneeds12methods.InsteadofaddwehaveaddFirstandaddLast.SimilarlywecanofferFirst,offerLastaswellaspeekFirst,peekLastandpollFirst,pollLast.ForsomereasonthemethodsthatimplementthefunctionalityoftheelementmethodonQueuearenamedgetFirstandgetLast.

SincethisinterfaceextendstheQueueinterfacethemethodsdefinedtherecanalsobeusedtoaccesstheheadofthequeue.InadditiontothesethisinterfacealsodefinesthemethodsremoveFirstOccurrenceandremoveLastOccurrencethatcanbeusedtoremoveaspecificelementinsidethequeue.Wecannotspecifytheindexoftheelementtoremoveandwealsocannotaccessanelementbasedonindex.TheremoveFirst/LastOccurrencemethods'argumentistheobjectthatistoberemoved.IfweneedthisfunctionalitywecanuseDequeevenifweaddandremoveelementsfromthesameendofthequeue.

WhyaretherethesemethodsinDequeandnotinQueue?ThesemethodshavenothingtodowithdoubleheadednessofDeque.Thereasonisthatmethodscannotbeaddedtointerfacesaftertheywerereleased.Ifweaddamethodtoaninterfacewebreakthebackwardcompatibilitybecauseallclassesthatimplementthatinterfacehavetoimplementthenewmethod.Java8introduceddefaultmethodsthateasedthisconstraint,buttheQueueinterfacewasdefinedinJava1.5andtheDequeinterfacewasdefinedinJava1.6.Therewasnowayatthattimetoaddthenewmethodstothealreadyexistinginterfaces.

Map

AMappairskeysandvalues.IfwewanttoapproachaMapfromtheCollectionpointofviewthenaMapisasetofkey/valuepairs.YoucanputkeyvaluepairsintoaMapandyoucangetavaluebasedonakey.KeysareuniquethesamewayaselementsinaSet.IfyoulookatthesourcecodeofthedifferentimplementationsoftheSetinterface,youmayseethatsomeofthemareimplementedasawrapperaroundaMapimplementationwherethevaluesaresimplydiscarded.

UsingMapsiseasyandalluring.Manylanguages,suchasPython,Go,JavaScript,Perl,andsoon,supportthisdatastructureonthelanguagelevel.However,usingaMapwhenanarraywouldbesufficientisabadpracticethatIhaveseenmanytimes,especiallyinscriptinglanguages.JavaisnotpronetothatnoviceprogrammererrorbutyoumaystillfindyourselfinasituationwhenyouwanttouseaMap,andstillthereisabettersolution.Itisageneralrulethatthesimplestdatastructureshouldbeusedthatissufficientfortheimplementationofthealgorithm.

HashMap

HashMapisahashtablebasedimplementationoftheMapinterface.Asthemapisbasedonahashtable,thebasicputandgetmethodsareperformedinconstanttime.Additionally,asMapisveryimportant,andbecausethemostfrequentlyusedimplementationintheJDKisHashMap,theimplementationisfairlyconfigurable.YoucaninstantiateHashMapusingthedefaultconstructorwithoutargument,butthereisalsoaconstructorthatdefinestheinitialcapacityandtheloadfactor.

IdentityHashMap

IdentityHashMapisaspecialMapthatimplementstheMapinterfaceliterally,butasamatteroffact,itviolatesthecontracttheMapinterfacedocumentationdefines.Itdoesitwithgoodreason.TheimplementationusesahashtablejustasHashMap,buttodecidetheequalityofthekeyfoundinthebucketcomparingwiththekeyelementprovidedasargumenttothegetmethoditusesObjectreference(==operator)andnotthemethodequals,whichisrequiredbydocumentationofMapinterface.

TheuseofthisimplementationisreasonablewhenwewanttodistinguishdifferentObjectinstancesaskeysthatotherwiseequaltoeachother.Usingthisimplementationforperformancereasonsisalmostcertainlyawrongdecision.Also,notethatthereisnoIdentityHashSetimplementationintheJDK.ProbablysuchcollectionissorarelyusedthatitsexistenceintheJDKwouldcausemoreharmthangoodalluringnoviceprogrammerstomisuse.

DependencyinjectionInthepreviouschapterwebrieflyalreadydiscusseddependencyinjection(DI).Nowwewilldigintoitabitmoredetail.

Objectsusuallydonotworkontheirown.Mostofthetimetheimplementationdependsontheservicesofotherclasses.WhenwewanttowritesomethingtotheconsoleweusetheSystemclass.WhenwemanagethetableofguessesweneedColorobjectsandColorManager.

IncaseofwritingtotheconsolewemaynotrealizethedependencybecausetheclassbeingpartoftheJDKclasslibraryisavailableallthetimeandallweneedtodoistowriteSystem.out.println.Inthiscasethisdependencyiswiredintothecode.Wecannotsendtheoutputsomewhereelseunlesswechangethecode.Thisisnottooflexibleandinmanycasesweneedasolutionthatcanworkwithdifferentoutput,differentcolormanagerordifferentwhateverserviceourcodedependson.Thefirststeptodothatistohaveafieldthathasareferenceoftheobjectthatgivesourclasstheservice.IncaseofoutputthetypeofthefieldcanbeoftypeOutputStream.Thenext,moreinterestingstepishowthisfieldgetsvalue.

OneofthesolutionistouseDI.Inthisapproachsomeexternalcodepreparesthedependenciesandinjectsthemintotheobject.Whenthefirstcalltoamethodoftheclassisissuedallthedependenciesarealreadyfilledandreadytobeused.

Inthisstructure,wehavefourdifferentplayers:

TheclientobjectistheonethatgetstheinjectedserviceobjectsduringtheprocessServiceobjectorobjectsareinjectedintotheclientobjectInjectoristhecodethatperformstheinjectionInterfacesdefinetheservicethattheclientneeds

Ifwemovethelogicofthecreationoftheserviceobjectsfromtheclientcodethecodebecomesshorterandcleaner.Theactualcompetencyoftheclientclassshouldhardlyevercoverthecreationoftheserviceobjects.ForexampleaGameclasscontainsaTableinstancebutagameisnotresponsibletocreatetheTable.Itisgiventoittoworkwithit,justasinreallifethatwemodel.

Thecreationofserviceobjectsissometimesassimpleasissuingthenewoperator.Sometimesserviceobjectsalsodependonotherserviceobjectsandthatwayalsoactasclientsintheprocessofdependencyinjection.Inthiscasethecreationoftheserviceobjectsmaybealotoflines.Thestructureofthedependenciescanbeexpressedinadeclarativefashionthatdescribeswhichserviceobjectneedswhichotherserviceobjectsandalsowhatimplementationoftheserviceinterfacesaretobeused.Dependencyinjectioninjectorsworkwithsuchdeclarativedescriptions.Whenthereisaneedforanobjectthatneedsserviceobjectsthatthemselvesneedagainotherserviceobjectstheinjectorcreatestheserviceinstancesintheappropriateorderusingtheimplementationsthatarematchingthedeclarativedescriptions.Theinjectordiscoversallthedependenciestransitivelyandcreatesatransitiveclosuregraphofthedependencies.

ThedeclarativedescriptionoftheneededdependenciescanbeXML,oraspeciallanguagedeveloped

especiallyforthedependencyinjectionoritcanevenbeJavaitselfusingspeciallydesignedfluentAPI(https://blog.jooq.org/2012/01/05/the-java-fluent-api-designer-crash-course/).XMLwasfirstusedinDIinjectors.LaterGroovybasedDomainSpecificLanguage(https://martinfowler.com/books/dsl.html)cameintopictureandJavafluentAPIapproach.WewilluseonlythelastonebeingthemostmodernandwewilluseSpringandGuiceDIcontainerssincetheyarethemostwell-knowninjectorimplementations.

ImplementingthegameCollectionswithoutexamplesareboring.Fortunately,wehaveourgamewhereweuseafewcollectionclassesandalsootheraspectsthatwewillexamineinthischapter.

ColorManagerWejumpedintothepoolfilledwithcollectionclassesfromtheimplementationoftheColorManagerclass.Let'srefreshthepartoftheclassthatisinterestingforusnow—theconstructor:

finalprotectedintnrColors;

finalprotectedMap<Color,Color>successor=newHashMap<>();

finalprivateColorfirst;

publicColorManager(intnrColors){

this.nrColors=nrColors;

first=newColor();

ColorpreviousColor=first;

for(inti=1;i<nrColors;i++){

finalColorthisColor=newColor();

successor.put(previousColor,thisColor);

previousColor=thisColor;

}

successor.put(previousColor,Color.none);

}

WewilluseHashMaptokeepthecolorsinanorderedlist.Atfirst,thechoiceofHashMapseemstobestrange.Verytrue,thatduringthecodingofColorManager,IalsoconsideredaList,whichseemedtobeamoreobviouschoice.WhenwehaveaList<Color>colorsvariable,thenthenextColormethodissomethinglikethis:

publicColornextColor(Colorcolor){

if(color==Color.none)

returnnull;

else

returncolors.get(colors.indexOf(color)+1);

}

Theconstructorwillbemuchsimpler,asshowninthefollowingpieceofcode:

finalList<Color>colors=newArrayList<>();

publicColorManager(intnrColors){

this.nrColors=nrColors;

for(inti=0;i<nrColors;i++){

colors.add(newColor());

}

colors.add(Color.none);

}

publicColorfirstColor(){

returncolors.get(0);

}

WhydidIchoosethemorecomplexsolutionandtheunobviousdatastructure?Thethingisperformance.WhenthenextColormethodisinvoked,thelistimplementationfirstfindstheelementcheckingalltheelementsinthelistandthenfetchesthenextelement.Thetimeisproportionaltothenumberofcolors.Whenournumberofcolorsincreases,thetimewillalsoincreasetojustgetthenextcolorhavingone.

Atthesametime,ifwefocusonnotthedatastructurethatcomesfromtheverbalexpressionofthetaskwewanttosolve(getthecolorsinasortedorder)butratherfocusontheactualmethodthatwewanttoimplement,nextColor(Color),thenwewilleasilycometotheconclusionthataMapismorereasonable.WhatweneedisexactlyaMap:havingoneelementwewantanotherrelatedtotheonewehave.Thekeyandthe

valueisalsoColor.GettingthenextelementisconstanttimeusingHashMap.ThisimplementationisprobablyfasterthantheonebasedonArrayList.

Theproblemisthatitisonlyprobablyfaster.Whenyouconsiderrefactoringacodetohavebetterperformance,yourdecisionshouldalwaysbebasedonmeasurements.Ifyouimplementacodethatyouonlythinkisfaster,practiceshows,youwillfail.Inbestcase,youwilloptimizeacodetobeblazingfastandrunsduringtheapplicationserversetup.Atthesametime,optimizedcodeisusuallylessreadable.Somethingforsomething.Optimizationshouldneverbedoneprematurely.Codeforreadabilityfirst.Then,assesstheperformance,andincasethereisproblemwiththeperformance,thenprofiletheexecutionandoptimizethecodewhereithurtsthemostoftheoverallperformance.Micro-optimizationswillnothelp.DidIdoprematureoptimizationselectingtheHashMapimplementationinsteadofList?IfIactuallyimplementedthecodeusingListandthenrefactored,thenyes.IfIwasthinkingabouttheListsolutionandthenitcametomethatMapsolutionisbetterwithoutpriorcoding,thenIdidnot.Byyears,suchconsiderationswillcomeeasier,asyouwillalsoexperience.

TheclasscolorWehavealreadylookedatthecodefortheclasscodeanditwasthesimplestclassintheworld.Inreality,asitisintheGitHubrepository(https://github.com/j9be/chapter04orhttps://github.com/PacktPublishing/Java-9-Programming-By-Example/tree/master/Chapter04),thecodeisabitmorecomplex:

packagepackt.java9.by.example.mastermind;

/**

*RepresentsacolorintheMasterMindtable.

*/

publicclassColor{

/**

*Aspecialobjectthatrepresentsa

*valuethatisnotavalidcolor.

*/

publicstaticfinalColornone=newColor();

}

WehaveaspecialcolorconstantnamednonethatweusetosignalareferencethatisoftypeColorbutisnotavalidColor.Inprofessionaldevelopment,weusedthenullvalueforalongtimetosignalinvalidreference,andbecausewearebackwardcompatible,westilluseit.However,itisrecommendedtoavoidthenullreferencewhereverpossible.

TonyHoare(https://en.wikipedia.org/wiki/Tony_Hoare),whoinventedthenullreferencein1965,admittedonetimethatthiswasamistakethatcostbillionsofdollarsintheITindustry.

Theproblemwiththenullvalueisthatittakesthecontrolawayfromtheclass,andthus,opensencapsulation.Ifamethodreturnsnullinsomesituation,thecallerisstrictlyrequiredtocheckthenullityandactaccordingtothat.Forexample,youcannotcallamethodonanullreferenceandyoucannotaccessanyfield.Ifthemethodreturns,aspecialinstanceoftheobjecttheseproblemsarelessserious.Ifthecallerforgetstocheckthespecialreturnvalueandinvokesmethodsonthespecialinstance,themethodsinvokedstillhavethepossibilitytoimplementsomeexceptionorerrorhandling.Theclasshasthecontrolencapsulatedandcanthrowaspecialexceptionthatmaygivemoreinformationabouttheerrorcausedbytheprogrammaticmistakebythecallernotcheckingthespecialvalue.

JavaDocandcodecommentsThereisalsoanotherdifferencebetweenwhatwepresentedhereearlierandthelisting.Thisisthecommentingofthecode.Codecommentsarepartoftheprogram,whichareignored,filteredoutbythecompiler.Thesecommentsaresolelyforthosewhomaintainorusethecode.

InJava,therearetwodifferentcomments.Thecodeenclosedbetween/*and*/arecomments.Thestartandtheendofthecommentdonotneedtobeonthesameline.Theothertypeofcommentstartswiththe//charactersandendsattheendoftheline.

Todocumentthecode,theJavaDoctoolcanbeused.JavaDocisaspecialtoolthatreadsthesourcecodeandextractsHTMLdocumentationabouttheclasses,methods,fields,andotherentitiesthathaveacommentstartingwiththe/**characters.ThedocumentationwillcontaintheJavaDoccommentsinaformattedwayandalsotheinformationthatisextractedfromtheprogramcode.

ThedocumentationalsoappearsasonlinehelpintheIDEwhenyoumovethemouseoveramethodcallorclassname,ifthereisany.TheJavaDoccommentcancontainHTMLcodes,butitgenerallyshouldnot.Ifreallyneeded,youcanuse<p>tostartanewparagraphorthe<pre>tagstoincludesomepreformattedcodesampleintothedocumentation,butnothingmoregivesrealbenefit.Documentationshouldbeasshortaspossibleandcontainasfewformattingaspossible.

TherearespecialtagsthatappearintheJavaDocdocumentation.TheseareprefilledbytheIDEswhenyoustarttotypeaJavaDocas/**andthenpressEnter.Theseareinsidethecommentandstartwiththe@character.Thereareapredefinedsetoftags:@author,@version,@param,@return,@exception,@see,@since,@serial,and@deprecated.Themostimportanttagsare@paramand@return.Theyareusedtodescribethemethodargumentsandthereturnvalue.Althoughwearenotthereyet,let'speekaheadtotheguessMatchmethodfromtheGuesserclass.

/**

*Aguessmatchesifallrowsinthetablematchestheguess.

*

*@paramguesstomatchagainsttherows

*@returntrueifallrowsmatch

*/

protectedbooleanguessMatch(Color[]guess){

for(Rowrow:table.rows){

if(!row.guessMatches(guess)){

returnfalse;

}

}

returntrue;

}

ThenameoftheparameterisautomaticallygeneratedbytheIDE.Whenyoucreatethedocumentation,writesomethingthatismeaningfulandnottautology.Manytimes,noviceprogrammersfeeltheurgetowriteJavaDoc,andthatsomethinghastobewrittenabouttheparameters.Theycreatedocumentationslikethis:

*@paramguessistheguess

Really?Iwouldneverhaveguessed.Ifyoudonotknowwhattowritetheretodocumenttheparameter,it

mayhappenthatyouwerechoosingthenameoftheparameterexcellent.Thedocumentationofourprecedingexamplewilllookasfollows:

Focusonwhatthemethod,class,andinterfacedoesandhowitcanbeused.Donotexplainhowitworksinternally.JavaDocisnottheplacefortheexplanationofthealgorithmorthecoding.Itisusedtohelpusethecode.However,ifsomebodyhappenstoexplainhowamethodworks,itisnotadisaster.Commentscaneasilybedeleted.

Thereis,however,acommentthatisworsethannothing:outdateddocumentationthatisnotvalidanymore.Whenthecontractoftheelementhaschanged,butthedocumentationdoesnotfollowthechangeandismisleadingtheuserwhowantstocallthemethod,interface,orclasswhateverwillfaceseriousbugsandwillbeclueless.

Fromnowon,JavaDoccommentswillnotbelistedinprinttosavetrees,andelectronsintheeBookversion,buttheyarethereintherepositoryandcanbeexamined.

RowNow,wehaveColorsandeveninstancesifweneedhavingaColorManager.ThisisthetimetostoreColorsinRows.TheRowclassisabitlonger,butnottoocomplex.

packagepackt.java9.by.example.mastermind;

importjava.util.Arrays;

publicclassRow{

finalColor[]positions;

privateintmatchedPositions;

privateintmatchedColors;

ARowcontainsthreefields.Oneisthepositionsarray.EachelementofthearrayisaColor.ThematchedPositionsisthenumberofpositionsthatarematchedandmatchedColorsisthenumberofcolorsthatmatchacolorinthehiddenrowbutisnotonthepositionasinthehiddenrow.

publicstaticfinalRownone=newRow(Guesser.none);

ThenoneisaconstantthatcontainsaspecialRowinstancethatwewillusewhereverwewouldusenull.Theconstructorgetsthecolorsinanarraythatshouldbeintherow.

publicRow(Color[]positions){

this.positions=Arrays.copyOf(positions,positions.length);

}

Theconstructormakesacopyoftheoriginalarray.Thisisanimportantcodethatwewillexamineabit.Let'sreiteratethatJavapassesargumentsbyvalue.Itmeansthatwhenyoupassanarraytoamethod,youwillpassthevalueofthevariablethatholdsthearray.However,anarrayinJavaisanObjectjustaswellasanythingelse(exceptprimitiveslikeint).Therefore,whatthevariablecontainsisareferencetoanobjectthathappenstobeanarray.Ifyouchangetheelementsofthearray,youactuallychangetheelementsoftheoriginalarray.Thearrayreferenceiscopiedwhentheargumentpasses,butthearrayitself,andtheelements,arenot.

Thejava.util.Arraysutilityclassprovidesalotofusefultools.WecaneasilycodethearraycopyinginJavabutwhytoreinventthewheel?Inadditiontothat,arraysarecontinuousareaofmemorythatcanveryeffectivelybecopiedfromoneplacetoanotherusinglow-levelmachinecode.ThecopyOfmethodthatweinvokecallsthemethodSystem.arraycopywhichisanativemethodandassuchexecutesnativecode.

NotethatthereisnoguaranteethatArrays.copyOfinvokesthenativeimplementationsandthatthiswillbeextremelyfastincaseoflargearrays.TheveryversionIwastestinganddebuggingwasdoingitthatway,andwecanassumethatagoodJDKdoessomethingsimilar,effectiveandfast.

Afterwecopiedthearray,itisnotaproblemifthecallermodifiesthearraythatwaspassedtotheconstructor.Theclasswillhaveareferencetoacopythatwillcontainthesameelements.However,notethatifthecallerchangesanyoftheobjectsthatarestoredinthearray(notthereferenceinthearray,buttheobjectitselfthatisreferencedbyanarrayelement),thenthesameobjectismodified.Arrays.copyOfdoesnotcopytheobjectsthatarereferencedbythearray,onlythearrayelements.

Therowiscreatedalongwiththecolorsandthus,weusedafinalfieldfortheColorarray.Thematches,however,cannotbeknownwhenaRowiscreated.OneoftheplayerscreatestheRowandafterthat,theotherplayerwilltellthetwointvalues.Wedonotcreatetwosettersforthetwovalues,however,becausetheyarealwaysdefinedatthesametimeinthegametogether.

publicvoidsetMatch(intmatchedPositions,intmatchedColors){

if(matchedColors+matchedPositions>positions.length){

thrownewIllegalArgumentException(

"Numberofmatchescannotbemorethattheposition.");

}

this.matchedColors=matchedColors;

this.matchedPositions=matchedPositions;

}

ThesetMatchmethoddoesnotonlysetthevalues,butalsochecksthatthevaluesareconsistent.Thesumofthetwovaluescannotbemorethanthenumberofthecolumns.Thischeckensuresthatthecaller,whousestheAPIoftheRowclass,doesnotuseitinconsistently.IfthisAPIisusedonlyfrominsideourcode,thisassertionshouldnotbepartofthecode.Agoodcodingstyle,inthatcase,willensurethatthemethodisneverinvokedinconsistentlyusingunittests.WhenwecreateAPItouseoutofourcontrol,weshouldcheckthattheuseisconsistent.Failingtodoso,ourcodemaybehavejustweirdwhenusedinconsistently.Whenthecallersetsmatchestovaluesthatdonotmatchanypossibleguess,thegamemayneverfinishandthecallermayhaveahardtimefiguringoutwhatisgoingon.Thisfiguringoutprobablywillneedthedebugexecutionofourcode.

Ifwethrowanexceptioninthiscase,theprogramstopswherethebugis.Thereisnoneedtodebugthelibrary.

publicbooleanguessMatches(Color[]guess){

returnnrMatchingColors(guess)==matchedColors&&

nrMatchingPositions(guess)==matchedPositions;

}

Thenextmethoddecidesifaguess,givenasanargument,matchestheactualrow.Thismethodchecksthattheanswerstotheguessintherowcanbevalidifthecurrentguesswasinthehiddenrow.Theimplementationisfairlyshortandsimple.Aguessmatchesarowifthenumberofthecolorsmatchingandthenumberofpositionsmatchingarethesameasthenumbergivenintherow.Donotbeshytowriteshortmethods.Donotthinkthataone-linemethodthatessentiallycontainsonestatementisuseless.Whereverweusethismethod,wecouldalsowritetheexpression,whichisrightafterthereturnstatement,butwedonotfortworeasons.Thefirstandmostimportantreasonisthatthealgorithm,whichdecidesthatarowmatchesaguessbelongstotheimplementationoftheclassRow.Ifevertheimplementationchanges,theonlylocationwherethecodeistobechangedishere.Theotherreasonisalsoimportant,andthatisreadability.Inourcodebase,wecallthismethodfromabstractclassGuesser.Itcontainsanifstatementwiththefollowingexpression:

if(!row.guessMatches(guess)){

Woulditbemorereadableinthefollowingway:

if(!(nrMatchingColors(guess)==matchedColors&&nrMatchingPositions(guess)==matchedPositions)){

Iamcertainthatthemajorityoftheprogrammersunderstandtheintentionofthefirstversioneasier.IwouldevenrecommendimplementingthedoesNotMatchGuessmethodtoimprovethereadabilityofthecode

evenmore.

publicintnrMatchingColors(Color[]guess){

intcount=0;

for(inti=0;i<guess.length;i++){

for(intj=0;j<positions.length;j++){

if(i!=j&&guess[i]==positions[j]){

count++;

}

}

}

returncount;

}

Thenumberofmatchingcolorsisthatwhichappearsbothintherowandtheguess,butnotinthesameposition.Thedefinition,andhowwecalculateit,isfairlysimpleandunambiguousincasenocolorcanappeartwiceinthehiddenrow.Incaseacolormayappearmultipletimesinthehiddenrow,thisimplementationwillcountalloccurrencesofthatcolorintheguessasmanytimesasitappearsinthehiddenrow.Ifwe,forexample,haveahiddenRRGBrowandtheguessisbYRR,thecalculationwillsay4.Itisamatterofagreementbetweentheplayershowtheycountinthiscase.Theimportantaspectisthattheyusethesamealgorithm,whichshouldbetrueinourcase,becausewewillasktheprogramtoplaybothplayers.Aswewillprogramthecodeourselves,wecantrustthatitwillnotcheat.

publicintnrMatchingPositions(Color[]guess){

intcount=0;

for(inti=0;i<guess.length;i++){

if(guess[i]==positions[i]){

count++;

}

}

returncount;

}

CountingthecolorsthatareOK,andalsoonthepositionwheretheyaresupposedtobe,isevensimpler.

publicintnrOfColumns(){

returnpositions.length;

}

ThismethodtellsthenumberofcolumnsintheRow.ThismethodisneededintheGameclassthatcontrolstheflowofawholegame.AsthisclassisinthesamepackageasRow,itcanaccessthefieldpositions.Icreatedthecodetogetthenumberofcolumnsasrow.positions.length.Butthen,Iwasreadingthecodenextdayandtoldmyself:Thisisuglyandunreadable!WhatIaminterestedinhereisnotsomemysteriouspositions'length;itisthenumberofcolumns.AndthenumberofcolumnsistheresponsibilityoftheRowclassandnotthebusinessofanyotherclass.IfIstarttostorethepositionsinaList,whichdoesnothavelength(ithasmethodsize),itisthesoleresponsibilityofRowandshouldnotaffectanyothercode.So,IcreatedthenrOfColumnsmethodtoimprovethecode.

Therestoftheclasscontainssomemoreverysimplemethodsthatareneededonlytodisplaythegameandnotforthealgorithmtoplay:

publicintnrColumns(){

returnpositions.length;

}

publicColorposition(inti){

returnpositions[i];

}

publicintmatchedPositions(){

returnmatchedPositions;

}

publicintmatchedColors(){

returnmatchedColors;

}

}

Ifyouareapurist,youcanencapsulatethesemethodsintoaninnerclassnamedOutputorPrintandcallthemthroughafinalinstanceofitcreatedasafieldintheRowclass.ItisalsopossibletochangethevisibilityofthesefieldsfromprivatetoprotectedandimplementthesemethodsinaPrintableRowthatcanbeinstantiatedfromanalreadyexistingRowandimplementthesemethods.

ThefirstversionofPrintableRowwilllooklikethis:

publicclassPrintableRowextendsRow{

publicPrintableRow(Rowrow){

super(row.positions);

super.setMatch(row.matchedPositions,row.matchedColors);

}

//themethodsaredeletedfromtheprint...

}

Themethodsareexactlythesameasintheprecedingprint;theyarecutandpasted,orrathermoved,usingtheIDErefactoringsupportfromoneclasstotheother.

Whenyouwriteacode,pleaseneverusecopyandpaste.Howeveryoucanusecutandpastetomovecodefragmentsaround.Thedangerisinthecopypasteuse.Manydevelopersclaimthattheiruseofactualcopyandpasteisnotcopypasteprogramming.Theirreasoningisthattheychangethepastedcodesomuchthatithaspracticallynothingtodowiththeoriginalcode.Really?Inthatcasewhydidyouneedthecopiedcodewhenyoustartedthemodificationofit?Whynotstartfromscratch?ThatisbecauseifyouusetheIDE'scopyandpastefunctionalitythen,nomatterwhat,youdocopypasteprogramming.

ClassPrintableRowisprettyneatandseparatestheoutputconcernfromthecorefunctionality.Whenyouneedaninstance,itisnotaproblemthatyouhaveaRowinstancealreadyinhand.Theconstructorwillessentiallyclonetheoriginalclassandreturnaprintableversion.Whatbothersmeistheimplementationofthecloning.ThecodeintheconstructorcallsthesuperconstructorandthenamethodandalltheseworkwiththeoriginalfunctionalityoftheRowclass.TheyhavenothingtodowiththeprintabilitythatPrintableRowimplements.ThisfunctionalityactuallybelongstotheRowclass.Weshouldcreateaprotectedconstructorthatdoesthecloning:

protectedRow(RowcloneFrom){

this(cloneFrom.positions);

setMatch(cloneFrom.matchedPositions,cloneFrom.matchedColors);

}

TheconstructorofPrintableRowshouldsimplycallsuper(row)andthatisit.

Codeisneverfinishedandneverperfect.Inaprofessionalenvironment,programmersmanytimestendtofinishpolishingthecodewhenitisgoodenough.Thereisnocodethatcannotbemadebetter,butthereisadeadline.Thesoftwarehastobepassedontothetestersandusersandhastobeusedtohelpeconomy.Afterall,thatisthefinalgoal

ofaprofessionaldeveloper:haveacodethatsupportsthebusiness.Acodethatneverrunsisworthnothing.IdonotwantyoutothinkthattheexamplesthatIprovidedherewerecreatedperfectupfront.Thereasonforthatis(didyoureadcarefully?)becausetheyarenotperfect.AsIsaid,codeisneverperfect.WhenIfirstcreatedRow,itcontainedtheprintingmethodsinaninnerclass.Ididnotlikeit.Thecodewassmelly.So,IdecidedtomovethefunctionalitytotheRowclass.However,Istilldidnotlikethesolution.Then,Iwenttobed,slept,worked,andreturnedtoitafewdayslater.WhatIcouldnotcreatethedaybeforenowseemedobvious—thesemethodshavetobemovedtoasubclass.Nowcomesanotherdilemma.ShouldIpresentthisfinalsolutionorshouldIhaveherethedifferentversions?Insomecases,Iwilljustpresentthefinalversion.Inothercases,likethis,therearethingstolearnfromthedevelopmentstep.Inthesecases,Ipresentnotonlythecode,butpartofitsevolutiononhowitwascreated.IfyouwanttoseethosethatIdidnotdarepublishing,lookattheGithistory.Iadmit,sometimes,Icreatecodethatevenmakesmefacepalmadaylater.

TableTableisasimpleclassthathasonlyoneverysimplefunctionality.

publicclassTable{

finalColorManagermanager;

finalintnrColumns;

finalList<Row>rows;

publicTable(intnrColumns,ColorManagermanager){

this.nrColumns=nrColumns;

this.rows=newLinkedList<>();

this.manager=manager;

}

publicvoidaddRow(Rowrow){

rows.add(row);

}

}

Thereisonethingtomention,whichisnothingnew,butworthrepeating.Therowsvariableisdeclaredasfinalanditgetsthevalueintheconstructor.ThisisaList<Row>typevariable.Thefactthatitisfinalmeansthatitwillholdthesamelistobjectduringitslifetime.Thelength,members,andotherfeaturesofthelistmayandwillchange.Wewilladdnewrowstothislist.Finalobjectvariablesreferenceanobject,butitdoesnotguaranteethattheobjectitselfisimmutable.Itisonlythevariablethatdoesnotchange.

Whenyoudocodereviewandexplaintoyourcolleagueswhataclassdoes,andyoufindyourselfstartingtheexplanation"thisclassisverysimple"manytimes,itmeansthecodeisgood.Well,itmaybewronginotheraspects,buttheclass'granularityseemstobeokay.

GuesserGuesserandtheUniqueGuesserandGeneralGuessersubclassesarethemostinterestingclassesoftheprogram.Theyactuallyperformthetaskthatisthecoreofthegame.GivenaTablewithahiddenrow,theguesserhastocreatenewerandnewerguesses.

Todothis,aGuesserneedstogetaTablewhenitiscreated.Thisispassedasaconstructorargument.Theonlymethoditshouldimplementisguess,whichreturnsanewguessbasedonthetableandonitsactualstate.

Aswewanttoimplementaguesserthatassumesthatallcolorsinthehiddenrowaredifferent,andalsoonethatdoesnotmakethisassumption,wewillimplementthreeclasses.Guesserisanabstractclassthatimplementsonlythelogicthatisindependentfromtheassumptions.Thesemethodswillbeinheritedbybothactualimplementations:UniqueGuesserandGeneralGuesser.

Let'sgothroughtheactualcodeoftheclass:

packagepackt.java9.by.example.mastermind;

publicabstractclassGuesser{

protectedfinalTabletable;

privatefinalColorManagermanager;

publicGuesser(Tabletable){

this.table=table;

this.lastGuess=newColor[table.nrColumns];

this.manager=table.manager;

}

Thestateoftheguesseristhelastguessitmade.Althoughthisisonthelastrowofthetable,itismoreofaninternalmatteroftheguesser.Theguesserhasallthepossibleguesses,oneaftertheother;lastGuessistheonewhereitleftofflasttimeanditshouldcontinuefromtherewhenitisinvokedagain.

abstractprotectedvoidsetFirstGuess();

Settingthefirstguessverymuchdependsontheassumptionofcoloruniqueness.Thefirstguessshouldnotcontainduplicatedcolorsincasethehiddenrowdoesnot(atleastinourimplementation),whileGeneralGuesserisfreetoguessanytime,evenasfirstGuessallcolorstobethesame.

protectedfinalColor[]lastGuess;

publicstaticfinalColor[]none=newColor[]{Color.none};

Again,noneinthisclassisjustanobjectthatwetrytouseinsteadofnull,wheneverweneedtoreturnsomethingthatisareferencetoaGuessbutisnotreallyaguess.

protectedColor[]nextGuess(){

if(lastGuess[0]==null){

setFirstGuess();

returnlastGuess;

}else{

returnnextNonFirstGuess();

}

}

ThenextGuessmethodisaninternalmethodthatgeneratesthenextguess,whichjustcomesasweorderthe

possibleguesses.ItdoesnotcheckanythingagainsttheTable;itonlygeneratesthenextguessalmostwithoutthinking.Theimplementationonhowwedothefirstguessandhowwedotheconsecutiveguessesaredifferent.Thus,wewillimplementthesealgorithmsindifferentmethodsandinvokethemfromhere.

ThenextNonFirstGuessmethodrepresentsthenextguessinthespecialcasewhentheguessisnotthefirstone:

privateColor[]nextNonFirstGuess(){

inti=0;

booleanguessFound=false;

while(i<table.nrColumns&&!guessFound){

if(manager.thereIsNextColor(lastGuess[i])){

lastGuess[i]=manager.nextColor(lastGuess[i]);

guessFound=true;

}else{

lastGuess[i]=manager.firstColor();

i++;

}

}

if(guessFound){

returnlastGuess;

}else{

returnnone;

}

}

Lookbackafewpageswherewedetailedhowthealgorithmworks.Wemadethestatementthatthiswayofworkingisverymuchlikethewaywecountwithdecimalnumbers.Bynow,youhaveenoughJavaknowledgeandprogrammingskilltounderstandwhatthemethoddoes.Itismoreinterestingtoknowwhyitiscodedthatway.

Hint:asalways,tobereadable.

ThereisthetemptationtoeliminatetheguessFoundvariable.Woulditnotbesimplertoreturnfromthemiddleofthemethodwhenwefindtheblessedguesses?Ifwedid,therewouldbenoneedtochecktheguessFoundvaluebeforereturningnonevalue.Thecodewouldnotgetthereifwereturnedfromthemiddleoftheloop.

Yes,itwouldbesimplertowrite.But,wecreatecodetobereadableandnotwritable.Yes,butlesscodeismorereadable.Notinthiscase!Returningfromaloopdegradesthereadability.Nottomention,thereturnstatementsarescatteredaroundinthemethodatdifferentstagesofexecution.

privateColor[]nextNonFirstGuess(){

inti=0;

while(i<table.nrColumns){

if(manager.thereIsNextColor(lastGuess[i])){

lastGuess[i]=manager.nextColor(lastGuess[i]);

returnlastGuess;

}else{

lastGuess[i]=manager.firstColor();

i++;

}

}

returnnone;

}

Whensomebodywritesacodeoptimizedinthatway,itissimilartoatoddlerwhomakeshisfirststepsandthenlooksproudlyatthemother.Okayboy/girl,youaregreat.Nowgoonandstartwalking.When

youarethepostman,walkingwillbeboring.Thatwillbeyourprofession.So,slideasidetheprideandwriteboringcode.Professionalswriteboringcode.Won'titbeslow?

No!Itwillnotbeslow.Firstofall,itisnotslowuntiltheprofilerprovesthatthecodedoesnotmeetthebusinessrequirements.Ifitdoes,itisfastenough,nomatterhowslowitis.Slowisgoodaslongasitisokayforthebusiness.Afterall,JITshouldhavesometaskoptimizingthecodetorun.

ThenextmethodchecksiftheguessmatchesthepreviousguessesandtheirresultsontheTable:

privatebooleanguessMatch(Color[]guess){

for(Rowrow:table.rows){

if(!row.guessMatches(guess)){

returnfalse;

}

}

returntrue;

}

privatebooleanguessDoesNotMatch(Color[]guess){

return!guessMatch(guess);

}

AswehavetheguessmatchingalreadyimplementedintheclassRow,allwehavetodoisinvokethatmethodforeachrowinthetable.Ifallrowsmatch,thentheguesscanbegoodforthetable.Ifanyoftheformerguessesdonotmatch,thenthisguessgoesdownthedrain.

Aswecheckthenegatedexpressionofmatching,wecreatedanEnglishversionofthemethod.

Insituationslikethis,itcouldbeenoughtocreatetheguessDoesNotMatchversionofthemethod.However,thelogicalexecutionofthecodeismorereadableifthemethodisnotnegated.Therefore,itismoreerrorpronetowritetheguessDoesNotMatchmethodalone.Instead,wewillimplementtheoriginal,readableversionandtheauxmethodtobenothingmorethananegation.

Afteralltheauxmethods,hereweareimplementingthepublicmethodoftheGuesser.

publicRowguess(){

Color[]guess=nextGuess();

while(guess!=none&&guessDoesNotMatch(guess)){

guess=nextGuess();

}

if(guess==none){

returnRow.none;

}else{

returnnewRow(guess);

}

}

}

ItjusttakesthenextGuessandagainandagainuntilitfindsonethatmatchesthehiddenrow,orthereisnomoreguess.Ifitfindsaproperguess,itencapsulateittoaRowobjectandreturnitsothatitcanlaterbeaddedtotheTablebytheGameobjects.

UniqueGuesserClassUniqueGuesserhastoimplementsetFirstGuess(allconcreteclassesextendinganabstractclassshouldimplementtheabstractmethodoftheparent)anditcanandwilloverridetheprotectednextGuessmethod:

packagepackt.java9.by.example.mastermind;

importjava.util.HashSet;

importjava.util.Set;

publicclassUniqueGuesserextendsGuesser{

publicUniqueGuesser(Tabletable){

super(table);

}

@Override

protectedvoidsetFirstGuess(){

inti=lastGuess.length-1;

for(Colorcolor=table.manager.firstColor();

i>=0;

color=table.manager.nextColor(color)){

lastGuess[i--]=color;

}

}

ThesetFirstGuessmethodselectsthefirstguessinsuchawaythatanypossiblecolorvariationsthatcomeafterthefirstonecreatetheguessesoneaftertheotherifwefollowthealgorithm.

TheauxisNotUniquemethodreturnstrueiftheguesscontainsduplicatecolors.Itisnotinterestingtoseehowmany.Ifallcolorsarethesame,oronlyonecolorappearstwice,itdoesnotmatter.Theguessisnotuniqueanddoesnotfitourguesser.Thismethodjudgesthat.

privatebooleanisNotUnique(Color[]guess){

finalSet<Color>alreadyPresent=newHashSet<>();

for(Colorcolor:guess){

if(alreadyPresent.contains(color)){

returntrue;

}

alreadyPresent.add(color);

}

returnfalse;

}

Todothis,itusesaSet,andanytimeanewcolorisfoundintheguessarray,thecolorisstoredintheset.Ifthesetcontainsthecolorwhenwefinditinthearray,itmeansthatthecolorwasalreadyusedbefore;theguessisnotunique.

@Override

protectedColor[]nextGuess(){

Color[]guess=super.nextGuess();

while(isNotUnique(guess)){

guess=super.nextGuess();

}

returnguess;

}

TheoverridingnextGuessmethodissimple.ItasksthesupernextGuessimplementationtomakeguessesbutthrowsawaythosethatitdoesnotlike.

packagepackt.java9.by.example.mastermind;<br/><br/>publicclassGeneralGuesserextendsGuesser{<br/><br/>publicGeneralGuesser(Tabletable){super(table);}<br/><br/>@Override<br/>protectedvoidsetFirstGuess(){<br/>inti=0;<br/>for(Colorcolor=table.manager.firstColor();<br/>i<lastGuess.length;){<br/>lastGuess[i++]=color;<br/>}<br/>}<br/><br/>}

SettingthelastGuessitjustputsthefirstcoloronallcolumns.Guesscouldnotbesimpler.EverythingelseisinheritedfromtheabstractclassGuesser.

packagepackt.java9.by.example.mastermind;<br/><br/>publicclassGame{<br/><br/>finalTabletable;<br/>finalprivateRowsecretRow;<br/>booleanfinished=false;<br/><br/>publicGame(Tabletable,Color[]secret){<br/>this.table=table;<br/>this.secretRow=newRow(secret);<br/>}<br/><br/>publicvoidaddNewGuess(Rowrow){<br/>if(isFinished()){<br/>thrownewIllegalArgumentException(<br/>"Youcannotguessonafinishedgame.");<br/>}<br/>finalintpositionMatch=secretRow.<br/>nrMatchingPositions(row.positions);<br/>finalintcolorMatch=secretRow.<br/>nrMatchingColors(row.positions);<br/>row.setMatch(positionMatch,colorMatch);<br/>table.addRow(row);<br/>if(positionMatch==row.nrOfColumns()){<br/>finished=true;<br/>}<br/>}<br/><br/>publicbooleanisFinished(){<br/>returnfinished;<br/>}<br/>}

ThinkaboutwhatIwroteearlieraboutshortmethods,andwhenyoudownloadthecodefromGitHubtoplaywithit,trytomakeitlookmorereadable.Youcan,perhaps,createanduseamethodnamedbooleanitWasAWinningGuess(intpositionMatch).

CreatinganintegrationtestWehavecreatedunittestsinthepreviouschapterandthereareunittestsforthefunctionalitiesimplementedintheclassesofthischapteraswell.Wewilljustnotprinttheseunittestshere.Insteadoflistingtheunittests,wewilllookatanintegrationtest.

Integrationtestsneedtheinvocationofmanyclassesworkingtogether.Theycheckthatthefunctionalitycanbedeliveredbythewholeapplication,oratleastalargerpartoftheapplication,anddonotfocusonasingleunit.Theyarecalledintegrationtestsbecausetheytesttheintegrationbetweenclasses.TheclassesaloneareallOK.Theyshouldnothaveanyproblemasitwasalreadyverifiedbytheunittests.Integrationfocusesonhowtheyworktogether.

IfwewanttotesttheGameclass,wewilleitherhavetocreatemocksthatmimicthebehavioroftheotherGameclasses,orwewilljustwriteanintegrationtest.Technically,anintegrationtestisverysimilartoaunittest.Manytimes,theverysameJUnitframeworkisusedtoexecutetheintegrationtests.Thisisthecasefortheintegrationtestofthisgame.

Thebuildtool,however,needstobeconfiguredtoexecutetheintegrationtestsonlywhenitisrequired.Usually,integrationtestexecutionsneedmoretime,andsometimesresources,suchasexternaldatabasethatmaynotbeavailableateachandeverydeveloperdesktop.Unittestsruneverytimetheapplicationiscompiledsotheyhavetobefast.Toseparatetheunitandintegrationtests,therearedifferenttechniquesandconfigurationoptions,butthereisnosuchmoreorlessde-factostandardlikethedirectorystructureintroducedbyMaven(lateradaptedbyGradle).

Inourcase,theintegrationtestdoesnotneedanyextraresourceanddoesnottakeenormoustimetorun.Itplaysagamefromthestarttotheendandplaystheroleofboththeplayers.Itisverymuchlikesomebodyplayingchesswiththemselves,makingastepandthenturningthetable.

Theaimofthiscodeistwofold.Ononehand,wewanttoseethatthecoderunsandplaysawholegame.Ifthegamefinishes,thenitisjustOK.Thisisaveryweakassertionandrealintegrationtestsperformlotsofassertions(onetesttestsonlyoneassertionthough).Wewillfocusontheotheraim—deliversomejoyandvisualizethegameontheconsoleintextformatsothatthereaderdoesnotgetbored.

Todothat,wewillcreateautilityclassthatprintsoutacolorandassignsletterstotheColorinstancesonthefly.ThisisthePrettyPrintRowclass.Thereareseverallimitationsinthisclassthatwehavetotalkaboutafterwelookatthecode.I'dsaythatthiscodeishereonlytodemonstratewhatnottodo,toestablishsomereasoningforthenextchapter,andwhyweneedtorefactorthecodewecreatedinthisone.

packagepackt.java9.by.example.mastermind;

importjava.util.HashMap;

importjava.util.Map;

publicclassPrettyPrintRow{

privatestaticfinalMap<Color,Character>

letterMapping=newHashMap<>();

privatestaticfinalStringletters="RGBYWb";

privatestaticintcounter=0;

privatestaticcharcolorToChar(Colorcolor){

if(!letterMapping.containsKey(color)){

letterMapping.put(color,letters.charAt(counter));

counter++;

}

returnletterMapping.get(color);

}

Thisistheheartofthisclass.Whenacoloristobeprinted,itgetsaletterassignedunlessitalreadyhasone.AstheMapcontainingtheassignmentsineachandeverygamethatisrunningintheJVMwillusethesamemapping,anewGameisstarted.ItallocatesnewColorsandwillsoonrunoutofthesixcharactersthatweallocatedhereintheStringconstant.

IftheGameinstancesarerunparallel,thenweareinevenmoretrouble.Theclassisnotthreadsafeatall.IftwothreadsconcurrentlycallthecolorToCharmethodforthesameColorinstance,(whichisnotlikelybecauseeachGameusesitsowncolor,butnotethatnotlikelyinprogrammingisverymuchlikeafamouslastwordsquoteonatombstone)thenboththreadsmayseeatthesametimethatthereisnoletterassignedtothecolorandbothwillassigntheletter(thesameletterortwodifferentletters,basedonluck)andincreasethecounteronceortwice.Atleast,whatwecansayisthattheexecutionisnondeterministic.

YoumayrecallthatIsaidviolatingthehashcontractisthesecondmostdifficulttofindbugaftermultithreadissues.Suchanondeterministiccodeisexactlythat:amultithreadissue.Thereisnoprizetofindthemostdifficultbug.Whentheapplicationdoesnotrun,andabugaffectstheproductionsystemforhoursordays,nobusinesspersonwillbehappy,andtheywillnotbeamazedafteryoufindthebug.Itmaybeanintellectualchallenge,buttherealvalueisnotcreatingthebugsinthefirstplace.

Asasummary,thiscodecanonlybeusedonceinaJVMbyasinglethread.Forthischapter,itisgood,thoughasmellyandshamefulcode,butitwillbeagoodexampleforthenextchapter,inwhichwewillsee,howtorefactortheapplicationsothatitwillnotneedsuchahackingtoprintoutthecolors.

CodesmellisatermmintedbyKentBack,accordingtoMartinFowler(http://martinfowler.com/bliki/CodeSmell.html).Itmeansthatsomecodelooksnotgood,norapparentlybad,butsomeconstructsmakethefeelinginthedeveloperthatitmaynotbegood.Asitisdefinedonthewebpage,"Acodesmellisasurfaceindicationthatusuallycorrespondstoadeeperprobleminthesystem."Thetermiswidelyacceptedandusedinsoftwaredevelopmentforthelast10years.

Therestofthecodeisplainandsimple:

publicstaticStringpprint(Rowrow){

Stringstring="";

PrintableRowpRow=newPrintableRow(row);

for(inti=0;i<pRow.nrOfColumns();i++){

string+=colorToChar(pRow.position(i));

}

string+="";

string+=pRow.matchedPositions();

string+="/";

string+=pRow.matchedColors();

returnstring;

}

Theintegrationtest,orratherthedemonstrationcode(asitdoesnotcontainanyassertionsotherthanitrunswithoutexception),definessixcolorsandfourcolumns.Thisisthesizeoftheoriginalgame.It

createsacolormanager,andthenitcreatesatableandasecret.Thesecretcouldbejustanyrandomcolorselectionfromthesixcolorsthatisavailable(thereare360differentpossibilitiestestedintheUniqueGuesserTestunittestavailablefromGitHub).AsweknowthattheGuesserimplementationstartsfromoneendofthecolorsetandcreatesthenewguessessystematically,wewanttosetasecretthatitwillguessthelast.Thisisnotbecauseweareevil,butratherbecausewewanttoseethatourcodereallyworks.

ThedirectorystructureofthecodeisverysimilartotheoneweusedincaseoftheMavenbuildtool,ascanbeseenonthefollowingscreenshotcreatedonaWindowsmachine:

Thesourcecodeisunderthedirectorysrcandthemainandtestsourcecodefilesareseparatedintotwosubdirectorystructures.ThecompiledfileswillbegeneratedinthedirectorybuildwhenweuseGradle.Thecodeoftheintegrationtestclassisthefollowing:

packagepackt.java9.by.example.mastermind.integration;

importorg.junit.Assert;

importorg.junit.Test;

importpackt.java9.by.example.mastermind.*;

publicclassIntegrationTest{

finalintnrColors=6;

finalintnrColumns=4;

finalColorManagermanager=newColorManager(nrColors);

privateColor[]createSecret(){

Color[]secret=newColor[nrColumns];

intcount=0;

Colorcolor=manager.firstColor();

while(count<nrColors-nrColumns){

color=manager.nextColor(color);

count++;

}

for(inti=0;i<nrColumns;i++){

secret[i]=color;

color=manager.nextColor(color);

}

returnsecret;

}

@Test

publicvoidtestSimpleGame(){

Tabletable=newTable(nrColumns,manager);

Color[]secret=createSecret();

System.out.println(

PrettyPrintRow.pprint(newRow(secret)));

System.out.println();

Gamegame=newGame(table,secret);

Guesserguesser=newUniqueGuesser(table);

while(!game.isFinished()){

Rowguess=guesser.guess();

if(guess==Row.none){

Assert.fail();

}

game.addNewGuess(guess);

System.out.println(PrettyPrintRow.pprint(guess));

}

}

}

TheeasiestwaytorunthetestisstartitfrominsidetheIDE.WhentheIDEimportstheprojectbasedonthebuildfile,beitaMavenpom.xmlorGradlebuild.gradle.IDEusuallyprovidesarunbuttonormenutostartthecode.Runningthegamewillprintoutthefollowingpieceofcodethatweworkedsohardoninthischapter:

RGBY0/0

GRWb0/2

YBbW0/2

BYGR0/4

RGYB2/2

RGBY4/0

Summary

Inthischapter,weprogrammedatablegame:Mastermind.Wenotonlyprogrammedthemodelofthegame,butalsocreatedanalgorithmthatcanguess.WerevisitedsomeOOprinciplesanddiscussedwhythemodelwascreatedthewayitwas.Whilewecreatedthemodelofthegame,whichwewillrefineinthenextchapter,youhavelearnedaboutJavacollections,whatanintegrationtestis,andhowtocreateJavaDoc.

ExtendingtheGame-RunParallel,RunFasterInthischapter,wewillextendtheMastermindgame.Asitisnow,itcanguessthesecretthatwashiddenandalsohidethepegs.Thetestcodecanevendobothatthesametime.Itcanplayagainstitselfleavingusonlywiththefunofprogramming.Whatitcannotdoismakeuseofalltheprocessorsthatwehaveintoday'snotebooksandservers.Thecoderunssynchronousandutilizesonlyasingleprocessorcore.

Wewillalterthecodeextendingtheguessingalgorithmtosliceuptheguessingintosubtasksandexecutethecodeinparallel.Duringthis,wewillgetacquaintedwithJavaconcurrentprogramming.Thiswillbeahugetopicwithmanysubtlecornersandcaveatslurkinginthedark.Wewillgetintothosedetailsthatarethemostimportantandwillformafirmbaseforfurtherstudieswheneveryouneedconcurrentprograms.

Astheoutcomeofthegameisthesameasitwas,onlyfaster,wehavetoassesswhatfasteris.Todothat,wewillutilizeanewfeatureintroducedinJava9:microbenchmarkingharness.

Inthischapter,wewillcoverthefollowingtopics:

Themeaningofprocesses,threadsandfibersMultithreadinginJavaIssueswithmultithreadprogrammingandhowtoavoidthemLocking,synchronization,andblockingqueuesMicrobenchmarking

HowtomakeMastermindparallelTheoldalgorithmwastogothroughallthevariationsandtrytofindaguessthatmatchesthecurrentstateofthetable.Assumingthatthecurrentlyexaminedguessisthesecret,willwegetthesameanswersfortheguessesthatarealreadyonthetableastheanswersareactuallyonthetable?Ifyes,thenthecurrentguesscanbethesecret,anditisjustasgoodaguessasanyotherguesses.

Amorecomplexapproachcanimplementthemin-maxalgorithm(https://en.wikipedia.org/wiki/Minimax).Thisalgorithmdoesnotsimplygetthenextpossibleguessbutalsolooksatallthepossibleguessesandselectstheonethatshortenstheoutcomeofthegamethemost.Ifthereisaguessthatcanbefollowedbythreemoreguessesintheworstcase,andthereisanotherforwhichthisnumberisonlytwo,thenmin-maxwillchoosethelatter.Itisagoodexercisefortheinterestedreaders.Inthecaseofthesixcolorsandfourcolumnsforthepegs,themin-maxalgorithmsolvesthegameinnomorethan5steps.Thesimplealgorithmweimplementedalsosolvesthegamein5steps.However,wedonotgointhatdirection.

Instead,wewanttohaveaversionofthegamethatutilizesmorethanoneprocessor.Howcanyoutransformthealgorithmintoaparallelone?Thereisnosimpleanswertothis.Whenyouhaveanalgorithm,youcananalyzethecalculationsandpartsofthealgorithm,andyoucantrytofinddependencies.IfthereissomecalculationBthatneedsthedata,whichistheresultofanothercalculationA,thenitisobviousthatAcanonlybeperformedwhenBisready.Iftherearepartsofthealgorithmthatdonotdependontheoutcomeoftheothers,thentheycanbeexecutedinparallel.

Forexample,thequick-sorthastwomajortasks:partitioningandthensortingofthetwoparts.Itisfairlyobviousthatthepartitioninghastofinishbeforewestartsortingthetwopartitionedparts.However,thesortingtasksofthetwopartsdonotdependoneachother,theycanbedoneindependently.Youcangivethemtotwodifferentprocessors.Onewillbehappysortingthepartcontainingthesmallerelements;theotheronewillcarrytheheavier,largerones.

IfyouturnthepagesbacktoChapter3,OptimizingtheSort-MakingCodeProfessionalwhereweimplementedquick-sortinanon-recursiveway,youcanseethatwescheduledsortingtasksintoastackandthenperformedthesortingbyfetchingtheelementsfromthestackinawhileloop.Insteadofexecutingthesortrightthereinthecoreoftheloop,wecouldpassthetasktoanasynchronousthreadtoperformitandgobackforthenextwaitingtask.Wejustdonotknowhow.Yet.Thatiswhywearehereinthischapter.

Processors,threads,andprocessesarecomplexandabstractthingsandtheyarehardtoimagine.Differentprogrammershavedifferenttechniquestoimagineparallelprocessingandalgorithms.IcantellyouhowIdoitbutitisnotaguaranteethatthiswillworkforyou.Othersmayhavedifferenttechniquesintheirmind.Asamatteroffact,IjustrealizedthatasIwritethis,Ihaveactuallynevertoldthistoanyonebefore.Itmayseemchildish,butanyway,hereitgoes.

WhenIimaginealgorithms,Iimaginepeople.Oneprocessorisoneperson.Thishelpsmeovercomethefreakingfactthataprocessorcanmakebillionsofcalculationsinasecond.Iactuallyimagineabureaucratwearingabrownsuitanddoingthecalculations.WhenIcreateacodeforaparallelalgorithm,Iimaginemanyofthemworkingbehindtheirdesks.Theyworkaloneandtheydonottalk.Itisimportant

thattheydonottalktoeachother.Theyareveryformal.Whenthereisaneedforinformationexchange,theystandupwithapieceofpapertheyhavewrittensomethingon,andtheybringittoeachother.Sometimes,theyneedapieceofpaperfortheirwork.Thentheystandup,gototheplacewherethepaperis,takeit,bringitbacktotheirdesk,andgoonworking.Whentheyareready,theygobackandbringthepaperback.Ifthepaperisnottherewhentheyneedit,theyqueueupandwaituntilsomeonewhohasthepaperbringsitthere.

HowdoesithelpwithMastermind?

Iimagineabosswhoisresponsiblefortheguesses.Thereisatableonthewallintheofficewiththepreviousguessesandtheresultsforeachrow.Thebossistoolazytocomeupwithnewguessessohegivesthistasktosubordinates.Whenasubordinatecomesupwithaguess,thebosscheckswhethertheguessisvalidornot.Hedoesnottrustthesubordinates,andiftheguessisgood,hemakesitasanofficialguess,puttingitonthetablealongwiththeresult.

Thesubordinatesdelivertheguesseswrittenonsmallpost-itnotes,andtheyputtheminaboxonthetableoftheboss.Thebosslooksattheboxfromtimetotime,andifthereisanote,thebosstakesit.Iftheboxisfullandasubordinatewantstoputapaperthere,thesubordinatestopsandwaitsuntilthebosstakesatleastonenotesothatthereissomeroomintheboxforanewnote.Ifthesubordinatesqueueuptodepositguessesinthebox,theyallwaitfortheirtime.

Thesubordinatesshouldbecoordinated;otherwise,theywilljustcomeupwiththesameguesses.Eachofthemshouldhaveanintervalofguesses.Forexample,thefirstoneshouldchecktheguessesfrom1234upuntil2134,thesecondshouldcheckfrom2134upuntil3124,andsoon,ifwedenotethecolorswithnumbers.

Willthisstructurework?Commonsensesaysthatitwill.However,bureaucrats,inthiscase,aremetaphorsandmetaphorsarenotexact.Bureaucratsarehuman,evenwhentheydonotseemlikeitmuchmorethanthreadsorprocessors.Theysometimesbehaveextremelystrangely,doingthingsthatnormalhumansdon'treallydooften.However,wecanstillusethismetaphorifithelpsusimaginehowparallelalgorithmswork.

Wecanimaginethatthebossgoesonholidayanddoesnottouchtheheapofpaperpilinguponthetable.Wecanimaginethatsomeoftheworkersareproducingresultsmuchfasterthantheothers.Asthisisonlyimagination,thespeedupcanbe1000times(thinkofatime-lapsevideo).Imaginingthesesituationsmayhelpusdiscoverspecialbehaviorthatrarelyhappens,butmaycauseproblems.Asthethreadsworkinparallel,manytimessubtledifferencesmayinfluencethegeneralbehaviorgreatly.

Insomeearlyversion,asIcodedtheparallelMastermindalgorithm,thebureaucratsstartedworkingandfilledtheboxofthebosswithguessesbeforethebosscouldputanyofthemonthetable.Astherewerenoguessesonthetable,thebureaucratssimplyfoundallpossiblevariationsintheirintervalbeingapossiblygoodguess.Thebossgainednothingbythehelpoftheparallelhelpers;theyhadtoselectthecorrectonesfromallpossibleguesses,whiletheguesserswerejustidle.

Anothertime,thebureaucratswerecheckingguessesagainstthetablewhilethebosswasputtingaguessononeofthemcreatedbeforehand.Andsomeofthebureaucratsfreakedoutsayingthatitisnotpossible

tocheckaguessagainstatableifsomeoneischangingit.Moreprecisely,thecodeexecutinginonethread,threwConcurrentModificationExceptionwhentheListofthetablewasmodified.

Anothertime,Itriedtoavoidthetoofastworkofbureaucrats,andIlimitedthesizeoftheboxwheretheycouldputtheirpaperscontainingtheguesses.Whenthebossfinallyfoundthesecret,andthegamefinished,thebosstoldthebureaucratsthattheycouldgohome.Thebossdidthatbycreatingasmallpaperwiththeinstruction:youcangohome,andputitonthetablesofthebureaucrats.Whatdidthebureaucratsdo?Keptwaitingfortheboxtohavespaceforthepaper!(Untiltheprocesswaskilled.ThisiskindofequivalentonMacOSandonLinuxasendingtheprocessfromthetaskmanageronWindows.)Suchcodingerrorshappenand,toavoidasmanyaspossible,wehavetodoatleasttwothings.Firstly,wehavetounderstandhowJavamultithreadingworksandsecondly,haveacodeascleanaspossible.Forthesecond,wewillcleanupthecodeevenmoreandthenwewilllookathowtheparallelalgorithmdescribedearliercanbeimplementedinJava,runningontheJVMinsteadofutilizingbureaucrats.

RefactoringWhenwefinishedthepreviouschapter,wehadtheclassesoftheMastermindgamedesignedandcodedinaniceandperfectlyobjectorientedwaythatdidnotbreakanyoftheOOprinciples.Didwe?Absurd.Thereisnocode,exceptsometrivialexamples,thatcannotbemadetolooknicerorbetter.Usually,whenwedevelopcodeandfinishthecoding,itlooksgreat.Itworks,thetestsallrun,anddocumentationisready.Fromtheprofessionalpointofview,itreallyisperfect.Well,itisgoodenough.Thebigquestionthatwehavenottestedyetismaintainability.Whatisthecosttoalterthecode?

Thatisnotaneasyquestion,especiallybecauseitisnotadefiniteone.Altertowhat?Whatisthemodificationthatistobemadetothecode?Wedonotknowthatwhenwecreatethecodeinthefirstplace.Ifthemodificationistofixabug,thenitisobviousthatwedidnotknowthatbeforehand.Ifweknew,wewouldnothaveintroducedthebuginthefirstplace.Ifthisisanewfeature,thenthereisapossibilitythatthefunctionwasforeseen.However,usuallyitisnotthecase.Whenadevelopertriestopredictthefuture,andwhatfeaturestheprogramwillneedinthefuture,theyusuallyfail.Itisthetaskofthecustomertoknowthebusiness.Featuresneededaredrivenbythebusinessincaseofprofessionalsoftwaredevelopment.Afterall,thatiswhatitmeanstobeprofessional.

Eventhoughwedonotexactlyknowwhatneedstobealteredlaterinthecode,therearecertainthingsthatmaygivehintstoexperiencedsoftwaredevelopers.Usually,theOOcodeiseasiertomaintainthanthead-hoccode,andtherearecodesmellsthatonecanspot.Forexample,takealookatthefollowingcodelines:

while(guesser.guess()!=Row.none){

while(guesser.nextGuess()!=Guesser.none){

publicvoidaddNewGuess(Rowrow){

Color[]guess=super.nextGuess();

Wemaysensetheodorofsomethingstrange.(EachoftheselinesisinthecodeoftheapplicationaswefinisheditinChapter4,Mastermind-CreatingaGame.)ThereturnvalueoftheguessmethodiscomparedtoRow.none,whichisaRow.Then,wecomparethereturnvalueofnextGuesstoGuesser.none,whichshouldbeaGuesser.Whenweaddanewguesstosomething,weactuallyaddaRow.Finally,wecanrealizethatnextGuessreturnsaguessthatisnotanobjectwithitsowndeclaredclass.Aguessisjustanarrayofcolors.

ShouldweintroduceanotherlayerofabstractioncreatingaGuessclass?Willitmakethecodemoremaintainable?Orwillitonlymakethecodemorecomplex?Itisusuallytruethatthelesscodelineswehave,thelesspossibilitywehaveforbugs.However,sometimes,thelackofabstractionwillmakethecodecomplexandtangled.Whatisthecaseinthissituation?Howcanwedecidethatgenerally?

Themoreexperienceyouhave,theeasieryouwilltellbylookingatthecodeandacutelyknowingwhatmodificationsyouwanttomake.Manytimes,youwillnotbothermakingthecodemoreabstract,andmanyothertimes,youwillcreatenewclasseswithouthesitation.Whenindoubt,docreatethenewclassesandseewhatcomesout.Theimportantthingisnottoruinthealreadyexistingfunctionality.Youcandothatonlyifyouhavesufficientunittests.

Whenyouwanttointroducesomenewfunctionalityorfixabug,butthecodeisnotappropriate,youwillhavetomodifyitfirst.Whenyoumodifythecodesothatthefunctionalitydoesnotchange,theprocessis

namedrefactoring.Youchangeasmallpartofthecodeinalimitedtime,andthenyoubuildit.Ifitcompilesandallunittestsrun,thenyoucangoon.Thehintistorunthebuildfrequently.Itislikebuildinganewroadnearanexistingone.Onceineveryfewmiles,youshouldmeettheoldline.Failingtodoso,youwillendupsomewhereinthemiddleofthedesertinatotallywrongdirection,andallyoucandoisreturntothestartingpoint—youroldto-be-refactoredcode.Effortwasted.

Itisnotonlythesafetythatadvisesustorunthebuildfrequently,itisalsotimelimitation.Refactoringdoesnotdirectlydeliverrevenue.Thefunctionalityoftheprogramistieddirectlytoincome.Nobodywillpayusforinfiniterefactoringwork.Refactoringhastostopsometimeanditisusuallynotthetimewhenthereisnothingtoberefactoredanymore.Thecodewillneverbeperfect,butyoumaystopwhenitisgoodenough.And,manytimes,programmersareneversatisfiedwiththequalityofthecode,andwhentheyarestoppedbysomeexternalfactor(usuallycalledprojectmanager),thecodeshouldcompileandtestsshouldrunsothatthenewfeatureandbugfixingcanbeperformedontheactualcodebase.

Refactoringisahugetopicandtherearemanytechniquesthatcanbefollowedduringsuchanactivity.ItissocomplexthatthereisawholebookaboutitbyMartinFowler(http://martinfowler.com/books/refactoring.html).

Inourcase,themodificationwewanttoapplytoourcodeistoimplementaparallelalgorithm.ThefirstthingwewillmodifyistheColorManager.Whenwewantedtoprintguessesandrowsontheterminal,wehadtoimplementsomebadtricks.Whynothavecolorimplementationsthatcanbeprinted?WecanhaveaclassthatextendstheoriginalColorclassandhasamethodthatreturnssomethingthatrepresentsthatcolor.Doyouhaveanycandidatenameforthatmethod?ItisthetoStringmethod.ItisimplementedintheObjectclassandanyclasscanfreelyoverrideit.Whenyouconcatenateanobjecttoastring,automatictypeconversionwillcallthismethodtoconverttheobjecttoString.Bytheway,itisanoldtricktouse""+objectinsteadofobject.toString()toavoidnullpointerexception.Needlesstosay,wedonotusetricks.ThetoStringmethodisalsoinvokedbytheIDEswhenthedebuggerwantstodisplaythevalueofsomeobject,soitisgenerallyrecommendedtoimplementtoStringiffornothingelse,thentoeasedevelopment.IfwehaveaColorclassthatimplementstoString,thenthePrettyPrintRowclassbecomesfairlystraightforwardandtricksless:

packagepackt.java9.by.example.mastermind;

publicclassPrettyPrintRow{

publicstaticStringpprint(Rowrow){

Stringstring="";

PrintableRowpRow=newPrintableRow(row);

for(inti=0;i<pRow.nrOfColumns();i++){

string+=pRow.pos(i);

}

string+="";

string+=pRow.full();

string+="/";

string+=pRow.partial();

returnstring;

}

}

Weremovedtheproblemfromtheprintingclass,butyoumayarguethattheissueisstillthere,andyouareright.Manytimes,whenthereisaprobleminaclassdesign,thewaytothesolutiontomovetheproblemfromtheclasstoanother.Ifitisstillaproblemthere,thenyoumaysplitthedesignmoreandmoreand,atthelaststage,youwillrealizethatwhatyouhaveisanissueandnotaproblem.

ToimplementaLetteredColorclassisalsostraightforward:

packagepackt.java9.by.example.mastermind.lettered;

importpackt.java9.by.example.mastermind.Color;

publicclassLetteredColorextendsColor{

privatefinalStringletter;

publicLetteredColor(Stringletter){

this.letter=letter;

}

@Override

publicStringtoString(){

returnletter;

}

}

Again,theproblemwaspushedforward.But,inreality,thisisnotaproblem.ItisanOOdesign.PrintingisnotresponsibleforassigningStringtocolorsfortheirrepresentation.Andthecolorimplementationitselfisalsonotresponsibleforthat.Theassignmenthastobeperformedwherethecolorismade,andthentheStringhastobepassedtotheconstructoroftheLetteredColorclass.ThecolorinstancesarecreatedinColorManagersowehavetoimplementthisintheColorManagerclass.Ornot?WhatdoesColorManagerdo?Itcreatesthecolorsand...

Whenyoucometoanexplanationordescriptionofaclassthatliststhefunctionalities,youmayimmediatelyseethatthesingleresponsibilityprinciplewasignored.ColorManagershouldmanagethecolors.Managingisprovidingawaytogetthecolorsinadefiniteorderandgettingthefirstandthenextwhenweknowonecolor.Weshouldimplementtheotherresponsibility—thecreationofacolorinaseparateclass.

Aclassthathasthesolefunctionalitytocreateaninstanceofanotherclassiscalledfactory.Thatisalmostthesameasusingthenewoperatorbutunlikenew,thefactoriescanbeusedmoreflexibly.Wewillseethatimmediately.TheColorFactoryinterfacecontainsasinglemethod,asfollows:

packagepackt.java9.by.example.mastermind;

publicinterfaceColorFactory{

ColornewColor();

}

Interfacesthatdefineonlyonemethodarenamedfunctionalinterfacesbecausetheirimplementationcanbeprovidedasalambdaexpressionattheplacewhereyouwoulduseanobjectthatisaninstanceofaclasswhichimplementsthefunctionalinterface.TheSimpleColorFactoryimplementationcreatesthefollowingColorobjects:

packagepackt.java9.by.example.mastermind;

publicclassSimpleColorFactoryimplementsColorFactory{

@Override

publicColornewColor(){

returnnewColor();

}

}

Itisverymuchlikehowwecreateaninterface,andthenanimplementation,insteadofjustwritingnewColor()inthecodeinColorManager.LetteredColorFactoryisabitmoreinteresting:

packagepackt.java9.by.example.mastermind.lettered;

importpackt.java9.by.example.mastermind.Color;

importpackt.java9.by.example.mastermind.ColorFactory;

publicclassLetteredColorFactoryimplementsColorFactory{

privatefinalStringletters="0123456789ABCDEFGHIJKLMNOPQRSTVWXYZabcdefghijklmnopqrstvwxzy";

privateintcounter=0;

@Override

publicColornewColor(){

Colorcolor=newLetteredColor(letters.substring(counter,counter+1));

counter++;

returncolor;

}

}

Now,herewehavethefunctionalitythatassignsStringstotheColorobjectswhentheyarecreated.Itisveryimportantthatthecountervariablethatkeepstrackofthealreadycreatedcolorsisnotstatic.Thesimilarvariableinthepreviouschapterwasstaticanditmeantthatitcouldrunoutofcharactersasever-newerColorManagerscreatedtoomanycolors.ItactuallydidhappentomeduringtheunittestexecutionwhenthetestseachcreatedColorManagersandnewColorinstances,andtheprintingcodetriedtoassignnewletterstothenewcolors.ThetestswererunninginthesameJVMunderthesameclassloaderandtheunfortunatestaticvariablehadnocluewhenitcouldjuststartcountingfromzeroforthenewtests.Thedrawbackisthatsomebody,somewhere,hastoinstantiatethefactoryanditisnottheColorManager.ColorManageralreadyhasaresponsibilityanditisnottocreateacolorfactory.TheColorManagerhastogettheColorFactoryinitsconstructor:

packagepackt.java9.by.example.mastermind;

importjava.util.HashMap;

importjava.util.List;

importjava.util.Map;

publicclassColorManager{

finalprotectedintnrColors;

finalprotectedMap<Color,Color>successor=newHashMap<>();

privateColorfirst;

privatefinalColorFactoryfactory;

publicColorManager(intnrColors,ColorFactoryfactory){

this.nrColors=nrColors;

this.factory=factory;

createOrdering();

}

privateColor[]createColors(){

Color[]colors=newColor[nrColors];

for(inti=0;i<colors.length;i++){

colors[i]=factory.newColor();

}

returncolors;

}

privatevoidcreateOrdering(){

Color[]colors=createColors();

first=colors[0];

for(inti=0;i<nrColors-1;i++){

successor.put(colors[i],colors[i+1]);

}

}

publicColorfirstColor(){

returnfirst;

}

publicbooleanthereIsNextColor(Colorcolor){

returnsuccessor.containsKey(color);

}

publicColornextColor(Colorcolor){

returnsuccessor.get(color);

}

publicintgetNrColors(){

returnnrColors;

}

}

YoumayalsonoticethatIcouldnotresistrefactoringthecreateColorsmethodintotwomethodstofollowthesingleresponsibilityprinciple.

Now,thecodethatcreatesColorManagerhastocreateafactoryandpassittotheconstructor.Forexample,theunittest'sColorManagerTestclasswillcontainthefollowingmethod:

@Test

publicvoidthereIsAFirstColor(){

ColorManagermanager

=newColorManager(NR_COLORS,Color::new);

Assert.assertNotNull(manager.firstColor());

}

Thisisthesimplestwayevertoimplementafactorydefinedbyafunctionalinterface.Justnametheclassandreferencethenewoperatorlikeitwasamethodbycreatingamethodreference.

ThenextthingwewillrefactoristheGuessclass,which,actually,wedidnothavesofar.AGuessclasscontainsthepegsoftheguessandcancalculatethenumberoffull(coloraswellasposition)andpartial(colorpresentbutinwrongposition)matches,andcanalsocalculatethenextGuessthatcomesafterthisguess.ThisfunctionalitywasimplementedintheGuesserclasssofar,butthisisnotreallythefunctionalityforhowweselecttheguesseswhencheckingthealreadymadeguessesonthetable.Ifwefollowthepatternwesetupforthecolors,wemayimplementthisfunctionalityinaseparateclassnamedGuessManager,butasfornow,itisnotneeded.Again,thisisnotblackandwhite.

ItisimportanttonotethataGuessobjectcanonlybemadeatonce.Ifitisonthetable,theplayerisnotallowedtochangeit.IfwehaveaGuessthatisnotyetonthetable,itisstilljustaGuessidentifiedbythecolorsandordersofthepegs.AGuessobjectneverchangesafteritwascreated.Suchobjectsareeasytouseinmultithreadprogramsandarecalledimmutableobjects:

packagepackt.java9.by.example.mastermind;

importjava.util.Arrays;

importjava.util.HashSet;

importjava.util.Set;

publicclassGuess{

finalstaticpublicGuessnone=newGuess(newColor[0]);

finalprivateColor[]colors;

privatebooleanuniquenessWasNotCalculated=true;

privatebooleanunique;

publicGuess(Color[]colors){

this.colors=Arrays.copyOf(colors,colors.length);

}

Theconstructoriscreatingacopyofthearrayofcolorsthatwerepassed.AsaGuessisimmutable,thisisextremelyimportant.Ifwejustkeeptheoriginalarray,anycodeoutsideoftheGuessclasscouldalterthe

elementsofthearray,essentiallychangingthecontentofGuessthatisnotsupposedtobechanging:

publicColorgetColor(inti){

returncolors[i];

}

publicintnrOfColumns(){

returncolors.length;

}

/**

*CalculatethenextguessandreturnanewGuessobject.

*Theguessesareorderedintheorderofthecolorsas

*specifiedbythecolormanager.

*

*@parammanagerthatspecifiestheorderofthecolors

*canreturnthenextcolorafteronecolor.

*@returntheguessthatcomesafterthisguess.

*/

publicGuessnextGuess(ColorManagermanager){

finalColor[]colors=Arrays.copyOf(

this.colors,nrOfColumns());

inti=0;

booleanguessFound=false;

while(i<colors.length&&!guessFound){

if(manager.thereIsNextColor(getColor(i))){

colors[i]=manager.nextColor(colors[i]);

guessFound=true;

}else{

colors[i]=manager.firstColor();

i++;

}

}

if(guessFound){

returnnewGuess(colors);

}else{

returnGuess.none;

}

}

Inthismethod,westarttocalculatethenextGuessstartingwiththecolorarraythatiscontainedintheactualobject.Weneedaworkarraythatismodified,sowewillcopytheoriginal.Thefinalnewobjectcan,thistime,usethearrayweuseduringthecalculation,sothatwillneedaseparateconstructorthatdoesnotcreateacopy.Itispossibleextracode,butweshouldconsidermakingthatonlyifweseethatthatisthebottleneckinthecodeandwearenotsatisfiedwiththeactualperformance.

ThenextmethodjustchecksifthepassedGuesshasthesamenumberofcolorsastheactualone.Thisisjustasafetycheckusedbythenexttwomethodsthatcalculatethematches:

privatevoidassertCompatibility(Guessguess){

if(nrOfColumns()!=guess.nrOfColumns()){

thrownewIllegalArgumentException("Cannotcomparedifferentlengthguesses");

}

}

/**

*Countthenumberofcolorsthatarepresentontheguess

*butnotontheposwheretheyareintheotherguess.

*Ifthesamecolorisonmultiplepositiscounted

*foreachposonce.Forexamplethesecretis

*<pre>

*RGRB

*</pre>

*andtheguessis

*<pre>

*YRPR

*</pre>

*thenthismethodwillreturn2.

*

*@paramguessistheactualguessthatweevaluate

*@returnthenumberofgoodcolorsnotinpos

*/

publicintnrOfPartialMatches(Guessguess){

assertCompatibility(guess);

intcount=0;

for(inti=0;i<nrOfColumns();i++){

for(intj=0;j<nrOfColumns();j++){

if(i!=j&&

guess.getColor(i)==this.getColor(j)){

count++;

}

}

}

returncount;

}

/**

*Countthenumberofcolorsthatarecorrectandareinpos.

*

*@paramguessistheactualguessthatweevaluate

*@returnthenumberofcolorsthatmatchinpos

*/

publicintnrOfFullMatches(Guessguess){

assertCompatibility(guess);

intcount=0;

for(inti=0;i<nrOfColumns();i++){

if(guess.getColor(i)==this.getColor(i)){

count++;

}

}

returncount;

}

TheisUniquemethodchecksifthereisanycolormorethanonceintheGuess.AstheGuessisimmutable,itmaynothappenthataGuessisuniqueonetimeandnotuniqueatanothertime.Thismethodshouldreturnthesameresultwheneveritiscalledonaspecificobject.Becauseofthat,itispossibletocachetheresult.Thismethoddoesthis,savingthereturnvaluetoaninstancevariable.

Youmaysaythatthisisprematureoptimization.Yes,itis.Idecidedtodoitforonereason.Itisdemonstration,andbasedonthat,youcantrytomodifythenextGuessmethodtodothesame:

/**

*@returntrueiftheguessdoesnot

*containanycolormorethanonce

*/

publicbooleanisUnique(){

if(uniquenessWasNotCalculated){

finalSet<Color>alreadyPresent=newHashSet<>();

unique=true;

for(Colorcolor:colors){

if(alreadyPresent.contains(color)){

unique=false;

break;

}

alreadyPresent.add(color);

}

uniquenessWasNotCalculated=false;

}

returnunique;

}

Methodsthatreturnthesameresultforthesameargumentsarecalledidempotent.Cachingthereturnvalueforsuchamethodcanbeveryimportantifthemethodiscalledmanytimesandthecalculationisusingalotofresources.Whenthemethodhasarguments,theresultcachingisnotsimple.Theobjectmethodhastoremembertheresultforallargumentsthatwerealreadycalculated,andthisstoragehastobeeffective.Ifittakesmoreresourcestofindthestoredresultthanthecalculationofit,thentheuseofcachenotonly

usesmorememorybutalsoslowsdowntheprogram.Ifthemethodiscalledforseveralargumentsduringthelifetimeoftheobject,thenthestoragememorymayjustgrowtoolarge.Someoftheelementshavetobepurged—thosethatwillnotbeneededanymoreinthefuture.However,wecannotknowwhichelementsofthecachearenotneeded,sowewillhavetoguess.

Asyoucansee,cachingcangetcomplexveryfastand,todothatprofessionally,itisalmostalwaysbettertousesomereadilyavailablecacheimplementation.Thecachingweusehereisonlythetipoftheiceberg.Or,itisevenonlythesunshineglimpsingonit.

Therestoftheclassisfairlystandardandsomethingwehavetalkedaboutindetail—agoodcheckofyourknowledgeistounderstandhowtheequals,hashCode,andtoStringmethodsareimplementedthisway.IimplementedthetoStringmethodtohelpmeduringdebugging,butitisalsousedinthefollowingexampleoutput:

@Override

publicbooleanequals(Objectother){

if(this==other)returntrue;

if(other==null||!(otherinstanceofGuess))

returnfalse;

Guessguess=(Guess)other;

returnArrays.equals(colors,guess.colors);

}

@Override

publicinthashCode(){

returnArrays.hashCode(colors);

}

@Override

publicStringtoString(){

if(this==none){

return"none";

}else{

Strings="";

for(inti=colors.length-1;i>=0;i--){

s+=colors[i];

}

returns;

}

}

}

ThisismainlythemodificationthatIneededwhileIdevelopedtheparallelalgorithm.Now,thecodeisfairlyup-to-dateanddescribedtofocusonthemaintopicofthischapter:howtoexecutecodeinJavainparallel.

TheparallelexecutionofthecodeinJavaisdoneinthreads.YoumayknowthatthereisaThreadobjectinJavaruntime,butwithoutunderstandingwhatathreadinthecomputeris,itmakesnosense.Inthefollowingsubsections,wewilllearnwhatthesethreadsare,howtostartanewthread,howtosynchronizedataexchangebetweenthreads,andfinallyputallthistogetherandimplementtheMastermindparallelguessingalgorithm.

ProcessesWhenyoustartyourcomputer,theprogramthatstartsistheoperatingsystem(OS).TheOScontrolsthemachinehardwareandtheprogramsthatyoucanrunonthemachine.Whenyoustartaprogram,theOScreatesanewprocess.ItmeansthattheOSallocatesanewentryinatable(array)whereitadministerstheprocessesandfillsintheparametersthatitknows,andneedstoknow,abouttheprocess.Forexample,itregisterswhatmemorysegmenttheprocessisallowedtouse,whattheIDoftheprocessis,andwhichuserstartedfromwhichotherprocess.Youcannotstartaprocessjustoutofthinair.Whenyoudouble-clickonanEXEfile,youactuallytellthefileexplorer,whichisaprogramrunningasaprocess,tostarttheEXEfileasaseparateprocess.TheexplorercallsthesystemviasomeAPIandkindlyaskstheOStodothat.TheOSwillregistertheexplorerprocessastheparentofthenewprocess.TheOSdoesnotactuallystarttheprocess,butcreatesallthedatathatitneedstostartitand,whenthereissomefreeCPUresource,thentheprocessgetsstarted,andthenitgetspausedverysoon.YouwillnotnoticeitbecausetheOSwillstartitagainandagainandisalwayspausingtheprocessrepeatedly.Itneedstodoittoproviderunpossibilitiestoallprocesses.Thatway,weexperienceallprocessesrunningatthesametime.Inreality,processesdonotrunatthesametimeonasingleprocessor,buttheygettimeslotstorunoften.

IfyouhavemorethanoneCPUinthemachine,thenprocessescanactuallyrunatthesametime,asmanyCPUsasthereare.Astheintegrationgetsmoreadvancedtoday,desktopcomputershaveCPUsthatcontainmultiplecoresthatfunctionalmostlikeseparateCPUs.Onmymachine,Ihavefourcores,eachcapableofexecutingtwothreadssimultaneously;so,myMacisalmostlikean8CPUmachine.

Processeshaveseparatememories.Theyareallowedtouseonepartofthememoryandifaprocesstriestouseanotherpartthatdoesnotbelongtoit,theprocessorwillstopdoingso.TheOSwillkilltheprocess.

JustimaginehowfrustratedthedevelopersoftheoriginalUNIXcouldhavebeenthattheynamedtheprogramtostopaprocesstokill,andstoppingaprocessiscalledkillingit.Itislikemedievalageswhentheycutoffthehandofafelon.Youtouchthewrongpartofthememoryandgetkilled.Iwouldnotliketobeaprocess.

Thememoryhandlingbytheoperatingsystemisverycomplexinadditiontoseparatingtheprocessesfromeachother.Whenthereisnotenoughmemory,theOSwritespartofthememorytodiskfreeingupthememoryandreloadingthatpartwhenitisneededagain.Thisisaverycomplex,low-levelimplementedandhighlyoptimizedalgorithmthatistheresponsibilityoftheOS.

ThreadsWhenIsaidthattheOSexecutestheprocessesintimeslots,Iwasnotabsolutelyprecise.Everyprocesshasoneormorethreads,andthreadsareexecuted.Athreadisthesmallestexecutionmanagedbyanexternalscheduler.Olderoperatingsystemsdidnothavethenotionofathreadandwereexecutingprocesses.Asamatteroffact,thefirstthreadimplementationsweresimplyduplicationsofprocessesthatweresharingthememory.

Youmayheartheterminology,lightweightprocess—itmeansathread.

Theimportantthingisthatthethreadsdonothavetheirownmemory.Theyusethememoryoftheprocess.Inotherwords,thethreadsthatruninthesameprocesshaveundistinguishedaccesstothesamememorysegment.Itisanextremelypowerfulpossibilitytoimplementparallelalgorithmsthatmakeuseofthemultiplecoresinthemachine,butatthesametime,itmayleadtobugs.

Imaginethattwothreadsincrementthesamelongvariable.Theincrementfirstcalculatestheincrementedvalueofthelower32bitsandthentheupper,iftherewereanyoverflowbits.ThesearetwoormorestepsthatmaybeinterruptedbytheOS.Itmayhappenthatonethreadincrementsthelower32bits,

remembersthatthereissomethingtodototheupper32bits,startsthecalculation,buthasnotimetostoretheresultbeforeitgetsinterrupted.Then,anotherthreadincrementsthelower32bits,theupper32bits,andthenthefirstthreadjustsavestheupper32bitsthatitcalculated.Theresultgetsgarbled.Onanolder32-bitJavaimplementation,itwasextremelyeasytodemonstratethiseffect.Ona64-bitJavaimplementation,allthe64bitsareloadedintoregistersandsavedbacktothememoryinonestepsoitisnotthateasytodemonstratemultithreadissues,butitdoesnotmeanthattherearenone.

Whenathreadispausedandanotherthreadisstarted,theoperatingsystemhastoperformacontextswitch.Itmeansthat,amongotherthings,theCPUregistershavetobesavedandthensettothevaluethattheyshouldhavefortheotherthread.Acontextswitchisalwayssavingthestateofthethreadandloadingthepreviouslysavedstateofthethreadtobestarted.ThisisonaCPUregisterlevel.Thiscontextswitchistimeconsuming;therefore,themorecontextswitchesthataredone,themoreCPUresourceisusedforthethreadadministrationinsteadoflettingthemrun.Ontheotherhand,iftherearenotenoughswitches,somethreadsmaynotgetenoughtimeslotstoexecute,andtheprogramhangs.

FibersJavadoesnothavefibers,butastherearesomelibrariesthatsupportfiberhandlings,itisworthmentioning.Afiberisafinerunitthanathread.Aprogramcodeexecutinginathreadmaydecidetogiveuptheexecutionandtellthefibermanagertojustexecutesomeotherfiber.Whatisthepointandwhyisitbetterthanusinganotherthread?Thereasonisthatthisway,fiberscanavoidpartofthecontextswitch.AcontextswitchcannotbeavoidedtotallybecauseadifferentpartofthecodethatstartstoexecuteitmayusetheCPUregistersinatotallydifferentway.Asitisthesamethread,thecontextswitchingisnotthetaskoftheOS,buttheapplication.

TheOSdoesnotknowifthevalueofaregisterisusedornot.Therearebitsintheregisters,andnoonecantellseeingonlytheprocessorstatewhetherthosebitsarerelevantforthecurrentcodeexecutionorjusthappentobethereinthatway.Theprogramgeneratedbyacompilerdoesknowwhichregistersareimportantandwhicharethosethatcanjustbeignored.Thisinformationchangesfromplacetoplaceinthecode,butwhenthereisaneedforaswitch,thefiberpassestheinformationofwhatisneededtobeswitchedatthatpointtothecodethatdoestheswitching.

Thecompilercalculatesthisinformation,butJavadoesnotsupportfibersinthecurrentversion.ThetoolsthatimplementfibersinJavaanalyzeandmodifythebytecodeoftheclassestodothisafterthecompilationphase.

Golang'sgoroutinesarefibersandthatiswhyyoucaneasilystartmanythousandgoroutinesinGo,butyoubetterlimitthenumberofthreadsinJavatoalowernumber.Theyarenotthesamethings.

Astheterminologylightweightprocessisfadingoutandusedbylessandlessfibers,manytimesarereferredtoaslightweightthreads.

java.lang.ThreadAseverythinginJava(well,almost)isobject,ifwewanttostartanewthread,wewillneedaclassthatrepresentsthethread.Thisclassisjava.lang.ThreadbuiltintotheJDK.WhenyoustartaJavacode,theJVMautomaticallycreatesafewThreadobjectsandusesthemtorundifferenttasksthatareneededbyit.IfyoustartupVisualVM,youcanselecttheThreadstabofanyJVMprocessandseetheactualthreadsthatareintheJVM.Forexample,theVisualVMasIstartedithas29livethreads.Oneofthemisthethreadnamedmain.Thisistheonethatstartstoexecutethemainmethod(surprise!).Themainthreadstartedmostoftheotherthreads.Whenwewanttowriteamultithreadapplication,wewillhavetocreatenewThreadobjectsandstartthem.ThesimplestwaytodothatisnewThread(),andthencallingthestartmethodonthethread.ItwillstartanewThreadthatwilljustfinishimmediatelyaswedidnotgiveitanythingtodo.TheThreadclass,asitisintheJDK,doesnotdoourbusinesslogic.Thefollowingarethetwowaystospecifythebusinesslogic:

CreatingaclassthatimplementstheRunnableinterfaceCreatingaclassthatextendstheThreadclassandoverridestherunmethod

Thefollowingblockofcodeisaverysimpledemonstrationprogram:

publicclassThreadIntermingling{

staticclassMyThreadextendsThread{

privatefinalStringname;

MyThread(Stringname){

this.name=name;

}

@Override

publicvoidrun(){

for(inti=1;i<1000;i++){

System.out.print(name+""+i+",");

}

}

}

publicstaticvoidmain(String[]args){

Threadt1=newMyThread("t1");

Threadt2=newMyThread("t2");

t1.start();

t2.start();

System.out.print("started");

}

}

Theprecedingcodecreatestwothreadsandstartsthemoneaftertheother.Whenthestartmethodiscalled,itschedulesthethreadobjecttobeexecutedandthenreturns.Asaresult,thenewthreadwillsoonstartexecutingasynchronouslywhilethecallingthreadcontinuesitsexecution.Thetwothreads,andthemainthread,runparallelinthefollowingexampleandcreateanoutputthatlookssomethinglikethis:

startedt21,t22,t23,t24,t25,t26,t27,t28,t11,t29,t210,t211,t212,...

Theactualoutputchangesfromruntorun.Thereisnodefiniteorderoftheexecutionorhowthethreadsgetaccesstothesinglescreenoutput.Thereisnotevenguaranteethatineachandeveryexecution,themessagestartedisprintedbeforeanyofthethreadmessages.

Togetabetterunderstandingofthis,wewillhavetolookatthestatediagramofthreads.AJavaThread

canbeinoneofthefollowingstates:

NEW

RUNNABLE

BLOCKED

WAITING

TIMED_WAITING

TERMINATED

ThesestatesaredefinedintheenumThread.State.Whenyoucreateanewthreadobject,itisintheNEWstate.Atthismoment,thethreadisnothingspecial,itisjustanobjectbuttheoperatingsystemexecution-schedulingdoesnotknowaboutit.Insomesense,itisonlyapieceofmemoryallocatedbytheJVM.

Whenthestartmethodisinvoked,theinformationaboutthethreadispassedtotheoperatingsystemandtheOSschedulesthethreadsoitcanbeexecutedbyitwhenthereisanappropriatetimeslot.Doingthisisaresourcefulactionandthatisthereasonwhywedonotcreateand,especially,donotstartnewThreadobjectsonlywhenitisneeded.InsteadofcreatingnewThreads,wewillkeeptheexistingthreadsforawhile,eveniftheyarenotneededatthemoment,andreuseanexistingoneifthereisonesuitable.

AthreadintheOScanalsobeinarunningstateaswellasrunnablewhentheOSschedulesandexecutesitatthemoment.JavaJDKAPIdoesnotdistinguishbetweenthetwoforgoodreason.Itwouldbeuseless.WhenathreadisintheRUNNABLEstateaskingifitisactuallyrunningfromthethreaditself,itwillresultinanobviousanswer:ifthecodejustreturnedfromthegetStatemethodimplementedintheThreadclass,thenitruns.Ifitwerenotrunning,itwouldnothavereturnedfromthecallinthefirstplace.IfthegetStatemethodwascalledfromanotherthread,thentheresultabouttheotherthreadbythetimethemethodreturnswouldbemeaningless.TheOSmayhavestopped,orstarted,thequeriedthreadseveraltimesuntilthen.

AthreadisinaBLOCKEDstatewhenthecodeexecutinginthethreadtriestoaccesssomeresourcethatisnotcurrentlyavailable.Toavoidconstantpollingofresources,theoperatingsystemprovideseffectivenotificationmechanismsothethreadsgetbacktotheRUNNABLEstatewhentheresourcetheyneedbecomesavailable.

AthreadisintheWAITorTIMED_WAITINGstatewhenitwaitsforsomeotherthreadorlock.TIMED_WAITINGisthestatewhenthewaitingstartedcallingaversionofamethodthathastimeout.

Finally,theTERMINATEDstateisreachedwhenthethreadfinishesitsexecution.Ifyouappendthefollowinglinestotheendofourpreviousexample,thenyouwillgetaTERMINATEDprintoutandalsoanexceptionthrownuptothescreencomplainingaboutillegalthreadstate,whichisbecauseyoucannotstartanalreadyterminatedthread:

System.out.println();

System.out.println(t1.getState());

System.out.println();

t1.start();

InsteadofextendingtheThreadclasstodefinewhattoexecuteasynchronously,wecancreateaclassthatimplementsRunnable.DoingthatismoreinlinewiththeOOprogrammingapproach.Thesomethingthatwe

implementintheclassisnotafunctionalityofathread.Itismoreofasomethingthatcanbeexecuted.Itissomethingthatcanjustrun.

Ifthisexecutionisasynchronousinadifferentthread,oritisexecutedinthesamethreadthatwascallingtherunmethod,isadifferentconcernthathastobeseparated.Ifwedoitthatway,wecanpasstheclasstoaThreadobjectasaconstructorargument.CallingstartontheThreadobjectwillstarttherunmethodoftheobjectwepassed.Thisisnotthegain.ThegainisthatwecanalsopasstheRunnableobjecttoanExecutor(dreadfulname,huhh!).Executorisaninterface,andimplementationsexecuteRunnable(andalsoCallable,seelater)objectsinThreadsinanefficientway.ExecutorsusuallyhaveapoolofThreadobjectsthatareprepared,andintheBLOCKEDstate.WhentheExecutorhasanewtasktoexecute,itgivesittooneoftheThreadobjectsandreleasesthelockthatisblockingthethread.TheThreadgetsintotheRUNNABLEstate,executestheRunnable,andgetsblockedagain.ItdoesnotterminateandthuscanbereusedtoexecuteanotherRunnablelater.Thatway,Executorsavoidtheresourceconsumingprocessofthreadregistrationintotheoperatingsystem.

ProfessionalapplicationcodenevercreatesanewThread.ApplicationcodeusessomeframeworktohandletheparallelexecutionofthecodeorusesExecutorsprovidedbysomeExecutorServicetostartRunnableorCallableobjects.

Pitfalls

Wehavealreadydiscussedmanyoftheproblemsthatwemayfacewhendevelopingparallelprogram.Inthissection,wewillsummarizethemwiththeusualterminologyusedfortheproblems.Terminologyisnotonlyinteresting,butitisalsoimportantwhenyoutalkwithcolleaguestoeasilyunderstandeachother.

DeadlocksDeadlockisthemostinfamousparallelprogrammingpitfall,andforthisreason,wewillstartwiththisone.Todescribethesituation,wewillfollowthemetaphorofbureaucrats.

Thebureaucrathastostampapaperhehasinhishand.Todothat,heneedsthestamp,andhealsoneedstheinkpad.First,hegoestothedrawerwherethestampisandtakesit.Then,hewalkstothedrawerwheretheinkpadisandtakestheinkpad.Heinksthestamp,pushesonthepaper.Then,heputsthestampbacktoitsplaceandthentheinkpadbackinitsplace.Everythingisnice,weareoncloud9.

Whathappensifanotherbureaucrattakestheinkpadfirstandthenthestampsecond?Theymaysoonendupasonebureaucratwiththestampinhandwaitingfortheinkpadandanotheronewiththeinkpadinhandwaitingforthestamps.And,theymayjuststaythere,frozenforever,andthenmoreandmorestarttowaitfortheselocks,thepapersnevergetstamped,andthewholesystemsinksintoanarchy.

Toavoidsuchsituations,thelockshavetobeorderedandthelocksshouldalwaysbeacquiredintheorder.Intheprecedingexample,thesimpleagreementthattheinkpadisacquiredfirstandthestampsecondsolvestheproblem.Whoeveracquiredthestampcanbesurethattheinkpadisfreeorwillsoonbefree.

RaceconditionsWetalkaboutraceconditionswhentheresultofacalculationmaybedifferentbasedonthespeedandCPUaccessofthedifferentparallelrunningthreads.Let'stakealookatthefollowingtwocodelines:

voidmethod1(){

1a=b;

2b=a+1;

}

voidmethod2(){

3c=b;

4b=c+2;

}

Ifthevalueofbatthestartoftheexecutionis0,andtwodifferentthreadsexecutethetwomethods,thentheorderofthelinescanbe1234,1324,1342,3412,3142,or3142.Anyexecutionorderofthefourlinesmayhappenwhichassuresthat1runsbefore2and3runsbefore4,butnootherrestrictions.Theoutcome,thevalueofb,iseither1or2attheendoftheexecutionofthesegments,whichmaynotbegoodandwhatwewantedwhencoding.

NotethattheimplementationoftheparallelMastermindgamealsohassomethinglikethis.Theactualguessesverymuchdependonthespeedofthedifferentthreads,butthisisirrelevantfromthefinalresultpointofview.Wemayhavedifferentguessesindifferentrunsandthatwaythealgorithmisnotdeterministic,butweareguaranteedtofindthefinalsolution.

Overusedlocks

Inmanysituations,itmayhappenthatthethreadsarewaitingonalock,whichprotectsaresourcefromconcurrentaccess.Iftheresourcecannotbeusedbymultiplethreadssimultaneously,andtherearemorethreadsthancanbeserved,thenthethreadsarestarving.However,inmanycases,theresourcecanbeorganizedinawaysothatthethreadscangetaccesstosomeoftheservicesthattheresourceprovides,andthelockingstructurecanbelessrestrictive.Inthatcase,thelockisoverusedandthesituationcanbemendedwithoutallocatingmoreresourceforthethreads.Itmaybepossibletouseseverallocksthatcontroltheaccesstothedifferentfunctionalityoftheresource.

Starving

Starvingisthesituationwhenseveralthreadsarewaitingforaresourcetryingtoacquirealockandsomethreadsgetaccesstothelockonlyafterextremelylongtimeornever.Whenthelockisreleasedandtherearethreadswaitingforit,thenoneofthethreadscangetthelock.Thereisusuallynoguaranteethatathreadgetsthelockifitwaitslongenough.Suchamechanismwouldrequireintensiveadministrationofthethreads,sortingtheminthewaitingqueue.Aslockingshouldbealowlatencyandhighperformanceaction,evenafewCPUclockcyclesaresignificant;therefore,thelocksdonotprovidethistypeoffairaccessbydefault.Notwastingtimewithfairnessinthreadschedulingisagoodapproach,incasethelockshaveonethreadwaiting.Themaingoaloflocksisnotschedulingthewaitingthreads,butratherpreventingparallelaccesstoresources.

Itislikeinashop.Ifthereissomebodyatthecashier,youwait.Itisalockbuiltinimplicitly.Itisnotaproblemifpeopledonotqueueupforthecashier,solongaslongthereisalmostalwaysonefree.However,whenthereareseveralqueuesbuiltupinfrontofthecashiers,thenhavingnoqueueandwaitingorderwillcertainlyleadtosomeverylongwaitingorderforsomeonewhoisslowtogetaccesstothecashier.Generally,thesolutionoffairnessandcreatingqueueofwaitingthreads(customers)isnotagoodsolution.Thegoodsolutionistoeliminatethesituationthatleadstowaitingqueues.Youcanemploymorecashiers,oryoucandosomethingtotallydifferentthatmakesthepeakloadsmaller.Inashop,youcangivediscounttodrivecustomerswhocomeinatoff-peakhours.Inprogramming,severaltechniquescanbeapplied,usually,dependingontheactualbusinesswecodeandfairschedulingoflocksisusuallyaworkaround.

ExecutorServiceExecutorServiceisaninterfaceintheJDK.AnimplementationoftheinterfacecanexecuteaRunnableorCallableclassinanasynchronousway.TheinterfaceonlydefinestheAPIfortheimplementationanddoesnotrequirethattheinvocationisasynchronousbut,inreality,thatisthemainpointimplementingsuchaservice.InvokingtherunmethodofaRunnableinterfaceinasynchronouswayissimplycallingamethod.Wedonotneedaspecialclassforthat.

TheRunnableinterfacedefinesonerunmethod.Ithasnoargumentsreturnsnovalueanddoesnotthrowanyexception.TheCallableinterfaceisparameterizedandtheonlymethoditdefines,call,hasnoargumentbutreturnsagenericvalueandmayalsothrowException.Inourcode,wewillimplementRunnableifwejustwanttorunsomething,andCallablewhenwewanttoreturnsomething.Bothoftheseinterfacesarefunctionalinterfaces,therefore,theyaregoodcandidatestobeimplementedusinglambda.

TohaveaninstanceofanimplementationofanExecutorService,wecanusetheutilityclassExecutors.ManytimeswhenthereisanXYZinterfaceintheJDK,therecanbeanXYZs(plural)utilityclassthatprovidesfactoryfortheimplementationsoftheinterface.Ifwewanttostartthet1taskmanytimes,wecandosowithoutcreatinganewThread.Weshouldusethefollowingexecutorservice:

publicclassThreadIntermingling{

staticclassMyThreadimplementsRunnable{

privatefinalStringname;

MyThread(Stringname){

this.name=name;

}

@Override

publicvoidrun(){

for(inti=1;i<1000;i++){

System.out.print(name+""+i+",");

}

}

}

publicstaticvoidmain(String[]args)

throwsInterruptedException,ExecutionException{

ExecutorServicees=Executors.newFixedThreadPool(2);

Runnablet1=newMyThread("t1");

Runnablet2=newMyThread("t2");

Future<?>f1=es.submit(t1);

Future<?>f2=es.submit(t2);

System.out.print("started");

f1.get();

f2.get();

System.out.println();

f1=es.submit(t1);

es.shutdown();

}

}

Thistime,wedonotgetanyexception.Instead,thet1taskrunssecondtime.Inthisexample,weareusingafixedsizethreadpoolthathastwoThreads.Aswewanttostartonlytwothreadssimultaneously,itisenough.Thereareimplementationsthatgrowandshrinkthesizeofthepooldynamically.Fixedsizepoolshouldbeusedwhenwewanttolimitthenumberofthethreadsorweknowfromsomeotherinformationsourcethenumberofthea-priorythreads.Inthiscase,itisagoodexperimenttochangethesizeofthepooltooneandseethatthesecondtaskwillnotstartinthiscaseuntilthefirstonefinishes.Theservicewillnothaveanotherthreadfort2andwillhavetowaituntiltheoneandonlyThreadinthepoolisfreed.

Whenwesubmitthetasktotheservice,itreturnsevenifthetaskcannotcurrentlybeexecuted.Thetasksareputinaqueueandwillstartexecutionassoonasthereisenoughresourcetostartthem.ThesubmitmethodreturnsaFutureobject,aswecanseeintheprecedingsample.

Itislikeaserviceticket.Youbringyourcartotherepairmechanic,andyougetaticket.Youarenotrequiredtostaythereuntilthecarisfixed,butatanytime,youcanaskifthecarisready.Allyouneedistheticket.Youcanalsodecidetowaituntilthecarisready.AFutureobjectisalsosomethinglikethat.Youdonotgetthevaluethatyouneed.Itwillbecalculatedasynchronously.However,thereisaFuturepromisethatitwillbethereandyourtickettoaccesstheobjectyouneedistheFutureobject.

WhenyouhaveaFutureobject,youcancalltheisDonemethodtoseeifitisready.Youcanstartwaitingforittocallgetwith,orwithout,sometimeout.Youcanalsocancelthetaskexecutingit,butinthatcase,theoutcomemaybequestionable.Justlike,incaseofyourcar,ifyoudecidetocancelthetask,youmaygetbackyourcarwiththemotordisassembled.Similarly,cancellingataskthatisnotpreparedforitmayleadtoresourceloss,openedandinaccessibledatabaseconnection(thisisapainfulmemoryforme,evenafter10years),orjustagarbledunusableobject.Prepareyourtaskstobecancelledordonotcancelthem.

Intheprecedingexample,thereisnoreturnvalueforFuturebecausewesubmittedaRunnableobjectandnotaCallableone.InthatcasethevaluepassedtotheFutureisnottobeused.Itisusuallynull,butthatisnothingtoleanon.

Thefinalandmostimportantthingthatmanydevelopersmiss,evenme,afternotwritingmultithreadJavaAPIusingcodeforyears,isshuttingdowntheExecutorService.TheExecutorServiceiscreatedandithasThreadelements.TheJVMstopswhenallnon-daemonthreadsarestopped.Itain'tovertillthefatladysings.

Athreadisadaemonthreadifitwassettobedaemon(invokingsetDaemon(true))beforeitwasstarted.Athreadisautomaticallydaemonofthestartingthreadisadaemonthread.DaemonthreadsarestoppedbytheJVMwhenallotherthreadsarefinishedandtheJVMwantstofinish.SomeofthethreadstheJVMexecutesitselfaredaemonthreads,butitislikelythatthereisnopracticaluseofcreatingdaemonthreadsinanapplicationprogram.

NotshuttingdowntheservicesimplypreventstheJVMfromstopping.Thecodewillhangafterthemainmethodfinishes.TotelltheExecutorServicethatthereisnoneedforthethreadsithas,wewillhavetoshutdowntheservice.Thecallwillonlystarttheshutdownandreturnimmediately.Inthiscase,wedonotwanttowait.TheJVMdoesanyway.Ifweneedtowait,wewillhavetocallawaitTermination.

ForkJoinPoolTheForkJoinPoolisaspecialExecutorServicethathasmethodstoexecuteForkJoinTaskobjects.Theseclassesareveryhandywhenthetaskthatwewanttoperformcanbesplitintomanysmalltasksandthentheresults,whentheyareavailable,aggregated.Usingthisexecutor,weneednotcareaboutthesizeofthethreadpoolandshuttingdowntheexecutor.Thesizeofthethreadpoolisadjustedtothenumberofprocessorsonthegivenmachinetohaveoptimalperformance.AstheForkJoinPoolisaspecialExecutorServicethatisdesignedforshortrunningtasks,itdoesnotexpectanytasktobetherelongerorbeingneededwhentherearenomoretaskstorun.Therefore,itisexecutedasadaemonthread;whentheJVMshutsdown,theForkJoinPoolautomaticallystopsandtheladydoesnotsinganymore.

Tocreateatask,theprogrammershouldextendeitherRecursiveTaskorRecursiveAction.Thefirstoneistobeusedwhenthereissomereturnvaluefromthetask,thesecondwhenthereisnocomputedvaluereturned.Theyarecalledrecursivebecausemanytimes,thesetaskssplittheproblemtheyhavetosolvesmallerproblemsandinvokethesetasksasynchronouslythroughthefork-joinAPI.

AtypicalproblemtobesolvedusingthisAPIisthequick-sort.IntheChapter3,OptimizingtheSort-MakingCodeProfessionalwecreatedtwoversionsofthequick-sortalgorithm.Oneusingrecursivecallsandonewithoutusingit.Wecanalsocreateanewone,which,insteadofcallingitselfrecursively,schedulethetasktobeexecuted,perhapsbyanotherprocessor.TheschedulingisthetaskoftheForkJoinPoolimplementationofExecutorService.

YoumayrevisitthecodeofQsort.javainChapter3,OptimizingtheSort-MakingCodeProfessional.HereistheversionthatisusingForkJoinPool:

publicclassFJQuickSort<E>{

finalprivateComparator<E>comparator;

finalprivateSwapperswapper;

publicFJQuickSort(Comparator<E>comparator,Swapperswapper){

this.comparator=comparator;

this.swapper=swapper;

}

publicvoidqsort(SortableCollection<E>sortable,

intstart,intend){

ForkJoinPoolpool=newForkJoinPool();

pool.invoke(newRASort(sortable,start,end));

}

privateclassRASortextendsRecursiveAction{

finalSortableCollection<E>sortable;

finalintstart,end;

publicRASort(SortableCollection<E>sortable,

intstart,intend){

this.sortable=sortable;

this.start=start;

this.end=end;

}

publicvoidcompute(){

if(start<end){

finalEpivot=sortable.get(start);

finalPartitioner<E>partitioner=

newPartitioner<>(comparator,swapper);

intcutIndex=partitioner.partition(

sortable,start,end,pivot);

if(cutIndex==start){

cutIndex++;

}

RecursiveActionleft=

newRASort(sortable,start,cutIndex-1);

RecursiveActionright=

newRASort(sortable,cutIndex,end);

invokeAll(left,right);

left.join();

right.join();

}

}

}

Wheneveryoucansplityourtasksintosubtaskssimilartothewayitwasdoneintheprecedingquick-sortexample,IrecommendthatyouuseForkJoinPoolasanExecutorService.YoucanfindgooddocumentationontheAPIandtheuseontheJavaDocdocumentationofOracle.

VariableaccessNowthatwecanstartthreadsandcreatecodethatrunsparallel,itistimetotalkalittlebitabouthowthesethreadscanexchangedatabetweeneachother.Atfirstglimpse,itseemsfairlysimple.Thethreadsusethesamesharedmemory;therefore,theyallcanreadandwriteallthevariablesthattheJavaaccessprotectionallowsthem.Thisistrue,exceptthatsomethreadsmayjustdecidenottoreadthememory.Afterall,iftheyhavejustrecentlyreadthevalueofsomevariable,whyreaditagainfromthememorytotheregistersifitwasnotmodified?Whowouldhavemodifiedthem?Let'sseethefollowingshortexample:

packagepackt.java9.by.example.thread;

publicclassVolatileDemonstrationimplementsRunnable{

privateObjecto=null;

privatestaticfinalObjectNON_NULL=newObject();

@Override

publicvoidrun(){

while(o==null);

System.out.println("oisnotnull");

}

publicstaticvoidmain(String[]args)

throwsInterruptedException{

VolatileDemonstrationme=newVolatileDemonstration();

newThread(me).start();

Thread.sleep(1000);

me.o=NON_NULL;

}

}

Whatwillhappen?Youmayexpectthatthecodestartsup,startsthenewthread,andoneminute,whenthemainthreadsetstheobjecttosomethingnotnull,willitstop?Itwillnot.

ItmaystoponsomeJavaimplementations,butinmostofthem,itwilljustkeepspinning.ThereasonforthatisthattheJITcompileroptimizesthecode.Itseesthattheloopdoesnothingandalsothatthevariablewilljustneverbenon-null.Itisallowedtoassumethatbecausethevariablesnotdeclaredvolatilearenotsupposedtobemodifiedbyanyotherthread,theJITiseligibletooptimize.IfwedeclaretheObjectovariabletobevolatile(withthevolatilekeyword),thenthecodewillstop.

Incaseyoutrytoremovethecalltosleep,thecodewillalsostop.This,however,doesnotfixtheissue.ThereasonisthatJIToptimizationkicksinonlyafterabout5000loopsofthecodeexecution.Beforethat,thecoderunsnaiveandstopsbeforetheoptimizationwilleliminatetheextraandregularlynotneededaccesstothenon-volatilevariable.

Ifthisissogruesome,thenwhydon'twedeclareallvariablestobevolatile?WhydoesJavanotdothatforus?Theanswerisspeed,andtounderstanditdeeper,wewilluseourmetaphor,theoffice,andthebureaucrat.

TheCPUheartbeatThesedaysCPUsrunon2to4GHzfrequencyprocessors.Itmeansthataprocessorgets2to4times109clocksignalstodosomethingeverysecond.Aprocessorcannotdoanyatomicoperationfasterthanthis,andalsothereisnoreasontocreateaclockthatisfasterthanwhataprocessorcanfollow.ItmeansthataCPUperformsasimpleoperation,suchasincrementingaregisterinhalforquarterofananosecond.Thisistheheartbeatoftheprocessor,andifwethinkofthebureaucratashumans,whotheyare,thenitisequivalenttoonesecond,approximately,ifandastheirheartbeat.

Processorshaveregistersandcachesonthechipondifferentlevels,L1,L2,andsometimesL3;thereismemory,SSD,disk,network,andtapesthatmaybeneededtoretrievedata.

AccessingdatathatisintheL1cacheisapproximately0.5ns.Youcangrabapaperthatisonyourdesk—halfofasecond.L2cacheis7ns.Thisisapaperinthedrawer.Youhavetopushthechairabitback,benditinasittingposition,pulloutthedrawer,takethepaper,pushthedrawerback,andraiseandputthepaperonthedesk;ittakes10seconds,giveortake.

Mainmemoryreadis100ns.Thebureaucratstandsup,goestothesharedfileatthewall,hewaitswhileotherbureaucratsarepullingtheirpapersorputtingtheirsback,selectsthedrawer,pullsitout,takesthepaper,andwalksbacktothedesk.Thisistwominutes.Thisisvolatilevariableaccesseverytimeyouwriteasinglewordonadocumentandithastobedonetwice.Oncetoread,andoncetowrite,evenifyouhappentoknowthatthenextthingyouwilldoisjustfillanotherfieldoftheformonthesamepaper.

Modernarchitectures,wheretherearenomultipleCPUsbutrathersingleCPUswithmultiplecores,areabitfaster.Onecoremaychecktheothercore'scachestoseeiftherewasanymodificationonthesamevariable,butthisspeedsthevolatileaccessto20nsorso,whichisstillamagnitudeslowerthannonvolatile.

Althoughtherestislessfocusedonmultithreadprogramming,itisworthmentioninghere,becauseitgivesgoodunderstandingonthedifferenttimemagnitudes.

ReadingablockfromanSSD(4Kblockusually)is150,000ns.Inhumanspeed,thatisalittlebitmorethan5days.ReadingorsendingsomethingtoaserveroverthenetworkontheGblocalEthernetis0.5ms,whichislikewaitingforalmostamonthforthemetaphoricbureaucrat.Ifthedataoverthenetworkisonaspinningmagneticdisk,thenseektimeaddsup(thetimeuntilthediskrotatessothatthepartofthemagneticsurfacegetsunderthereadinghead)to20ms.Itis,approximately,ayearinhumanterms.

IfwesendanetworkpacketovertheAtlanticontheInternet,itisapproximatelyis150ms.Itislike14years,andthiswasonlyonesinglepackage;ifwewanttosenddataovertheocean,itmaybesecondsthatcountuptohistorictimes,thousandsofyears.Ifwecountoneminuteforamachinetoboot,itisequivalenttothetimespanofourwholecivilization.

WeshouldconsiderthesenumberswhenwewanttounderstandwhattheCPUisdoingmostofthetime:itwaits.Additionally,italsohelpscoolyournerveswhenyouthinkaboutthespeedofareal-lifebureaucrat.Theyarenotthatslowafterall,ifweconsidertheirheartbeat,whichimpliestheassumption

thattheyhaveaheart.However,let'sgobacktoreallife,CPUs,andL1,L2cachesandvolatilevariables.

VolatilevariablesLet'smodifythedeclarationoftheovariableinoursamplecodeasfollows:

privatevolatileObjecto=null;

Theprecedingcoderunsfineandstopsafterasecondorso.AnyJavaimplementationhastoguaranteethatmultiplethreadscanaccessvolatilefieldsandthevalueofthefieldisconsistentlyupdated.Thisdoesnotmeanthatvolatiledeclarationwillsolveallsynchronizationissues,butguaranteesthatthedifferentvariablesandtheirvaluechangerelationsareconsistent.Forexample,let'sconsiderwehavethefollowingtwofieldsincrementedinamethod:

privatevolatileinti=0,j=0;

publicvoidmethod(){

i++;j++;

}

Intheprecedingcode,readingiandjfromanotherthreadwillneverresultani>j.Withoutthevolatiledeclaration,thecompilerisfreetoreorganizetheexecutionoftheincrementoperationsifitneedsandthus,itwillnotguaranteethatanasynchronousthreadreadsconsistentvalues.

SynchronizedblockDeclaringvariablesarenottheonlytooltoensuretheconsistencybetweenthreads.ThereareothertoolsintheJavalanguageandoneofthemisthesynchronizedblock.Thesynchronizedkeywordispartofthelanguageanditcanbeusedinfrontofamethodoraprogramblockinsideamethod.

EveryobjectintheJavaprogramhasamonitorthatcanbelockedandunlockedbyanyrunningthread.Whenathreadlocksamonitor,itissaidthatthatthreadholdsthelock,andnotwothreadscanholdthelockofamonitoratatime.Ifathreadtriestolockamonitorthatisalreadylocked,itgetsBLOCKEDuntilthemonitorisreleased.Asynchronizedblockstartswiththesynchronizedkeyword,andthenanobjectinstancespecifiedbetweenparenthesesandtheblockcomes.Thefollowingsmallprogramdemonstratesthesynchronizedblock:

publicclassSynchronizedDemoimplementsRunnable{

publicstaticfinalintN=1000;

publicstaticfinalintMAX_TRY=1_000_000;

privatefinalcharthreadChar;

privatefinalStringBuffersb;

publicSynchronizedDemo(charthreadChar,StringBuffersb){

this.threadChar=threadChar;

this.sb=sb;

}

@Override

publicvoidrun(){

for(inti=0;i<N;i++){

synchronized(sb){

sb.append(threadChar);

sleep();

sb.append(threadChar);

}

}

}

privatevoidsleep(){

try{

Thread.sleep(1);

}catch(InterruptedExceptionignored){}

}

publicstaticvoidmain(String[]args){

booleanfailed=false;

inttries=0;

while(!failed&&tries<MAX_TRY){

tries++;

StringBuffersb=newStringBuffer(4*N);

newThread(newSynchronizedDemo('a',sb)).start();

newThread(newSynchronizedDemo('b',sb)).start();

failed=sb.indexOf("aba")!=-1||

sb.indexOf("bab")!=-1;

}

System.out.println(failed?

"failedafter"+tries+"tries":"notfailed");

}

}

Thecodestartstwodifferentthreads.OneofthethreadsappendsaatotheStringBuffer.Theotheroneappendsbb.Thisappendingisdoneintwoseparatestepswithasleepinbetween.ThesleepisneededtoavoidJITthatoptimizesthetwoseparatestepsintoone.Eachthreadexecutestheappend1000timeseachtimeappendingaorbtwotimes.AsthetwoappendsoneaftertheotherareinsideasynchronizedblockitcannothappenthatanabaorbabsequencegetsintotheStringBuffer.Whileonethreadexecutesthesynchronizedblock,theotherthreadcannotexecuteit.

IfIremovethesynchronizedblock,thentheJVMIusedtotestJavaHotSpot(TM)64-BitServerVM(build9-ea+121,mixedmode)printsoutthefailurewithatry-countaroundafewhundreds.

Itclearlydemonstrateswhatthesynchronizationmeans,butitdrawsourattentiontoanotherimportantphenomena.Theerroroccursonlyaroundeveryfewhundredthousandexecutionsonly.Itisextremelyrare,eventhoughthisexamplewasfurnishedtodemonstratesuchamishap.Ifabugappearssorare,itisextremelyhardtoreproduceand,evenmore,todebugandfix.Mostofthesynchronizationerrorsmanifestinmysteriouswaysandtheirfixingusuallyistheresultofmeticulouscodereviewratherthandebugging.Therefore,itisextremelyimportanttoclearlyunderstandthetruenatureofJavamultithreadbehaviorbeforestartingcommercialmultithreadapplication.

Thesynchronizedkeywordcanalsobeusedinfrontofamethod.Inthiscase,theobjecttoacquirethelockofistheobject.Incaseofastaticmethod,thesynchronizationisdoneonthewholeclass.

WaitandnotifyTherearefivemethodsimplementedintheclassObjectthatcanbeusedtogetfurthersynchronizationfunctionality:waitwiththreedifferenttimeoutargumentsignature,notify,andnotifyAll.Tocallwait,thecallingthreadshouldhavethelockoftheObjectonwhichwaitisinvoked.Itmeansthatyoucanonlyinvokewaitfrominsideasynchronizedblock,andwhenitiscalled,thethreadgetsBLOCKEDandreleasesthelock.WhenanotherthreadcallsnotifyallonthesameObject,thethreadgetsintotheRUNNABLEstate.Itcannotcontinueexecutionimmediatelyasitcannotgetthelockontheobject.ThelockisheldatthatmomentbythethreadthatjustcallednotifyAll.However,sometimeaftertheotherthreadreleases,thelockgetsoutofthesynchronizedblock,andthewaitingthreadcontinuestheexecution.

Iftherearemorethreadswaitingonanobject,allofthemgetoutoftheBLOCKEDstate.Thenotifymethodwakesonlyoneofthewaitingthreads.Thereisnoguaranteewhichthreadisawakened.

Thetypicaluseofwait,notify,andnotifyAlliswhenoneormorethreadsarecreatingObjectsthatareconsumedbyotherthread,orthreads.Thestoragewheretheobjectstravelbetweenthethreadsissomekindofqueue.Theconsumerwaitsuntilthereissomethingtoreadfromthequeue,andtheproducerputstheobjectsintothequeueoneaftertheother.Theproducernotifiestheconsumerswhenitstoressomethingintothequeue.Ifthereisnoroomleftinthequeue,theproducerhastostopandwaituntilthequeuehassomespace.Inthiscase,theproducercallsthewaitmethod.Towaketheproducerup,theconsumercallsnotifyAllwhenitreadssomething.

Theconsumerconsumestheobjectsfromthequeueinaloopandcallswaitonlyifthereisnothingtobereadfromthequeue.WhentheproducercallsnotifyAll,andthereisnoconsumerwaiting,thenotificationisjustignored.Itfliesaway,butthisisnotaproblem;consumersarenotwaiting.WhentheconsumerconsumesanobjectandcallsnotifyAllandthereisnoproducerwaiting,thesituationisthesame.Itisnotaproblem.

Itcannothappenthattheconsumerconsumes,callsnotifyAll,andafterthenotificationwasflyingintheairnotfindinganywaitingproducer,aproducerstartstowait.Thiscannothappenbecausethewholecodeisinasynchronizedblockanditensuresthatnoproducerisinthecriticalsection.Thisisthereasonwhywait,notify,andnotifyAllcanonlybeinvokedwhenthelockoftheObjectclassisacquired.

Iftherearemanyconsumers,whichareexecutingthesamecodeandareequivalentlygoodinconsumingtheobjects,thenitisanoptimizationtocallnotifyinsteadofnotifyAll.Inthatcase,notifyAllwilljustawakeallconsumerthreadsandall,buttheluckyonewillrecognizethattheywerewokenupbutsomebodyelsealreadygotawaywiththebait.

IrecommendthatyoupracticeatleastoncetoimplementablockingqueuethatcanbeusedtopassObjectsbetweenthreads.However,neverusethatcodeinproduction:startingwithJava1.5,thereareimplementationsoftheBlockingQueueinterface.Useonethatfitsyourneeds.Wewilltoo,inourexamplecode.

FeelluckythatyoucancodeinJava9.IstartedusingJavaprofessionallywhenitwas1.4andonceIhadtoimplementablockingqueue.Lifegetsjustbetterandeasierallthe

timewithJava.

Inprofessionalcode,weusuallyavoidusingsynchronizedmethodsorblocksandvolatilefieldsaswellasthewaitandnotifymethods,notifyAlltoo,ifpossible.Wecanuseasynchronouscommunicationbetweenthreads,orpassthewholemultithreadingtotheframeworkforhandling.Synchronizedandvolatilecannotbeavoidedinsomespecialcaseswhentheperformanceofthecodeisimportant,orwecannotfindabetterconstruct.Sometimes,thedirectsynchronizationonspecificcodeanddatastructuresismoreefficientthantheapproachdeliveredbyJDKclasses.Itistonote,however,thatthoseclassesalsousetheselow-levelsynchronizationconstructs,soitisnotmagichowtheywork;andtodevelopyourself,youcanlookintothecodeoftheJDKclassesbeforeyouwanttoimplementyourownversion.Youwillrealizethatitisnotthatsimpletoimplementthesequeues;thecodeoftheclassesisnotcomplexandcompoundwithoutreason.Ifyoufindthecodesimple,itmeansthatyouareseniorenoughtoknowwhatnottoreimplement.Or,perhaps,youdonotevenrealizewhatcodeyouread.

LockLocksarebuiltinJava;everyObjecthasalockthatathreadmayacquirewhenitentersasynchronizedblock.Wediscussedthatalready.Insomeprogrammingcode,therearesituationswhenthiskindofstructureisnotoptimal.

Insomesituations,thestructureoflocksmaybelineduptoavoiddeadlock.ItmaybeneededtoacquirelockAbeforeBandtoacquireBbeforeC.However,Ashouldbereleasedassoonaspossible,nottopreventaccesstoresourceprotectedbylockD,butalsoneedinglockAbeforeit.Incomplexandhighlyparallelstructures,thelocksarestructuredmanytimesintotreeswhereaccessingaresourceathreadshouldclimbdownalongthetreetoaleafrepresentingtheresource.Inthisclimbing,thethreadgetsholdofalockonanode,thenalockonanodebelowit,andthenreleasesthelockabove,justlikearealclimberdescending(orclimbingupifyouimaginethetreewiththeleafsatthetop,whichismorerealistic,neverthelessgraphsusuallyshowtreesupsidedown).

Youcannotleaveasynchronizedblockremaininginanotherthatisinsidethefirstone.Synchronizedblocksarenested.Thejava.util.concurrent.LockinterfacedefinesmethodstohandlethatsituationandtheimplementationsarealsothereintheJDKtobeusedinourcode.Whenyouhavealock,youcancallthemethodslockandunlock.Theactualorderisinyourhandandyoucanwritethefollowinglineofcodetogetthelockingsequence:

a.lock();b.lock();a.unlock();c.lock()

Thefreedom,however,comeswithresponsibilityaswell.Thelocksandunlocksarenottiedtotheexecutionsequenceofthecode,likeincaseofsynchronizedblock,anditmaybeveryeasytocreatecodethatinsomecasejustlosesalocknotunlockingitrenderingsomeresourceunusable.Thesituationissimilartoamemoryleak:youwillallocate(lock)somethingandforgettorelease(unlock)it.Afterawhile,theprogramwillrunoutofresource.

Mypersonalrecommendationistoavoidusinglocksifpossibleandusehigher-levelconstructsandasynchronouscommunicationsbetweenthreads,suchasblockingqueues.

Condition

Thejava.util.concurrent.Conditioninterfaceinfunctionalityissimilartothebuilt-inwait,notify,andnotifyAll.AnyimplementationofLockshouldcreatenewConditionobjectsandreturnasaresulttotheinvocationofthenewConditionmethod.WhenthethreadhasaCondition,itcancallawait,signal,andsignalAllwhenthethreadhasthelockthatcreatedtheconditionobject.

ThefunctionalityisverysimilartothemethodsofObjectmentioned.However,thebigdifferenceisthatyoucancreatemanyConditionforasingleLockandtheywillworkindependentofeachother,butnotindependentoftheLock.

ReentrantLock

ReentrantLockisthesimplestimplementationoftheinterfacelockintheJDK.Therearetwowaystocreatethistypeoflock:withandwithoutfairnesspolicy.IftheReentrantLock(Booleanfair)constructoriscalledwiththetrueargument,thenthelockwillbeassignedtothethreadthatiswaitingforthelockthelongesttimeincasetherearemanythreadswaiting.Thiswillavoidathreadmadetowaitforinfinitetimeandstarving.

ReentrantReadWriteLockThisclassisanimplementationofReadWriteLock.ReadWriteLockisalockthatcanbeusedforparallelreadaccessandexclusivewriteaccess.Itmeansthatseveralthreadscanreadtheresourceprotectedbythelock,butwhenathreadwritestheresource,nootherthreadcangetaccesstoit,notevenreadduringthatperiod.AReadWriteLockissimplytwoLockobjectsreturnedbythereadLockandwriteLockmethods.TogetreadaccessonReadWriteLock,thecodehastoinvokemyLock.readLock().lock(),andtogetaccesstowritelock,myLock.writeLock().lock().Acquiringoneofthelocksandreleasingitintheimplementationiscoupledwiththeotherlock.Toacquireawritelock,nothreadshouldhaveanactivereadlock,forexample.

Thereareseveralintricaciesintheuseofthedifferentlock.Forexample,youcanacquireareadlock,butyoucannotgetawritelocksolongasyouhavethereadlock.Youhavetoreleasethereadlockfirsttoacquireawritelock.Thisisjustoneofthesimpledetails,butthisistheonethatnoviceprogrammershavetroublewithmanytimes.Whyisitimplementedthisway?Whyshouldtheprogramgetawritelock,whichismoreexpensive—insenseofhigherprobabilitylockingotherthreads—whenitstillisnotsurethatitwantstowritetheresource?Thecodewantstoreaditand.basedonthecontent.itmaylaterdecidethatitwantstowriteit.

Theissueisnotwiththeimplementation.Thedevelopersofthelibrarydecidedthisrule,notbecausetheyjustlikeditthatwayorbecausetheywereawareofparallelalgorithmsanddeadlockpossibilities.Whentwothreadshavereadlockandeachdecidestoupgradethelocktowritelock,thentheywouldintrinsicallycreateadeadlock.Eachwouldholdthereadlockwaitingforthewriteandnoneofthemwouldgetitever.

Ontheotherend,youcandowngradeawritelocktoareadlockwithoutriskingthatinthemeantimesomebodyacquiresawritelockandmodifiestheresource.

AtomicclassesAtomicclassesencloseprimitivevaluesintoobjectsandprovideatomicoperationsonthem.Wediscussedraceconditionsandvolatilevariables.Forexample,ifwehaveanintvariabletobeusedasacounterandwewanttoassignauniquevaluetoobjectsthatweworkwith,wecanincrementthevalueandusetheresultasauniqueID.However,whenmultiplethreadsusethesamecode,wecannotbesureaboutthevaluewereadaftertheincrement.Itmayhappenthatanotherthreadalsoincrementedthevalueinthemeantime.Toavoidthat,wewillhavetoenclosetheincrementandtheassignmentoftheincrementedvaluetoanobjectintoasynchronizedblock.ThiscanalsobedoneusingAtomicInteger.

IfwehaveavariableofAtomicInteger,thencallingincrementAndGetincrementsthevalueofintenclosedintheclassandreturnstheincrementedvalue.Whydoitinsteadofusingsynchronizedblock?ThefirstansweristhatifthefunctionalityisthereintheJDK,thenusingitislesslinethanimplementingitagain.DevelopersmaintainingthecodeyoucreateareexpectedtoknowtheJDKlibrariesbuthavetostudyyourcode,andthistakestimeandtimeismoney.

Theotherreasonisthattheseclassesarehighlyoptimizedand,manytimes,theyimplementthefeaturesusingplatformspecificnativecodethatgreatlyoverperformstheversionwecanimplementusingsynchronizedblocks.Worryingaboutperformancetooearlyisnotgood,butparallelalgorithmsandsynchronizationbetweenthreadsareusuallyusedwhenperformanceiscrucial;thus,thereisagoodchancethattheperformanceofthecodeusingtheatomicclassesisimportant.

Inthejava.util.concurrent.atomicpackage,thereareseveralclasses,AtomicInteger,AtomicBoolean,AtomicLong,andAtomicReferenceamongthem.Theyallprovidemethodsthatarespecifictotheencapsulatedvalue.

Themethod,whichisimplementedbyeveryatomicclass,iscompareAndSet.Thisisaconditionalvalue-settingoperationthathasthefollowingformat:

booleancompareAndSet(expectedValue,updateValue);

Whenitisappliedonanatomicclass,itcomparestheactualvaluewiththeoneexpectedValue,andiftheyarethesame,thenitsetsthevaluetoupdateValue.Ifthevaluewasupdated,themethodreturnstrueanditdoesallthisinanatomicaction.

Youmayaskthequestionthatifthismethodisinalloftheseclasses,whyistherenoInterfacedefiningthismethod?Thereasonforthisisthattheargumenttypesaredifferentbasedontheencapsulatedtype,andthesetypesareprimitives.Asprimitivescannotbeusedasgenerictypes,notevenagenericinterfacecanbedefined.IncaseofAtomicXXXArray,themethodhasanextrafirstargument,whichistheindexofthearrayelementhandledinthecall.

Thevariablesencapsulatedarehandledthesamewayasvolatile,asfarasthereorderingisconcerned,buttherearespecialmethodsthatloosentheconditionsabittobeusedwhenpossible,andperformanceiskey.

Thegeneraladviceistoconsiderusingatomicclasses,ifthereisoneusable,andyouwillfindyourself

creatingasynchronizedblockforcheck-and-set,atomicincrement,oradditionoperations.

BlockingQueue

BlockingQueueisaninterfacethatextendsthestandardQueueinterfacewithmethodsthataresuitabletobeusedbymultithreadapplications.Anyimplementationofthisinterfaceprovidesmethodsthatallowdifferentthreadstoputelementintothequeue,pullelementsoffthequeue,andwaitforelementsthatareinthequeue.

Whenthereisanewelementthatistobestoredinthequeue,youcanaddit,offerit,orputit.Thesearethenameofthemethodsthatstoreelementsandtheydothesamething,butabitdifferently.Theaddelementthrowsanexceptionifthequeueisfullandthereisnoroomfortheelement.Theofferelementdoesnotthrowexceptionbutreturnseithertrueorfalse,basedonthesuccess.Ifitcanstoretheelementinthequeue,itreturnstrue.Thereisalsoaversionofofferthatspecifiesatimeout.Thatversionofthemethodwaits,andreturnsonlyfalseifitcannotstorethevalueintothequeueduringtheperiod.Theputelementisthedumbestversion;itwaitsuntilitcandoitsjob.

Whentalkingaboutavailableroominaqueue,donotgetpuzzledandmixitwithgeneralJavamemorymanagement.Ifthereisnomorememory,andthegarbagecollectorcanalsonotreleaseany,youwillcertainlygetOutOfMemoryError.Exceptionisthrownbyadd,andfalseisreturnedbyoffer,whenthequeuelimitsarereached.SomeoftheBlockingQueueimplementationscanlimitthenumberofelementsthatcanbestoredatatimeinaqueue.Ifthatlimitisreached,thenthequeueisfullandcannotacceptmoreelements.

FetchingelementsfromaBlockingQueueimplementationalsohasfourdifferentways.Inthisdirection,thespecialcaseiswhenthequeueisempty.Inthatcase,removethrowsanexceptioninsteadofreturningtheelement,pollreturnsnullifthereisnoelement,andtakejustwaitsuntilitcanreturnanelement.

Finally,therearetwomethodsinheritedfromtheinterfaceQueuesthatdonotconsumetheelementfromthequeueonlylookat.Theelementreturntheheadofthequeueandthrowsanexceptionifthequeueisempty,andpeekreturnsnullifthereisnoelementinthequeue.Thefollowingtablesummarizestheoperationsborrowedfromthedocumentationoftheinterface:

Throwsexception Specialvalue Blocks Timesout

Insert add(e) offer(e) put(e) offer(e,time,unit)

Remove remove() poll() take() poll(time,unit)

Examine element() peek() notapplicable notapplicable

LinkedBlockingQueue

ThisisanimplementationoftheBlockingQueueinterface,whichisbackedupbyalinkedlist.Thesizeofthequeueisnotlimitedbydefault(tobeprecise,itisInteger.MAX_VALUE)butitcanoptionallybelimitedinaconstructorargument.Thereasontolimitthesizeinthisimplementationistoaidtheusewhentheparallelalgorithmperformsbetterwithlimitedsizequeue,buttheimplementationdoesnothaveanyrestrictiononthesize.

LinkedBlockingDeque

ThisisthesimplestimplementationoftheBlockingQueueandalsoitssubinterfaceBlockingDeque.Aswediscussedinthepreviouschapter,aDequeisadouble-endedqueuethathasadd,remove,offer,andsoon,typeofmethodsintheformofxxxFirstandxxxLasttodotheactwithoneortheotherendofthequeue.TheDequeinterfacedefinesgetFirstandgetLastinsteadofconsistentlynamingelementFirstandelementLast,sothisissomethingyoushouldgetusedto.Afterall,theIDEshelpwithautomaticcodecompletionsothisshouldnotbeareallybigproblem.

ArrayBlockingQueue

ArrayBlockingQueueimplementstheBlockingQueueinterface,hencetheQueueinterface.Thisimplementationmanagesaqueuewithfixedsizeelements.ThestorageintheimplementationisanarrayandtheelementsarehandledinaFIFOmanner:first-infirst-out.ThisistheclassthatwewillalsouseintheparallelimplementationofMastermindforthecommunicationbetweenthebossandthesubordinatedbureaucrats.

LinkedTransferQueue

TheTransferQueueinterfaceisextendingBlockingQueueandtheonlyimplementationofitintheJDKisLinkedTransferQueue.ATransferQueuecomeshandywhenathreadwantstohandoversomedatatoanotherthreadandneedstobesurethatsomeotherthreadtakestheelement.ThisTransferQueuehasamethodtransferthatputsanelementonthequeuebutdoesnotreturnuntilsomeotherthreadremoves(orpolls)it.Thatwaytheproducingthreadcanbesurethattheobjectputonthequeueisinthehandsofanotherprocessingthreadanddoesnotwaitinthequeue.ThemethodtransferalsohasaformattryTransferinwhichyoucanspecifysometimeoutvalue.Ifthemethodtimesouttheelementisnotputintothequeue.

IntervalGuesserWediscussedthedifferentJavalanguageelementsandJDKclassesthatareallavailabletoimplementparallelalgorithms.Now,wewillseehowtousetheseapproachestoimplementtheparallelguesserfortheMasterrmindgame.

TheclassthatperformsthecreationoftheguessesisnamedIntervalGuesser.ItcreatestheguessesbetweenastartandanendguessandsendsthemtoaBlockingQueue.TheclassimplementsRunnablesoitcanruninaseparateThread.ThepuristimplementationwillseparatetheRunnablefunctionalityfromtheintervalguessing,butasthewholeclassishardlymorethan50lines,itisforgivablesinimplementingthetwofunctionalitiesinasingleclass.

publicclassIntervalGuesserextendsUniqueGuesserimplementsRunnable{

privatefinalGuessstart;

privatefinalGuessend;

privateGuesslastGuess;

privatefinalBlockingQueue<Guess>guessQueue;

publicIntervalGuesser(Tabletable,Guessstart,Guessend,BlockingQueue<Guess>guessQueue){

super(table);

this.start=start;this.end=end;

this.lastGuess=start;

this.guessQueue=guessQueue;

nextGuess=start;

}

@Override

publicvoidrun(){

Guessguess=guess();

try{

while(guess!=Guess.none){

guessQueue.put(guess);

guess=guess();

}

}catch(InterruptedExceptionignored){

}

}

@Override

protectedGuessnextGuess(){

Guessguess;

guess=super.nextGuess();

if(guess.equals(end)){

guess=Guess.none;

}

lastGuess=guess;

returnguess;

}

publicStringtoString(){

return"["+start+","+end+"]";

}

}

TheimplementationisverysimpleasmostofthefunctionalityisalreadyimplementedintheabstractGuesserclass.ThemoreinterestingcodeistheonethatinvokedtheIntervalGuesser.

ParallelGamePlayerTheParallelGamePlayerclassimplementsthePlayerinterfacethatdefinestheplaymethod:

@Override

publicvoidplay(){

Tabletable=newTable(NR_COLUMNS,manager);

Secretsecret=newRandomSecret(manager);

GuesssecretGuess=secret.createSecret(NR_COLUMNS);

Gamegame=newGame(table,secretGuess);

finalIntervalGuesser[]guessers=createGuessers(table);

startAsynchronousGuessers(guessers);

finalGuesserfinalCheckGuesser=newUniqueGuesser(table);

try{

while(!game.isFinished()){

finalGuessguess=guessQueue.take();

if(finalCheckGuesser.guessMatch(guess)){

game.addNewGuess(guess);

}

}

}catch(InterruptedExceptionie){

}finally{

stopAsynchronousGuessers(guessers);

}

}

ThismethodcreatesaTable,aRandomSecretthatcreatestheguessusedasasecretinarandomway,aGameobject,IntervalGuessers,andaUniqueGuesser.TheIntervalGuessersarethebureaucrats;theUniqueGuesseristhebosswhocrosscheckstheguessesthattheIntervalGuesserscreate.ThemethodstartsofftheasynchronousguessersandthenreadstheguessesinaloopfromthemandputsthemonthetableiftheyareOKuntilthegamefinishes.Attheendofthemethod,inthefinallyblock,theasynchronousguessersarestopped.

ThestartandthestopmethodfortheasynchronousguessersuseExecutorService.

privateExecutorServiceexecutorService;

privatevoidstartAsynchronousGuessers(

IntervalGuesser[]guessers){

executorService=Executors.newFixedThreadPool(nrThreads);

for(IntervalGuesserguesser:guessers){

executorService.execute(guesser);

}

}

privatevoidstopAsynchronousGuessers(

IntervalGuesser[]guessers){

executorService.shutdown();

guessQueue.drainTo(newLinkedList<>());

}

Thecodeisquitestraightforward.Theonlythingthatmayneedmentionisthatthequeueoftheguessesisdrainedintoacollectionthatwedonotuseafterward.ThisisneededtohelpanyIntervalGuesserthatiswaitingwithasuggestedguessinhand,tryingtoputitintothequeue.Whenwedrainthequeue,theguesserthreadreturnsfromthemethodputintheguessQueue.put(guess);lineinIntervalGuesserandcancatchtheinterrupt.TherestofthecodedoesnotcontainanythingthatwouldberadicallydifferentfromwhatwehavealreadyseenandyoucanfinditonGitHub.

Thelastquestionthatwestillwanttodiscussinthischapterishowmuchspeeddidwegainmakingthecodeparallel?

MicrobenchmarkingMicrobenchmarkingismeasuringtheperformanceofasmallcodefragment.Whenwewanttooptimizeourcode,wewillhavetomeasureit.Withoutmeasurement,codeoptimizationislikeshootingblindfolded.Youwillnothitthetarget,butyoulikelywillshootsomebodyelse.

Shootingisagoodmetaphorbecauseyoushouldusuallynotdoit,butwhenyoureallyhavetothenyouhavenochoice.Ifthereisnoperformanceissueandthesoftwaremeetstherequirements,thenanyoptimization,includingspeedmeasurement,isawasteofmoney.Thisdoesnotmeanthatyouareencouragedtowriteslowandsloppycode.Whenwemeasureperformance,wewillcompareitagainstarequirement,andtherequirementisusuallyontheuserlevel.Somethinglike,theresponsetimeoftheapplicationshouldbelessthan2seconds.Todosuchameasurement,weusuallycreateloadtestsinatestenvironmentandusedifferentprofilingtoolsthattelluswhatisconsumingthemosttimeandwhereweshouldoptimize.Manytimes,itisnotonlyJavacode,butconfigurationoptimization,usinglargerdatabaseconnectionpool,morememory,andsimilarthings.

Microbenchmarkingisadifferentstory.ItisabouttheperformanceofasmallJavacodefragmentand,assuch,closertotheJavaprogramming.

Itisrarelyused,andbeforestartingtodoamicrobenchmarkforrealcommercialenvironment,wewillhavetothinktwice.Microbenchmarkisaluringtooltooptimizesomethingsmallwithoutknowingifitisworthoptimizingthatcode.Whenwehaveahugeapplicationthathasseveralmodulesrunonseveralservers,howcanwebesurethatimprovingsomespecialpartoftheapplicationdrasticallyimprovestheperformance?Willitpaybackinincreasedrevenuethatgeneratessomuchprofitthatwillcoverthecostweburnedintotheperformancetestinganddevelopment?Statistically,almostsurethatsuchanoptimizationincludingmicrobenchmarkingwillnotpayoff.

OnceIwasmaintainingthecodeofasenior'scolleague.Hecreatedahighlyoptimizedcodetorecognizeconfigurationkeywordsthatwerepresentinafile.Hecreatedaprogramstructurethatrepresentedadecisiontreebasedonthecharactersinthekeystring.Iftherewasakeywordintheconfigurationfilethatwasmisspelled,thecodethrewanexceptionattheveryfirstcharacterwhereitcoulddecidethatthekeywordcouldnotbecorrect.Toinsertanewkeyword,itneededtogetthroughthecodestructuretofindtheoccasioninthecodewherethenewkeywordwasfirstdifferentfromalreadyexistingonesandextendthedeeplynestedif/elsestructures.Toreadthelistofthehandledkeywordswaspossiblefromthecommentsthatlistedallthekeywordsthathedidnotforgettodocument.Thecodewasworkingblazinglyfast,probablysavingafewmillisecondsoftheservletapplicationstartuptime.Theapplicationwasstarteduponlyaftersystemmaintenanceeveryfewmonth.Youfeeltheirony,don'tyou?Seniorityisnotalwaysthenumberofyears.Luckyonescansavetheirinnerchild.

Sowhentousemicrobenchmarking?Icanseetwoareas:

YouidentifiedthecodesegmentthateatsmostoftheresourcesinyourapplicationandtheimprovementcanbetestedbymicrobenchmarksYoucannotidentifythecodesegmentthatwilleatmostoftheresourcesinanapplicationbutyoususpectit

Thefirstistheusualcase.Thesecondiswhenyoudevelopalibrary,andyoujustdonotknowalltheapplicationsthatwilluseit.Inthiscase,youwilltrytooptimizethepartthatyouthinkisthemostcrucialformostoftheimagined,suspectedapplications.Eveninthatcase,itisbettertotakesomesampleapplicationsthatarecreatedbyusersofyourlibraryandcollectsomestatisticsabouttheuse.

Whyshouldwetalkaboutmicrobenchmarkingindetails?Whatarethepitfalls?Benchmarkingisanexperiment.ThefirstprogramsIwrotewasaTIcalculatorcodeandIcanjustcountthenumberofstepstheprogrammadetofactortwolarge(10digitsthosedays)primenumbers.Evenatthattime,IwasusinganoldRussianstopwatchtomeasurethetime,beinglazytocalculatethenumberofsteps.Experimentandmeasurementwaseasier.

Today,youcannotcalculatethenumberofstepstheCPUmakesevenifyouwanted.Therearesomanysmallfactorsthatmaychangetheperformanceoftheapplicationsthatareoutofcontroloftheprogrammer,whichmakesitimpossibletocalculatethesteps.Wehavethemeasurementleftforus,andwewillgainalltheproblemsofmeasurements.

Whatisthebiggestproblem?Weareinterestedinsomething,sayX,andweusuallycannotmeasurethat.So,wewillmeasureYinsteadandhopethatthevaluesofYandXarecoupledtogether.Wewanttomeasurethelengthoftheroom,butinsteadwemeasurethetimeittakesforthelaserbeamtotravelfromoneendtotheother.Inthiscase,thelengthXandthetimeYarestronglycoupled.Manytimes,XandYonlycorrelatemoreorless.Mostofthetimes,whenpeopledomeasurement,theXandYvalueshavenorelationtoeachotheratall.Still,peopleputtheirmoneyandmoreondecisionsbackedbysuchmeasurements.

Microbenchmarkingisnodifferent.Thefirstquestionishowtomeasuretheexecutiontime?SmallcoderunsshorttimesandSystem.currentTimeMillis()mayjustreturnthesamevaluewhenthemeasurementstartsandwhenitends,becausewearestillinthesamemillisecond.Eveniftheexecutionis10ms,theerrorofthemeasurementisstillatleast10%purelybecauseofthequantizationofthetimeaswemeasure.Luckily,thereisSystem.nanoTime().Butisthere?Justbecausethenamesaysitreturnsthenumberofnanosecondsfromaspecificstarttime,itdoesnotnecessarilymeanitreallycan.

ItverymuchdependsonthehardwareandtheimplementationofthemethodintheJDK.Itiscallednanobecausethisistheprecisionthatwecannotcertainlyreach.Ifitwasmicroseconds,thensomeimplementationmaybelimitedbythedefinition,evenifonthespecifichardware,thereisamorepreciseclock.However,thisisnotonlytheprecisionofanavailablehardwareclock;itisabouttheprecisionofthehardware.

Let'sremembertheheartbeatofthebureaucrats,andthetimeneededtoreadsomethingfrommemory.Callingamethod,suchasSystem.nanoTime(),islikeaskingthebellboyinahoteltorundownfromthesecondfloortothelobbyandpeekouttolookattheclockonthetowerontheothersideoftheroad,comeback,andtellsecondsprecisionwhatthetimewasitwhenweasked.Nonsense.Weshouldknowthe

precisionoftheclockonthetowerandthespeedofthebellboyrunningfromthefloortothelobbyandback.ThisisabitmorethanjustcallingnanoTime.Thisiswhatamicrobenchmarkingharnessdoesforus.

TheJavaMicrobenchmarkingHarness(JMH)isavailableforsometimeasalibrary.ItisdevelopedbyOracleandusedtotunetheperformanceofsomecoreJDKclasses,andwithJava9,theseperformancemeasurementsandresultsbecomepartofthedistributedJDK.ThisisgoodnewsforthosewhodevelopJavaplatformfornewhardware,butalsofordevelopers,becauseitmeansthattheJMHisandwillbesupportedbyOracle.

"JMHisaJavaharnesstobuild,run,andanalyzenano/micro/milli/macrobenchmarkswritteninJavaandotherlanguagestargetingtheJVM."(quotefromtheofficialsiteofJMH,http://openjdk.java.net/projects/code-tools/jmh/).

Youcanrunjmhasaseparateprojectindependentfromtheactualprojectyoumeasure,oryoucanjuststorethemeasurementcodeinaseparatedirectory.Theharnesswillcompileagainsttheproductionclassfilesandwillexecutethebenchmark.Theeasiestway,asIsee,istousetheGradleplugintoexecuteJMH.Youcanstorethebenchmarkcodeinadirectorycalledjmh(thesamelevelasmainandtest)andcreateamainthatcanstartthebenchmark.

TheGradlebuildscriptisextendedwiththefollowinglines:

buildscript{

repositories{

jcenter()

}

dependencies{

classpath"me.champeau.gradle:jmh-gradle-plugin:0.2.0"

}

}

applyplugin:"me.champeau.gradle.jmh"

jmh{

jmhVersion='1.13'

includeTests=true

}

Andthemicrobenchmarkclassisasfollows:

publicclassMicroBenchmark{

publicstaticvoidmain(String...args)

throwsIOException,RunnerException{

Optionsopt=newOptionsBuilder()

.include(MicroBenchmark.class.getSimpleName())

.forks(1)

.build();

newRunner(opt).run();

}

@State(Scope.Benchmark)

publicstaticclassThreadsAndQueueSizes{

@Param(value={"1","4","8"})

StringnrThreads;

@Param(value={"-1","1","10","100","1000000"})

StringqueueSize;

}

@Benchmark

@Fork(1)

publicvoidplayParallel(ThreadsAndQueueSizest3qs)throwsInterruptedException{

intnrThreads=Integer.valueOf(t3qs.nrThreads);

intqueueSize=Integer.valueOf(t3qs.queueSize);

newParallelGamePlayer(nrThreads,queueSize).play();

}

@Benchmark

@Fork(1)

publicvoidplaySimple(){

newSimpleGamePlayer().play();

}

}

ParallelGamePlayeriscreatedtoplaythegamewith-1,1,4,and8IntervalGuesserthreads,andineachcase,thereisatestrunningwithaqueueoflength1,10,100,and1million.Theseare16testexecutions.Whenthenumberofthreadsisnegative,thentheconstructorusesLinkedBlockingDeque.Thereisanotherseparatemeasurementthatmeasuresthenonparallelplayer.Thetestwasexecutedwithuniqueguessesandsecrets(nocolorusedmorethanonce)andtencolorsandsixcolumns.

Whentheharnessstarts,itdoesallthecalibrationsautomaticallyandrunsthetestsformanyiterationstolettheJVMstartup.Youmayrecallthecodethatjustneverstoppedunlessweusedthevolatilemodifierinforthevariablethatwasusedtosignalthecodetostop.ThathappenedbecausetheJITcompileroptimizedthecode.Thisisdoneonlywhenthecodewasalreadyrunafewthousandtimes.TheharnessmakestheseexecutionstowarmupthecodeandensurethatthemeasurementisdonewhenJVMisatfullspeed.

Runningthisbenchmarktakesapproximately15minutesonmymachine.Duringtheexecution,itisrecommendedtostopallotherprocessesandletthebenchmarkuseallavailableresources.Ifthereisanythingusingresourcesduringthemeasurement,thenitwillbereflectedintheresult.

Benchmark(nrThreads)(queueSize)ScoreError

playParallel1-115,636&pm;1,905

playParallel1115,316&pm;1,237

playParallel11015,425&pm;1,673

playParallel110016,580&pm;1,133

playParallel1100000015,035&pm;1,148

playParallel4-125,945&pm;0,939

playParallel4125,559&pm;1,250

playParallel41025,034&pm;1,414

playParallel410024,971&pm;1,010

playParallel4100000020,584&pm;0,655

playParallel8-124,713&pm;0,687

playParallel8124,265&pm;1,022

playParallel81024,475&pm;1,137

playParallel810024,514&pm;0,836

playParallel8100000016,595&pm;0,739

playSimpleN/AN/A18,613&pm;2,040

Theactualoutputoftheprogramisabitmoreverbose;itwaseditedforprintingpurposes.TheScorecolumnshowshowmanytimesthebenchmarkcanruninasecond.TheErrorshowsthatthemeasurementshowslessthan10%scattering.

Thefastestperformancewehaveiswhenthealgorithmrunsoneightthreads,whichisthenumberofthreadstheprocessorcanindependentlyhandleonmymachine.Itisinterestingthatlimitingthesizeofthequeuedidnothelptheperformance.Iactuallyexpectedittobedifferent.Usingaonemillionlengtharrayasablockingqueuehasahugeoverheadandthisisnotasurprisethat,inthiscase,theexecutionisslowerthanwhenwehaveonly100elementsinthequeue.Theunlimitedlinkedlist-basedqueuehandling,ontheotherhand,fairlyfastandclearlyshowsthattheextraspeedatthelimitedqueuefor100elementsdoesnotcomefromthefactthatthelimitdoesnotallowtheIntervalThreadstoruntoofar.

Whenwestartonethread,thenweexpectsimilarresults,aswhenweruntheserialalgorithm.Thefactthattheserialalgorithmbeatstheparallelalgorithmrunningononethreadisnotasurprise.Thethreadcreationandthecommunicationbetweenthemainthreadandtheextraonethreadhaveoverhead.Theoverheadissignificant,especiallywhenthequeueisunnecessarilylarge.

SummaryInthischapter,youlearnedalotofthings.Firstofall,werefactoredthecodetobereadyforfurtherdevelopmentthatusesparallelguessing.Wegotacquaintedwithprocessesandthreads,andweevenmentionedfibers.Afterthat,welookedathowJavaimplementsthreadsandhowtocreatecodethatrunsonmultiplethreads.Additionally,wesawthedifferentmeansthatJavaprovidestoprogrammersneedingparallelprograms,startingthreads,orjuststartingsometasksinalreadyexistingthreads.

Perhapsthemostimportantpartofthischapterthatyoushouldrememberisthemetaphorofbureaucratsandthedifferentspeeds.Thisisextremelyimportantwhenyouwanttounderstandtheperformanceofconcurrentapplications.AndIhopethatthisisacatchypicture,whichiseasytoremember.

TherewasahugetopicaboutthedifferentsynchronizationmeansthatJavaprovides,andyouhavealsolearnedaboutthepitfallsthatprogrammerscanfallintowhenprogrammingconcurrentapplications.

Lastbutnotleast,wecreatedtheconcurrentversionoftheMastermindguesserandalsomeasuredthatitisindeedfasterthantheversionthatusesonlyoneprocessor(atleastonmymachine).WeusedtheJavaMicrobenchmarkHarnesswiththeGradlebuildtoolanddiscussed,abit,howtoperformmicrobenchmarking.

Thiswasalongchapterandnotaneasyone.Imaytendtothinkthatthisisthemostcomplexandmosttheoreticalone.Ifyouunderstoodhalfofitatfirstread,youcanbeproud.Ontheotherhand,beawarethatthisisonlyagoodbasetostartexperimentingwithconcurrentprogrammingandthereisalongwaytobeingseniorandprofessionalinthisarea.And,itisnotaneasyone.Butfirstofall,beproudofyourselfattheendofthischapter.

Inthefollowingchapterswewilllearnmoreaboutwebandwebprogramming.Intheverynextchapterwewilldevelopourlittlegamesothatitcanruninaserverandtheplayercanplaywithitusingawebbrowser.Thiswillestablishthebasicknowledgeforwebprogramming.Laterwewillbuildonthisdevelopingwebbasedserviceapplications,reactiveprogrammingandallthetoolsandareasthatwillmakeaprofessionalJavadeveloper.

MakingOurGameProfessional-DoitasaWebappInthischapter,wewillprogramawebapplication.WewillbuildonwhatwehaveachievedalreadyandcreateawebversionoftheMastermindgame.Thistime,itwillnotonlyrunalone,guessingandansweringthenumberofpositionsandmatchedcolors,butalsocommunicatewiththeuseraskingfortheanswerstotheguesses.Thiswillbearealgame.WebprogrammingisextremelyimportantforJavaprogrammers.Mostoftheprogramsarewebapplications.TheuniversalclientavailableontheInternetisthewebbrowser.Thethin-client,webbrowser-basedarchitectureiswidelyacceptedinenterprisesaswell.Thereareonlysomeexceptionswhenthearchitecturehassomethingelsebutthewebclient.IfyouwanttobecomeaprofessionalJavadeveloper,youmustbefamiliarwithwebprogramming.Anditisalsofun!

Therearealotoftechnicaltopicsthatwewillvisitduringthedevelopment.Firstofall,wewilldiscussnetworkingandwebarchitecture.Thisistheconcretebaseofthewholebuilding.Itisnottoosexy,justlikewhenyouconstructthebuilding.Youspendalotofmoneyandeffortdiggingtrenches,andthenyouburytheconcreteandendupattheendofthephasewithwhatyouseeminglyhadbefore:flatground.Exceptthatthereisthebase.Buildingwithoutthisbase,thehousewouldeithercollapsesoonafterorduringtheprocessofbuilding.Networkingisjustasimportantforwebprogramming.Therearealotoftopicsthatseeminglyhavenothingtodowithprogramming.Still,itisthebaseofthebuildingandwhenyouprogramwebapplications,youwillalsofindthefunpartinit.

WewillalsotalkabitaboutHTML,CSS,andJavaScript,butnottoomuch.Wecannotavoidthembecausetheyarealsoimportantforwebprogramming,buttheyaretopicsthatyoucanlearnfromsomewhereelseaswell.Incaseyouarenotanexpertinsomeoftheseareas,thereareusuallyotherexpertsinenterpriseprojectteamswhocanextendyourknowledge.(Inthecaseofnetworking,thereisnomercy.)Inadditiontothat,JavaScriptisatopicsocomplexandhugethatitdeservesawholebooktostartwithit.ThereareonlyveryfewexpertswhodeeplyunderstandbothJavaandJavaScript.Iunderstandthegeneralstructureofthelanguageandtheenvironmentitrunsin,butIcannotkeepupwiththenewframeworksthatarereleasedeveryweekthesedays,havingmyfocusonotherareas.

YouwilllearnhowtocreateJavaapplicationsthatruninanapplicationserver,thistimeinJetty,andwewillseewhataservletis.Wewillcreateawebhelloworldapplicationtostartupfast,andthenwewillcreatetheservletversionofMastermind.Notethatwehardlyeverprogramservletsdirectlywithouttheaidofsomeframeworkthatimplementsthecodetohandleparameters,authentication,andmanyotherthingsthatarenotapplication-specific.Wewillstillsticktoanakedservletinthischapterbecauseitisnotpossibletoeffectivelyuseframeworks,suchasSpring,withoutfirstunderstandingwhataservletis.Springwillcomeinthenextchapter.

WewillmentionJavaServerPages(JSP)onlybecauseyoumaymeetsomelegacyapplication,whichwasdevelopedusingthattechnology,butmodernwebapplicationsdonotuseJSP.Still,JSPisapartoftheservletstandardandisavailableforuse.Thereareothertechnologiesthatweredevelopedintherecentpastbutdonotseemtobefuture-proofthesedays.Theyarestillusablebutappearonlyinlegacy

applications,andchoosingthemforanewprojectisfairlyquestionable.Wewilltalkaboutthesetechnologiesshortlyinaseparatesection.

Bytheendofthischapter,youwillunderstandhowthebasicwebtechnologyworksandwhatthemajorarchitecturalelementsare,andyouwillbeabletocreatesimplewebapplications.ThisisnotenoughtobeaprofessionalJavawebdeveloperbutwillbeagoodgroundingforthenextchapter,wherewewillhavealookattheprofessionalframeworksusedintoday'senterprisesforrealapplicationdevelopments.

Webandnetwork

Programsrunoncomputers,andcomputersareconnectedtotheInternet.Thisnetworkwasdevelopedinthelast60years,firsttoprovidemilitarydatacommunicationthatisresilienttorocketattack,thenitwasextendedtobeanacademicnetwork,andlateritbecameacommercialnetworkusedbyanyoneandavailablealmostubiquitouslyallovertheEarth.

Thedesignofthenetwork,andtheresearch,startedasaresponsetotheflightofGagarinovertheEarthinthefifties.SendingGagarintospaceandtravellingovertheEarthwasademonstrationthatRussiacouldsendarocketanywhereontheglobe,possiblywithatomicexplosives.Itmeantthatanydatanetworkthatneededsomecentralcontrolwasnotresilienttosuchanattack.Itwasnotfeasibletohaveanetworkwithacentrallocationasasinglepointoffailure.Therefore,researchwasstartedtocreateanetworkthatgoesonworkingevenifanypartofitisbroughtdown.

IPThenetworkdeliversdatapacketsbetweenanytwocomputersconnectedtoit.TheprotocolusedonthenetworkisIP,whichissimplyanabbreviationofInternetProtocol.UsingIP,acomputercansendadatapackettoanother.Thepackagecontainsaheaderandthedatacontent.TheheadercontainstheInternetaddressesofthesenderandthetargetmachine,otherflags,andinformationaboutthepackage.Sincethemachinesarenotconnectedtoeachotherdirectly,routersforwardthepackets.Itislikepostofficessendingmailstoeachothertillitgetsintothehandsofthepostmanyouknow,whocandirectlydeliverittoyourmailbox.Todothat,theroutersusetheinformationintheheader.ThealgorithmandorganizationofhowtheroutersinteractarecomplexandsomethingweneednotknowtobeJavaprofessionals.

IfyoueverneedtoprograminordertosendIPpacketsdirectly,youshouldlookatjava.net.DatagramPacket,andtherestisimplementedintheJDK,theoperatingsystem,andonthefirmwareofthenetworkcard.Youcancreateadatapacket;sendingitandchangingthemodulatedvoltageonthenetworkcardoremittingphotonstothefiberisnotyourheadache.However,youwillallknowwhetheryoureallyneedtoprogramdatagramsdirectly.

IPhastwoversions.TheoldversionstillinuseisIPv4.ThenewversionthatcoexistswiththeoldoneisIPv6orIPng(ngstandsfornewgeneration).ThemajordifferencethatmayconcernaJavadeveloperisthatversion4uses32-bitaddressesandversion6uses128-bitaddresses.Whenyouseeaversion-4address,youwillseesomethinglike192.168.1.110,whichcontainsthefourbytesinadecimalformatseparatedbydots.IPv6addressesareexpressedas2001:db8:0:0:0:0:2:1,aseight16-bitnumbersexpressedinhexadecimalseparatedbycolons.

TheWebisabitmorecomplexthansendingdatapackets.Ifsendingadatapacketislikesendingaone-pageletter,thenawebpagedownloadislikediscussingacontractinpapermail.Thereshouldbeanagreementintheinitialpapermailastowhattosend,whattoanswer,andsoon,untilthecontractissigned.OntheInternet,thatprotocoliscalledTransmissionControlProtocol(TCP).Whileitishighlyunlikely(butpossible)thatyouwillmeetIProutingissues,beingaJavadeveloper,youcertainlymaymeetTCPprogramming.Therefore,wewillcovershortlyhowtheTCPworks.Beawarethatthisisverybrief.Really.YouwillnotbecomeaTCPexpertreadingthenextsection,butyouwillgetaglimpseofthemostimportantissuesthataffectwebprogramming.

TCP/IPTheTCPprotocolisimplementedintheoperatingsystemandprovidesahigherlevelofinterfacethanIP.WhenyouprogramTCP,youdonotdealwithdatagrams.Instead,youhaveachannelofbytestreamswhereyoucanputbytestobedeliveredtotheothercomputer,andyoucanreadbytesfromthechannelthatweresentbytheothercomputer,exactlyintheorderastheyweresent.Thisisakindofconnectionbetweentwocomputersand,what'smore,betweentwoprograms.

ThereareotherprotocolsthatareimplementedoverIPandwhicharenotconnection-oriented.OneofthemisUserDatagramProtocol(UDP),usedforserviceswhenthereisnoneedforconnections,whenthedatamaybelostanditismoreimportantthatthedatagetstothedestinationinatimelymannerthanlosingsomeofthepackets(videostreaming,telephony).Whenthedataamountissmallandincaseitisnotdelivered,itcanberequestedagain;thecostoflosingitischeap(DNSrequest,seethenextsection).

Whenapacketislostonthenetwork,orwhenitissenttwice,orwhenitisdeliveredsoonerthanalaterpackage,itishandledbytheTCPsoftwarelayerimplementedbytheoperatingsystem.ThislayerisalsopopularlycalledtheTCPstack.

SincetheTCPisaconnectedprotocol,thereisaneedforsomethingthattellstheTCPstackwhichstreamadatagrambelongstowhenitarrives.Thestreamisidentifiedbytwoports.Aportisa16-bitinteger.Oneidentifiestheprogramthatinitiatestheconnection,calledthesourceport.Theotheroneidentifiesthetargetprogram:thedestinationport.ThesearecontainedineachandeveryTCPpacketdelivered.WhenamachinerunsaSecureShell(SSH)serverandawebserver,theyusedifferentports,usuallyport22and80.Whenapackagecomesthatcontainsthedestinationportnumber22intheTCPheader,theTCPstackknowsthatthedatainthepacketbelongstothestreamhandledbytheSSHserver.Likewise,ifthedestinationportis80,thenthedatagoestothewebserver.

Whenweprogramaserver,weusuallyhavetodefinetheportnumber;otherwise,thereisnowaytheclientswillfindtheserverprogram.Webserversareusuallylistenonport80,andclientstrytoconnecttothatport.Theclientportisusuallynotimportantandnotspecified;itisallocatedbytheTCPstackautomatically.

Toconnectfromaclientcodetoaserveriseasy:onlyafewlinesofcode.Sometimes,itisonlyonelineofcode.However,underthehood,thereisalotofworkthattheTCPstackdoesthatweshouldcareabout—ittakestimetobuildupaTCPconnection.

Tohaveaconnection,theTCPstackhastosendadatagramtothedestinationtoknowthatitexists.Ifthereisnoserverlisteningontheport,sendingthedataoverthenetworkhasnoresult,exceptforwastingthenetworkbandwidth.Forthisreason,theclientfirstsendsanemptydatapacketcalledSYN.Whentheothersidereceivesit,itsendsbackasimilarpackagecalledSYN-ACK.Finally,theclientsendsapackagecalledACK.IfthepacketsgothroughtheAtlantic,thisisapproximately45msforeachpackage,whichisequivalentto45millionsecondsinbureaucrattime.Thisisalmostoneandahalfyears.Weneedthreeofthosetosetuptheconnection,andthereismore.

WhenaTCPconnectionstarts,theclientdoesnotstarttosendthedatawithoutcontrol.Itsendssomedatapacketsandthenitwaitsfortheservertoacknowledgetheirreceipt.Itwouldnotonlybeuseless,butalsonetworkwasting,tosenddatathattheserverisnotpreparedtoacceptandhastothrowaway.TheTCPisdesignedtooptimizethenetworkusage.Therefore,theclientsendssomedata,andthenitwaitsfortheacknowledgement.TheTCPstackautomaticallymanagesthis.Iftheacknowledgementarrives,itsendsmorepackets,andifacarefullydesignedoptimizationalgorithm,implementedintheTCPstack,believesthatitisgoodtosendmore,itwillsendabitmoredatathaninthefirststep.Iftherearenegativeacknowledgementstellingtheclientthattheservercouldnotacceptsomeofthedataandhadtothrowitaway,thentheclientwilllowerthenumberofpacketsitsendswithoutacknowledgement.Butfirstitstartsslowandcautious.ThisiscalledTCPslowstartandwehavetobeawareofit.AlthoughitisalowlevelnetworkingfeatureithasconsequencesthatwehavetoconsiderinourJavacode:weusedatabaseconnectionpoolsinsteadofcreatinganewconnectiontothedatabaseeachtimethereisaneedforsomedata;wetrytomanagetohaveasfewconnectionstowebserversaspossibleusingtechniquessuchaskeep-alive,SPDYprotocol,orhttp/2.0(alsoreplacingSPDY).

Forastart,itisenoughthatTCPisconnection-orientedwhereyoubuildupaconnectiontoaserver,sendandreceivebytes,andfinallyclosetheconnection.Whenyouhaveanetworkperformanceproblem,youhavetolookattheissuesIlisted.

DNS

TheTCPprotocolcreatesachannelusingtheIPaddressesofmachines.WhenyoutypeaURLinthebrowser,itusuallydoesnotcontainIPnumbers.Itcontainsmachinenames.ThenameisconvertedtoIPnumbersusingadistributeddatabasecalledDomainNameSystem(DNS).Thisdatabaseisdistributed,andwhenaprogramneedstoconvertanametoanaddress,itsendsDNSrequesttooneoftheDNSserversitknows.Theseserversqueryeachotherortelltheclientwhomtoask,untiltheclientknowstheIPaddressassignedtothename.Theserversandtheclientalsocachetherecentlyrequestednames,soansweringisfast.Ontheotherhand,whentheIPaddressofaserverchangesthisname,notallclientswillimmediatelyseetheaddressassignmentovertheglobe.TheDNSlookupcanbeeasilyprogrammed,andthereareclassesandmethodsinJDKthatsupportthis,butusuallyweneednotcareaboutthat;whenweprogram,itisdoneautomaticallyinwebprogramming.

TheHTTPprotocolTheHypertextTransportProtocol(HTTP)isbuiltontopoftheTCP.WhenyoutypeaURLinabrowser,thebrowseropensaTCPchanneltotheserver(afterDNSlookup,ofcourse)andsendsaHTTPrequesttothewebserver.Theserver,afterreceivingtherequest,producesaresponseandsendsittotheclient.Afterthat,theTCPchannelmaybeclosedorkeptaliveforfurtherHTTPrequest-responsepairs.

Boththerequestandtheresponsecontainaheaderandanoptional(possiblyzero-length)body.Theheaderisinthetextformat,anditisseparatedfromthebodybyanemptyline.

Morepreciselytheheaderandthebodyareseparatedbyfourbytes:0x0D,0x0A,0x0D,and0x0A,whicharetwoCR,LFlineseparators.TheHTTPprotocolusescarriagereturnandlinefeedtoterminatelinesintheheader,andthus,anemptylineistwoCRLFfollowingeachother.

Thestartoftheheaderisastatuslineplusheaderfields.ThefollowingisasampleHTTPrequest:

GET/html/rfc7230HTTP/1.1

Host:tools.ietf.org

Connection:keep-alive

Pragma:no-cache

Cache-Control:no-cache

Upgrade-Insecure-Requests:1

User-Agent:Mozilla/5.0(Macintosh;IntelMacOSX10_11_6)AppleWebKit/537.36(KHTML,likeGecko)Chrome/52.0.2743.116Safari/537.36

Accept:text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8

DNT:1

Referer:https://en.wikipedia.org/

Accept-Encoding:gzip,deflate,sdch,br

Accept-Language:en,hu;q=0.8,en-US;q=0.6,de;q=0.4,en-GB;q=0.2

Thefollowingistheresponse:

HTTP/1.1200OK

Date:Tue,04Oct201613:06:51GMT

Server:Apache/2.2.22(Debian)

Content-Location:rfc7230.html

Vary:negotiate,Accept-Encoding

TCN:choice

Last-Modified:Sun,02Oct201607:11:54GMT

ETag:"225d69b-418c0-53ddc8ad0a7b4;53e09bba89b1f"

Accept-Ranges:bytes

Cache-Control:max-age=604800

Expires:Tue,11Oct201613:06:51GMT

Content-Encoding:gzip

Strict-Transport-Security:max-age=3600

X-Frame-Options:SAMEORIGIN

X-Xss-Protection:1;mode=block

X-Content-Type-Options:nosniff

Keep-Alive:timeout=5,max=100

Connection:Keep-Alive

Transfer-Encoding:chunked

Content-Type:text/html;charset=UTF-8

<!DOCTYPEhtmlPUBLIC"-//W3C//DTDXHTML1.0Transitional//EN"

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<htmlxmlns="http://www.w3.org/1999/xhtml"xml:lang="en"lang="en">

<headprofile="http://dublincore.org/documents/2008/08/04/dc-html/">

<metahttp-equiv="Content-Type"content="text/html;charset=utf-8"/>

<metaname="robots"content="index,follow"/>

Therequestdoesnotcontainabody.Thestatuslineisasfollows:

GET/html/rfc7230HTTP/1.1

Itcontainstheso-calledmethodoftherequest,theobjectthatisrequested,andtheprotocolversionusedbytherequest.Therestoftherequestoftheheadercontainsheaderfieldsthathavetheformat,label:value.Someofthelinesarewrappedintheprintedversion,butthereisnolinebreakinaheaderline.

Theresponsespecifiestheprotocolituses(usuallythesameastherequest),thestatuscode,andthemessageformatofthestatus:HTTP/1.1200OK

Afterthis,theresponsefieldscomewiththesamesyntaxasintherequest.Oneimportantheaderisthecontenttype:

Content-Type:text/html;charset=UTF-8

Itspecifiesthattheresponsebody(truncatedintheprintout)isHTMLtext.

TheactualrequestwassenttotheURL,https://tools.ietf.org/html/rfc7230,whichisthestandardthatdefinesthe1.1versionofHTTP.Youcaneasilylookintothecommunicationyourself,startingupthebrowserandopeningthedevelopertools.Suchatoolisbuiltintoeverybrowserthesedays.YoucanuseittodebugtheprogrambehavioronthenetworkapplicationlevellookingattheactualHTTPrequestsandresponsesonthebytelevel.Thefollowingscreenshotshowshowthedevelopertoolshowsthiscommunication:

HTTPmethodsThemethodthatisthefirstwordinthestatuslineoftherequesttellstheserverwhattodowiththerequest.Thestandarddefinesdifferentmethods,suchasGET,HEAD,POST,PUT,DELETE,andsomeothers.

TheclientusestheGETmethodwhenitwantstogetthecontentofaresource.InthecaseofaGETrequest,thebodyoftherequestisempty.Thisisthemethodusedbythebrowserwhenwedownloadawebpage.Itisalso,manytimes,themethodusedwhensomeprogramimplementedinJavaScriptandrunninginthebrowserasksforsomeinformationfromawebapplication,butitdoesnotwanttosendmuchinformationtotheserver.

WhentheclientusesPOST,theintentionisusuallytosendsomedatatotheserver.Theserverdoesreplyand,manytimes,thereisalsoabodyinthereply,butthemainpurposeoftherequest/replycommunicationistosendsomeinformationfromtheclienttotheserver.ThisistheoppositeoftheGETmethodinsomesense.

TheGETandPOSTmethodsarethemostfrequentlyusedmethods.AlthoughthereisageneralguidelinetouseGETtoretrievedataandPOSTtosenddatatotheserver,itisonlyarecommendation,andthereisnocleanseparationofthetwocases.Manytimes,GETisusedtosendsomedatatotheserver.Afterall,itisanHTTPrequestwithastatuslineandheaderfields,andalthoughthereisnobodyintherequest,theobject(partoftheURL)thatfollowsthemethodinthestatuslineisstillabletodeliverparameters.Manytimes,itisalsoeasytotestaservicethatrespondstoaGETrequestbecauseyouonlyneedabrowserandtotypeintheURLwiththeparameters,andlookattheresponseinthebrowserdevelopertools.YoushouldnotbesurprisedifyouseeanapplicationthatusesGETrequeststoexecuteoperationsthatmodifythestateonawebserver.However,notbeingsurpriseddoesnotmeanapproval.Youshouldbeawarethatinmostcases,thesearenotgoodpractices.WhenwesendsensitiveinformationusingtheGETrequest,theparametersintheURLareavailabletotheclientintheaddresslineofthebrowser.WhenwesendusingPOST,theparametersarestillreachablebytheclient(afterall,theinformationtheclientsendsisgeneratedbytheclientand,assuch,cannotbeunavailable),butnotthateasyforasimplesecurity-unawareusertocopy-pastetheinformationandsend,perhaps,toamalevolentthirdparty.ThedecisionbetweenusingGETandPOSTshouldalwaysconsiderpracticalitiesandsecurityissues.

TheHEADmethodisidenticaltoaGETrequest,buttheresponsewillnotcontainabody.Thisisusedwhentheclientisnotinterestedintheactualresponse.Itmayhappenthattheclientalreadyhastheobjectandwantstoseeifitwaschanged.TheLast-Modifiedheaderwillcontainthetimewhentheresourcewaslastchanged,andtheclientcandecideifithasaneweroneorneedstoaskfortheresourceinanewrequest.

ThePUTmethodisusedwhentheclientwantstostoresomethingontheserverandDELETEwhentheclientwantstoerasesomeresource.ThesemethodsareusedonlybyapplicationsusuallywritteninJavaScriptandnotdirectlybythebrowser.

Thereareothermethodsdefinedinthestandard,butthesearethemostimportantandfrequentlyusedones.

Statuscodes

Theresponsestartswiththestatuscode.Thesecodesarealsodefinedandtherearealimitednumberofcodesusableinaresponse.Themostimportantis200,whichsaysallisOK;theresponsecontainswhattherequestwanted.Thecodesarealwaysintherangeof100to599,containthreedigits,andaregroupedbythefirstdigit.

1xx:Thesecodesareinformationcodes.Theyarerarelyusedbutcanbeveryimportantinsomecases.Forexample,100meanscontinue.AservercansendthiscodewhenitgetsaPOSTrequestandtheserverwantstosignaltheclienttosendthebodyoftherequestbecauseitcanprocessit.Usingthiscode,andtheclientwaitingforthiscode,maysavealotofbandwidthifproperlyimplementedontheserverandalsoontheclient.2xx:Thesecodesmeansuccess.Therequestisansweredproperly,ortherequestedservicewasdone.Therearecodes,suchas200,201,202,andsoon,definedinthestandardandthereisadescriptionaboutwhentouseoneortheother.3xx:Thesecodesmeanredirection.OneofthesecodesissentwhentheservercannotdirectlyservicetherequestbutknowstheURLthatcan.Theactualcodescandistinguishbetweenapermanentredirect(whenitisknownthatallfuturerequestsshouldbesenttothenewURL)andtemporaryredirect(whenanylaterrequestshouldbesenthereandpossiblyservedorredirected),butthedecisioniskeptontheserverside.4xx:Theseareerrorcodes.Themostfamouscodeis404,whichmeansNotFound,thatis,theserverisnotabletorespondtotherequestbecausetheresourceisnotfound.401meansthattheresourcetoservetherequestmaybeavailablebutitrequiresauthentication.403isacodethatsignalsthattherequestwasvalidbutisstillrefusedtobeservedbytheserver.5xx:Thesecodesareservererrorcodes.Whenaresponseholdsoneoftheseerrorcodes,themeaningisthatthereissomeerrorontheserver.Thiserrorcanbetemporary,forexample,whentheserverisprocessingtoomanyrequestsandcannotrespondtoanewrequestwithacalculation-intensiveresponse(thisisusuallysignaledbyerrorcode503)orwhenthefeatureisnotimplemented(code501).Thegeneralerrorcode500isinterpretedasInternalError,whichmeansthatnoinformation,whatsoever,isavailableaboutwhatwasgoingwrongontheserver,butitwasnotgoingwellandhence,nomeaningfulresponse.

HTTP/2.0Afteralmost20yearssincethelastreleaseofHTTP,thenewversionofHTTPwasreleasedin2015.Thisnewversionoftheprotocolhasseveralenhancementsoverthepreviousversions.Someoftheseenhancementswillalsoaffectthewayserverapplicationswillbedeveloped.

ThefirstandmostimportantenhancementisthatthenewprotocolwillmakeitpossibletosendseveralresourcesparallellyinasingleTCPconnection.Thekeep-aliveflagisavailabletoavoidtherecreationoftheTCPchannel,butitdoesnothelpwhenaresponseiscreatedslowly.Inthenewprotocol,otherresourcescanalsobedeliveredinthesameTCPchannelevenbeforeonerequestisfullyserved.Thisrequirescomplexpackagehandlingintheprotocol,butthisishiddenfromtheserverapplicationprogrammeraswellasthebrowserprogrammer.Theapplicationserver,servletcontainer,andbrowserimplementthistransparently.

HTTP/2.0willalwaysbeencrypted,thereforeitwillnotbepossibletousehttpasaprotocolinthebrowserURL.Itwillalwaysbehttps.

Thefeaturethatwillneedchangesinservletprogrammingtoleveragetheadvantagesofthenewversionoftheprotocolisserverpush.Version4.0oftheservletspecificationincludessupportforHTTP/2.0,andthisversionisstillindraft.

ServerpushisanHTTPresponsetoarequestthatwillcomeinthefuture.Howcanaserveranswerarequestthatisnotevenissued?Well,theserveranticipates.Forexample,theapplicationsendsanHTMLpagethathasreferencestomanysmallpicturesandicons.TheclientdownloadstheHTMLpage,buildstheDOMstructure,analyzesit,andrealizesthatthepicturesareneeded,andsendstherequestforthepictures.Theapplicationprogrammerknowswhatpicturesarethereandmaycodetheservertosendthepicturesevenbeforethebrowserrequestsforit.EverysuchresponseincludesaURLthatthisresponseisfor.Whenthebrowserwantstheresource,itrealizesthatitisalreadythereanddoesnotissueanewrequest.InHttpServlet,theprogramshouldaccessPushBuilderviatherequest'snewgetPushBuildermethodandusethattopushdownresourcestotheclient.

CookiesCookiesaremaintainedbythebrowserandaresentintheHTTPrequestheaderusingtheCookieheaderfield.Eachcookiehasaname,value,domain,path,expirationtime,andsomeotherparameters.WhenarequestissenttoaURLthatmatchesthedomain,thepathofanon-expiredcookie,theclientsendsthecookietotheserver.Cookiesareusuallystoredinsmallfilesontheclientbythebrowserorinalocaldatabase.Theactualimplementationisthebusinessofthebrowser,andweneednotworryaboutit.Itisjustthetextinformationthatisnotexecutedbytheclient.Itisonlysentbacktotheserverwhensomerules(mainlydomainandpath)match.CookiesarecreatedbyserversandaresenttotheclientinHTTPresponsesusingtheSet-Cookieheaderfield.Thus,essentiallytheservertellstheclient,Hey,hereisthiscookie,wheneveryoucometomenexttime,showmethispieceofinformation,soIwillknowitisyou.

Cookiesareusuallytorememberclients.Advertisersandonlineshopsthatneedtorememberwhotheyaretalkingtoheavilyuseit.Butthisisnottheonlyuse.Thesedays,anyapplicationthatmaintainsusersessionsusescookiestochainuptheHTTPrequeststhatcomefromthesameuser.Whenyoulogintoanapplication,theusernameandpasswordyouusetoidentifyyourselfaresenttotheserveronlyonce,andinsubsequentrequests,onlyaspecialcookieissenttotheserverusedtoidentifythealreadyloggedinuser.Thisuseofcookiesemphasizeswhyitisimportanttousecookievaluesthatcannotbeeasilyguessed.Ifthecookieusedtoidentifyauseriseasilyguessable,thenanattackercouldjustcreateacookieandsendittotheservermimickingtheotheruser.Cookievalues,forthepurpose,areusuallylongrandomstrings.

Cookiesarenotalwayssentbacktotheserverwheretheyoriginate.Whenthecookieisset,theserverspecifiesthedomainoftheURLwherethecookieshouldbesentback.Thisisusedwhenadifferentserverfromtheoneprovidingtheservicesneedingauthenticationdoestheuserauthentication.

Applicationssometimesencodevaluesintocookies.Thisisnotnecessarilybad,thoughinmostactualcases,itis.Whenencodingsomethingintoacookie,weshouldalwaysconsiderthefactthatthecookietravelsthroughthenetworkandcangohugeasmoreandmoredataisencodedinitandcancreateunnecessaryburdenonthenetwork.Usually,itisbettertosendonlysomeunique,otherwisemeaningless,randomkey,andstorethevaluesinsomedatabase,beitondiskorinthememory.

ClientserverandwebarchitectureTheapplicationswedevelopedsofarwererunningonasingleJVM.Wealreadyhavesomeexperiencewithconcurrentprogrammingandthisissomethingthatwillcomehandynow.Whenweprogramawebapplication,apartofthecodewillrunontheserverandapartoftheapplicationlogicwillexecuteinthebrowser.TheserverpartwillbewritteninJava,thebrowserpartwillbeimplementedinHTML,CSS,andJavaScript.SincethisisaJavabookwewillfocusmainlyontheserverpart,butweshouldstillbeawareofthefactthatmanyofthefunctionalitiescanbeandshouldbeimplementedtoruninthebrowser.ThetwoprogramscommunicatewitheachotherovertheIPnetwork,thatis,theInternet,orinthecaseofanenterpriseinternalapplication,thenetworkofthecompany.

Today,abrowseriscapableofrunningverypowerfulapplications,allimplementedinJavaScript.Afewyearsago,suchapplicationsneededclientapplicationimplementedinDelphi,C++,orJava,usingthewindowingcapabilitiesoftheclientoperatingsystem.

Originally,theclient-serverarchitecturemeantthatthefunctionalityoftheapplicationwasimplementedontheclient,andtheprogramwasusinggeneralservicesonlyfromtheserver.Theserverprovideddatabaseaccessandfilestoragebutnothingmore.Later,thethree-tierarchitectureputthebusinessfunctionalityontheserversthatusedotherserversfordatabaseandothergeneralservices,andtheclientapplicationimplementedtheuserinterfaceandlimitedbusinessfunctionality.

Whenthewebtechnologystartedtopenetrateenterprisecomputing,thewebbrowserstartedtoreplacetheclientapplicationsinmanyusecases.Previously,thebrowsercouldnotruncomplexJavaScriptapplications.TheapplicationwasexecutedonthewebserverandtheclientdisplayedtheHTMLthattheservercreatedasapartoftheapplicationlogic.Everytimesomethingwaschangedontheuserinterface,thebrowserstartedacommunicationwiththeserver,andinaHTTPrequest-responsepair,thebrowsercontentwasreplaced.Awebapplicationwasessentiallyaseriesofformfillingandformdatasendingtotheserver,andtheserverrespondedwithHTML-formattedpages,presumablycontainingnewforms.

JavaScriptinterpretersweredevelopedandbecamemoreandmoreeffectiveandstandardized.Today,modernwebapplicationscontainHTML(whichisapartoftheclientcodeandisnotgeneratedbytheserveronthefly),CSS,andJavaScript.Whenthecodeisdownloadedfromthewebserver,theJavaScriptstartstoexecuteandcommunicatewiththeserver.ItisstillHTTPrequestsandresponses,buttheresponsesdonotcontainHTMLcode.Itcontainspuredata,usuallyintheJSONformat.ThisdataisusedbytheJavaScriptcodeandsomeofthedata,ifneeded,isdisplayedonthewebbrowserdisplayalsocontrolledbyJavaScript.Thisisfunctionallyequivalenttoathree-tierarchitecturewithsomeslightbutveryimportantdifferences.

Thefirstdifferenceisthatthecodeisnotinstalledontheclient.Theclientdownloadstheapplicationfromawebserver,andtheonlythingthatisinstalledisthemodernbrowser.Thisremovesalotofenterprisemaintenanceburdenandcost.

Theseconddifferenceisthattheclientisnotable,orislimited,toaccesstheresourcesoftheclientmachine.Thickclientapplicationscouldsaveanythinginalocalfileoraccessalocaldatabase.Thisisverylimited,forsecurityreasons,comparedtoaprogramrunningonthebrowser.Atthesametimethisis

ahandylimitationbecauseclientsaren'tandshouldn'tbeatrustedpartofthearchitecture.Thediskintheclientcomputerishardandexpensivetobackup.Itcanbestolenwithanotebook,andencryptingitiscostly.Therearetoolstoprotectclientstorage,butmostofthetime,storingthedataontheserveronlyisamoreviablesolution.

Itisalsoacommonprogramdesignerrortotrusttheclientapplication.Theclientphysicallycontrolstheclientcomputerandalthoughitcanbemadetechnicallyverydifficult,theclientcanstillovercomethesecuritylimitationsoftheclientdeviceandclientcode.Ifitisonlytheclientapplicationthatchecksthevalidityofsomefunctionalityordata,thenthephysicalsecurityprovidedbythephysicalcontroloftheserverisnotused.Wheneverdataissentfromtheclienttotheserver,thedatahastobecheckedinregardsofvalidity,nomatterwhattheclientapplicationis.Actually,sincetheclientapplicationcanbechanged,wejustdon'treallyknowwhattheclientapplicationreallyis.

Inthischapterand,asamatteroffact,intheentirebook,wefocusonJavatechnologies;thereforethesampleapplicationwillnotcontainalmostanyclienttechnology.IcouldnothelpbutcreatesomeCSS.Ontheotherhand,IdefinitelyavoidedJavaScript.Therefore,Ihavetoemphasizeagainthattheexampleistodemonstratetheprogrammingoftheserversideandstillprovidingsomethingthatreallyworks.AmodernapplicationwoulduseRESTandJSONcommunicationsandwouldnotplayaroundcreatingHTMLontheflyontheserverside.Originally,IwantedtocreateaJavaScriptclientandRESTserverapplication,butthefocuswasmovedsomuchfromserver-sideJavaprogrammingthatIdroppedthisidea.Ontheotherhand,youcanextendtheapplicationtobeonelikethat.

WritingservletsServletsareJavaclassesthatareexecutedinawebserverthatimplementstheservletcontainerenvironment.ThefirstwebserverscouldonlydeliverstaticHTMLfilestothebrowsers.ForeachURL,therewasanHTMLpageonthewebserverandtheserverdeliveredthecontentofthisfile,inresponsetoarequestsentbythebrowser.Verysoon,therewasaneedtoextendthewebserverstobeabletostartsomeprogramthatcalculatesthecontentoftheresponse,onthefly,whentherequestisprocessed.

ThefirststandardtodothatdefinedCGI.Itstartedanewprocesstorespondtoarequest.Thenewprocessgottherequestonitsstandardinput,andthestandardoutputwassentbacktotheclient.Thisapproachwastesalotofresources.Startinganewprocess,asyoulearnedinthepreviouschapter,iswaytoocostlyjusttorespondtoanHTTPrequest.Evenstartinganewthreadseemstobeunnecessary,butwiththat,weranabitahead.

ThenextapproachwasFastCGI,executingtheexternalprocesscontinuallyandreusingit,andthencamedifferentotherapproaches.Theapproachesafter

FastCGIallusein-processextensions.Inthesecases,thecodecalculatingtheresponserunsinsidethesameprocessasthewebserver.SuchstandardsorextensioninterfaceswereISAPIfortheMicrosoftIISserver,NSASPIfortheNetscapeserver,andtheApachemoduleinterface.Eachofthesemadeitpossibletocreateadynamicallyloadedlibrary(DLLonWindowsorSOfilesonUnixsystems)tobeloadedbythewebserverduring

startupandtomapcertainrequeststobehandledbythecodeimplementedintheselibraries.

WhensomebodyprogramsPHP,forexample,theApachemoduleextensionisthePHPinterpreterthatreadsthePHPcodeandactsuponit.WhensomebodyprogramsASPpagesfortheMicrosoftIIS,theISAPIextensionimplementingtheASPpageinterpreterisexecuted(well,thisisabitsloppyandoversimplifiedtosaybutworksasanexample).

ToJava,theinterfacedefinitionisaservletdefinedinJSR340asofversion3.1.

JSRstandsforJavaSpecificationRequest.ThesearerequestsformodificationoftheJavalanguage,libraryinterfaces,andothercomponents.Therequestsgothroughanevaluationprocess,andwhentheyareaccepted,theybecomeastandard.TheprocessisdefinedbytheJavaCommunityProcess(JCP).JCPisalsodocumentedandhasversions.Thecurrentversionis2.10andcanbefoundathttps://jcp.org/en/procedures/overview.TheJSR340standardcanbefoundathttps://jcp.org/en/jsr/detail?id=340.

Aservletprogramimplementstheservletinterface.UsuallythisisdoneviaextendingHttpServlet,theabstractimplementationoftheServletinterface.Thisabstractclassimplementsmethods,suchasdoGet,doPost,doPut,doDelete,doHead,doOption,anddoTrace,freetobeoverriddenbytheactualclassextendingit.Ifaservletclassdoesnotoverrideoneofthethesemethods,sendingthecorrespondingHTTPmethod,GET,POST,andsoon,willreturnthe405NotAllowedstatuscode.

HelloworldservletBeforegettingintothetechnicaldetails,let'screateanextremelysimplehelloworldservlet.Todoit,wesetupaGradleprojectwiththebuildfile,build.gradle,theservletclassinthefile,src/main/java/packt/java9/by/example/mastermind/servlet/HelloWorld.java,andlastbutnotleast,wehavetocreatethefilesrc/main/webapp/WEB-INF/web.xml.Thegradle.buildfilewilllookthefollowing:

applyplugin:'java'

applyplugin:'jetty'

repositories{

jcenter()

}

dependencies{

providedCompile"javax.servlet:javax.servlet-api:3.1.0"

}

jettyRun{

contextPath'/hello'

}

TheGradlebuildfileusestwoplugins,javaandjetty.Wehavealreadyusedthejavaplugininthepreviouschapter.ThejettypluginaddstaskssuchasjettyRunthatloadtheJettyservletcontainerandstartuptheapplication.ThejettypluginisalsoanextensionofthewarpluginthatcompileswebapplicationsintoaWebArchive(WAR)packagingformat.

TheWARpackagingformatispracticallythesameasJAR;itisazipfileanditcontainsalibdirectorythatcontainsalltheJARfilesthatthewebapplicationdependson.Theclassesoftheapplicationareinthedirectory,WEB-INF/classes,andthereisaWEB-INF/web.xmlfilethatdescribesservletURLmapping,whichwewillexploreindetailsoon.

Sincewewanttodevelopanextremelysimpleservlet,weaddtheservletAPIasadependencytotheproject.Thisis,however,notacompiledependency.TheAPIisavailablewhentheservletrunsinthecontainer.Still,ithastobeavailablewhenthecompilercompilesourcode;therefore,adummyimplementationisprovidedbytheartifactspecifiedasprovidedCompile.Becauseitisspecifiedthatway,thebuildprocesswillnotpackagethelibraryintothegeneratedWARfile.ThegeneratedfilewillcontainnothingthatisspecifictoJettyoranyotherservletcontainer.

Theservletcontainerwillprovidetheactualimplementationoftheservletlibrary.WhentheapplicationisdeployedandstartedinaJetty,theJetty-specificimplementationoftheservletlibrarywillbeavailableontheclasspath.WhentheapplicationisdeployedtoaTomcat,theTomcatspecificimplementationwillbeavailable.

Wecreateaclassinourproject,asfollows:

packagepackt.java9.by.example.mastermind.servlet;

importjavax.servlet.ServletException;

importjavax.servlet.http.HttpServlet;

importjavax.servlet.http.HttpServletRequest;

importjavax.servlet.http.HttpServletResponse;

importjava.io.IOException;

importjava.io.PrintWriter;

publicclassHelloWorldextendsHttpServlet{

privateStringmessage;

@Override

publicvoidinit()throwsServletException{

message="Hello,World";

}

@Override

publicvoiddoGet(HttpServletRequestrequest,

HttpServletResponseresponse)

throwsServletException,IOException{

response.setContentType("text/html");

PrintWriterout=response.getWriter();

out.println("<h1>"+message+"</h1>");

}

@Override

publicvoiddestroy(){

}

}

Whentheservletisstarted,theinitmethodisinvoked.Whenitisputoutofservice,thedestroymethodiscalled.Thesemethodscanbeoverriddenandprovideamorefine-grainedcontrolthantheconstructorandotherfinalizationpossibilities.Aservletobjectmaybeputintoservicemorethanonce,andaftercallingdestroy,theservletcontainermayinvokeinitagain;thus,thiscycleisnotstrictlytiedtothelifecycleoftheobject.Usually,thereisnotmuchthatwedointhesemethods,butsometimes,youmayneedsomecodeinthem.

Also,notethatasingleservletobjectmaybeusedtoservemanyrequests,evenatthesametime;thus,theservletclassesandmethodsinitshouldbefairlythread-safe.Thespecificationdemandsthataservletcontainerusesonlyoneservletinstanceincasethecontainerrunsinanon-distributedenvironment.Incasethecontainerrunsonthesamemachineinseveralprocesses,eachexecutingaJVM,orevenondifferentmachines,therecanbemanyservletinstancesthathandletherequests.Generally,theservletclassesshouldbedesignedsuchthattheydonotassumethatonlyonethreadisexecutingthem,butatthesametime,theyshouldalsonotassumethattheinstanceisthesamefordifferentrequests.Wejustcannotknow.

Whatdoesitmeaninpractice?Youshouldnotuseinstancefieldsthatarespecifictoacertainrequest.Intheexample,thefieldinitializedtoholdthemessageholdsthesamevalueforeachandeveryrequest;essentially,thevariableisalmostafinalconstant.Itisusedonlytodemonstratesomefunctionalityfortheinitmethod.

ThedoGetmethodisinvokedwhentheservletcontainergetsanHTTPrequestwiththeGETmethod.Themethodhastwoarguments.Thefirstonerepresentstherequest,andthesecondonerepresentstheresponse.Therequestcanbeusedtocollectallinformationthatcomesintherequest.Intheprecedingexample,thereisnothinglikethat.Wedonotuseanyoftheinputs.Ifarequestcomestoourservlet,thenweanswertheHello,Worldstring,nomatterwhat.Later,wewillseeexampleswhenwereadtheparametersfromtherequest.Theresponsegivesmethodsthatcanbeusedtohandletheoutput.Intheexample,wefetchPrintWriter,whichistobeusedtosendcharacterstothebodyoftheHTTPresponse.Thisisthecontentthatappearsinthebrowser.Themimetypewesendistext/html,andthisissetbycallingthesetContentTypemethod.ThiswillgetintotheHTTPheaderfield,Content-Type.ThestandardandtheJavaDocdocumentationoftheclassesdefineallthemethodsthatcanbeused,andalsohowthese

shouldbeused.

Finally,wehaveaweb.xmlfilethatdeclarestheservletsthatareimplementedinourcode.Thisis,justasthenameofthefileindicates,anXMLfile.Itdeclarativelydefinesalltheservletsthatareincludedinthearchiveandalsootherparameters.Intheexample,theparametersarenotdefined,onlytheservletandthemappingtotheURL.Sincewehaveonlyonesingleservletinthisexample,theWARfile,itismappedtotherootcontext.AllandeveryGETrequestthatarrivestotheservletcontainerandtothisarchivewillbeservedbythisservlet:

<?xmlversion="1.0"encoding="UTF-8"?>

<web-appversion="2.5"

xmlns="http://java.sun.com/xml/ns/javaee"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xsi:schemaLocation="http://java.sun.com/xml/ns/javaeehttp://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">

<servlet>

<display-name>HelloWorldServlet</display-name>

<servlet-name>HelloWorldServlet</servlet-name>

<servlet-class>packt.java9.by.example.mastermind.servlet.HelloWorld</servlet-class>

</servlet>

<servlet-mapping>

<servlet-name>HelloWorldServlet</servlet-name>

<url-pattern>/</url-pattern>

</servlet-mapping>

</web-app>

JavaServerPagesIpromisedyouthatIwouldnotboreyouwithJavaServerPagesbecausethatisatechnologyofthepast.Eventhoughitisthepast,itisstillnothistoryastherearemanyprogramsrunningthatstilluseJSPandcontainJSPcode.

JSPpagesarewebpagesthatcontainHTMLandJavacodemixed.WhenanHTTPrequestisservedbyaJSPpage,theservletcontainerreadstheJSPpage,executestheJavaparts,takestheHTMLpartsastheyare,andinthisway,mixingthetwotogether,createsanHTMLpagethatissenttothebrowser.

<%@pagelanguage="java"

contentType="text/html;charset=UTF-8"

pageEncoding="UTF-8"%>

<html>

<body>

<%for(inti=0;i<5;i++){%>

hallo<br/>

<%}%>

</body>

</html>

TheprecedingpagewillcreateanHTMLpagethatcontainsthetexthallofivetimes,eachinanewlineseparatedbythetagbr.Behindthescenes,theservletcontainerconvertstheJSPpagetoaJavaservlet,thencompilestheservletusingtheJavacompiler,andthenrunstheservlet.ItdoesiteverytimethereissomechangeinthesourceJSPfile;therefore,itisveryeasytoincrementallycraftsomesimplecodeusingJSP.ThecodethatisgeneratedfromtheprecedingJSPfileis138lineslong(ontheTomcat8.5.5version),whichissimplylongandboringtolisthere,butthepartthatmayhelptounderstandhowtheJavafilegenerationworksisonlyafewlines.

Ifyouwanttoseeallthelinesofthegeneratedservletclass,youcandeploytheapplicationintoaTomcatserverandlookatthedirectorywork/Catalina/localhost/hello/org/apache/jsp/.Itisararelyknownfactamongdevelopersthatthiscodeisactuallysavedtodiskandisavailable.SometimesithelpswhenyouneedtodebugsomeJSPpages.

Herearethefewinterestinglinesgeneratedfromtheprecedingcode:

out.write("\n");

out.write("<html>\n");

out.write("<body>\n");

for(inti=0;i<5;i++){

out.write("\n");

out.write("hallo<br/>\n");

}

out.write("\n");

out.write("</body>\n");

out.write("</html>\n");

TheJSPcompilermovestheinsideoftheJSPcodeoutandtheoutsidein.IntheJSPcode,JavaissurroundedbyHTML,andinthegeneratedservletJavasource,theHTMLissurroundedbyJava.Itislikewhenyouwanttomendclothes:thefirstthingistoturnthedressinsideout.

ItisnotonlytheJavacodethatyoucanmixintoHTMLintheJSPpagesbutalsotheso-calledtags.Tags

arecollectedintotaglibraries,implementedinJava,andpackagedintoJARfiles,andtheyshouldbeavailableontheclasspathtobeused.TheJSPpageusingthetagsfromsomelibraryshoulddeclaretheuse:

<%@taglibprefix="c"

uri="http://java.sun.com/jsp/jstl/core"%>

ThetagslooklikeHTMLtags,buttheyareprocessedbytheJSPcompilerandexecutedbythecodeimplementedinthetagliblibrary.JSPmayalsorefertothevalueoftheJavaobjectsthatareavailableinthescopeoftheJSP.TodothisinsidetheHTMLpage,theJSPexpressionlanguagecouldbeused.

JSPwasoriginallycreatedtoeasethedevelopmentofawebapplication.Themainadvantageisthefaststartupofdevelopment.Thereisnolengthytimeforconfiguration,setup,andsooninthedevelopment,andwhenthereisanychangeintheJSPpage,thereisnoneedtocompilethewholeapplicationagain:theservletcontainergeneratestheJavacode,compilesittoclassfile,loadsthecodeintomemory,andexecutes.JSPwasacompetitorofMicrosoftASPpages,whichmixedHTMLwithVisualBasiccode.

Whentheapplicationstartstogrowbig,usingtheJSPtechnologycausesmoreproblemsthanaregood.Thecodethatmixesthebusinesslogicandtheviewoftheapplication,howitisrenderedinthebrowser,becomesmessy.DevelopingJSPrequiresfrontendtechnologyknowledge.AJavadeveloperisexpectedtoknowsomefrontendtechnologybutisrarelyadesignexpertandCSSguru.ModerncodealsocontainsJavaScript,manytimesembeddedintheHTMLpage.Afterall,thebigadvantageofJSPisthatitcontainscodethatrunsontheserveraswellasontheclient-sidecode.Thedevelopersfollowtheparadigmmanytimes,sodonotbesurprisedtoseesomelegacycodethatcontainsJava,HTML,CSS,andJavaScriptallmixedinaJSPfile.SinceJavaandJavaScriptaresyntacticallysimilarsometimes,itisnotobvioustoseewhatisexecutedontheserverandwhatisexecutedontheclient.IhaveevenseencodethatcreatedJavaScriptcodefromJavacodeinaJSPfile.Thatisatotalmixofdifferentresponsibilitiesandamessthatisnearlyimpossibletomaintain.ThisledtothetotaldeprecationofJSPasoftoday.

ThedeprecationofJSPisnotofficial.Itismyexpertopinion.YoumaymeetsomeexperienceddeveloperswhoarestillinlovewithJSP,andyoumayfindyourselfinprojectswhereyouarerequiredtodevelopprogramsinJSP.Itisnotshamefuldoingthat.Somepeopledoworseformoney.

Tomendthemessysituation,thereweretechnologiesthatadvocatedtheseparationoftheservercodeandtheclientfunctionalitymoreandmore.ThesetechnologiesincludeWicket,Vaadin,JSF,anddifferentJavatemplatingengines,suchasFreemarker,ApacheVelocity,andThymeleaf.TheselattertechnologiescanalsobeinterestingwhenyougeneratetextualoutputfromJavaevenwhenthecodeisnotweb-relatedatall.

Thesetechnologies,withdiscipline,helpedcontrolthedevelopmentandmaintenancecostsofmoderateandlargewebprojects,butthebasicproblemofthearchitecturewasstillthere:noclearseparationofconcerns.

Today,modernapplicationsimplementthecodeofawebapplicationinseparateprojects:onefortheclient,usingHTML,CSSandJavaScript,andaseparateonetoimplementserverfunctionalityinJava(orinsomethingelse,butwefocushereonJava).ThecommunicationbetweenthetwoistheRESTprotocol,

whichwewillcoverinthesubsequentchapters.

HTML,CSS,andJavaScriptHTML,CSS,andJavaScriptareclient-sidetechnologies.Theseareextremelyimportantforwebapplications,andaprofessionalJavadevelopershouldhavesomeknowledgeaboutthem.NobodyexpectsyoutobeanexpertinJavaandinweb-clienttechnologiesatthesametime,thoughthisisnotimpossible.Acertainunderstandingisdesirable.

HTMListhetextualrepresentationofastructuredtext.Thetextisgivenascharacters,asinanytextfile.Tagsrepresentthestructure.Astarttagstartswitha<character,thenthenameofthetag,then,optionally,name="value"attributes,andfinallyaclosing>character.Anendtagstartswith</,thenthenameofthetag,andthen>.Tagsareenclosedintohierarchies;thus,youshouldnotcloseatagsoonerthantheonethatwasopenedlater.First,thetagthatwasopenedlasthastobeclosed,thenthenext,andsoon.Thisway,anyactualtagintheHTMLhasalevel,andalltagsthatarebetweenthestartandendtagsarebelowthistag.Sometagsthatcannotencloseothertagsortextdonothaveendtagsandstandontheirown.Considerthefollowingsample:

<html>

<head>

<title>thisisthetitle</title>

</head>

</html>

Thetagheadisunderhtml,andtitleisunderhead.Thiscanbestructuredintoatree,asfollows:

html

+head

+title

+"thisisthetitle"

ThebrowserstorestheHTMLtextinatreestructure,andthistreeistheobjectmodelofthewebpagedocument,thusthename,DocumentObjectModel(DOM)tree.

TheoriginalHTMLconceptmixedformattingandstructure,andevenwiththecurrentversionofHTML5,westillhavetagssuchasb,i,ttthatsuggestthebrowsertodisplaythetextbetweenthestartandendtagsinbold,italics,andteletype,respectively.

AsthenameHTML,standingforHypertextMarkupLanguage,suggests,thetextcancontainreferencestootherwebpagesintheformofhyperlinks.Theselinksareassignedtotextsusingtheatag(standingforanchor)ortosomeformthatmayconsistofdifferentfields,andwhenthesubmitbuttonoftheformispressed,thecontentofthefieldsissenttotheserverinaPOSTrequest.Whentheformissent,thecontentofthefieldsisencodedintheso-calledapplication/x-www-form-urlencodedform.

TheHTMLstructurealwaystriedtopromotetheseparationofstructureandformatting.Todoso,formattingwasmovedtostyles.StylesdefinedinCascadingStyleSheets(CSS)providemuchmoreflexibilityforformattingthanHTML;theformatofaCSSismoreeffectiveforformatting.TheaimtocreateCSSwasthatthedesigncanbedecoupledfromthestructureofthetext.IfIhadtochooseoneofthethree,IwouldoptforCSSastheonethatisleastimportantforJavaserver-sidewebdevelopersand,atthesametime,themostimportantfortheusers(thingsshouldlooknice).

JavaScriptisthethirdpillarofclient-sidetechnologies.JavaScriptisafullyfunctional,interpretedprogramminglanguageexecutedbythebrowser.ItcanaccesstheDOMtree,andreadandmodifyit.WhentheDOMtreeismodified,thebrowserautomaticallydisplaysthemodifiedpage.JavaScriptfunctionscanbescheduledandregisteredtobeinvokedwhensomeeventoccurs.Forexample,youcanregisterafunctiontobeinvokedwhenthedocumentisfullyloaded,whentheuserpressesabutton,clicksonalink,orjusthoversthemouseoversomesection.AlthoughJavaScriptwasfirstonlyusedtocreatefunnyanimationsonthebrowser,todayitispossible,andisausualpractice,toprogramfullyfunctionalclientsusingthecapabilitiesofthebrowser.TherearereallypowerfulprogramswritteninJavaScript,evensuchpower-hungryapplicationsasPCemulators.

Inthisbook,wefocusonJavaandusetheclient-sidetechnologiesasmuchasisneededfordemonstrationtechnologies.However,beingaJavawebdeveloperprofessional,youhavetolearnthesetechnologiesaswell,tosomeextentatleast,tounderstandwhataclientcandoandtobeabletocooperatewiththeprofessionalsresponsibleforfrontendtechnologies.

MastermindservletPlayingtheMastermindgameviatheWebisabitdifferentfromwhatitusedtobe.Tillnow,wedidnothaveanyuserinteractionandourclassesweredesignedaccordingly.Forexample,wecouldaddanewguesstothetable,alongwiththepartialandfullmatchescalculatedbytheprogram.Nowwehavetoseparatethecreationofanewguess,addittothegame,andsetthefullandpartialmatches.Thistime,wehavetodisplaythetablefirst,andtheuserhastocalculateandprovidethenumberofmatches.

Wehavetomodifysomeoftheclassestobeabletodothat.WeneedtoaddanewmethodtoGame.java:publicRowaddGuess(Guessguess,intfull,intpartial){assertNotFinished();finalRowrow=newRow(guess,full,partial);table.addRow(row);if(itWasAWinningGuess(full)){finished=true;}returnrow;}

Tillnow,wehadonlyonemethodthatwasaddinganewguess,andsincetheprogramknewthesecret,itwasimmediatelycalculatingthevalueoffullandpartial.ThenameofthemethodcouldbeaddNewGuess,overloadingtheoriginalmethod,butthistime,themethodisusednotonlytoaddanewguessbutalsotoaddoldguessestorebuildthetable.

Whentheprogramstarts,therearenoguesses.Theprogramcreatesone,thefirstone.Lateron,whentheusertellstheprogramthefullandpartialmatches,theprogramneedstheGamestructurewithTableandRowobjectscontainingGuessobjectsandthefullandpartialmatchvalues.Thesewerealreadyavailable,butwhenthenewHTTPhitcomesin,wehavetopullitfromsomewhere.Programmingaservlet,wehavetostorethestateofthegamesomewhereandrestoreitwhenanewHTTPrequesthitstheserver.

StoringstateStoringthestatecanbedoneintwoplaces.Oneplace,whichwewillfirstdoinourcode,istheclient.Whentheprogramcreatesanewguess,itaddsittothetableandsendsanHTMLpagethatcontainsnotonlythenewguessbutalsoallthepreviousguessesandthefullandpartialmatchvaluesthattheusergaveforeachoftherows.Tosendthedatatotheserver,thevaluesarestoredinthefieldsofaform.Whentheformissubmitted,thebrowsergatherstheinformationinthefields,createsanencodedstringfromthecontentofthefields,andputsthecontentintothebodyofaPOSTrequest.

Theotherpossibilityforstoringtheactualstateisintheserver.Theservercanstorethestateofthegame,anditcanreconstructthestructurewhenitcreatesanewguess.Theprobleminthiscaseisknowingwhichgametouse.Theservercanandshouldstoremanygames,oneforeachuser,andusersmayusetheapplicationconcurrently.Itdoesnotnecessarilymeanstrongconcurrencyinthesamemeaningasweexaminedinthepreviouschapter.

Eveniftheusersarenotservedatthesametimeinmultiplethreads,therecanbegamesthatareactive.Imaginecnn.comtellingyouthatyoucannotreadthenewsatthemomentbecausesomebodyelseisreadingit.Therecanbemultipleusersplayingmultiplegames,andwhileservinganHTTPrequest,weshouldknowwhichuserweareserving.

Servletsmaintainsessionsthatcanbeusedforthispurposeaswewillseeinthenextsection.

HTTPsessionWhenaclientsendsrequestsfromthesamebrowsertothesameservlet,theseriesofrequestsbelongtoonesession.Toknowthattherequestsbelongtothesamesession,theservletcontainerautomaticallysendsacookienamedJSESSIONIDtotheclient,andthiscookiehasalong,random,hard-to-guessvalue(tkojxpz9qk9xo7124pvanc1zasIruntheapplicationinJetty).TheservletmaintainsasessionstorethatcontainstheHttpSessioninstances.ThekeystringthattravelsinthevalueoftheJSESSIONIDcookieidentifiestheinstances.WhenanHTTPrequestarrivesattheservlet,thecontainerattachesthesessiontotherequestobjectfromthestore.Ifthereisnosessionforthekey,thenoneiscreated,andthecodecanaccessthesessionobjectbycallingtherequest.getSession()method.

AHttpSessionobjectcanstoreattributes.TheprogramcancallthesetAttribute(String,Object),getAttribute(String),andremoveAttribute(String)methodstostore,retrieve,ordeleteanattributeobject.EachattributeisassignedtoaStringandcanbeanyObject.

AlthoughthesessionattributestoreessentiallylooksassimpleasaMap<String,?>object,itisnot.Thevaluesstoredinthesessioncanbemovedfromonenodetoanotherwhentheservletcontainerrunsinaclusteredorotherdistributedenvironment.Todothat,thevaluesareserialized;therefore,thevaluesstoredinthesessionshouldbeSerializable.Failingtodosoisaverycommonnoviceerror.Duringdevelopment,executingthecodeinasimpledevelopmentTomcatorJettycontainerpracticallyneverserializesthesessiontodiskandneverloadsitfromtheserializedform.ThismeansthatthevaluessetusingsetAttributewillbeavailablebycallinggetAttribute.Werunintotroublethefirsttimetheapplicationgetsinstalledinaclusteredenvironment.AssoonasaHTTPrequestarrivesondifferentnodesgetAttributemayreturnnull.ThemethodsetAttributeiscalledononenodeandduringtheprocessingofthenextrequestgetAttributeonadifferentnodecannotdeserializetheattributevaluefromthedisksharedamongthenodes.Thisisusually,andsadly,theproductionenvironment.

You,asadeveloper,shouldalsobeawarethatserializingandde-serializinganobjectisaheavyoperationthatcostsseveralCPUcycles.IfthestructureoftheapplicationusesonlyapartoftheclientstateservingmostoftheHTTPrequests,thenthisisawasteofCPUtocreatethewholestateinmemoryfromaserializedformandthenserializingitagain.Insuchcases,itismoreadvisabletostoreonlyakeyinthesessionandusesomedatabase(SQLorNoSQL)orsomeotherservicetostoretheactualdatareferencedbythekey.Enterpriseapplicationsalmostexclusivelyusethisstructure.

packagepackt.java9.by.example.mastermind.servlet;<br/><br/>importpackt.java9.by.example.mastermind.Color;<br/>importpackt.java9.by.example.mastermind.Table;<br/><br/>importjavax.inject.Inject;<br/>importjavax.inject.Named;<br/><br/>publicclassHtmlTools{<br/>@Inject<br/>Tabletable;<br/><br/>@Inject<br/>@Named("nrColumns")<br/>privateintNR_COLUMNS;<br/><br/>publicStringtag(StringtagName,String...attributes){<br/>StringBuildersb=newStringBuilder();<br/>sb.append("<").append((tagName));<br/>for(inti=0;i<attributes.length;i+=2){<br/>sb.append("").<br/>append(attributes[i]).<br/>append("=\"").<br/>append(attributes[i+1]).<br/>append("\"");<br/>}<br/>sb.append(">");<br/>returnsb.toString();<br/>}<br/><br/>publicStringinputBox(Stringname,Stringvalue){<br/>returntag("input","type","text","name",name,"value",value,"size","1");<br/>}<br/><br/>publicStringcolorToHtml(Colorcolor,introw,intcolumn){<br/>returntag("input","type","hidden","name",paramNameGuess(row,column),<br/>"value",color.toString())+<br/>tag("div","class","color"+color)+<br/>tag("/div")+<br/>tag("div","class","spacer")+<br/>tag("/div");<br/>}<br/><br/><br/>publicStringparamNameFull(introw){<br/>return"full"+row;<br/>}<br/><br/>publicStringparamNamePartial(introw){<br/>return"partial"+row;<br/>}<br/><br/>publicStringparamNameGuess(introw,intcolumn){<br/>return"guess"+row+column;<br/>}<br/><br/>publicStringtableToHtml(){<br/>StringBuildersb=newStringBuilder();<br/>sb.append("<html><head>");<br/>sb.append("<linkrel=\"stylesheet\"type=\"text/css\"href=\"colors.css\">");<br/>sb.append("<title>Mastermindguessing</title>");<br/>sb.append("<body>");<br/>sb.append(tag("form","method","POST","action","master"));<br/><br/>for(introw=0;row<table.nrOfRows();row++){<br/>for(intcolumn=0;column<NR_COLUMNS;column++){<br/>sb.append(colorToHtml(table.getColor(row,column),row,column));<br/>}<br/><br/>sb.append(inputBox(paramNameFull(row),""+table.getFull(row)));<br/>sb.append(inputBox(paramNamePartial(row),""+table.getPartial(row)));<br/>sb.append("<p>");<br/>}<br/>returnsb.toString();<br/>}<br/>}

<html><br/><head><br/><linkrel="stylesheet"type="text/css"href="colors.css"><br/><title>Mastermindguessing</title><br/><body><br/><formmethod="POST"action="master"><br/><inputtype="hidden"name="guess00"value="3"><br/><divclass="color3"></div><br/><divclass="spacer"></div><br/><inputtype="hidden"name="guess01"value="2"><br/><divclass="color2"></div><br/><divclass="spacer"></div><br/><inputtype="hidden"name="guess02"value="1"><br/><divclass="color1"></div><br/><divclass="spacer"></div><br/><inputtype="hidden"name="guess03"value="0"><br/><divclass="color0"></div><br/><divclass="spacer"></div><br/><inputtype="text"<br/>name="full0"value="0"size="1">

<br/><inputtype="text"<br/>name="partial0"value="2"size="1"><br/><p><br/><inputtype="hidden"name="guess10"value="5"><br/><divclass="color5"></div><br/><br/>...deletedcontentthatjustlooksalmostthesame...<br/><br/><p><br/><inputtype="submit"value="submit"><br/></form><br/></body><br/></head><br/></html>

.color0{<br/>background:red;<br/>width:20px;<br/>height:20px;<br/>float:left<br/>}<br/>.color1{<br/>background-color:green;<br/>width:20px;<br/>height:20px;<br/>float:left<br/>}<br/>....color2to.color5isdeleted,contentisthesameexceptdifferentcolors...<br/><br/>.spacer{<br/>background-color:white;<br/>width:10px;<br/>height:20px;<br/>float:left<br/>}

DependencyinjectionwithGuiceTheservletclassisverysimpleasshowninthefollowing:

packagepackt.java9.by.example.mastermind.servlet;

importcom.google.inject.Guice;

importcom.google.inject.Injector;

importorg.slf4j.Logger;

importorg.slf4j.LoggerFactory;

importjavax.servlet.ServletException;

importjavax.servlet.http.HttpServlet;

importjavax.servlet.http.HttpServletRequest;

importjavax.servlet.http.HttpServletResponse;

importjava.io.IOException;

publicclassMastermindextendsHttpServlet{

privatestaticfinalLoggerlog=LoggerFactory.getLogger(Mastermind.class);

publicvoiddoGet(HttpServletRequestrequest,

HttpServletResponseresponse)

throwsServletException,IOException{

doPost(request,response);

}

publicvoiddoPost(HttpServletRequestrequest,

HttpServletResponseresponse)

throwsServletException,IOException{

Injectorinjector=

Guice.createInjector(newMastermindModule());

MastermindHandlerhandler=

injector.getInstance(MastermindHandler.class);

handler.handle(request,response);

}

}

Becausemanythreadsuseservletsconcurrently,andthuswecannotuseinstancefieldsholdingdataforasinglehit,theservletclassdoesnothingelsebutcreateanewinstanceofaMastermindHandlerclassandinvokeitshandlemethod.SincethereisanewinstanceofMastermindHandlerforeachrequest,itcanstoreobjectsinfieldsspecifictotherequest.Tocreateahandler,weusetheGuicelibrarycreatedbyGoogle.

Wehavealreadytalkedaboutdependencyinjection.ThehandlerneedsaTableobjecttoplay,aColorManagerobjecttomanagethecolors,andaGuesserobjecttocreateanewguess,butcreatingtheseorfetchingsomeprefabricatedinstancesfromsomewhereisnotthecorefunctionalityofthehandler.Thehandlerhastodoonething:handletherequest;theinstancesneededtodothisshouldbeinjectedfromoutside.ThisisdonebyaGuiceinjector.

TouseGuice,wehavetolistthelibraryamongthedependenciesinbuild.gradle:

applyplugin:'java'

applyplugin:'jetty'

repositories{

jcenter()

}

dependencies{

providedCompile"javax.servlet:javax.servlet-api:3.1.0"

testCompile'junit:junit:4.12'

compile'org.slf4j:slf4j-api:1.7.7'

compile'ch.qos.logback:logback-classic:1.0.11'

compile'com.google.inject:guice:4.1.0'

}

jettyRun{

contextPath'/hello'

}

Thenwehavetocreateaninjectorinstancethatwilldotheinjection.Theinjectoriscreatedwiththefollowinglineintheservlet:

Injectorinjector=Guice.createInjector(newMastermindModule());

TheinstanceofMastermindModulespecifieswhattoinjectwhere.ThisisessentiallyaconfigurationfileintheJavaformat.Otherdependencyinjectorframeworksused,anduse,XMLandannotationstodescribetheinjectionbindingandwhattoinjectwhere,butGuicesolelyusesJavacode.ThefollowingistheDIconfigurationcode:

publicclassMastermindModuleextendsAbstractModule{

@Override

protectedvoidconfigure(){

bind(int.class)

.annotatedWith(Names.named("nrColors")).toInstance(6);

bind(int.class)

.annotatedWith(Names.named("nrColumns")).toInstance(4);

bind(ColorFactory.class).to(LetteredColorFactory.class);

bind(Guesser.class).to(UniqueGuesser.class);

}

}

ThemethodsusedintheconfiguremethodarecreatedinafluentAPImannersothatthemethodscanbechainedoneaftertheotherandthatthecodecanbereadalmostlikeEnglishsentences.AgoodintroductiontofluentAPIcanbefoundathttps://blog.jooq.org/2012/01/05/the-java-fluent-api-designer-crash-course/.Forexample,thefirstconfigurationlinecouldbereadinEnglishas

Bindtotheclassintwhereveritisannotatedwiththe@Nameannotationhavingvalue"nrColor"totheinstance6.

(Notethattheintvalue6isautoboxedtoanIntegerinstance.)

TheMastermindHandlerclasscontainsfieldsannotatedwith@Injectannotation:

@Inject

@Named("nrColors")

privateintNR_COLORS;

@Inject

@Named("nrColumns")

privateintNR_COLUMNS;

@Inject

privateHtmlToolshtml;

@Inject

Tabletable;

@Inject

ColorManagermanager;

@Inject

Guesserguesser;

ThisannotationisnotGuice-specific.@Injectisapartofthejavax.injectpackageandisastandardpartofJDK.JDKdoesnotprovidethedependencyinjector(DI)frameworkbutsupportsthedifferentframeworkssothattheycanusestandardJDKannotations,andincasetheDIframeworkisreplaced,theannotationsmayremainthesameandnotframework-specific.

WhentheinjectoriscalledtocreateaninstanceofMastermindHandler,itlooksattheclassandseesthatithasanintfieldannotatedwith@Injectand@Named("nrColors"),andfindsintheconfigurationthatsuchafieldshouldhavethevalue6.ItinjectsthevaluetothefieldbeforereturningtheMastermindHandlerobject.Similarly,italsoinjectsthevaluesintotheotherfields,andifitshouldcreateanyoftheobjectstobeinjected,itdoes.Iftherearefieldsintheseobjects,thentheyarealsocreatedbyinjectingotherobjectsandsoon.

ThiswaytheDIframeworkremovestheburdenfromtheprogrammers'shouldertocreatetheinstances.Thisissomethingfairlyboringandisnotthecorefeatureoftheclassesanyway.Instead,itcreatesalltheobjectsneededtohaveafunctionalMastermindHandlerandlinksthemtogetherviatheJavaobjectreferences.Thisway,thedependenciesofthedifferentobjects(MastermindHandlerneedsGuesser,ColorManager,andTable;ColorManagerneedsColorFactory;andTablealsoneedsColorManager,andsoon)becomeadeclaration,specifiedusingannotationsonthefields.Thesedeclarationsareinsidethecodeoftheclasses,anditistherightplaceforthem.Whereelsecouldwespecifywhattheclassneedstoproperlyfunctionthanintheclassitself?

TheconfigurationinourexamplespecifiesthatwhereverthereisaneedforColorFactory,wewilluseLetteredColorFactory,andthatwhereverweneedGuesser,wewilluseUniqueGuesser.Thisisseparatedfromthecodeandithastobelikethat.Ifwewanttochangetheguessingstrategy,wereplacetheconfigurationandthecodeshouldworkwithoutmodifyingtheclassesthatusetheguesser.

GuiceiscleverenoughandyouneednotspecifythatwhereverthereisaneedforTable,wewilluseTable:thereisnobind(Table.class).to(Table.class).FirstIcreatedalinelikethatintheconfiguration,butGuicerewardedmewithanerrormessage,andnow,writingitagaininplainEnglish,Ifeelreallystupid.IfIneedatableIneedatable.Really?

TheMastermindHandlerclassWehavealreadystartedthelistingoftheMastermindHandlerclass,andsincethisclassismorethanahundredlines,Iwillnotincludeithereasawhole.Themostimportantmethodofthisclassishandle:

publicvoidhandle(HttpServletRequestrequest,

HttpServletResponseresponse)

throwsServletException,IOException{

Gamegame=buildGameFromRequest(request);

GuessnewGuess=guesser.guess();

response.setContentType("text/html");

PrintWriterout=response.getWriter();

if(game.isFinished()||newGuess==Guess.none){

displayGameOver(out);

}else{

log.debug("Addingnewguess{}tothegame",newGuess);

game.addGuess(newGuess,0,0);

displayGame(out);

}

bodyEnd(out);

}

Weperformthreesteps.Step1iscreatingthetableandwedoitfromtherequest.Ifthisisnotthestartofthegame,thereisalreadyatableandtheHTMLformcontainsallpreviousguesscolorsandtheanswerstothose.Then,asthesecondstep,wecreateanewguessbasedonthat.Step3istosendthenewHTMLpagetotheclient.

Again,thisisnotamodernapproach,creatingHTMLontheservletcode,butdemonstratingpureservletfunctionalitywithREST,JSON,andJavaScriptwithsomeframeworkwouldmakethischapteraloneafewhundredpageslong,anditwoulddefinitelydistractourfocusfromJava.

PrintingHTMLtexttoaPrintWriterisnotsomethingthatshouldbenewtoyouatthispointinthisbook;therefore,wewillnotlistthatcodehere.YoucandownloadtheworkingexampleonGitHub.Thebranchforthisversionofthecodeisnosession.Insteadofprinting,wewillfocusontheservletparameterhandling.

TherequestparametersareavailableviathegetParametermethod,whichreturnsthestringvalueofaparameter.Thismethodassumesthatanyparameter,beitGETorPOST,appearsonlyonceintherequest.Incasethereareparametersthatappearmultipletimes,thevalueshouldhavebeenastringarray.Insuchacase,weshouldusegetParameterMap,whichreturnsthewholemapwiththeStringkeysandString[]values.Eventhoughwedonothavemultiplevaluesforanykeythistime,andwealsoknowthevaluesofthekeyscomingasPOSTparameters,wewillstillusethelattermethod.Thereasonforthisisthatwewilllaterusethesessiontostorethesevalues,andwewanttohaveamethodthatisreusableinthatcase.

IfyoulookattheearliercommitsintheGitrepository,youwillseethatthefirstversionusedgetParameterandIrefactoreditonlylaterwhenIcreatedthesecondversionoftheprogram,whichstoresthestateinasession.Neverbelieveifanyonetellsyouthatprogramsarecreatedperfectlyupfrontwithoutanyrefactoringduringdevelopment.Donotfeelashamedtocreatefoolishcodeandrefactoritlater.Itisonlyshamefulifyoudonotrefactorit.

Togettothatweconverttherequest'sMap<String,String[]>toMap<String,String>:

privateGamebuildGameFromRequest(HttpServletRequestrequest){

returnbuildGameFromMap(toMap(request));

}

privateMap<String,String>toMap(HttpServletRequestrequest){

log.debug("convertingrequesttomap");

returnrequest.getParameterMap().entrySet().

stream().collect(

Collectors.toMap(

Map.Entry::getKey,

e->e.getValue()[0]));

}

Then,weusethatmaptore-createthegame:

privateGamebuildGameFromMap(Map<String,String>params){

finalGuesssecret=newGuess(newColor[NR_COLUMNS]);

finalGamegame=newGame(table,secret);

for(introw=0;

params.containsKey(html.paramNameGuess(row,0));

row++){

Color[]colors=getRowColors(params,row);

Guessguess=newGuess(colors);

finalintfull=Integer.parseInt(params.get(html.paramNameFull(row)));

finalintpartial=Integer.parseInt(params.get(html.paramNamePartial(row)));

log.debug("Addingguesstogame");

game.addGuess(guess,full,partial);

}

returngame;

}

TheconversionfromStringtointisdoneviathemethodparseInt.ThismethodthrowsNumberFormatExceptionwhentheinputisnotanumber.Trytorunthegame,usethebrowser,andseehowJettyhandlesthecasewhentheservletthrowsanexception.Howmuchvaluableinformationdoyouseeinthebrowserthatcanbeusedbyapotentialhacker?Fixthecodesothatitaskstheuseragainifanyofthenumbersarenotwellformatted!

StoringstateontheserverTheapplicationstateshouldusuallynotbesavedontheclient.Theremaybesomespecialcaseinadditiontotheonewhereyouwriteeducationalcodeandwanttodemonstratehowtodoit.Generally,thestateoftheapplicationrelatedtotheactualuseisstoredinthesessionobjectoronsomedatabase.Thisisespeciallyimportantwhentheapplicationrequeststheusertoenteralotofdataanddoesnotwanttheusertolosetheworkifthereissomehiccupintheclientcomputer.

Youspendalotoftimeselectingtheappropriateitemsinanonlineshop,choosingtheappropriateitemsthatworktogether,creatingaconfigurationofyournewmodelairplane,andallofasudden,thereisablackoutinyourhome.Ifthestatewerestoredontheclientyou'dhavehadtostartfromscratch.Ifthestateisstoredontheserver,thestateissavedtodisk;theserversareduplicated,fedbybattery-backedpowersupplies,andwhenyourebootyourclientmachinewhenthepowercomesbackinyourhome,youlogin,andmiraculously,theitemsareallthereinyourshoppingbasket.Well,itisnotamiracle;itiswebprogramming.

Inourcase,thesecondversionwillstorethestateofthegameinthesession.Thiswilllettheuserhavethegamerestoresolongasthesessionisthere.Iftheuserquitsandrestartsthebrowser,thesessiongetslostandanewgamecanstart.

Sincethereisnoneedtosendtheactualcolorsandmatchinginhiddenfieldsthistime,theHTMLgenerationismodifiedabit,andthegeneratedHTMLwillalsobesimpler:

<html>

<head>

<linkrel="stylesheet"type="text/css"href="colors.css">

<title>Mastermindguessing</title>

<body>

<formmethod="POST"action="master">

<divclass="color3"></div>

<divclass="spacer"></div>

<divclass="color2"></div>

<divclass="spacer"></div>

<divclass="color1"></div>

<divclass="spacer"></div>

<divclass="color0"></div>

<divclass="spacer"></div>0

<divclass="spacer"></div>2<p>

<divclass="color5"></div>

...

<divclass="spacer"></div>

<divclass="color1"></div>

<divclass="spacer"></div>

<inputtype="text"name="full2"value="0"size="1"><inputtype="text"name="partial2"value="0"size="1">

<p>

<inputtype="submit"value="submit">

</form>

</body>

</head></html>

Thenumberoffullandpartiallymatchingcolorsisdisplayedasasimplenumber,sothisversiondoesnotallowcheatingorchangingpreviousresults.(Thesearethenumbers0and2afterthedivtagsthathavetheCSSclassspacer.)

ThehandlemethodinMastermindHandleralsochanges,asshowninthefollowing:

publicvoidhandle(HttpServletRequestrequest,

HttpServletResponseresponse)

throwsServletException,IOException{

Gamegame=buildGameFromSessionAndRequest(request);

GuessnewGuess=guesser.guess();

response.setContentType("text/html");

PrintWriterout=response.getWriter();

if(game.isFinished()||newGuess==Guess.none){

displayGameOver(out);

}else{

log.debug("Addingnewguess{}tothegame",newGuess);

game.addGuess(newGuess,0,0);

sessionSaver.save(request.getSession());

displayGame(out);

}

bodyEnd(out);

}

ThisversionoftheclassgetsaSessionSaverobjectinjectedbytheGuiceinjector.Thisisaclassthatwecreate.ThisclasswillconvertthecurrentTableintosomethingthatisstoredinthesession,anditalsorecreatesthetablefromthedatastoredinthesession.ThehandlemethodusesthebuildGameFromSessionAndRequestmethodtorestorethetableandtoaddthefullandpartialmatchanswersthattheuserjustgaveintherequest.Whenthemethodcreatesanewguessandfillsitinthetable,andalsosendsittotheclientintheresponse,itsavesthestateinthesessionbycallingthesavemethodviathesessionSaverobject.

ThebuildGameFromSessionAndRequestmethodreplacestheotherversion,whichwenamedbuildGameFromRequest:

privateGamebuildGameFromSessionAndRequest(HttpServletRequestrequest){

Gamegame=buildGameFromMap(sessionSaver.restore(request.getSession()));

Map<String,String>params=toMap(request);

introw=getLastRowIndex(params);

log.debug("lastrowis{}",row);

if(row>=0){

finalintfull=Integer.parseInt(params.get(html.paramNameFull(row)));

finalintpartial=Integer.parseInt(params.get(html.paramNamePartial(row)));

log.debug("settingfull{}andpartial{}forrow{}",full,partial,row);

table.setPartial(row,partial);

table.setFull(row,full);

if(full==table.nrOfColumns()){

game.setFinished();

}

}

returngame;

}

NotethatthisversionhasthesameillnessofusingtheparseIntmethodfromtheIntegerclassinJDK,whichthrowsanexception.

TheGameSessionSaverclassThisclasshasthreepublicmethods:

save:Thissavesatabletotheusersessionrestore:Thisgetsatablefromtheusersessionreset:Thisdeletesanytablethatmaybeinthesession

Thecodeoftheclassisthefollowing:

publicclassGameSessionSaver{

privatestaticfinalStringSTATE_NAME="GAME_STATE";

@Inject

privateHtmlToolshtml;

@Inject

Tabletable;

@Inject

ColorManagermanager;

publicvoidsave(HttpSessionsession){

Map<String,String>params=convertTableToMap();

session.setAttribute(STATE_NAME,params);

}

publicvoidreset(HttpSessionsession){

session.removeAttribute(STATE_NAME);

}

publicMap<String,String>restore(HttpSessionsession){

Map<String,String>map=

(Map<String,String>)

session.getAttribute(STATE_NAME);

if(map==null){map=newHashMap<>();}

returnmap;

}

privateMap<String,String>convertTableToMap(){

Map<String,String>params=newHashMap<>();

for(introw=0;row<table.nrOfRows();row++){

for(intcolumn=0;

column<table.nrOfColumns();column++){

params.put(html.paramNameGuess(row,column),

table.getColor(row,column).toString());

}

params.put(html.paramNameFull(row),

""+table.getFull(row));

params.put(html.paramNamePartial(row),

""+table.getPartial(row));

}

returnparams;

}

}

Whenwesavethesessionandconvertthetabletoamap,weuseaHashMap.Theimplementationinthiscaseisimportant.TheHashMapclassimplementstheSerializableinterface;therefore,wecanbesafeputtingittothesession.ThisalonedoesnotguaranteethateverythinginHashMapisSerializable.ThekeysandthevaluesinourcaseareStrings,andfortunately,theStringclassalsoimplementstheSerializableinterface.Thisway,theconvertedHashMapobjectcanbesafelystoredinthesession.

Alsonotethat,althoughserializationcanbeslow,storingHashMapinasessionissofrequentthatitimplementsitsownserializationmechanism.Thisimplementationisoptimizedandavoidsserializationbeingdependentontheinternalstructureofthemap.

ItistimetothinkaboutwhywehavetheconvertTableToMapmethodinthisclassandbuildGameFromMapinMastermindHandler.ConvertingthegameandthetableinittoaMapandtheotherwayroundshouldbeimplementedtogether.Theyarejusttwodirectionsofthesameconversion.Ontheotherhand,theimplementationoftheTabletoMapdirectionshoulduseaMapversionthatisSerializable.Thisisverymuchrelatedtosessionhandling.ConvertingaMapobject,ingeneral,toaTableobjectisonelevelhigher,restoringthetablefromwhereveritwasstored:client,session,database,orinthemoistureofthecloud.Sessionstorageisonlyonepossibleimplementation,andmethodsshouldbeimplementedintheclassthatmeetstheabstractionlevel.Thebestsolutioncouldbetoimplementtheseinaseparateclass.Youhavehomework!

Theresetmethodisnotusedfromthehandler.ThisisinvokedfromtheMastermindclass,thatis,theservletclasstoresetthegamewhenwestartit:

publicvoiddoGet(HttpServletRequestrequest,

HttpServletResponseresponse)

throwsServletException,IOException{

GameSessionSaversessionSaver=newGameSessionSaver();

sessionSaver.reset(request.getSession());

doPost(request,response);

}

Withoutthis,playingthegameagainstthemachineoncewouldjustdisplaythefinishedgameeverytimewewanttostartitagain,untilweexitthebrowserandrestartitorexplicitlydeletetheJSESSIONIDcookiesomewhereintheadvancedmenuofthebrowser.Callingresetdoesnotdeletethesession.Thesessionremainsthesame,andthusthevalueofJSESSIONIDtoo,butthegameisdeletedfromthesessionobjectthattheservletcontainermaintains.

RunningtheJettywebservletSincewehaveincludedtheJettypluginintoourGradlebuild,thetargetsofthepluginareavailable.TostartJettyisaseasyastypingthefollowing:gradlejettyRun

Thiswillcompilethecode,buildtheWARfile,andstarttheJettyservletcontainer.Tohelpusremember,italsoprintsthefollowingonthecommandline:Runningathttp://localhost:8080//hello

WecanopenthisURLandseetheopeningscreenofthegamewiththecolorsthattheprogramcreatedasafirstguess:

Nowitistimetohavesomefunandplaywithourgame,givinganswerstotheprogram.Donotmakeit

easyforthecode!Refertothefollowingscreenshot:

Atthesametime,ifyoulookattheconsolewhereyouhavetypedgradlejettyRun,youwillseethatthecodeisprintingoutlogmessages,asshowninthefollowingscreenshot:

Theseprintoutscomethroughtheloggerthatwehaveinourcode.Inthepreviouschapters,weusedtheSystem.out.printlnmethodcallstosendinformationalmessagestotheconsole.Thisisapracticethatshouldnotbefollowedinanyprogramthatismorecomplexthanahelloworld

Logging

ThereareseveralloggingframeworksavailableforJavaandeachhasadvantagesanddisadvantages.ThereisonebuiltintoJDKinthejava.util.loggingpackageandaccessingtheloggerissupportedbytheSystem.getLoggermethod:theSystem.LoggerandSystem.LoggerFinderclasses.Eventhoughjava.util.logginghasbeenavailableinJavasinceJDK1.4,alotofprogramsuseotherloggingsolutions.Inadditiontothebuilt-inlogging,wehavetomentionlog4j,slf4jandApacheCommonsLogging.Beforegettingintothedetailsofthedifferentframeworks,let'sdiscusswhyitisimportanttouselogginginsteadofjustprintingtothestandardoutput.

ConfigurabilityThemostimportantreasonisconfigurabilityandeaseofuse.Weuseloggingtorecordinformationabouttheoperationofcode.Thisisnotthecorefunctionalityoftheapplicationbutisinevitabletohaveaprogramthatcanbeoperated.Therearemessagesweprintouttothelog,whichcanbeusedbytheoperatingpersonneltoidentifyenvironmentalissues.Forexample,whenanIOExceptionisthrownanditgetslogged,theoperationmaylookatthelogsandidentifythatthediskgotfull.Theymaydeletefiles,oraddanewdiskandextendthepartition.Withoutthelogs,theonlyinformationwouldbethattheprogramdoesnotwork.

Thelogsarealsousedmanytimestohuntdownbugs.Someofthebugsdonotmanifestinthetestenvironmentandareverydifficulttoreproduce.Insuchacase,thelogsthatprintoutdetailedinformationabouttheexecutionofthecodearetheonlysourceoffindingtherootcauseofsomeerror.

SinceloggingneedsCPU,IObandwidth,andotherresources,itshouldbecarefullyexaminedwhatandwhentolog.Thisexaminationandthedecisionscouldbedoneduringprogramming,andasamatteroffact,thatistheonlypossibilityifweusedSystem.out.printlnforlogging.Ifweneedtofindabug,weshouldlogalot.Ifwelogalot,theperformanceofthesystemwillgodown.Theconclusionisthatwehavetologonlyifitisneeded.Ifthereisabuginthesystemthatcannotbereproduced,thedevelopersasktheoperationtoswitchondebugloggingforashortperiod.SwitchingonandoffdifferentpartsofloggingisnotpossiblewhenSystem.out.printlnisused.Whenthedebuglevellogisswitchedon,theperformancemaygodownforawhile,butatthesametime,thelogsbecomeavailableforanalysis.Atthesametime,theanalysisissimplerwhenwehavetofindtheloglinesthatarerelevant(andyoudonotknowbeforehandwhicharerelevant)ifthereisasmall(afewhundredmegabyteslogfile)ratherthanalotof2-GBcompressedlogfilestofindthelinesin.

Usingalogframework,youcandefineloggersthatidentifythesourceofthelogmessagesandloglevels.Astringusuallyidentifiesthelogger,anditisacommonpracticetousethenameoftheclassfromwhichthelogmessageiscreated.Thisissuchacommonpracticethatthedifferentlogframeworksprovidefactoryclassesthatgettheclassitself,insteadofitsname,togetalogger.

Thepossiblelogginglevelsmaybeslightlydifferentindifferentloggingframeworks,butthemostimportantlevelsareasfollows:

FATAL:Thisisusedwhenthelogmessageisaboutsomeerrorthatpreventstheprogramfromcontinuingitsexecution.ERROR:Thisisusedwhenthereissomesevereerror,buttheprogramcanstillgoonfunctioningalthough,probably,insomelimitedmanner.WARNING:Thisisusedwhenthereissomeconditionthatisnotadirectproblembutmaylaterleadtoanerrorifnotattended.Forexample,theprogramrecognizesthatadiskisnearfull,somedatabaseconnectionsanswerwithinlimitsbutclosetothetimeoutvalue,andsimilarsituations.INFO:Thisisusedtocreatemessagesaboutnormaloperationsthatmaybeinterestingtooperateandarenotanerrororwarning.Thesemessagesmayhelptheoperationtodebugtheoperationalenvironmentsettings.DEBUG:Thisisusedtologinformationabouttheprogramthatisdetailedenough(hopefully)tofinda

buginthecode.Thetrickisthatwhenweputthelogstatementintothecode,wedonotknowwhatbugitcouldbe.Ifweknew,webetterfixedit.TRACE:Thisisevenmoredetailedinformationabouttheexecutionofthecode.

Thelogframeworksareusuallyconfiguredusingsomeconfigurationfile.Theconfigurationmaylimitthelogging,switchingoffcertainlevels.Inanormaloperationalenvironment,thefirstthreelevelsareusuallyswitchedon,andINFO,DEBUG,andTRACEareswitchedonwhenreallyneeded.Itisalsopossibletoswitchonandoffcertainlevelsonlyforcertainloggers.IfweknowthattheerroriscertainlyintheGameSessionSaverclass,thenwecanswitchontheDEBUGlevelonlyforthatclass.

Logfilesmayalsocontainotherinformationthatwedidnotdirectlycodeandwouldbeverycumbersometoprinttothestandardoutput.Usually,eachlogmessagecontainstheprecisetimewhenthemessagewascreated,thenameofthelogger,and,manytimes,theidentifierofthethread.Imagineifyouwereforcedtoputallthistoeachandeveryprintlnargument;youwouldprobablysoonwritesomeextraclasstodothat.Don't!Ithasalreadybeendoneprofessionally:itistheloggerframework.

Loggerscanalsobeconfiguredtosendthemessagetodifferentlocations.Loggingtotheconsoleisonlyonepossibility.Loggingframeworksarepreparedtosendmessagestofiles,database,WindowsEventRecorder,syslogservice,ortoanyothertarget.Thisflexibility,whichmessagetoprint,whatextrainformationtoprint,andwheretoprintisreachedbyseparatingthedifferenttasksthattheloggerframeworkdoesintoseveralclassesfollowingthesingleresponsibilityprinciple.

Theloggerframeworksusuallycontainloggersthatcreatethelogs,formattersthatformatthemessagefromtheoriginalloginformation,manytimes,addinginformationsuchasthreadIDandtimestamp,andappendersthatappendtheformattedmessagetosomedestination.Theseclassesimplementinterfacesdefinedintheloggingframeworkandnothingbutthesizeofthebookstopsusfromcreatingourownformattersandappenders.

Whenalogisconfigured,theappendersandformattersareconfigured,giventheclassthatimplementsthem.Therefore,whenyouwanttosendsomelogstosomespecialdestination,youarenotlimitedtotheappendersthatareprovidedbytheauthorsoftheframework.Therearealotofindependentopen-sourceprojectsfordifferentloggingframeworksprovidingappendersfordifferenttargets.

PerformanceThesecondreasontousealoggingframeworkisperformance.Althoughitisnotgoodtooptimizeforperformancebeforeweprofilethecode(prematureoptimization),usingsomemethodologyknowntobeslowandinsertingseverallinesintoourperformance-criticalcode,invokingslowmethodsisnotreallyprofessionaleither.Usingawell-established,highlyoptimizedframeworkinawaythatisindustrybestpracticeshouldnotbequestionable.

UsingSystem.out.printlnsendsthemessagetoastreamandreturnsonlywhentheIOoperationisdone.Usingreallogginghandlestheinformationtotheloggerandletstheloggerdotheloggingasynchronously,anditdoesnotwaitforcompletion.Itisreallyadrawbackthatloginformationmaybelostifthereissomesystemfailure,butthisisusuallynotaseriousissueconsideringhowrarelythathappensandwhatisontheothersideofthewage:performance.Whatdoweloseifthereisamissingdebugloglinewhenthediskgotfull,anywayrenderingthesystemunusable?

Thereisoneexceptiontothis:auditlogging—whensomeloginformationaboutthetransactionsofthesystemhastobesavedforlegalreasonssothattheoperationandtheactualtransactionscanbeaudited.Insuchacase,theloginformationissavedinatransactionalmanner,makingthelogpartofthetransaction.Becausethatisatotallydifferenttypeofrequirement,auditloggingisnotusuallydonewithanyoftheseframeworks.

Also,System.out.printlnisnotsynchronizedandthatwaydifferentthreadsmayjustgarbletheoutput.Logframeworkspayattentiontothisissue.

LogframeworksThemostwidelyusedloggingframeworkisApachelog4j.Itcurrentlyhasasecondversionthatisatotalrewriteofthefirstversion.Itisveryversatileandhasmanyappendersandformatters.Theconfigurationoflog4jcanbeinXMLorpropertiesfileformat,anditcanalsobeconfiguredthroughAPI.

Theauthoroflog4jversion1createdanewloggingframework:slf4j.Thislogginglibraryisessentiallyafaçadethatcanbeusedtogetherwithanyotherloggingframework.Thus,whenyouuseslf4jinalibraryyoudevelop,andyourcodeisaddedtoaprogramasadependencythatusesadifferentloggingframework,itiseasytoconfigureslf4jtosendthelogstotheloggersoftheotherframework.Thus,thelogswillbehandledtogetherandnotinseparatefile,whichisdesirabletodecreasethecostofoperation.Whendevelopingyourlibrarycodeoranapplicationthatusesslf4j,thereisnoneedtoselectanotherlogframeworktoslf4j.Ithasitsownsimpleimplementationcalledbacklog.

ApacheCommonsLoggingisalsoafaçadewithitsownloggingimplementationifnothingelsefails.Themajordifferencefromslf4jisthatitismoreflexibleinconfigurationandwhatunderlyingloggingtouse,anditimplementsarun-timealgorithmtodiscoverwhichloggingframeworkisavailableandistobeused.Theindustrybestpracticeshowsthatthisflexibility,whichalsocomeswithhighercomplexityandcost,isnotneeded.

Java9loggingJava9includesafacadeimplementationforlogging.Theuseisverysimpleandwecanexpectthatloggingframeworkswillverysoonstarttosupportthisfaçade.ThefactthatthisfaçadeisbuiltintotheJDKhastwomajoradvantage:

Thelibrariesthatwanttologdonotneedtohaveanydependencyonanyloggingframeworkorloggingfaçadeanymore.TheonlydependencyistheJDKlogfaçadethatisthereanyway.TheJDKlibrariesthatlogthemselvesusethisfaçadeandthustheywilllogintothesamelogfileastheapplication.

IfweusetheJDK-providedloggingfaçadethestartoftheColorManagerclasswillbechangedtothefollowing:

packagepackt.java9.by.example.mastermind;

importjavax.inject.Inject;

importjavax.inject.Named;

importjavax.inject.Singleton;

importjava.util.HashMap;

importjava.util.Map;

importjava.lang.System.Logger;

importstaticjava.lang.System.Logger.Level.DEBUG;

@Singleton

publicclassColorManager{

protectedfinalintnrColors;

protectedfinalMap<Color,Color>successor=newHashMap<>();

privateColorfirst;

privatefinalColorFactoryfactory;

privatestaticfinalLoggerlog=System.getLogger(ColorManager.class.getName());

@Inject

publicColorManager(@Named("nrColors")intnrColors,

ColorFactoryfactory){

log.log(DEBUG,"creatingcolorManagerfor{0}colors",

nrColors);

Inthisversionwedonotimporttheslf4jclasses.Insteadweimportthejava.lang.System.Loggerclass.

NotethatwedonotneedtoimporttheSystemclass,becausetheclassesfromthejava.langpackageareautomaticallyimported.ThisisnottruefortheclassesthatarenestedclassesintheSystemclass.

TogetaccesstotheloggertheSystem.getLoggerstaticmethodiscalled.Thismethodfindstheactualloggerthatisavailableandreturnsoneforthenamethatwepassasargument.ThereisnoversionofthemethodgetLoggerthatacceptstheclassastheargument.IfwewanttosticktotheconventionthenwehavetowriteColorManager.class.getName()togetthenameoftheclassorwecanwritetherethenameoftheclassasastring.Thesecondapproachhasthedrawbackthatitdoesnotfollowthechangeofthenameoftheclass.IntelligentIDEslikeIntelliJ,Eclipse,orNetbeansrenamethereferencestoclassesautomaticallybuttheyhaveahardtimewhenthenameoftheclassisusedinastring.

TheinterfaceSystem.Loggerdoesnotdeclaretheconveniencemethodserror,debug,warning,andsoon,thatarefamiliarfromotherloggingframeworksandfaçade.Thereisonlyonemethodnamedlogandthefirst

argumentofthismethodistheleveloftheactuallogweissue.Thereareeightlevelsdefined:ALL,TRACE,DEBUG,INFO,WARNING,ERROR,andOFF.Whencreatingalogmessagewearesupposedtouseoneofthemiddlesixlevels.ALLandOFFaremeanttobepassedtotheisLoggablemethod.Thismethodcanbeusedtocheckiftheactuallogginglevelgetsloggedornot.Forexample,ifthelevelissettoINFOthenmessagessentwithDEBUGorTRACEwillnotbeprinted.

TheactualimplementationislocatedbytheJDKusingtheserviceloaderfunctionality.Thelogimplementationhastobeinamodulethatprovidestheinterfacejava.lang.System.LoggerFinderviasomeimplementation.InotherwordsthemoduleshouldhaveaclassthatimplementstheLoggerFinderinterfaceandthemodule-info.javashoulddeclarewhichclassitisusingthecode:

providesjava.lang.System.LoggerFinderwith

packt.java9.by.example.MyLoggerFinder;

TheMyLoggerFinderclasshastoextendtheLoggerFinderabstractclasswiththemethodgetLogger.

LoggingpracticeThepracticeofloggingisverysimple.Ifyoudonotwanttospendtoomuchtimeexperimentingwithdifferentloggingsolutionsandyoudonothavesomespecialrequirement,thensimplygowithslf4j,addtheJARtothedependencylistasacompiledependency,andstartusinglogginginthesourcecode.

Sinceloggingisnotinstance-specific,andloggersimplementthreadsafety,thelogobjectsthatweusuallyusearestoredinastaticfield,andsincetheyareusedaslongastheclassisused,theprogramrunningthefieldisalsofinal.Forexampleusingtheslf4jfaçadewecangetaloggerwiththefollowingcommand:privatestaticfinalLoggerlog=LoggerFactory.getLogger(MastermindHandler.class);

Togetthelogger,theloggerfactoryisused,whichjustcreatestheloggerorreturnstheonethatisalreadyavailable.

Thenameofthevariableisusuallylogorlogger,butdonotbesurprisedifyouseeLOGorLOGGER.Thereasonforuppercasingthenameofthevariableisthatsomestaticcodeanalysischeckerstreatstaticfinalvariablesasconstants,astheyreallyare,andtheconventionintheJavacommunityistouseuppercasenamesforsuchvariables.Itisamatteroftaste;manytimeslogandloggerareusedinlowercase.

Tocreatealogitemthemethodstrace,debug,info,warn,anderrorcreateamessagewiththerespectivelevelasthenameimplies.Forexample,considerthefollowingline:

log.debug("Addingnewguess{}tothegame",newGuess);

Itcreatesadebugmessage.Slf4jhassupportforformattingusingthe{}literalinsidestrings.Thisway,thereisnoneedtoappendthestringfromsmallparts,andincasetheactuallogitemisnotsenttothelogdestination,theformattingwillnotperform.IfweuseStringconcatenationinanyformtopassastringasanargument,thentheformattingwillhappenevenifdebugloggingisnotdesiredaspertheexample.

Theloggingmethodsalsohaveaversionthatgetsonlytwoarguments:aStringmessageandThrowable.Inthiscase,theloggingframeworkwilltakecareoftheoutputoftheexceptionandthestacktracealongwithit.Ifyoulogsomethinginexceptionhandlingcode,logtheexceptionandlettheloggerformatit.

OthertechnologiesWediscussedtheservlettechnology,abitofJavaScript,HTML,andCSS.Whenprogramminginarealprofessionalenvironment,thesetechnologiesaregenerallyused.Thecreationoftheuserinterfaceofapplications,however,wasnotalwaysbasedonthesetechnologies.Olderoperatingsystem-nativeGUIapplicationsaswellasSwing,AWT,andSWTuseadifferentapproachtocreateUI.TheybuilduptheUIfacingtheuserfromprogramcode,andtheUIisbuiltasahierarchicalstructureofcomponents.Whenwebprogrammingstarted,Javadevelopershadexperiencewithtechnologiesliketheseandprojectscreatedframeworksthattriedtohidethewebtechnologylayer.

OnetechnologyworthmentioningisGoogleWebToolkit,whichimplementstheserveraswellasthebrowsercodeinJava,butsincethereisnoJavaenvironmentimplementedinthebrowsers,ittranspiles(converts)theclientpartofthecodefromJavatoJavaScript.Thelastreleaseofthetoolkitwascreatedtwoyearsagoin2014andsincethenGooglehasreleasedothertypesofwebprogrammingtoolkitsthatsupportnativeJavaScript,HTML,andCSSclientdevelopment.

Vaadinisalsoatoolkitthatyoumaycomeacross.ItletsyouwriteGUIcodeontheserverinJava.ItisbuiltontopofGWTandiscommerciallysupported.ItmaybeagoodchoiceincasetherearedevelopersavailablewhohaveexperiencewithGUIdevelopmentinJavabutnotinwebnativetechnologies,andtheapplicationdoesnotrequirespecialusabilitytuningontheclientside.Atypicalintranetcorporateapplicationcanselectitasatechnology.

JavaServerFaces(JSF)isatechnologythattriestooffloadtheclient-sidedevelopmentoftheapplicationfromthedevelopersprovidingwidgetsreadytobeusedandtheserverside.ItisacollectionofseveralJavaSpecificationRequests(JSR)andthereareseveralimplementations.ThecomponentsandtheirrelationsareconfiguredinXMLfilesandtheservercreatestheclientnativecode.Inthistechnology,thereisnotranspilationfromJavatoJavaScript.Itismorelikeusingalimitedbuthugesetofwidgets,limitingtheusetothoseonly,andgivingupdirectprogrammingofthewebbrowser.Ifonehastheexperienceandknowledge,however,theycancreatenewwidgetsinHTML,CSS,andJavaScript.

TherearemanyothertechnologiesthatweredevelopedtosupportwebapplicationsinJava.Themodernapproachadvocatedbymostofthebigplayersistodeveloptheserversideandtheclientsideusingseparatetoolsetsandmethodologies,andconnectthetwousingRESTcommunication.

Summary

Inthischapter,youlearntthestructureofwebprogramming.ThiswasnotpossiblewithoutunderstandingthebasicsofTCP/IPnetworking,whichistheprotocoloftheInternet.TheapplicationlevelprotocolthatisusedoverthatisHTTP,currentlyinaverynewversion2.0,whichisstillnotsupportedbytheservletstandard.WecreatedaversionoftheMastermindgamethat,thistime,canreallybeplayedusingthebrowserandwestarteditinadevelopmentenvironmentusingJetty.Weexaminedhowtostorethegamestateandimplementedtwoversions.Finally,welearnedthebasicsofloggingandwelookedatothertechnologies.Atthesametime,wealsolookedatthedependencyinjectionimplementationGuicefromGoogle,andwestudiedhowitworksunderthehood,andwhyandhowtouseit.

Afterthischapter,youwillbeabletostartthedevelopmentofawebapplicationinJavaandwillunderstandthearchitectureofsuchaprogram.YouwillunderstandwhatisunderthehoodwhenyoustartlearninghowtoprogramwebapplicationsusingtheSpringframework,whichhidesmanyofthecomplexitiesofwebprogramming.

BuildingaCommercialWebApplicationUsingRESTWewereplayingaroundtillnow,butJavaisnotatoy.WewanttouseJavaforsomethingrealandserious,commercialandprofessional.Inthischapter,wewilldothat.Theexampleisnotsomethingthatisonlyinterestingtoplaywith,suchasMastermindinthepreviousthreechapters,butratherarealcommercialapplication.Notareal-lifeapplicationactually.Youshouldnotexpectanythinglikethatinabook.Itwouldbetoolongandnoteducatingenough.However,theapplicationthatwewilldevelopinthischaptercanbeextendedandcanbeusedasacoreforareal-lifeapplicationincaseyoudecidedtodoso.

Inthepreviouschapter,wecreatedservlets.Todoso,weusedtheservletspecification,andwehand-implementedservlets.Thatissomethingyouwillrarelydothesedays.Instead,wewilluseareadilyavailableframework,thistime,Spring.ItisthemostwidelyusedframeworkforJavacommercialapplications,andIdaresayitisthedefactostandard.Itwilldoallthetediousworkthatwehadtodo(atleasttounderstandandlearnhowaservletworks)inthepreviouschapter.WewillalsouseSpringfordependencyinjection(whyusetwoframeworkswhenonedoesitall?),andwewilluseTomcat.

Inthepreviouschapter,weusedGuiceasaDIframeworkandJettyasaservletcontainer.Theycanbeaperfectlygoodchoiceforsomeprojects.Forotherprojects,otherframeworksdobetter.Tohavetheopportunitytolookatdifferenttoolsinthisbook,wewillusedifferentframeworkseventhoughalltheexamplescouldbecreatedbysimplyusingTomcatandSpring.

Thecommercialapplicationwewilldevelopwillbeanorderingsystemtargetingresellers.Theinterfacewewillprovidetotheuserswillnotbeawebbrowser;rather,itwillbeREST.Theuserswillthemselvesdevelopapplicationsthatcommunicatewithoursystemandplaceordersfordifferentproducts.Thestructureoftheapplicationwewilldevelopwillbemicroservicesarchitecture,andwewillusesoapUItotesttheapplication,inadditiontothestandardChromedevelopertoolfeatures.

TheMyBusinesswebshop

Imaginethatwehaveahugetradingandlogisticscompany.Therearetensofthousandsofdifferentproductsontheshelves;hundredsoflorriescometoourwarehousebringingnewgoods,andhundredsoflorriesdelivergoodstoourcustomers.Tomanagetheinformation,wehaveaninventorysystemthatkeepstrackofthegoodseveryday,hour,andminutetoknowwhatweactuallyhaveinthewarehouse.Weserveourcustomerswithouthumansmanagingthewarehouseinformation.Formerly,therewerephones,faxmachines,andeventelex,buttoday,allweuseistheInternetandwebservices.Wedonotprovideawebsiteforourcustomers.Wehaveneverdirectlyservedtheendusersinourimaginedbusiness,butthesedays,wehaveasubsidiarythatwestartedoffasaseparatecompanytodojustthat.Theyhaveawebsite,anditistotallyindependentfromus.Theyarejustoneofourhundredsofregisteredpartnerswhoeachuseawebserviceinterfacetoseetheproductswehave,orderproducts,andtracktheorderstatus.

SamplebusinessarchitectureOurpartnersarealsolargecompanieswithautomatedadministration,withseveralprogramsrunningonseveralmachines.Wehavenointerestintheirarchitectureandthetechnologytheyuse,butwewanttointegratetheiroperations.Wewanttoservetheminawaythatdoesn'trequireanyhumaninteractionfortheadministrationtoordergoodsoneitherofoursides.Todoso,awebserviceinterfaceisprovidedthatcanbeutilizednomatterwhatITinfrastructuretheyuse.

Onourside,asweimaginetheexample,werecentlyreplacedourmonolithicapplicationwithmicroservicesarchitecture,andthoughtherearestillsomeSOAP-basedsolutionsinthesystem,mostofthebackendmodulescommunicateusingHTTPSandRESTprotocols.SomeofthemodulesstillrelyonasynchronousfiletransfersdoneonadailybasisusingFTPstartedfromaUNIXcronjob.TheGeneralLedgersystemwasprogrammedinCOBOL.Fortunately,wedonotneedtodealwiththesedinosaurs.

Allthisstructureisanimaginedsetupbutarealisticone.Imadeupanddescribedthesepartstogiveyouapictureofhowyoumayseemixedtechnologiesinalargeenterprise.WhatIdescribedhereisaverysimplesetup.Therearecompaniesthathavemorethanathousandsoftwaremodulesintheirsystemsusingdifferenttechnologiesandtotallydifferentinterfaces,allinterconnectedwitheachother.Thisisnotbecausetheylikethemess,butthatisthewayitbecomesafter30yearsofcontinuousITdevelopment.Newtechnologiescomeandoldtechnologiesfadeout.Thebusinesschangesandyoucannotsticktotheoldtechnologiesifyouwanttostaycompetitive.Atthesametime,youjustcannotreplacetheentireinfrastructureinstantaneously.Theresultisthatweseeinanenterprisefairlyoldtechnologiesstillrunningand,manytimes,newtechnologies.Oldtechnologiesgetrolledoutbytime.Theydonotstayforever,andstill,wearesurprisedsometimeswhenadinosaurcomesinfrontofus.

Whatwehavetodealwithisthetwofrontendcomponentsthatwewilldevelop.Theseareasfollows:

ProductInformationOrderPlacementandTracking

Inthefollowingimage,youcanseethearchitecturalUMLdiagramofthestructurethatwelookat.Thepartswewillhaveinteractionwithareonlythefrontendcomponents,butithelpsunderstandtheworkingandtheirroleifwehaveabiggerpicture:

ProductInformationdeliversinformationaboutasingleproduct,butitcanalsodeliveralistofproductsbasedonthequerycriteria.OrderPlacementandTrackingprovidesfunctionstoplaceanorderandalsoletstheclienttoquerythestateofpastorders.

Toprovideproductinformation,weneedaccesstotheProductCatalogmodulethatholdstheactualproductdetails.

TherecanbealotofothertasksthattheProductCatalogdoes,andthatisthereasonitisaseparatemodule.Itcanhave,forexample,aworkflowandapprovalenginethatletsproductadministratorstoenterproductdataandmanagerstocheckandapprovethedata.Approvalisusuallyacomplexprocess,consideringtyposandlegalquestions(wedonotwanttotradeunlicenseddrugs,explosives,andsoon),andcheckingthequalityandapprovalstateofthesourceofthegoods.Manycomplextasksareincludedthatmakeitabackendmodule.Inlargeenterpriseapplications,thefrontendsystemsrarelydoanythingelseotherthantheverybasicfunctionalityofservingtheoutsideparties.Butthisisgoodforus;wecanfocusontheservicethatwehavetodeliver.Andthisisalsogoodforthearchitecture.Itisthesameprincipleasinobject-orientedprogramming:singleresponsibility.

TheProductInformationmodulealsohastoconsultwiththeAccessControlmoduletoseeifacertainproductcanbedeliveredtotheactualcustomer,andwiththeinventorytoseeifthereisanyproductleft,sowedonotofferaproductthatisoutofstock.

TheOrderPlacementandTrackingalsoneedsaccesstoProductInventoryandAccessControlmodulestocheckwhethertheordercanbefulfilled.Atthesametime,italsoneedsservicesfromthePricingmodule,whichcancalculatethepricefortheorder,andfromtheLogisticsmodule,whichtriggersthecollectionofgoodsfromtheinventorylocationsandshipmenttothecustomer.Logisticsalsohasaconnectionto

invoicing,whichhasaconnectiontotheGeneralLedger,buttheseareonthepictureonlytoshowthatthetravelofinformationdoesnotendthere.Therearemanyothermodulesthatrunthecompany,allofwhicharenoneofourinterestatthemoment.

MicroservicesThearchitecturedescribedinthepreviouschapterisnotacleanmicroservicearchitecture.Youwillnevermeetoneinitspureforminanyenterprise.Itismorelikesomethingthatwereallymeetinarealcompanymovingfrommonolithictomicroservices.

WetalkaboutthemicroservicearchitecturewhentheapplicationisdevelopedintheformofmanysmallservicesthatcommunicatewitheachotherusingsomesimpleAPI,usuallyoverHTTPandREST.Theservicesimplementbusinessfunctionalitiesandcanbedeployedindependently.Manytimes,itisdesirablethattheservicedeploymentisautomated.

Theindividualservicescanbedevelopedusingdifferentprogramminglanguages,canusedifferentdatastorage,andcanrunondifferentoperatingsystems;thus,theyarehighlyindependentofeachother.Theycanbe,andusuallyare,developedbydifferentteams.Theimportantrequirementisthattheycancooperate;thus,theAPIoneserviceimplementsisusablebytheotherservicesthatbuilduponit.

ThemicroservicearchitectureisnottheHolyGrailofallarchitectures.Itgivesdifferentanswerstosomeproblemsfrommonolithicarchitectures,andmanytimes,theseanswersworkbetterusingmoderntools.Theapplicationsstillhavetobetestedanddebugged,performancehastobemanaged,andbugsandissueshavetobeaddressed.Thedifferenceisthattestingcanbeseparatedalongdifferenttechnologies;debuggingmayneedmorenetwork-relatedwork.Thesemaybegood,bad,orbothatthesametime.Forthedevelopers,however,theadvantageisclear.Theycanworkonasmallerunitindependentlyandcanseetheresultoftheirworkfaster.Whendevelopingasinglemoduleofamonolithicapplication,theresultcanbeseenwhentheentireapplicationgetsdeployed.Inthecaseofalargeapplication,thatmayberare.Atypicaldeploymentcycleinalargecorporatedevelopingmonolithiciseveryfewmonths,say3,butitisnotraretoseethereleasedevelopmenttwiceorevenonceayear.Developingmicroservices,thenewmodulecanbedeployedassoonasitisreadyandtested.

Ifyouwanttoreadmoreonmicroservices,thefirstandthemostauthenticsourceisthearticlebyMartinFowler(http://www.martinfowler.com/articles/microservices.html).

ServiceinterfacedesignWedesignthetwointerfacesthatwewillimplement.Whenwedesigninterfaces,wefocusonthefunctionalityfirst.Formattingandprotocolcomelater.Interfacesgenerallyshouldbesimpleand,atthesametime,shouldaccommodatethefuturechange.Thisisahardproblembecausewecannotseethefuture.Business,logistics,andallotherexpertsmayseesomepartofthefuture:howtheworldwillchangeandwhatitwillimposeontheoperationofthecompanyand,especially,ontheinterfaceweprovideforourpartners.

Thestabilityofaninterfaceisofutmostimportancebecausethepartnersareoutsideentities.Wecannotrefactorthecodetheyuse.WhenwechangeaJavainterfaceinourcode,thecompilerwillcomplainatallthecodelocationswherethechangeshouldbefollowed.Incaseofaninterfacethatisusedoutsideofourrealm,thisisnotthecase.EvenifitisonlyaJavainterfacethatwepublishasopensourceonGitHub,weshouldbepreparedthatouruserswillfaceissuesifwechangethelibraryinanincompatibleway.Inthatcase,theirsoftwarewillnotcompileandworkwithourlibrary.Inthecaseofanorderingsystem,itmeansthattheywillnotorderfromusandwewillsoonbeoutofbusiness.

Thisisoneofthereasonswhyinterfacesshouldbesimple.Althoughthisisgenerallytrueformostofthethingsinlife,itisextremelyimportantforsuchinterfaces.Itistemptingtoprovideconveniencefeaturesforthepartnersbecausetheyareeasytoimplement.Inthelongrun,however,thesefeaturesmaybecomeveryexpensiveastheyneedmaintenance,shouldbekeptbackwardcompatible,and,inthelongrun,maynotgainasmuchastheycost.

Toaccessproductinformation,weneedtwofunctions.Oneofthemlistscertainproductsandanotherreturnsthedetailsofaspecificproduct.IfitwereaJavaAPI,itwouldlookasfollows:

List<ProductId>query(Stringquery);

ProductInformationbyId(ProductIdid);

Similarly,orderplacementmaylookasshowninthefollowing:

OrderIdplaceOrder(Orderorder);

Weprovidethesefunctionalitiesinourapplicationviaawebserviceinterfaceand,morespecifically,RESTusingJSON.WewilldiscussthesetechnologiesinabitmoredetailedmanneralongwiththeSpringframeworkandModelViewControllerdesignpattern,butfirstlet'slookattheproductinformationcontrollertogetsomefeelingofhowourprogramwilllook:

packagepackt.java9.by.example.mybusiness.productinformation;

import...

@RestController

publicclassProductInformationController{

@Autowired

ProductLookuplookup;

@RequestMapping("/pi/{productId}")

publicProductInformation

getProductInformation(@PathVariableStringproductId){

ProductInformationproductInformation=

lookup.byId(productId);

returnproductInformation;

}

@RequestMapping("/query/{query}")

publicList<String>lookupProductByTitle(@PathVariableStringquery,HttpServletRequestrequest){

//tobedevelopedlater

}

}

Ifyoucomparethecodeoftheservletwiththeprecedingcode,youcanseethatthisismuchsimpler.WedonotneedtodealwiththeHttpServletRequestobject,callanAPItogetaparameter,orcreateanHTMLoutputandwriteittotheresponse.Theframeworkdoesthis.Weannotatethe@RestControllerclass,tellingSpringthatthisisacontrollerthatutilizestheRESTwebservices;thus,itwillbydefaultcreateaJSONresponsefromtheobjectwereturn.WedonotneedtocareabouttheconversionoftheobjecttoJSON,althoughwecanifthereisreallyaneed.TheobjectwillautomaticallybeconvertedtoJSONusingthefieldnamesusedintheclassandthefieldvaluesoftheinstancewereturn.IftheobjectcontainsmorecomplexstructuresthanjustplainString,int,anddoublevalues,thentheconverterispreparedfornestedstructuresandthemostcommondatatypes.

TohavedifferentcodehandlinganddifferentURLsontheservlet,allweneedtodoistoannotatethemethodwith@RequestMapping,providingthepathpartoftheURL.The{productId}notationinsidethemappingstringisreadableandeasytomaintain.SpringjustcutsthevaluefromthereandputsitforusintheproductIdvariable,asrequestedbythe@PathVariableannotation.

Theactuallookupoftheproductisnotimplementedinthecontroller.Thatisnotthefunctionofthecontroller.Thecontrolleronlydecideswhatbusinesslogictoinvokeandwhatviewtouse.Apartofitisimplementedintheframework,andtheverysmallpartyoucanseetheprecedingcode.Thebusinesslogicisimplementedinaserviceclass.Aninstanceofthisclassisinjectedtothelookupfield.ThisisalsodonebySpring.Theactualworkwehavetodoistoinvokethebusinesslogic,whichthistime,sincewehaveonlyone,isfairlyeasy.

Mostofthesethingsseemmagicwithoutsomemoredetailsaboutwhattheframeworkdoesforus.Therefore,beforegoingon,wewillhavealookatthebuildingblocks:JSON,REST,MVC,andabitoftheSpringframework.

JSONJSONstandsforJavaScriptObjectNotation.Itisdefinedonthesite,http://www.json.org/.ThisisatextualnotationinthesamewayastheobjectliteralsaredefinedinJavaScript.Anobjectrepresentationstartswiththe{characterandendswiththe}character.Thetextinbetweendefinesthefieldsoftheobjectsintheform,string:value.Thestringisthenameofthefield,andsinceJSONwantstobelanguageagnostic,itallowsanycharacterstobeapartofthenameofafield,andthusthisstring(aswellasanystringinJSON)shouldstartandendwiththe"characters.

Thismayseemstrangeand,manytimes,whenyoustartworkingwithJSON,itiseasytoforgetandwrite{myObject:"hasastring"}insteadofthecorrect{"myObject":"hasastring"}notation.

Commasseparatethefields.YoucanalsohavearraysinJSON.Theystartandendwith[and]characters,respectively,andtheycontaincomma-separatedvalues.Thevalueinanobjectfieldorinanarraycanbeastring,anumber,anobject,anarray,oroneoftheconstants,true,false,andnull.

Generallyspeaking,JSONisaverysimplenotationtodescribedatathatcanbestoredinanobject.Itiseasytowriteusingtexteditorsandeasytoread,andthusitiseasiertodebuganycommunicationthatusesJSONinsteadofmorecomplexformats.WaystoconvertJSONtoaJavaobjectandtheotherwayround,arereadilyavailableinlibrariesthatwewilluseinthischapter.AsampleJSONobjectthatdescribesaproductfromoursamplecodeisalsoavailableinthesourcecodeoftheprogram,asfollows:

{"id":"125","title":"BarStool","description":"anotherfurniture","size":[20.0,2.0,18.0],"weight":300.0}

NotethattheformattingofJSONdoesnotrequireanewline,butatthesametime,thisisalsopossible.Program-generatedJSONobjectsareusuallycompactandarenotformatted.Whenweeditsomeobjectusingatexteditor,wetendtoformattheindentationofthefieldsinthesamewayasweusuallydoinJavaprogramming.

RESTThereisnoexactdefinitionoftheRESTprotocol.ItstandsforRepresentationalstatetransfer,whichprobablydoesnotmeanathingtosomeonewhohasneverheardofit.WhenweprogramtheRESTAPI,weusetheHTTP(S)protocol.Wesendsimplerequeststotheserver,andwegetsimpleanswersthatweprogram.Thisway,theclientofthewebserverisalsoaprogram(bytheway,thebrowserisalsoaprogram)thatconsumestheresponsefromtheserver.Theformatoftheresponse,therefore,isnotHTMLformattedusingCSSandenrichedbyclient-sidefunctionalitiesbyJavaScript,butrathersomedatadescriptiveformatsuchasJSON.RESTdoesnotsetrestrictionsontheactualformat,butthesedays,JSONisthemostwidelyused.

ThewikipagethatdescribesRESTisavailableathttps://en.wikipedia.org/wiki/Representational_state_transfer.RESTinterfacesareusuallymadesimple.TheHTTPrequestsalmostalwaysusetheGETmethod.ItalsomakesthetestingofRESTservicessimplesincenothingiseasierthanissuingaGETrequestfromabrowser.POSTrequestsareonlyusedwhentheserviceperformssometransactionorchangeontheserver,andthatway,therequestissendingdatatotheserverratherthangettingsomedata.

Inourapplication,wewillusetheGETmethodtoqueryalistofproductsandgetinformationaboutaproduct,andwewillonlyusePOSTtoorderproducts.Theapplicationthatservestheserequestswillruninaservletcontainer.Youhavelearnthowtocreateanakedservletwithouttheuseofaframework.Inthischapter,wewillusetheSpringframework,whichoffloadsmanyofthetasksfromthedeveloper.Therearemanyprogramconstructsinservletprogrammingthatarejustthesamemostofthetime.Theyarecalledboilerplatecode.TheSpringframeworkutilizestheModelViewControllerdesignpatterntodevelopwebapplications;thus,wewilllookatitinbrief,beforediscussingSpringingeneral.

ModelViewControllerModelViewController(MVC)isadesignpattern.Designpatternsareprogrammingconstructs:simplestructuresthatgivesomehintonhowtosolvesomespecificproblems.Theterm,designpatternwascoinedandformallydescribedinthebook,DesignPatterns,ElementsofReusableObject-OrientedSoftware,writtenbyErichGamma,RichardHelm,RalphJohnson,andJohnVlissides.Thebookdefinesadesignpatternasastructurewithaname,aproblem,andasolution.Thenamedescribesthepatternandgivesthevocabularyforthedevelopercommunitytousewhentalkingaboutthesepatterns.Itisimportantthatdifferentdevelopersusethesamelanguageterminologyinordertounderstandeachother.Theproblemdescribesthesituation,thatis,thedesignproblemwherethepatterncanbeapplied.Thesolutiondescribesclassesandobjectsandtherelationsbetweenthem,whichcontributetoagooddesign.

OneofthemisMVC,whichissuitableforprogrammingwebapplicationsbutgenerallyforanyapplicationthathasauserinterface.Inourcase,wedonothaveaclassicaluserinterfacebecausetheclientisalsoaprogram;still,MVCcanbeandisagoodchoicetouse.

TheMVCpattern,asthenamealsoindicates,hasthreeparts:amodel,aview,andacontroller.Thisseparationfollowsthesingleresponsibilityprinciple,requiringonepartforeachdistinctresponsibility.Thecontrollerisresponsibleforhandlingtheinputsofthesystem,anditdecideswhatmodelandviewtouse.Itcontrolstheexecutionbutusuallydoesnotdoanybusinesslogic.Themodeldoesthebusinesslogicandcontainsthedata.Viewconvertsthemodeldatatoarepresentationthatisconsumablebytheclient.

MVCisawell-knownandwidelyuseddesignpattern,anditisdirectlysupportedbySpringinsucha

waythatwhenyoucreateawebapplication,youprogramthecontrollerbuiltintotheframework,usingannotations;thus,youessentiallyconfigureit.Youcanprogramtheview,butitismorelikelythatyouwilluseonethatisbuiltintotheframework.YouwillwanttosenddatatotheclientinXML,JSON,orHTML.Ifyouareveryexotic,youmaywanttosendYAML,butgenerally,thatisit.Youdonotwanttoimplementanewformatthatneedstobeprogrammedontheserverand,sincethisisnew,alsoontheclient.

Wecreatethemodel,andthistime,wealsoprogramit.Afterall,thatisthebusinesslogic.Frameworkscandomanythingsforus,mainlythethingsthatarethesameformostoftheapplicationsbutforthebusinesslogic.Businesslogicisthecodethatdistinguishesourcodefromotherprograms.Thatiswhatwehavetoprogram.

Ontheotherhand,thatiswhatweliketodo.Focusonthebusinesscodeandavoidallboilerplateprovidedbytheframework.

NowthatweknowwhatJSON,REST,andthegeneralModelViewControllerdesignpatternare,let'slookathowthesearemanagedbySpringandhowwecanputthesetechnologiesintoaction.

Springframework

TheSpringframeworkisahugeonewithseveralmodules.Thefirstversionoftheframeworkwasreleasedin2003,andsincethen,therehavebeenfourmajorreleasesdeliveringnewandenhancedfeatures.Currently,Springisthedefactoenterpriseframeworkused,perhapsmorewidelythanthestandardEJB3.0

Springsupportsdependencyinjection,Aspect-OrientedProgramming(AOP),persistenceforSQLandNoSQLdatabasesinaconventionalandObjectRelationalMappingway,transactionalsupport,messaging,webprogramming,andmanyotherfeatures.YoucanconfigureitusingXMLconfigurationfiles,annotations,orJavaclasses.

ArchitectureofSpringSpringisnotmonolithic.Youcanuseapartofit,oronlysomeofthefeatures.YoucanincludesomeofthemodulesofSpringthatyouneedandleaveoutothers.Somemodulesdependonsomeothers,butGradle,Maven,orsomeotherbuildtoolhandlesthat.

ThefollowingimageshowsyouthemodulesoftheSpringframeworkforversion4:

Springisconstantlydevelopingsinceitsfirstrelease,anditisstillconsideredasamodernframework.Thecoreoftheframeworkisadependencyinjectioncontainersimilartotheonewesawinthepreviouschapter.Astheframeworkdeveloped,italsosupportedAOPandmanyotherenterprisefunctionalities,suchasmessageorientedpatternsandwebprogrammingwithanimplementationofModelViewController,supportingnotonlyservletsbutalsoportletsandWebSockets.SinceSpringtargetstheenterpriseapplicationarena,italsosupportsdatabasehandlinginmanydifferentways.ItsupportsJDBCusingtemplates,ObjectRelationalMapping(ORM),andtransactionmanagement.

Inthesampleprogram,weuseafairlyrecentmodule:Springboot.Thismodulemakesitextremelyeasytostartwritingandrunningapplications,assumingalotofconfigurationsthatareusuallythesameformanyprograms.ItcontainsanembeddedservletcontainerthatitconfiguresfordefaultsettingsandconfiguresSpringwhereveritispossible,sowecanfocusontheprogrammingaspectratherthanontheSpringconfiguration.

SpringcoreThecentralelementofthecoremoduleisthecontext.WhenaSpringapplicationstarts,thecontainerneedsacontextinwhichthecontainercancreatedifferentbeans.Thisisverygeneralandtrueforanydependencyinjectioncontainer.Ifweprogrammaticallycreatetwodifferentcontexts,theymayliveindependentofeachotherinthesameJVM.Ifthereisabeandeclaredasasingletonsothatthereshouldbeonlyonesingleinstanceofit,thenthecontainerwillcreateasingleinstanceofitforacontextwhenweneedoneinstance.Theobjectsrepresentingthecontexthaveareferencetotheobjectthatwehavealreadycreated.Iftherearemultiplecontexts,however,theywillnotknowthatthereisanothercontextintheJVMthatalreadyhasaninstance,andthecontainerwillcreateanewinstanceofthesingletonbeanfortheothercontext.

Usually,wedonotusemorethanonecontextinaprogram,buttherearenumerousexamplesoftherebeingmanycontextsinasingleJVM.Whendifferentservletsruninthesameservletcontainer,theyruninthesameJVMseparatedbytheclassloaderandtheymayeachuseSpring.Inthiscase,thecontextwillbelongtotheservletandeachwillhaveanewcontext.

Inthepreviouschapter,weusedGuice.TheSpringcontextissimilartotheGuiceinjector.Inthepreviouschapter,IwascheatingabitbecauseIwasprogrammingGuicetocreateanewinjectorforeachrequest.Thisisfarfromoptimal,andGuiceprovidesaninjectorimplementationthatcanhandleservletenvironments.ThereasonforcheatingwasthatIwantedtofocusmoreontheDIarchitectureessentials,andIdidnotwanttocomplicatethecodebyintroducingacomplex(well,morecomplex)implementationoftheinjector.

IntheSpringcontextbehavior,whatitdoes,isdefinedbytheinterfaceApplicationContext.Therearetwoextensionsofthisinterfaceandmanyimplementations.ConfigurableApplicationContextextendsApplicationContext,definingsetters,andConfigurableWebApplicationContextdefinesmethodsneededinthewebenvironment.Whenweprogramwebapplications,weusuallydonotneedtointerferedirectlywiththecontext.Theframeworkconfigurestheservletcontainerprogrammatically,anditcontainsservletsthatcreatethecontextandinvokeourmethods.Thisisallboilerplatecodecreatedforus.

Thecontextkeepstrackofthebeanscreated,butitdoesnotcreatethem.Tocreatebeans,weneedbeanfactories(atleastone).ThetopmostinterfaceofbeanfactoriesinSpringisBeanFactory.Thedifferencebetweenanobjectandabeanisthatabeanfactorycreatesthebean,itisregisteredinthecontext,andithasaStringname.Thisway,theprogramcanrefertothebeanbythename.

DifferentbeanscanbeconfiguredinseveraldifferentwaysinSpring.TheoldestapproachistocreateanXMLfilethatdescribesthedifferentbeans,specifyingthenames,theclassthathastobeinstantiatedtocreatethebean,andfieldsincasethebeanneedsotherbeanstobeinjectedforitscreation.

Themotivationbehindthisapproachisthatthisway,thebeanwiringandconfigurationcanbetotallyindependentoftheapplicationcode.Itbecomesaconfigurationfilethatcanbemaintainedseparately.Ifwehavealargeapplicationthatmayworkinseveraldifferentenvironments,theaccesstoinventorydatamaybeavailableinmultitudeways.Inoneenvironment,theinventoryisavailablebycallingSOAP

services.Inanotherenvironment,thedataisaccessibleinanSQLdatabase.Inthethirdenvironment,itcanbeavailableinsomeNoSQLstore.Eachoftheseaccessesisimplementedasaseparateclass,implementingacommoninventoryaccessinterface.Theapplicationcodedependsonlyontheinterface,anditisthecontainerthathastoprovideoneortheotherimplementation.

WhentheconfigurationofthebeanwiringisinXML,thenonlythisXMLfileistobeedited,andthecodecanbestartedwiththeimplementationoftheinterfacethatissuitableforthatenvironment.

Thenextpossibilityistoconfigurethebeansusingannotations.Manytimes,weusebeansandSpringnotbecausetherearemanyimplementationsforabeanfunctionality,butbecausewewanttoseparatethecreationoftheobjectinstancefromthefunctionality.Thisisagoodstyle:separationoftheconcernseveniftheimplementationissinglewithoutalternatives.However,inthiscase,creatingtheXMLconfigurationisredundant.Ifthereisaninterfaceandasingleimplementationofitinourcode,thenwhyshouldIspecifyinanXMLthatbycreatinganobjectwithaclassthatimplementsthatinterface,Ishouldusetheclassthatimplementsthatinterface?Quiteobvious,isn'tit?Wedonotlikeprogrammingthingsthatcanbeautomated.

Tosignalthataclasscanbeusedasabean,andtopossiblyprovideaname,wecanusethe@Componentannotation.Wedonotneedtoprovideanameasanargument.Inthatcase,thenamewillbeanemptystring,butwhyhaveanameifwedonotrefertoit?Springscansalltheclassesthatareontheclasspathandrecognizestheclassesannotated,anditknowsthattheyarethecandidatestobeusedforbeancreation.Whenacomponentneedsanotherbeantobeinjected,thefieldcanbeannotatedwith@Autowiredor@Inject.The@AutowiredannotationisaSpringannotationandexistedbeforethe@Injectannotationwasstandardized.IfyouintendtouseyourcodeoutsideoftheSpringcontainer,itisrecommendedtousestandardannotations.Functionally,theyareequivalent.

Inourcode,whenSpringcreatesaninstanceoftheProductInformationControllercomponent,itseesthatitneedsaninstanceofProductLookup.Thisisaninterface,andthus,Springstartstolookforsomeclassthatimplementsthisinterface,createsaninstanceofit,possiblyfirstcreatingotherbeans,andtheninjectsit,settingthefield.Youcandecidetoannotatethesetterofthefieldinsteadofthefielditself.Insuchacase,Springwillinvokethesetterevenifthesetterisprivate.Youcaninjectdependenciesthroughconstructorarguments.Themajordifferencebetweenthesetter,fieldinjection,andconstructorinjectionisthatyoucannotcreateabeanwithoutdependencyincaseyouuseconstructorinjection.Whenthebeanisinstantiated,itshouldandwillhaveallotherbeansinjectedsothatitdependsonusingtheconstructorinjection.Atthesametime,thedependenciesthatneedtobeinjectedthroughthesetterinjection,ordirectlyintoafield,couldbeinstantiatedlaterbythecontainersometimebetweeninstantiatingtheclassandreadyingthebean.

Thisslightdifferencemaynotseeminterestingorimportantuntilyourconstructorcodemaybecomemorecomplexthanthesimpledependencysettingsoruntilthedependenciesbecomecomplex.Inthecaseofacomplexconstructor,thecodeshouldpayattentiontothefactthattheobjectisnotfullycreated.Thisisgenerallytrueforanyconstructorcode,butinthecaseofbeanscreatedbyadependencyinjectioncontainer,itisevenmoreimportant.Thus,itmaybeadvisabletouseconstructorinjection.Inthatcase,thedependenciesarethere;ifaprogrammermakesamistake,forgettingthattheobjectisnotfullyinitialized,andusesitintheconstructororamethodthatiscalledfromaconstructor,thedependencyisthere.Also,itiscleanandwell-structuredtousetheconstructortoinitializethedependenciesandhave

thosefieldsdeclaredfinal.

Ontheotherhand,constructorinjectionhasitsdownsides.

Ifdifferentobjectsdependoneachotherandthereisaringinthedependencygraph,thenSpringwillfaceahardtimeifyouuseconstructordependencies.WhenclassAneedsclassBandtheotherwayround,asthesimplestcircle,thenneitherAnorBcanbecreatedwithouttheotherifthedependencyinjectionisconstructordependency.Insituationslikethis,theconstructorinjectioncannotbeused,andthecircleshouldbebrokenatleastasasingledependency.Insituationslikethis,setterinjectionisinevitable.

Setterinjectionmayalsobebetterwhenthereareoptionaldependencies.Manytimes,someclassmaynotneedallitsdependenciesatthesametime.SomeclassmayuseadatabaseconnectionoraNoSQLdatabasehandlebutnotbothatthesametime.AlthoughitmayalsobeacodesmellandprobablyasignofpoorOOdesign,itmayhappen.ItmaybeadeliberatedecisiontodothatbecausethepureOOdesignwouldresultintoodeepobjecthierarchiesandtoomanyclasses,beyondthemaintainablelimit.Ifsuchisthesituation,theoptionaldependenciesmaybebetterhandledusingsetterinjection.Someareconfiguredandset;someareleftwithdefaultvalues,usuallynull.

Lastbutnotleast,wecanconfigurethecontainerusingJavaclassesincasetheannotationsarenotenough.Forexample,therearemultipleimplementationsoftheProductLookupinterface,asitis,inourcodebase.(Don'tworryifyoudidnotrecognizethat;Ihavenottoldyouaboutthatyet.)ThereisaResourceBasedProductLookupclassthatreadspropertiesfilesfromthepackageandismainlytotesttheapplication,andthereisRestClientProductLookup,whichisaproduction-likeimplementationoftheinterface.IfIhavenootherconfigurationthanannotatingthelookupfieldwith@Autowired,Springwillnotknowwhichimplementationtouseandwillrewardtheuseduringstartupwiththefollowingerrormessage:

ErrorstartingApplicationContext.Todisplaytheauto-configurationreportre-runyourapplicationwith'debug'enabled.

2016-11-0307:25:01.217ERROR51907---[restartedMain]o.s.b.d.LoggingFailureAnalysisReporter:

***************************

APPLICATIONFAILEDTOSTART

***************************

Description:

Parameter0ofconstructorinpackt.java9.by.example.mybusiness.productinformation.ProductInformationControllerrequiredasinglebean,but2werefound:

-resourceBasedProductLookup:definedinfile[/.../sources/ch07/productinformation/build/classes/main/packt/java9/by/example/mybusiness/productinformation/lookup/ResourceBasedProductLookup.class]

-restClientProductLookup:definedinfile[/.../sources/ch07/productinformation/build/classes/main/packt/java9/by/example/mybusiness/productinformation/lookup/RestClientProductLookup.class]

Action:

Considermarkingoneofthebeansas@Primary,updatingtheconsumertoacceptmultiplebeans,orusing@Qualifiertoidentifythebeanthatshouldbeconsumed

Thisisafairlyself-explanatoryerrormessage;ittellsalot.NowisthetimewhenwecanconfigurethebeaninXML,butatthesametime,wecanalsoconfigureitusingJava.

Manydevelopersdonotgetthepointthefirsttime.Ididnotgetiteither.ThewholeXMLconfigurationwastoseparatetheconfigurationfromthecode.Itwastocreatethepossibilitythatasystemadministratorchangestheconfigurationandisfreetoselectoneortheotherimplementationofsomeinterface,wiringtheapplicationtogether.NowSpringtellsmethatitisbettertoreturntotheprogrammaticway?

Atthesametime,IcouldhearconcernsformanyyearsthatXMLisnotreallyanybetterthanJavacode.XMLwritingisessentiallyprogramming,exceptthatthetoolingandIDEsupportisnotasgoodforXML

asitisforJavacode(thelatterdevelopedalotinrecentyears,althoughforSpringXMLconfiguration).

TounderstandtheconceptofreturningtoJavacodefromXML,wehavetogetbacktothepurereasonandaimoftheXMLwayofconfiguration.ThemainadvantageofXMLSpringconfigurationisnotthattheformatisnotprogrammaticbutratherthattheconfigurationcodeisseparatedfromapplicationcode.IfwewritetheconfigurationinJavaandkeepthoseconfigurationclassestothebareminimum,andtheystayastheyshould,thentheseparationofapplicationversusconfigurationcodestillstands.ItisonlytheformatoftheconfigurationthatwechangefromXMLtoJava.Theadvantagesarenumerous.OneisthatthenamesoftheclassesarerecognizedbytheIDEasweeditandwecanhaveautocompleteinJava(notethatthisalsoworksusingXMLinsomeoftheIDEsutilizingsomeoftheextensionsofplugins).InthecaseofJava,IDEsupportisubiquitous.JavaismorereadablethanXML.Well,thisisamatteroftaste,butmostofuslikeJavamorethanXML.

SystemadministratorscanalsoeditJavacode.WhentheyedittheXMLconfiguration,theyusuallyhavetoextractitfromaJARorWARfile,editit,andthenpackagethearchiveagain.InthecaseofJavaediting,theyalsohavetoissueagradlewarcommandorsomethingsimilar.ThisshouldnotbeashowstopperforasystemmanagerwhorunsJavaapplicationsonaserver.Andagain,itisnotJavaprogramming.ItisonlyeditingsomeJavacodefilesandreplacingsomeclassnameliteralsandstringconstants.

Wefollowthisapproachinoursampleapplicationcode.Wehavetwoconfigurationfilesintheapplication:oneforlocaldeploymentandtestingandanotherforproduction.The@Profileannotationspecifieswhichprofiletheconfigurationshoulduse.Theprofile,whenthecodeisexecuted,canbespecifiedonthecommandlineasasystemproperty,asfollows:

$gradle-Dspring.profiles.active=localbootRun

Theconfigurationclassisannotatedwith@Configuration.Themethodsthatarebeanfactoriesareannotatedwith@Bean:

packagepackt.java9.by.example.mybusiness.productinformation;

import...

@Configuration

@Profile("local")

publicclassSpringConfigurationLocal{

@Bean

@Primary

publicProductLookupproductLookup(){

returnnewResourceBasedProductLookup();

}

@Bean

publicProductInformationServiceUrlBuilderurlBuilder(){

returnnull;

}

}

ThebeanfactorysimplyreturnsanewinstanceoftheResourceBasedProductLookupclassthatimplementstheProductLookupinterface.Thisimplementationcanbeusedtoruntheapplicationforlocaltestingwhentherearenoexternalservicestorelyon.ThisimplementationreadstheproductdatafromlocalresourcefilespackagedintotheJARapplication.

Theproductionversionoftheconfigurationisnotmuchdifferent,butasitmaybeexpected,thereareafewmorethingstoconfigure:

@Configuration

@Profile("production")

publicclassSpringConfiguration{

@Bean

@Primary

publicProductLookupproductLookup(){

returnnewRestClientProductLookup(urlBuilder());

}

@Bean

publicProductInformationServiceUrlBuilderurlBuilder(){

returnnewProductInformationServiceUrlBuilder(

"http://localhost");

}

}

ThisversionoftheProductLookupserviceclassusesanexternalRESTservicetoretrievethedatathatitwillpresenttotheclients.Todoso,itneedstheURLsoftheseservices.SuchURLsshouldusuallybeconfigured.Inourexample,weimplementasolutionwheretheseURLscanbecomputedonthefly.Itriedtomakeupasituationwhereitmaybeneededinreallife,butallreasoningwascontortedandIgaveup.Therealreasonisthat,thisway,wecanseecodethatcontainsabeanthatneedsanotherbeantobeinjected.Fornow,notethattheProductInformationServiceUrlBuilderinstancebeanisdefinedinthesamewayastheProductLookupbean,andwhenithastobeinjectedintotheconstructoroftheProductLookupbean,itsdefiningbeanmethodisusedandnotthefollowingexpressiondirectly:

newProductInformationServiceUrlBuilder("http://localhost");

Thelattermaywork,butnotinallsituationsandweshouldnotuseit.Forthereasons,wewillreturnwhenwediscussAOPwithSpringinasubsequentsection.

Alsonotethatthereisnoneedtodefineaninterfacetodefineabean.Thetypethatthebeanmethodreturnscanalsobeaclass.Thecontextwillusethemethodthatfitstheneededtype,andiftherearemorethanonesuitabletypesandtheconfigurationisnotpreciseenough,aswesaw,thecontainerwillloganerrorandwillnotwork.

Intheconfigurationthatservesthelocalprofile,wecreateanullvalueforProductInformationServiceBuilder.Thisisbecausewedonotneeditwhenweuselocaltesting.Also,ifanymethodfromthisclassisinvoked,itwillbeanerror.Errorsshouldbedetectedassoonaspossible;thus,anullvalueisagoodchoice.

TheProductInformationServiceUrlBuilderclassisverysimple:

packagepackt.java9.by.example.mybusiness.productinformation;

publicclassProductInformationServiceUrlBuilder{

privatefinalStringbaseUrl;

publicProductInformationServiceUrlBuilder(StringbaseUrl){

this.baseUrl=baseUrl;

}

publicStringurl(Stringservice,Stringparameter){

finalStringserviceUrl;

switch(service){

case"pi":

serviceUrl=

baseUrl+":8081/product/{id}";

break;

case"query":

serviceUrl=

baseUrl+":8081/query/{query}";

break;

case"inventory":

serviceUrl=

baseUrl+":8083/inventory/{id}";

break;

default:

serviceUrl=null;

break;

}

returnserviceUrl;

}

}

Thisbeanalsoneedsaconstructorparameter,andweusedastringconstantintheconfiguration.Thisclearlyshowsthatitispossibletouseasimpleobjecttoinitializesomeofthedependencies(whatwouldstopus,itispureJavaafterall),butitmayhindertheworkingofsomeSpringfeatures.

ServiceclassesWehavetwoserviceclasses.Theseclassesservethecontrollerswithdataandimplementthebusinesslogic,nomatterhowsimpletheyare.OneoftheserviceclassimplementationscallsREST-basedservices,whiletheotheronereadsdatafrompropertiesfiles.Thelattercanbeusedtotesttheapplicationoffline.TheonethatcallsRESTservicesisusedintheproductionenvironment.BothofthemimplementtheProductLookupinterface:

packagepackt.java9.by.example.mybusiness.productinformation;

importjava.util.List;

publicinterfaceProductLookup{

ProductInformationbyId(Stringid);

List<String>byQuery(Stringquery);

}

ResourceBasedProductLookupstoresthewholedatabaseinamapcalledproducts.Itisfilledfromthepropertiesfileswhenoneoftheservicemethodsisinvoked.TheprivatemethodloadProductsisinvokedfromeachoftheservicemethodswhentheystart,butitloadsthedataonlyifitisnotloadedyet:

packagepackt.java9.by.example.mybusiness.productinformation.lookup;

import...

@Service

publicclassResourceBasedProductLookupimplementsProductLookup{

privatestaticLoggerlog=LoggerFactory.getLogger(ResourceBasedProductLookup.class);

Theclassisannotatedusing@Service.Thisannotationispracticallyequivalenttothe@Componentannotation.Thisisonlyanalternativenametothesameannotation.Springalsohandlesthe@Componentannotationsuchthatifanannotationinterfaceisannotatedusingthe@Componentannotation,theannotationcanalsobeusedtosignalthataclassisaSpringcomponent.Youcanwriteyourownannotationinterfacesifyouwanttosignalforbetterreadabilitythataclassisnotasimplecomponentbutsomeotherspecialtype.

Forexample,startupyourIDEandnavigatetothesourcecodeoftheorg.springframework.stereotype.Serviceinterface:

privateProductInformation

fromProperties(Propertiesproperties){

finalProductInformationpi=newProductInformation();

pi.setTitle(properties.getProperty("title"));

pi.setDescription(properties.getProperty("description"));

pi.setWeight(

Double.parseDouble(properties.getProperty("weight")));

pi.getSize()[0]=

Double.parseDouble(properties.getProperty("width"));

pi.getSize()[1]=

Double.parseDouble(properties.getProperty("height"));

pi.getSize()[2]=

Double.parseDouble(properties.getProperty("depth"));

returnpi;

}

ThefromPropertiesmethodcreatesaninstanceofProductInformationandfillsitfromtheparametersgiveninthePropertiesobject.ThePropertiesclassisanoldandwidelyusedtype.Althoughtherearemoremodernformatsandclasses,thisisstillwidelyusedanditislikelythatyouwillmeetthisclass.Thisistheveryreasonweuseithere.

ProductInformationisasimpleDataTransferObject(DTO)thatcontainsnologic,onlyfields,setters,andgetters.Italsocontainsaconstant,emptyProductInformation,holdingareferencetoaninstanceoftheclasswithemptyvalues.

APropertiesobjectissimilartoaMapobject.ItcontainsStringvaluesassignedtoStringkeys.Therearemethods,aswewillseeinourexamples,thathelptheprogrammertoloadaPropertiesobjectfromaso-calledpropertiesfile.Suchafileusuallyhasthe.propertiesextension,anditcontainskeyvaluepairsinthefollowingformat:

key=value

Forexample,the123.propertiesfilecontainsthefollowing:

id=123

title=BookJava9byExample

description=anewbooktolearnJava9

weight=300

width=20

height=2

depth=18

Thepropertiesfilesareusedtostoresimpleconfigurationvaluesandarealmostexclusivelyusedtocontainlanguage-specificconstants.ThisisaverycontortedusebecausepropertiesfilesareISOLatin-1encodedfiles,andincaseyouneedtousesomespecialUTF-8characters,youhavetotypethemusingthe\uXXXXformatorusingthenative2asciiconverterprogram.YoucannotsavethemsimplyasUTF-8.Nevertheless,thisisthefileformatusedforlanguage-specificstringsusedforprograminternationalization(alsoabbreviatedasi18nbecausethereare18charactersbetweenthestartingiandthelastn).TogetthePropertiesobject,wehavetoreadthefilesintheprojectandgetthempackagedintoaJARfile.TheSpringclass,PathMatchingResourcePatternResolver,helpsusindoingso.

Gosh,yes,Iknow!WehavetogetusedtotheselongnameswhenweuseSpring.Anyway,suchlonganddescriptivenamesarewidelyusedinanenterpriseenvironmentandtheyareneededtoexplainthefunctionalityoftheclasses.

Wedeclareamapthatwillcontainalltheproductsduringthetesting:

finalprivateMap<String,ProductInformation>

products=newHashMap<>();

ThekeyistheproductID,whichisastringinourexample.ThevaluesaretheProductInformationobjectsthatwefillupusingthefromPropertiesmethod:

privatebooleanproductsAreNotLoaded=true;

Thenextfieldsignalsthattheproductsarenotloaded:

NoviceprogrammersusuallyusetheoppositevaluewiththenameproductsAreLoadedandsettofalsebydefault.Inthatcase,theonlyplacewherewewillreadavaluewillnegatethevalue,orthemainbranchoftheifcommandbecomesthedonothingpart.Neitherisabestpractice.

privatevoidloadProducts(){

if(productsAreNotLoaded){

try{

Resource[]resources=

newPathMatchingResourcePatternResolver()

.getResources(

"classpath:products/*.properties");

for(Resourceresource:resources){

loadResource(resource);

}

}

productsAreNotLoaded=false;

}catch(IOExceptionex){

log.error("Testresourcescannotberead",ex);

}

}

}

ThegetResourcesmethodreturnsalltheresources(files)thatareontheclasspathundertheproductsdirectoryandthathavea.propertiesextension:

privatevoidloadResource(Resourceresource)throwsIOException{

finalintdotPos=resource.getFilename().lastIndexOf('.');

finalStringid=resource.getFilename().substring(0,dotPos);

Propertiesproperties=newProperties();

properties.load(resource.getInputStream());

finalProductInformationpi=fromProperties(properties);

pi.setId(id);

products.put(id,pi);

}

TheproductIDisgivenbythenameofthefile.Thisiscalculatedusingsimplestringmanipulation,cuttingofftheextension.TheResourcecanalsoprovideaninputstreamthatthePropertiesclass'sloadmethodcanusetoloadallthepropertiesatoncefromthefile.Finally,wesavethenewProductInformationobjectinthemap.

WealsohaveaspecialnoProductlistthatisempty.Thisisreturnedifthereisnoproductforthequerywhenwewanttosearchforproducts:

privatestaticfinalList<String>noProducts=

newLinkedList<>();

TheproductlookupservicejusttakesaproductfromtheMapandreturnsit,orifitdoesnotexist,itreturnsanemptyproduct:

@Override

publicProductInformationbyId(Stringid){

loadProducts();

if(products.containsKey(id)){

returnproducts.get(id);

}else{

returnProductInformation.emptyProductInformation;

}

}

Thequeryisabitmorecomplex.Itimplementssearchingforaproductbytitle.Real-lifeimplementationsmayimplementamorecomplexlogic,butthisversionisforlocaltestingonly;thus,thesearchbytitleisenough,perhapsevenmorecomplexthanwouldbereallynecessary:

@Override

publicList<String>byQuery(Stringquery){

loadProducts();

List<String>pis=newLinkedList<>();

StringTokenizerst=newStringTokenizer(query,"&=");

while(st.hasMoreTokens()){

finalStringkey=st.nextToken();

if(st.hasMoreTokens()){

finalStringvalue=st.nextToken();

log.debug("processing{}={}query",key,value);

if(!"title".equals(key)){

returnnoProducts;

}

for(Stringid:products.keySet()){

ProductInformationpi=products.get(id);

if(pi.getTitle().startsWith(value)){

pis.add(id);

}

}

}

}

returnpis;

}

Theserviceclassthatimplementstheproductionfunctionalityismuchsimpler.Strange,butmanytimesthetestcodeismorecomplexthantheproductioncode:

packagepackt.java9.by.example.mybusiness.productinformation.lookup;

import...

@Component

publicclassRestClientProductLookupimplementsProductLookup{

privatestaticLoggerlog=LoggerFactory.getLogger(RestClientProductLookup.class);

finalprivateProductInformationServiceUrlBuilderpiSUBuilder;

publicRestClientProductLookup(

ProductInformationServiceUrlBuilderpiSUBuilder){

this.piSUBuilder=piSUBuilder;

}

TheconstructorisusedtoinjecttheURLbuilderbeanandthisisalltheauxiliarycodetheclasshas.Therestarethetwoservicemethods:

@Override

publicProductInformationbyId(Stringid){

Map<String,String>uriParameters=newHashMap<>();

uriParameters.put("id",id);

RestTemplaterest=newRestTemplate();

InventoryItemAmountamount=rest.getForObject(

piSUBuilder.url("inventory"),

InventoryItemAmount.class,

uriParameters);

if(amount.getAmount()>0){

returnrest.getForObject(piSUBuilder.url("pi"),

ProductInformation.class,

uriParameters);

}else{

returnProductInformation.emptyProductInformation;

}

}

ThebyIdmethodfirstcallstheinventoryservicetoseeifthereareanyproductsontheinventory.ThisRESTservicereturnsaJSONthathastheformat,{amount:nnn};thus,weneedaclass(sosimplethatwedonotlisthere)thathastheintamountfield,thesetter,andthegetter.

TheSpringRestTemplateprovidesaneasywaytoaccessaRESTservice.AllitneedsistheURLtemplate,atypethatisusedtoconverttheresult,andaMapobjectwiththeparameters.TheURLtemplatestringmaycontainparametersinthesamewayastherequestmappingintheSpringcontrollers,thenameoftheparameterbeingbetweenthe{and}characters.Thetemplateclassprovidessimplemethodstoaccess

RESTservices.Itautomaticallydoesmarshaling,sendingparameters,andun-marshaling,receivingtheresponse.InthecaseofaGETrequest,themarshalingisnotneeded.ThedataisintherequestURL,andthe{xxx}placeholdersarereplacedwiththevaluesfromthemapsuppliedasathirdargument.Theun-marshalingisreadilyavailableformostoftheformats.Inourapplication,theRESTservicesendsJSONdata,anditisindicatedintheresponseContent-TypeHTTPheader.RestTemplateconvertstheJSONtothetypeprovidedasargument.IfevertheserverdecidestosendtheresponseinXML,anditwillalsobeindicatedintheHTTPheader,RestTemplatewillhandlethesituationautomatically.Asamatteroffact,lookingatthecode,wecannottellhowtheresponseisencoded.Thisisalsonicebecauseitmakestheclientflexibleandatthesametime,wedonotneedtodealwithsuchtechnicaldetails.Wecanfocusonthebusinesslogic.

Atthesametime,theclassalsoprovidesconfigurationparametersinthecaseofmarshalingorsomeotherfunctionalitysothatitautomaticallyneedsthat.Youcan,forexample,providemarshalingmethods,thoughIrecommendthatyouusewhateverisavailablebydefault.Inmostcases,whenadeveloperthinksthatthereisaneedforaspecialversionofanyofthesefunctions,theoriginaldesignoftheircodeisflawed.

Thebusinesslogicisverysimple.Wefirstasktheinventoryifthereisanyproductinstock.Ifthereis(morethanzero),thenwequerytheproductinformationserviceandreturnthedetails.Ifthereisnone,thenwereturnanemptyrecord.

Theotherserviceisevensimpler.Itsimplycallstheunderpinningserviceandreturnstheresult:

@Override

publicList<String>byQuery(Stringquery){

Map<String,String>uriParameters=newHashMap<>();

uriParameters.put("query",query);

RestTemplaterest=newRestTemplate();

returnrest.getForObject(

piSUBuilder.url("query"),

List.class,

uriParameters);

}

}

CompilingandrunningtheapplicationWeusegradletocompileandruntheapplication.Sincetheapplicationdoesnothaveanyspecificconfigurationthatwouldnotappearinmostsimilarapplications,itiswisetousetheSpringboot.TheSpringbootmakesitextremelysimpletocreateandrunawebapplication.WeneedaJavastandardpublicstaticvoidmainmethodthatstartsuptheapplicationviaSpring:packagepackt.java9.by.example.mybusiness.productinformation;import...@SpringBootApplication(scanBasePackageClasses=packt.java9.by.example.mybusiness.SpringScanBase.class)publicclassApplication{publicstaticvoidmain(String[]args){SpringApplication.run(Application.class,args);}}

ThemethoddoesnothingbutstarttheStringApplicationclass'srunmethod.Itpassestheoriginalargumentsandalsotheclassthattheapplicationisin.Springusesthisclasstoreadtheannotation.The@SpringBootApplicationannotationsignalsthatthisclassisaSpringbootapplicationandprovidesargumentstoconfigurethepackagesthatcontaintheapplication.Todoso,youcanprovidethenameofthepackagethatcontainstheclasses,butyoucanalsoprovideaclassinthebasepackagethatcontainsalltheclassesthatSpringhastobeawareof.Youmaynotbeabletousetheclassversionoftheannotationparameterbecausetherootpackagemaynotcontainanyclass,onlysub-packages.Atthesametime,providingthenameoftherootpackageasString,willnotrevealanytypoormisalignmentduringcompiletime.SomeIDEmayrecognizethattheparameterissupposedtobeapackagename,oritmayscanthestringsoftheprogramforpackagenameswhenyourefactororrenameapackageandgiveyousupportforthat,butthisismoreheuristicsonly.Itisacommonpracticetocreateaplaceholderclassthatdoesnothingintherootpackageincasethereisnoclassthere.ThisclasscanbeusedtospecifyscanBasePackageClassesasanannotationparameterinsteadofscanBasePackagesthatneedsString.Inourexample,wehaveanemptyinterface,SpringScanBase,asaplaceholder.

Springscansalltheclassesthatareontheclasspath,recognizesthecomponentsandfieldannotationsthatitcaninterpret,andusesthisknowledgetocreatebeanswithoutconfigurationwhenneeded.

Notethattheabstractclass,ClassLoader,includedintheJDKdoesnotprovideanyclassscanningmethod.SinceJavaenvironmentsandframeworkscanimplementtheirownClassLoaders,itispossible(butveryunlikely)thatsomeimplementationdoesnotprovidethescanningfunctionalityprovidedbytheURLClassLoader.URLClassLoaderisanon-abstractimplementationoftheclassloadingfunctionalityandisapartoftheJDKjustaswellasClassLoader.Wewilldiscusstheintricaciesoftheclassloadingmechanisminthesubsequentchapters.

Thegradlebuildfilecontainstheusualthings.Itspecifiestherepositories,thepluginsforJava,theIDEs,

andalsoforSpringboot.ItalsospecifiesthenameoftheJARfilethatitgeneratesduringbuild.Themostimportantpartisthedependencylist:buildscript{repositories{mavenCentral()}dependencies{classpath("org.springframework.boot:spring-boot-gradle-plugin:1.4.1.RELEASE")}}

applyplugin:'java'applyplugin:'eclipse'applyplugin:'idea'applyplugin:'spring-boot'

jar{baseName='packt-ch07-microservice'version='1.0.0'}

repositories{mavenCentral()}

bootRun{systemPropertiesSystem.properties}

sourceCompatibility=1.9targetCompatibility=1.9

dependencies{compile("org.springframework.boot:spring-boot-starter-web")compile("org.springframework.boot:spring-boot-devtools")compile("org.springframework:spring-aop")compile("org.springframework:spring-aspects")testCompile("org.springframework.boot:spring-boot-starter-test")}

WedependonSpringbootpackages,sometestpackages,AOPsupport(whichwewilllookatsoon),andalsoonSpringbootdevtools.

Springbootdevtoolsmakeitpossibletorestartawebapplicationwheneveritisrecompiled,withoutrestartingthebuilt-inTomcatserver.Suppose,westarttheapplicationusingthefollowingcommandline:gradle-Dspring.profiles.active=productionbootRun

TheGradlestartsuptheapplicationandwheneveritseesthattheclassesitrunsaremodified,itreloadsthem,andwecantestthemodifiedapplicationwithinafewseconds.

The-Dspring.profiles.active=productionargumentspecifiesthattheproductionprofileshouldbeactive.Tobeabletousethiscommandlineparameter,wewillalsoneedthebootRun{}configurationclosureinthebuildfile.

Testingtheapplication

Theapplicationshouldhaveunittestsforeachandeveryclassithasexcept,perhaps,fortheDTOclassesthatcontainnofunctionality.ThesettersandgettersarecreatedbytheIDEandarenottypedinbytheprogrammer,soitisunlikelythattherewillbeanyerrorsinthose.Ifthereissomeerrorrelatedtothoseclasses,itismorelikelythatitissomeintegrationproblemthatcannotbediscoveredusingunittests.Sincewediscussedunittestsinthepreviouschaptersindetail,wewillfocusmoreonintegrationtestsandapplicationtestshere.

IntegrationtestIntegrationtestsareverysimilartounittests,andmanytimes,noviceprogrammersclaimtheydounittestingwhentheyactuallydointegrationtesting.

Integrationtestsdrivethecodebutdonottesttheindividualclasses(units)inisolation,mockingeverythingthattheclassmayuse.Rather,theytestthefunctionalityofmostoftheclassesthatareneededtoperformatest.Thisway,theintegrationtestdoestestthattheclassesareabletoworktogetherandnotonlysatisfytheirownspecificationsbutalsoensurethatthesespecificationsworktogether.

Inintegrationtest,theexternalworld(likeexternalservices)andaccesstodatabasearemockedonly.Thatisbecausetheintegrationtestsaresupposedtorunonintegrationservers,inthesameenvironmentwheretheunittestsareexecuted,andtheretheseexternalinterfacesmaynotbeavailable.Manytimes,databasesaremockedusingin-memorySQL,andexternalservicesaremockedusingsomemockclasses.

Springprovidesaniceenvironmenttoexecutesuchintegrationtests.Inourproject,wehaveasampleintegrationtest:

packagepackt.java9.by.example.mybusiness.productinformation;

import...

@RunWith(SpringRunner.class)

@SpringBootTest(classes=Application.class)

@AutoConfigureMockMvc

@ActiveProfiles("local")

publicclassProductInformationControllerTest{

@Autowired

privateMockMvcmockMvc;

@Test

publicvoidnoParamGreetingShouldReturnDefaultMessage()

throwsException{

this.mockMvc.perform(get("/pi")).andDo(print())

.andExpect(status().isNotFound());

}

@Test

publicvoidparamGreetingShouldReturnTailoredMessage()

throwsException{

this.mockMvc.perform(get("/pi/123"))

.andDo(print()).andExpect(status().isOk())

.andExpect(jsonPath("$.title")

.value("BookJava9byExample"));

}

}

Thisisfarfrombeingacompleteandfull-fledgedintegrationtest.Therearemanysituationsthatarenottested,buthereitisgoodasanexample.TohaveallthesupportfortheSpringenvironment,wehavetousetheSpringRunnerclass.The@RunWithannotationishandledbytheJUnitframework,allotherannotationsareforSpring.WhentheJUnitframeworkseesthatthereisa@RunWithannotationandarunnerclassspecified,itstartsthatclassinsteadofthestandardrunner.SpringRunnersetsupaSpringcontextforthetestandhandlestheannotations.

@SpringBootTestspecifiestheapplicationsthatweneedtotest.ThishelpsSpringtoreadthatclassandtheannotationonthatclass,identifyingthepackagestobescanned.

@AutoConfigureMockMvctellsSpringtoconfigureamockversionoftheModelViewControllerframework,

whichcanbeexecutedwithoutaservletcontainerandwebprotocol.Usingthat,wecantestourRESTserviceswithoutreallygoingtothenetwork.

@ActiveProfilestellsSpringthattheactiveprofileislocalandthatSpringhastousetheconfigurationthatisdenotedbytheannotation,@Profile("local").Thisisaversionthatusesthe.propertiesfilesinsteadofexternalHTTPservices;thus,thisisappropriateforintegrationtesting.

ThetestperformsGETrequestsinsidethemockingframework,executesthecodeinthecontroller,andteststhereturnedvalueusingthemockingframeworkandfluentAPIinaveryreadableway.

Notethatusingthepropertiesfilesandhavingtheserviceimplementationbasedonpropertiesfileisabitofanoverkill.Icreatedthissothatitispossibletostartuptheapplicationinteractivelywithoutanyrealbackingservice.Considerthefollowingcommand:gradle-Dspring.profiles.active=localbootRun.Ifweissuetheprecedingcommand,thentheserverstartsupusingthislocalimplementation.Ifweonlyaimforintegrationtesting,thenthelocalimplementationoftheserviceclassesshouldbeunderthetestdirectoryandshouldbemuchsimpler,mainlyonlyreturningconstantresponsesforanyexpectedrequestandthrowingerrorsifanynon-expectedrequestcomes.

ApplicationtestConsiderthefollowingcommand:

gradle-Dspring.profiles.active=productionbootRun

IfwestartuptheapplicationissuingtheprecedingcommandandfireupthebrowsertotheURL,http://localhost:8080/pi/123,wewillgetafaterrormessageonthebrowserscreen.Ouch...

ItsaysInternalServerError,status=500orsomethingsimilar.Thatisbecauseourcodewantstoconnecttothebackingservices,butwedonothaveanyyet.Tohavesometotesttheapplicationonthislevel,weshouldcreatethebackingservicesoratleastsomethingthatmocksthem.TheeasiestwayistousethesoapUIprogram.

ThesoapUIisaJavaprogramavailablefromhttps://www.soapui.org/.Thereisanopensourceandfreeversionofit,andthereisacommercialversion.Forourpurposes,thefreeversionisenough.Wecaninstallitinthesimplestclick-forwardwayasithasasetupwizard.Afterthat,wecanstartitupandusethegraphicaluserinterface.

Wecreateanewtestproject,Catalogandinventory,andsetuptwoRESTmockservicesinit:CatalogandInventory,asshowninthefollowingscreenshot:

Wesetup,foreachofthemockservices,requeststobematchedandresponses.Thecontentoftheresponseistextandcanbetypedintothetextfieldontheuserinterface.Itisimportantthatwedonotforgettosetthemediatypeoftheresponsetoapplication/json(thedefaultisXML).

Beforestartingtheservices,wehavetosettheportnumbersbyclickingonthecogwheeltosomethingthatisavailableontheserver.Since8080isusedbytheTomcatserverexecutedbyGradle,and8082isusedbysoapUItolistthemockservicesthatarecurrentlyrunning,Isetthecatalogtolistenon8081andinventoryon8083.YoucanalsoseetheseportnumbersinthelistingoftheProductInformationServiceUrlBuilderclass.

ThesoapUIsavestheprojectinanXMLfile,anditisavailableforyouonGitHubintheprojectdirectory.

Afterstartingthemockservices,theerrormessagedisappearsfromthebrowserscreenwhenwepressrefresh:

WhatweseeisexactlywhatwetypedintosoapUI.

IfnowIchangetheinventorymockservicetoreturn0insteadof100,asintheoriginalversion,whatIgetisthefollowingemptyrecord:

{"id":"","title":"","description":"","size":[0.0,0.0,0.0],"weight":0.0}

Thetestingevenonthislevelcanbeautomated.Now,wewereplayingaroundusingthebrowserandthisissomethingnice.Somehow,IfeelIamproducingsomethingwhenthereisaprogramthatisreallydoingsomething,whenIcanseethatthereissomeresponseinthebrowserwindow.However,afterawhile,thisbecomesboringandtestingmanuallythattheapplicationisstillworkingiscumbersome.Itisespeciallyboringforthosefunctionsthatwerenotchanged.Thefactisthattheydochangemiraculouslymanytimesevenwhenwedonottouchthecodethatinfluencesthem.Wetouchthecodethatdoesinfluencethefunctionexceptthatwearenotawareofit.Poordesign,poorcoding,ormaybewejustforgot,butithappens.Regressiontestisinevitable.

Althoughbrowsertestinguserinterfacescanalsobeautomated,thistime,wearehavingaRESTservicethatwecantestandthatiswhatsoapUIisfor.Wehavealreadyinstalledthetool,wehavealreadystartedit,andwehavesomemockservicesrunninginit.ThenextthingistoaddaNewRESTservicefromURItotheprojectandspecifytheURL,http://localhost:8080/pi/{id},exactlythesamewayaswedidforSpring:

WhenwehaveaRESTservicedefinedintheproject,wecancreateanewTestSuiteandaTestCaseinsidethesuite.WecanthenaddasteptotheTestCasethatwillcalltheRESTserviceusingtheparameter123ifwemodifythedefaultvalue,whichisthesameasthenameoftheparameter,inthiscase,id.Wecanruntheteststepusingthegreentriangleontheupper-leftcornerofthewindow,andsincewehavethetestedapplicationandthesoapUImockservicesrunning,weshouldgetananswerinJSON.WehavetoselectJSONontheresponseside;otherwise,soapUItriestointerprettheresponseasXML,andsincewehaveaJSONresponse,itisnottoofruitful.Whatweseeisthefollowingwindow:

Itisthesameresponsethatwesawinthebrowser.Therearenomiracleswhenweprogramcomputers.Sometimes,wedonotunderstandwhathappens,andsomethingsaresocomplexthattheyseemtobeamiracle,buttheyareactuallynot.Thereisanexplanationforeverything,itmayjustnotbeknowntous.Inthiscase,wecertainlyknowwhatishappening,butwhyisitanybettertoseetheJSONonthescreenofsoapUIthanitisonthebrowser?ThereasonisthatsoapUIcanexecuteassertionsandinsomecases,furtherteststepsbasedontheresultoftheRESTinvocation,andthefinalresultisasimpleYESorNO.ThetestisOK,oritFAILS.

Toaddanassertion,clickontheAssertionstextonthelower-leftcornerofthewindow.Asyoucanseeintheprecedingscreenshot,Ihavealreadyaddedonethatcomparesthe"title"fieldofthereturnedJSONwiththetext"BarStool".Whenweaddtheassertion,thedefaultvalueitsuggestsistheonethatwasactuallyreturned,whichisjustaveryhandyfeature.

Afterthis,runningthewholetestsuiteagainwillrunallthetestcases(wehaveonlyone),andalltheteststeps,oneaftertheother(weagainhaveonlyone),andfinallyitwilldisplayagreenFINISHEDbarontheUI,asshowninthefollowingscreenshot:

ThisisnotallthatsoapUIcando.Thisisawell-developedtesttoolthathasbeeninthemarketformanyyears.soapUIcantestSOAPservicesandRESTservices,anditcanhandleJMSmessages.Youcancreatetestsofmanystepswiththesecalls,loops,andassertionsincallsorinseparatetests,andincaseallelsefails,youcandojustanythingbycreatingprogrammedstepsintheGroovylanguageorcreatingextensionsinJava.

ServletfiltersTheservicesworkfinebynowandanyonecanquerythedetailsofourproducts.Thatmaybeaproblem.Thedetailsoftheproductsarenotnecessarilypublicinformation.Wehavetoensurethatweservethedataonlytopartnerswhoareeligibletoseeit.

Toensurethat,weneedsomethingintherequestthatprovesthattherequestcomesfromapartner.Thisinformationistypicallyapasswordorsomeothersecret.ItcouldbeplacedintotheGETrequestparametersorintotheHTTPrequestheader.Itisbettertoputitintotheheaderbecausetheinformationissecretandnottobeseenbyanybody.

TheGETparametersareapartoftheURL,andthebrowserhistoryremembersthat.Itisalsoveryeasytoenterthisinformationintothebrowserlocationwindow,copypasteit,andsenditoverachatchannelorovere-mail.Thisway,auseroftheapplication,whoisnotsoeducatedandconcernedaboutsecurity,maydisclosesecretinformation.AlthoughitisnotimpossibletodothesamewithinformationthatissentinanHTTPheader,itisnotlikelytohappen.Iftheinformationisintheheaderandsomebodysendstheinformationinane-mail,theyprobablyknowwhattheyaredoing;theycrossasecurityborderwillinglyandnotbysimplenegligence.

TosendauthenticationinformationalongtheHTTPrequest,SpringprovidesasecuritymodulethatcaneasilybeconfiguredwithannotationsandconfigurationXMLsand/orclasses.Thistime,wewilldoitabitdifferentlytointroduceservletfilters.

WewillrequirethatthevendorsinserttheX-PartnerSecretheaderintotherequest.Thisisanon-standardheader,andthusitmusthavetheX-prefix.Followingthisapproachisalsosomeextrasecurityfeature.Thisway,wecanpreventtheuserfromreachingtheserviceusingasimplebrowser.Thereis,atleast,aneedforsomeextrapluginthatcaninsertacustomheaderorsomeotherprogramsuchassoapUI.Thisway,itwillensurethatourpartnerswillusetheinterfaceprogrammatically,orifevertheyneedtotesttheinterfaceadhoc,onlyuserswithacertainleveloftechnologycandoso.Thisisimportanttokeepthesupportcostscontrolled.

Sincethissecrethastobecheckedinthecaseofeachandeveryservice,webetternotinsertthecheckingcodeintoeachandeveryservicecontroller.Evenifwecreatethecodeproperlyandfactorthecheckforthesecretintoaseparateclass,theinvocationofthemethodassertingthatthesecretisthereandiscorrectwillhavetobeinsertedineachandeverycontroller.Thecontrollerdoestheservice;checkingtheclientauthenticityisaninfrastructureissue.Theyaredifferentconcerns,andthus,theyhavetobeseparated.

Thebestwaythattheservletstandardprovidesforusisaservletfilter.Aservletfilterisaclassinvokedbytheservletcontainerbeforetheservletitselfifthefilterisconfigured.Thefiltercanbeconfiguredintheweb.xmlconfigurationfileoftheservletcontainerorbyusinganannotationwhenweusetheSpringboot.ThefilterdoesnotonlygettherequestandresponseasparametersbutalsoathirdargumentoftheFilterChaintypethatitshouldusetocalltheservletorthenextfilterinthechain.

Therecanbemorethanonefilterdefinedandtheygetchainedup.Thefiltermay,atitsdiscretion,decide

tocallornottocallthenextinthechain.

Weputourservletfilterintotheauthsub-packageofourapplication:

packagepackt.java9.by.example.mybusiness.productinformation.auth;

import...

@Component

publicclassAuthFilterimplementsFilter{

privatestaticLoggerlog=

LoggerFactory.getLogger(AuthFilter.class);

publicstaticfinalintNOT_AUTHORIZED=401;

@Override

publicvoidinit(FilterConfigfilterConfig)

throwsServletException{

}

@Override

publicvoiddoFilter(ServletRequestrequest,

ServletResponseresponse,

FilterChainchain)

throwsIOException,ServletException{

HttpServletRequesthttpRequest=

(HttpServletRequest)request;

finalStringsecret=

httpRequest.getHeader("X-PartnerSecret");

log.info("Partnersecretis{}",secret);

if("packt".equals(secret)){

chain.doFilter(request,response);

}else{

HttpServletResponsehttpResponse=

(HttpServletResponse)response;

httpResponse.sendError(NOT_AUTHORIZED);

}

}

@Override

publicvoiddestroy(){

}

}

ThefilterimplementstheFilterinterfacethatdefinesthreemethods.Inourcase,wedonothaveanyparameterstoconsiderinthefilter,andwedonotallocateanyresourcestorelease;thus,bothinitanddestroymethodsareempty.ThemainworkofthefilteristhedoFiltermethod.Ithasthreeparameters,twoofthemarethesameastheparametersofaservletandthethirdisFilterChain.

TherequestisconvertedtoHttpServletRequest,sowecangetaccesstotheX-PartnerSecretheaderthroughthegetHeadermethod.Ifthevaluesentinthisheaderfieldisgood,wecallthenextinthechain.Inourapplication,therearenomorefiltersconfigured;therefore,thenextinthechainistheservlet.Ifthesecretisnotacceptable,thenwedonotcallthenextinthechain.Instead,wereturnthe401NotAuthorizedHTTPerrortotheclient.

Inthisapplication,thesecretisverysimple.Thisistheconstantstringpackt.Thisisnotreallyabigsecret,especiallynowthatitispublishedinthisbook.Areal-lifeapplicationwouldrequiresomethingmorecrypticandlessknown.Itisveryprobablethateachpartnerwouldusedifferentsecretsandthatthesecrethastochangefromtimetotime.

Whenthereisanerrorconditioninaservletthatourprogramhandles,itisagoodpracticetousetheHTTPerrorhandlingmechanism.Insteadofsendingbackamessagewiththestatuscode200OKandexplaining,forexample,inaJSONformatthattheauthenticationwasnotsuccessful,wehavetosendbackthe401code.Thisisdefinedbythestandardanddoesnotneedanyfurtherexplanationordocumentation.

Thereisonethingleftinourprogram,andthatisauditlogging.

AuditloggingandAOPWehavelogginginoursamplecodeandforthatweuseslf4j,whichwecoveredinthepreviouschapter.Loggingismoreorlessthedecisionofthedeveloperandsupportstechnicallevelsofoperation.There,wealsotouchedonafewsentenceauditloggings.Thistypeofloggingisusuallyexplicitlyrequiredinafunctionalrequirement.

Generally,AOPisseparatingthedifferentaspectsofcodefunctionalityintoseparatecodefragments,andimplementingthemindependentofeachother.Thisisverymuchthesingleresponsibilityprinciple.Thistime,itisimplementedinawaythatnotonlythedifferentfunctionalitiesareimplementedseparatelybutalsohowweconnectthemtogetherisdefinedseparately.WhatisexecutedbeforeandafterwhatotherpartsareencodedseparatelygetstotheSpringconfiguration.Wehaveseensomethingsimilaralready.Thedependenciesthataclassneedstoproperlyoperatearedefinedinaseparatesegment(XMLorJavacode).ItisnotasurprisethatinthecaseofAOP,thesameisdoneusingSpring.Aspectsareconfiguredintheconfigurationfileorclass.

Atypicalaspectisauditlogging,andwewillusethisasanexample.Therearemanytopicsthatcanbeimplementedusingaspects,andsomeofthemareevenworthimplementingthatway.

Wedonotwanttoimplementtheauditloggingcodeineachbusinessmethodorclassthatneedsit.Instead,weimplementageneralaspectandconfigurethewiringsuchthatwheneverabeanmethodthatneedsauditloggingisinvoked,Springinvokestheauditlogging.

ThereareotherimportantterminologiesthatweshouldunderstandforAOPandespeciallyhowAOPcanbeconfiguredinSpring.

Thefirstandmostimportantistheaspect.Thisisthefunctionalitythatwewanttoimplement,inourexample,theauditlogging.

Joinpointisthepointinexecutionwhenanaspectisinvoked.Whenusingafull-scaleaspectsolutioninJavathatmodifiesthebytecodeofthegeneratedclass,ajoinpointcanbealmostanything.Itcanbeaccesstoafield,readorwrite;itcanbetheinvocationofamethodorexceptionthrowing.InthecaseofSpring,theclassbytecodeisnotmodified;thus,Springisnotabletoidentifytheaccessofafieldoranexceptionthrowing.UsingSpring,ajoinpointisalwaysusedwhenamethodisinvoked.

Anadviceishowtheaspectisinvokedatthejoinpoint.Itcanbebeforeadvice,afteradvice,oraroundadvice.Whentheadviceisbefore,theaspectisinvokedbeforethemethodiscalled.Whentheadviceisafter,theaspectisinvokedafterthemethodisinvoked.Aroundmeansthattheaspectisinvokedbeforethemethodcall,andtheaspectalsohasanargumenttocallthemethodandstillperformsomeactionsafterthemethodiscalled.Thisway,thearoundadviceisverysimilartoservletfilters.

Thebeforeadviceiscalledbeforethemethodcall,andafteritreturns,theframeworkwillinvokethemethod.Thereisnowayfortheaspecttopreventtheinvocationoftheoriginalmethod.Theonlyexceptioniswhentheaspect,well,throwsanexception.

Theafteradviceisalsoaffectedbyexceptions.Therecanbeanafterreturningadvicethatisinvokedwhenthemethodisreturning.Theafterthrowingisinvokedonlyifthemethodwerethrowinganexception.Afterfinallyisinvokedinthecaseofanexceptionorreturn.

Pointcutisaspecialstringexpressionthatidentifiesjoinpoints.Apointcutexpressionmaymatchzero,one,ormorejoinpoints.Whentheaspectisassociatedwithapointcutexpression,theframeworkwillknowthejoinpointsandwhenandwheretoinvoketheaspect.Inotherwords,pointcutisthestringthattellswhenandforwhichmethodtoinvoketheaspect.

EventhoughSpringimplementationofAOPdoesnotuseAspectJanddoesnotmodifythebytecodethatwascreatedfortheclasses,itsupportsthepointcutexpressionlanguage.AlthoughthisexpressionlanguageprovidesmorefeaturesthanwhatSpringimplements,itisawell-establishedandwidelyusedandacceptedexpressionlanguagetodescribepointcuts,anditjustwouldnotmakesensetoinventsomethingnew.

Introductionisaddingmethodsorfieldstoatypethatalreadyexistsanddoingitduringruntime.SpringallowsthisAOPfunctionalitytoaddaninterfacetoanexistingtypeandaddanimplementationoftheinterfaceintheformofanadviceclass.Inourexample,wedonotusethisfunctionality.

Targetobjectistheobjectthatisbeingadvisedbytheaspect.Thisisthebeanthatcontainsthemethodaroundtheaspect,thatis,beforeoraftertheaspectisinvoked.

Thatwasjustacondensedsetofdefinitions,almostlikeinamathbook.Ifyoudidnotgetthepointjustreadingit,don'tworry.Ididnotunderstanditeither.Thatiswhywehavethefollowingexample,afterwhichallwejustcoveredwillmakemoresense:

packagepackt.java9.by.example.mybusiness.productinformation;

import...

@Configuration

@Aspect

publicclassSpringConfigurationAspect{

privatestaticLoggerlog=

LoggerFactory.getLogger("AUDIT_LOG");

@Around("execution(*byId(..))")

publicProductInformationbyIdQueryLogging(

ProceedingJoinPointjp)

throwsThrowable{

log.info("byIdqueryisabouttorun");

ProductInformationpi=

(ProductInformation)jp.proceed(jp.getArgs());

log.info("byIdquerywasexecuted");

returnpi;

}

@Around("execution(*url(..))")

publicStringurlCreationLogging(ProceedingJoinPointjp)

throwsThrowable{

log.info("urlistobecreated");

Stringurl=(String)jp.proceed(jp.getArgs());

log.info("urlcreatedwas"+url);

returnurl;

}

}

Theclassisannotatedwiththe@ConfigurationannotationsothatSpringknowsthatthisclasscontainstheconfiguration.The@Aspectannotationdenotesthatthisconfigurationmayalsocontainaspectdefinitions.

The@Aroundannotationonthemethodsgivesthetypeofadvice,andtheargumentstringfortheannotationisthepointcutexpression.Ifthetypeofadviceisdifferent,oneoftheannotations,@Before,@After,@AfterReturning,or@AfterThrowing,shouldbeused.

Inourexample,weusethe@Aroundaspecttodemonstratethemostcomplexscenario.Welogtheexecutionofthetargetmethodbeforeandaftertheexecutionofthemethod,andwealsocalltheoriginalmethodthroughtheProceedingJoinPointobject.Becausethetwoobjectsreturndifferenttypesandwewanttologdifferently,wedefinetwoaspectmethods.

Theargumentoftheadviceannotationisthepointcutstring.Inthiscase,itisasimpleone.Thefirstone,execution(*byId(..)),saysthattheaspectshouldbeinvokedforanyexecutionofanymethodthathasthenamebyIdandhasanyarguments.Thesecondisverysimilar,exceptthenameofthemethodisdifferent.Thesearesimplepointcutexpressions,butinalargeapplicationthatheavilyusesAOP,theycanbeverycomplex.

ThepointcutexpressionsyntaxinSpringmainlyfollowsthesyntaxusedbyAspectJ.Theexpressionusesthenotionofpointcutdesignator(PCD)thatisusuallyexecution.Itisfollowedbythepatternthatdefineswhichmethodtointercept.Thegeneralformatisasfollows:

execution(modifiers-pattern?ret-type-patterndeclaring-type-pattern?name-pattern(param-pattern)throws-pattern?)

Exceptforthereturntypepart,allotherpartsareoptional.Forexample,wecanwritethefollowing:

execution(public**(..))

Thiswillinterceptallpublicmethods.Thefollowingexpressioninterceptsallmethodsthathaveanamestartingwithset:

execution(*set*(..))

Wecanusethe*characterasajokerinthesamewayaswecanuseitonthecommandlineinWindowsorUnixshell.Theargumentmatchingdefinitionisabitmorecomplex.(..)meansanyarguments,()meansnoarguments,and(*)meansexactlyoneargumentofanytype.Thelastonecanalsobeusedwhentherearemorearguments;forexample,(*,Integer)meansthattherearetwoarguments,thesecondbeinganInteger,andwejustdonotcarewhatthetypeofthefirstoneis.

Pointcutexpressionscanbemorecomplex,joiningtogethermatchexpressionswiththe&&(and)and||(or)logicaloperators,orusingthe!(negation)unaryoperator.

Usingthe@Pointcut()annotation,theconfigurationcandefinepointcutsputtingtheannotationsonmethods.Forexample,considerthefollowing:

@Pointcut("execution(*packt.java.9.by.example.service.*.*(..))")

publicvoidbusinessService(){}

Itwilldefineajoinpointforanymethodthatisimplementedinanyclassinthepackt.java.9.by.example.servicepackage.ThismerelydefinesthepointcutexpressionandassignsittothenamebusinessService,whichisgivenbythenameofthemethod.Later,wecanrefertothisexpressioninaspectannotations,forexample:

@After("businessService()")

Notethattheuseofthemethodispurelyforitsname.ThismethodisnotinvokedbySpring.Itisonlyusedtoborrowitsnametotheexpressionthatisdefinedonitusingthe@Pointcutannotation.Thereisaneedforsomething,suchasamethod,toputthisannotationon,andsincemethodshavenames,whynotuseit:Springdoesit.Whenitscanstheconfigurationclassesandseestheannotation,itassignsitinitsinternalstructurestothenameofthemethod,andwhenthatname(alongwiththeparenthesis,toconfusethenoviceprogrammermimickingamethodcall)isused,itlooksuptheexpressionforthatname.

AspectJdefinesotherdesignators.SpringAOPrecognizessomeofthem,butitthrowsIllegalArgumentExceptionbecauseSpringimplementsonlymethodexecutionpointcuts.AspectJ,ontheotherhand,canalsointerceptobjectcreationforwhichthePCDisinitialization,asanexample.SomeotherPCDs,inadditiontoexecution,canlimitanexecutionPCD.Forexample,thePCD,within,canbeusedtolimittheaspecttojoinpointsbelongingtoclasseswithincertainpackages,orthe@targetPCDcanbeusedtolimitthematchingtomethodsinobjectsthathavetheannotationgivenbetween(and)afterthekeyword@targetinthepointcutexpression.

ThereisaPCDthatSpringusesthatdoesnotexistinAspectJ.Thisisabean.Youcandefineapointcutexpressionthatcontainsbean(namepattern)tolimitthejoinpointtomethodexecutionsthatareinthenamedbean.Thepatterncanbetheentirenameoritcanbe,asalmostanyPCDexpressionmatching,*asajokercharacter.

Dynamicproxy-basedAOPSpringAOP,whenfirstpresentedtoJavaprogrammers,seemslikemagic.HowdoesithappenthatwehaveavariableofclassXandwecallsomemethodonthatobject,butinstead,itexecutessomeaspectbeforeorafterthemethodexecution,orevenaroundit,interceptingthecall

ThetechniquethatSpringdoesiscalleddynamicproxy.Whenwehaveanobject,whichimplementsaninterface,wecancreateanotherobject—theproxyobject—thatalsoimplementsthatinterface,buteachandeverymethodimplementationinvokesadifferentobjectcalledhandler,implementingtheJDKinterface,InvocationHandler.Whenamethodoftheinterfaceisinvokedontheproxyobject,itwillcallthefollowingmethodonthehandlerobject:

publicObjectinvoke(Objecttarget,Methodm,Object[]args)

Thismethodisfreetodoanything,evencallingtheoriginalmethodonthetargetobjectwiththeoriginalormodifiedargument.

Whenwedonothaveaninterfaceathandthattheclasstobeproxiedimplements,wecannotuseJDKmethods.Luckily,therearewidelyusedlibraries,suchascglib,whicharealsousedbySpringandthatcandosomethingsimilar.Cglibcancreateaproxyobjectthatextendstheoriginalclassandimplementsitsmethods,invokingthehandlerobject'sinvokemethodinawaysimilartohowtheJDKversiondoesfortheinterfacemethods.

ThesetechnologiescreateandloadclassesintotheJavamemoryduringruntime,andtheyareverydeeptechnicaltools.Theyareadvancedtopics.IdonotsaynottoplaywiththemwhilebeinganoviceJavaprogrammer.Afterall,whatcanhappen?Javaisnotaloadedgun.Itis,however,importantthatyoudonotloseyourinterestwhenyoudonotunderstandsomeofthedetailsorsomethingdoesnotworkfirst.Orsecond.Orthird...Keepswimming.

AOPimplementationinSpringworksbygeneratingproxyobjectsforthetargetobjects,andthehandlersinvoketheaspectsthatwedefineintheSpringconfiguration.Thisisthereasonyoucannotputaspectsonfinalclassesoronfinalmethods.Also,youcannotconfigureaspectsonprivateorprotectedmethods.Theprotectedmethodscouldbeproxiedinprinciple,butthisisnotagoodpractice,andthusSpringAOPdoesnotsupportit.Similarly,youcannotputaspectsonclassesthatarenotSpringbeans.Theyarecreatedby

thecodedirectlyandnotthroughSpringandhavenochancetoreturnaproxyinsteadoftheoriginalobjectwhentheobjectiscreated.Simplyput,ifSpringisnotaskedtocreatetheobject,itcannotcreateacustomone.Thelastthingwewanttodoistoexecutetheprogramandseehowtheaspectsperform.Theimplementationofourauditloggingisextremelysimple.Weusethestandardlogging,whichisnotreallysufficientforareal-lifeapplicationofauditlogging.Theonlyspecialthingwedoisthatweusealoggeridentifiedbythename,AUDIT_LOGandnotbythenameofaclass.Thisisalegitimateuseoftheloggersinmostoftheloggingframeworks.Inspiteofthefactthatweusuallyusetheclasstoidentifythelogger,itisabsolutelypossibletouseastringtoidentifyalogger.Inthecaseofourlogging,thisstringwillalsobeprintedontheconsoleintheloglines,anditwillvisuallystandout.

Considerthefollowingcommand:

gradle-Dspring.profiles.active=productionbootRun

Ifweagainstarttheapplicationwiththeprecedingcommand,startsoapUIfortheproject,startthemockservices,andexecutethetest,wewillseeontheconsolethefollowingloglinesthattheaspectsprint:

2016-11-1019:14:09.559INFO74643---[nio-8080-exec-1]o.a.c.c.C.[Tomcat].[localhost].[/]:InitializingSpringFrameworkServlet'dispatcherServlet'

2016-11-1019:14:09.567INFO74643---[nio-8080-exec-1]o.s.web.servlet.DispatcherServlet:FrameworkServlet'dispatcherServlet':initializationstarted

2016-11-1019:14:09.626INFO74643---[nio-8080-exec-1]o.s.web.servlet.DispatcherServlet:FrameworkServlet'dispatcherServlet':initializationcompletedin59ms

2016-11-1019:14:09.629INFO74643---[nio-8080-exec-1]p.j.b.e.m.p.auth.AuthFilter:Partnersecretispackt

2016-11-1019:14:09.655INFO74643---[nio-8080-exec-1]AUDIT_LOG:byIdqueryisabouttorun

2016-11-1019:14:09.666INFO74643---[nio-8080-exec-1]AUDIT_LOG:urlistobecreated

2016-11-1019:14:09.691INFO74643---[nio-8080-exec-1]AUDIT_LOG:urlcreatedwashttp://localhost:8083/inventory/{id}

2016-11-1019:14:09.715INFO74643---[nio-8080-exec-1]p.j.b.e.m.p.l.RestClientProductLookup:amount{id:123,amount:100}.

2016-11-1019:14:09.716INFO74643---[nio-8080-exec-1]p.j.b.e.m.p.l.RestClientProductLookup:Thereitemsfrom123.Weareoffering

2016-11-1019:14:09.716INFO74643---[nio-8080-exec-1]AUDIT_LOG:urlistobecreated

2016-11-1019:14:09.716INFO74643---[nio-8080-exec-1]AUDIT_LOG:urlcreatedwashttp://localhost:8081/product/{id}

2016-11-1019:14:09.725INFO74643---[nio-8080-exec-1]AUDIT_LOG:byIdquerywasexecuted

SummaryInthischapter,webuiltasimplebusinessapplicationthatsupportsbusiness-to-businesstransactions.WeimplementedaRESTserviceinamicroservices(almost)architectureusingthefeaturesthatareprovidedbythedefactostandardenterpriseframework:Spring.Lookingbackatthechapter,itisamazinghowfewlinesofcodewewrotetoachieveallthefunctionality,andthatisgood.Thelesscodeweneedtodevelopwhatwewant,thebetter.Thisprovesthepoweroftheframework.

Wediscussedmicroservices,HTTP,REST,JSON,andhowtousethemusingtheMVCdesignpattern.WelearnedhowSpringisbuiltup,whatmodulesarethere,howdependencyinjectionworksinSpring,andweeventouchedabitofAOP.ThiswasveryimportantbecausealongwithAOP,wediscoveredhowSpringworksusingdynamicproxyobjects,andthisissomethingthatisveryvaluablewhenyouneedtodebugSpringorsomeotherframeworkthatusesasimilarsolution(andthereareafewfrequentlyused).

Westartedtotestourcodeusingasimplebrowser,butafterthatwerealizedthatRESTservicesarebettertestedusingsomeprofessionaltestingtool,andforthatweusedsoapUIandbuiltupasimpleRESTtestsuitewithRESTteststepsandmockservices.

Havinglearntallthat,nothingwillstopusfromextendingthisapplicationusingverymodernandadvancedJavatechnologies,suchasreflection(whichwehavealreadytouchedonabitwhenwediscussedtheJDKdynamicproxy),Javastreams,lambdaexpressions,andscriptingontheserverside.

ExtendingOurE-CommerceApplication

Inthelastchapter,westarteddevelopingane-commerceapplicationandwecreatedthefunctionalitytolookupproductsbasedontheirIDand,also,bysomeparameters.Inthischapter,wewillextendthefunctionalitysothatwecanalsoordertheproductsweselected.Whiledoingso,wewilllearnnewtechnologies,focusingonfunctionalprogramminginJavaandonsomeotherlanguagefeatures,suchasreflectionandannotationhandlingduringruntime,andscriptinginterface.

Aswedidinthepreviouschapters,wewilldeveloptheapplicationstepbystep.Aswediscoverthenewlylearnttechnologies,wewillrefactorthecodetoenrollthenewtoolsandmethodstoproducemorereadableandeffectivecode.Wewillalsomimicthedevelopmentofreal-lifeprojectsinthesensethatatthestart,wewillhavesimplerequirements,andlater,newrequirementswillbesetasourimaginedbusinessdevelopsandsellsmoreandmoreproducts.Wewillbecomeimaginedmillionaires.

Wewillusethecodebaseofthepreviouschapter,andwewilldevelopitfurther,though,inanewproject.WewilluseSpring,Gradle,Tomcat,andsoapUI,whicharenotnewafterwegotacquaintedwiththeseinthepreviouschapter.Inthischapter,youwilllearnthefollowingtopics:

AnnotationprocessingUsingreflectionFunctionalprogramminginJavausing:

LambdaexpressionsStreamsInvokingscriptsfromJava

TheMyBusinessorderingTheorderingprocessisalittlebitmorecomplicatedthanjustlookingupproducts.Theorderformitselflistsproductsandamounts,andidentifieswhothecustomerforthatorderis.Identifiersgivetheproducts.Allthatwehavetodoischeckthattheproductsareavailableinourstore,andwecandeliverthemtothegivencustomer.Thisisthesimplestapproach;however,withsomeproducts,therearemorerestrictions.Forexample,whensomebodyordersadesk-sidelamp,wedeliverthepowercordseparately.Thereasonforthisisthatthepowercordisspecifictothecountry.WedeliverdifferentpowercordstotheUnitedKingdomandtoGermany.Onepossibleapproachcouldbetoidentifythecountryofthecustomer.Butthisapproachdoesnottakeintoaccountthatourcustomersareresellers.AllcustomerscouldbelocatedintheUnitedKingdom,andatthesametimetheymaywanttodeliverthelampwiththepowercabletoGermany.Toavoidsuchsituationsandambiguity,itwouldbeaptthatourcustomersorderthedesk-sidelampandthepowercordasseparateitemsinthesameorder.Insomecases,wedeliverthedesk-sidelampwithoutthepowercord,butthisisaspecialcase.Weneedsomelogictoidentifythesespecialcases.Therefore,wehavetoimplementlogictoseeifthereisapowercordforadesk-sidelampandifthereisnoautomatichandlingoftheorder,itisrefused.Itdoesnotmeanthatwewillnotdelivertheproduct.Wewillonlyputtheorderinaqueueandsomeoperatorwillhavetolookatit.

Theproblemwiththisapproachisthatthedesk-sidelampisonlyoneproductthatneedsconfigurationsupport.Themoreproductswehave,themorespecialitiestheymayhave,andthepieceofcodethatcheckstheconsistencyofanorderbecomesmoreandmorecomplexuntilitreachesalevelofcomplexitythatisnotmanageable.Whenaclassormethodbecomestoocomplex,theprogrammersrefactorit,splittingupthemethodorclassintosmallerpieces.Wehavetodothesamewiththeproductchecking.Weshouldn'ttrytocreateonehugeclassthatchecksfortheproductandallthepossibleorderconstellations,butratherweshouldhavemanysmallercheckssothateachchecksonlyonesmallset.

Checkingforconsistencyissimplerinsomecases.Checkingwhetherthelamphasapowercordhasacomplexityanynoviceprogrammercanprogram.Weusethisexampleinourcodebecausewewanttofocusontheactualstructureofthecode,andnotonthecomplexnatureofthecheckitself.Inreallife,however,thecheckscanbefairlycomplex.Imagineashopthatsellscomputers.Itputsaconfigurationtogether:powersupply,graphiccards,andmotherboard,theappropriateCPU,andthememory.Therearemanychoicesandsomeofthemmaynotworktogether.Inareal-lifesituation,weneedtocheckthatthemotherboardiscompatiblewiththememoryselected,thatithasasmanybanksasareintheorder,thattheyareappropriatelypaired(somememoriescanonlybeinstalledinpairs),thatthereisacompatibleslotforthegraphicscard,andthatthepowerhasenoughwattstoreliablyrunthewholeconfiguration.Thisisverycomplexandisbetternotmixedupwiththecodethatchecksifthereisapowercordforalamp.

Settinguptheproject

SincewearestillusingSpringboot,thebuildfiledoesnotneedanymodification;wewilluseitaswewillusethesamefileasinthelastchapter.Thepackagestructure,however,isabitdifferent.Thistime,wedosomethingmorecomplicatedthangettingarequestandrespondingtowhateverthebackendservicesdelivertous.Now,wehavetoimplementcomplexbusinesslogicthat,aswewillsee,needsmanyclasses.Whenwehavemorethan10classes,giveortake,inacertainpackage,itistimetothinkaboutputtingthemintoseparatepackages.Theclassesthatarerelatedtoeachotherandhaveasimilarfunctionalityshouldbeputintoonepackage.Thisway,wewillhaveapackageforthefollowing:

Thecontrollers(thoughwehaveonlyoneinthisexample,butusuallytherearemore)Datastoringbeansthathavenomorefunctionalitythanstoringdata,thus,fields,setters,andgettersCheckersthatwillhelpuscheckpowercordswhenadesk-sidelampisorderedServicesthatperformdifferentservicesforthecontrollerThemainpackageforourprogramthatcontainstheApplicationclass,SpringConfiguration,andsomeinterfaces

OrdercontrollerandDTOsWhenarequestcomestotheservertoorderabunchofproducts,itcomesinanHTTPSPOSTrequest.ThebodyoftherequestisencodedinJSON.Tillnow,wehadcontrollersthatwerehandlingGETparameters,buthandlingPOSTrequestsisnotmuchmoredifficultwhenwecanrelyonthedatamarshallingofSpring.Thecontrollercodeitselfissimple:

packagepackt.java9.by.example.mybusiness.bulkorder.controllers;

import...

@RestController

publicclassOrderController{

privateLoggerlog=

LoggerFactory.getLogger(OrderController.class);

privatefinalCheckerchecker;

publicOrderController(@AutowiredCheckerchecker){

this.checker=checker;

}

@RequestMapping("/order")

publicConfirmationgetProductInformation(@RequestBodyOrderorder){

if(checker.isConsistent(order)){

returnConfirmation.accepted(order);

}else{

returnConfirmation.refused(order);

}

}

}

Thereisonlyonerequestthatwehandleinthiscontroller:order.ThisismappedtotheURL,/order.TheorderisautomaticallyconvertedfromJSONtoanorderobjectfromtherequestbody.Thisiswhatthe@RequestBodyannotationasksSpringtodoforus.Thefunctionalityofthecontrollersimplycheckstheconsistencyoftheorder.Iftheorderisconsistent,thenweaccepttheorder;otherwise,werefuseit.Thereal-lifeexamplewillalsocheckthattheorderisnotonlyconsistentbutalsocomesfromacustomerwhoiseligibleforbuyingthoseproductsandthattheproductsareavailableinthewarehouseor,atleast,canbedelivered,basedonthepromisesandleadtimefromtheproducers.

Tochecktheconsistencyoftheorder,weneedsomethingthatdoesthisjobforus.Asweknowthatwehavetomodularizethecodeandnotimplementtoomanythingsinasingleclass,weneedacheckerobject.Thisisprovidedautomaticallybasedontheannotationontheclassandalsoontheconstructorofthecontrollerby@Autowired.

TheOrderclassisasimplebean,simplylistingtheitems:

packagepackt.java9.by.example.mybusiness.bulkorder.dtos;

import...;

publicclassOrder{

privateStringorderId;

privateList<OrderItem>items;

privateStringcustomerId;

...settersandgetters...

}

Thenameofthepackageisdtos,whichstandsforthepluralofDataTransferObject(DTO).DTOsareobjectsthatareusedtotransferdatabetweendifferentcomponents,

usuallyoverthenetwork.Sincetheothersidecanbeimplementedinanylanguage,themarshalingcanbeJSON,XML,orsomeotherformatthatiscapableofdeliveringnothingbutdata.Theseclassesdonothaverealmethods.DTOsusuallyhaveonlyfields,setters,andgetters.

Thefollowingistheclassthatcontainsoneiteminanorder:

packagepackt.java9.by.example.mybusiness.bulkorder.dtos;

publicclassOrderItem{

privatedoubleamount;

privateStringunit;

privateStringproductId;

...settersandgetters...

}

Theorderconfirmationisalsointhispackage,andthoughthisisalsoatrueDTO,ithassomesimpleauxiliarymethods:

packagepackt.java9.by.example.mybusiness.bulkorder.dtos;

publicclassConfirmation{

privatefinalOrderorder;

privatefinalbooleanaccepted;

privateConfirmation(Orderorder,booleanaccepted){

this.order=order;

this.accepted=accepted;

}

publicstaticConfirmationaccepted(Orderorder){

returnnewConfirmation(order,true);

}

publicstaticConfirmationrefused(Orderorder){

returnnewConfirmation(order,false);

}

publicOrdergetOrder(){

returnorder;

}

publicbooleanisAccepted(){

returnaccepted;

}

}

Weprovidetwofactorymethodsfortheclass.Thisisalittleviolationofthesingleresponsibilityprinciplethatpuristshate.Mostofthetime,whenthecodebecomesmorecomplex,suchshortcutsbiteback,andthecodehastoberefactoredtobecleaner.Thepuristsolutionwouldbetocreateaseparatefactoryclass.Theuseofthefactorymethodseitherfromthisclassorfromaseparatedclassmakesthecodeofthecontrollermorereadable.

Themajortaskwehaveistheconsistencycheck.Thecode,tillthispoint,isalmosttrivial.

ConsistencycheckerWehaveaconsistencycheckerclass,andaninstanceofitisinjectedintothecontroller.Thisclassisusedtochecktheconsistency,butitdoesnotactuallyperformthecheckitself.Itonlycontrolsthedifferentcheckersthatweprovideandinvokesthemonebyonetodotherealwork.

Werequirethataconsistencychecker,suchastheonethatcheckswhethertheordercontainsapowercordwhenadesk-sidelampisordered,implementstheConsistencyCheckerinterface:

packagepackt.java9.by.example.mybusiness.bulkorder;

import...

publicinterfaceConsistencyChecker{

booleanisInconsistent(Orderorder);

}

ThemethodisInconsistentshouldreturntrueiftheorderisinconsistent.Itreturnsfalseifitdoesnotknowwhethertheorderisinconsistentornot,butfromtheaspectthattheactualcheckerexaminestheorder,thereisnoinconsistency.HavingseveralConsistencyCheckerclasses,wehavetoinvokeoneaftertheotheruntilonereturnstrueorweareoutofthem.Ifnoneofthemreturnstrue,thenwecansafelyassume,atleastfromtheautomatedcheckers'pointofview,thattheorderisconsistent.

Weknowatthestartofthedevelopmentthatwewillreallyhavealotofconsistencycheckersandnotallarerelevantforalloftheorders.Wewanttoavoidtheinvocationofeachcheckerforeachorder.Todoso,weimplementsomefiltering.Weletproductsspecifywhattypeofcheckstheyneed.Thisisapieceofproductinformation,suchasthesizeorthedescription.Toaccommodatethis,weneedtoextendtheProductInformationclass.

WewillcreateeachConsistencyCheckerinterface,implementingtheclasstobeaSpringbean(annotatedwiththe@Componentannotation),andatthesametime,wewillannotatethemwithanannotationthatspecifieswhattypeofcheckstheyimplement.Atthesametime,ProductInformationisextended,containingasetofAnnotationclassobjectsthatspecifywhichcheckerstoinvoke.Wecouldsimplylistthecheckerclassesinsteadoftheannotations,butthisgivesussomeextrafreedominconfiguringthemappingbetweentheproductsandtheannotations.Theannotationspecifiesthetypeoftheproducts,andthecheckerclassesareannotated.Thedesk-sidelamphasthePoweredDevicetype,andthecheckerclass,NeedPowercord,isannotatedwiththe@PoweredDeviceannotation.Ifthereisanyothertypeofproductsthatalsoneedsapowercord,thentheannotationofthattypeshouldbeaddedtotheNeedPowercordclass,andourcodewillwork.Sincewestartdivingdeepintoannotationsandannotationhandling,wehavetofirstlearnwhatannotationsreallyare.WehavealreadyusedannotationssinceChapter3,OptimizingtheSort,MakingCodeProfessionalbutallweknewwashowtousethem,andthatisusuallydangerouswithoutunderstandingwhatwedid.

AnnotationsAnnotationsareusedwiththe@characterinfrontofthemandcanbeattachedtopackages,classes,interfaces,fields,methods,methodparameters,generictypedeclarationanduse,and,finally,toannotations.Annotationscanbeusedalmosteverywhereandtheyareusedtodescribesomeprogrammetainformation.Forexample,the@RestControllerannotationdoesnotdirectlyalterthebehavioroftheOrderControllerclass.ThebehavioroftheclassisdescribedbytheJavacodethatisinside.TheannotationhelpsSpringtounderstandwhattheclassisandhowitcanandshouldbeused.WhenSpringscansallthepackagesandclassestodiscoverthedifferentSpringbeans,itseestheannotationontheclassandtakesitintoaccount.TherecanbeotherannotationsontheclassthatSpringdoesnotunderstand.Theymaybeusedbysomeotherframeworkorprogramcode.Springignoresthemasanywell-behavingframework.Forexample,aswewillseelater,wehaveinourcodebase,theNeedPowercordclass,whichisaSpringbeanand,assuch,annotatedwiththe@Componentannotation.Atthesametime,itisalsoannotatedwiththe@PoweredDeviceannotation.Springhasnoideaaboutwhatapowereddeviceis.Thisissomethingthatwedefineanduse.Springignoresthis.

Packages,classes,interfaces,fields,andsoon,canhavemanyannotationsattachedtothem.Theseannotationsshouldsimplybewritteninfrontofthedeclarationofthesyntacticalunittheyareattachedto.

Inthecaseofpackages,theannotationhastobewritteninfrontofthepackagenameinthepackage-info.javafile.ThisfilecanbeplacedinthedirectoryofthepackageandcanbeusedtoedittheJavaDocforthepackageandalsotoaddanannotationtothepackage.ThisfilecannotcontainanyJavaclasssincethename,package-info,isnotavalididentifier.

Wecannotjustwriteanythinginfrontofanythingasanannotation.Annotationsshouldbedeclared.TheyareintheruntimeofJavaspecialinterfaces.TheJavafilethatdeclaresthe@PoweredDeviceannotation,forexample,lookslikethis:

packagepackt.java9.by.example.mybusiness.bulkorder.checkers;

importjava.lang.annotation.Retention;

importjava.lang.annotation.RetentionPolicy;

@Retention(RetentionPolicy.RUNTIME)

public@interfacePoweredDevice{

}

The@characterinfrontoftheinterfacekeywordshowsusthatthisisaspecialone:anannotationtype.Therearesomespecialrules;forexample,anannotationinterfaceshouldnotextendanyotherinterface,notevenanannotationone.Ontheotherhand,thecompilerautomaticallymakestheannotationinterfacesothatitextendstheJDKinterface,java.lang.annotation.Annotation.

Annotationsareinthesourcecode,andthus,theyareavailableduringthecompilationprocess.Theycanalsoberetainedbythecompilerandputintothegeneratedclassfiles,andwhentheclassloaderloadstheclassfile,theymayalsobeavailableduringruntime.Thedefaultbehavioristhatthecompilerstorestheannotationalongwiththeannotatedelementintheclassfile,buttheclassloaderdoesnotkeepitavailableforruntime.

Tohandleannotationsduringthecompilationprocess,theJavacompilerhastobeextendedusingannotationprocessors.ThisisafairlyadvancedtopicandthereareonlyafewexamplesyoucanmeetwhileworkingwithJava.AnannotationprocessorisaJavaclassthatimplementsaspecialinterfaceandisinvokedbythecompilerwhenitprocessesanannotationinthesourcefilethattheprocessorisdeclaredtohaveaninterestin.

AnnotationretentionSpringandotherframeworksusuallyhandleannotationsduringruntime.Thecompilerandtheclassloaderhavetobeinstructedthattheannotationistobekeptavailableduringruntime.Todoso,theannotationinterfaceitselfhastobeannotatedusingthe@Retentionannotation.ThisannotationhasoneparameteroftheRetentionPolicytype,whichisanenum.Wewillsoondiscusshowannotationparametersshouldbedefined.

Itisinterestingtonotethatthe@Retentionannotationontheannotationinterfacehastobeavailableintheclassfile;otherwise,theclassloaderswouldnotknowhowtotreatanannotation.Howdowesignalthatanannotationistobekeptbythecompilerafterthecompilationprocess?Weannotatetheannotationinterfacedeclaration.Thus,thedeclarationof@Retentionisannotatedbyitselfanditisdeclaredtobeavailableinruntime.

Theannotationdeclarationcanbeannotatedusing@Retention(RetentionPolicy.SOURCE),@Retention(RetentionPolicy.CLASS),or@Retention(RetentionPolicy.RUNTIME).

Annotationtarget

Thelastretentiontypewillbethemostfrequentused.Therearealsootherannotationsthatcanbeusedonannotationdeclarations.The@Targetannotationcanbeusedtorestricttheuseoftheannotationtocertainlocations.Theargumenttothisannotationiseitherasinglejava.lang.annotation.ElementTypevalueoranarrayofthesevalues.Thereisagoodreasontorestricttheuseofannotations.Itismuchbettertogetacompilationtimeerrorwhenweplaceanannotationinthewrongplacethanhuntingduringruntimewhytheframeworkignoresourannotation.

AnnotationparametersAnnotations,aswesaw,canhaveparameters.Todeclaretheseparametersinthe@interfacedeclarationoftheannotation,weusemethods.Thesemethodshaveanameandareturnvalue,buttheyshouldnothaveanargument.Youmaytrytodeclaresomeparameters,buttheJavacompilerwillbestrictandwillnotcompileyourcode.

Thevaluescanbedefinedattheplacewheretheannotationisused,usingthenameofthemethodandwiththe=character,assigningtothemsomevaluethatiscompatiblewiththetypeofthemethod.Forexample,let'ssupposethatwemodifythedeclarationoftheannotationPoweredDevicetothefollowing:

public@interfaceParameteredPoweredDevice{

StringmyParameter();

}

Insuchacase,attheuseoftheannotation,weshouldspecifysomevaluefortheparameter,suchasthefollowing:

@Component

@ParameteredPoweredDevice(myParameter="1966")

publicclassNeedPowercordimplementsConsistencyChecker{

...

Ifthenameoftheparameterisavalueandattheplaceofuseoftheannotationthereisnootherparameterdefined,thenthename,"value",maybeskipped.Forexample,modifyingthecodeasfollowsisahandyshorthandwhenwehaveonlyoneparameter:

public@interfaceParameteredPoweredDevice{

Stringvalue();

}

...

@Component

@ParameteredPoweredDevice("1966")

publicclassNeedPowercordimplementsConsistencyChecker{

...

Wecandefineoptionalparametersalsousingthedefaultkeywordfollowingthemethoddeclaration.Inthiscase,wehavetodefineadefaultvaluefortheparameter.Modifyingthesampleannotationwehavefurther,westillcan,butneednot,specifythevalue.Inthelattercase,itwillbeanemptystring:

public@interfaceParameteredPoweredDevice{

Stringvalue()default"";

}

Sincethevaluewespecifyshouldbeconstantandcalculableduringcompiletime,thereisnotmuchuseofcomplextypes.Annotationparametersareusuallystrings,integers,andsometimes,doubles,orotherprimitivetypes.Theexactlistofthetypesgivenbythelanguagespecificationisasfollows:

Primitive(double,int,andsoon)StringClassAnenumAnotherannotation

Anarrayofanyoftheaforementionedtypes

WehaveseenexamplesofStringandalsothatenum:RetentionandTargetbothhaveenumparameters.Theinterestingpartwewanttofocusonisthelasttwoitemsoftheprecedinglist.

Whenthevalueoftheparameterisanarray,thevaluecanbespecifiedascomma-separatedvaluesbetweenthe{and}characters.Forexample:

String[]value();

Thiscanthenbeaddedtothe@interfaceannotationwecanwrite:

@ParameteredPoweredDevice({"1966","1967","1991"})

However,incasethereisonlyonevaluewewanttopassastheparametervalue,wecanstillusetheformat:

@ParameteredPoweredDevice("1966")

Inthiscase,thevalueoftheattributewillbeanarrayoflength1.Whenthevalueofanannotationisanarrayofannotationtypes,thingsgetabitmorecomplex.Wecreatean@interfaceannotation(notethepluralinthename):

@Retention(RetentionPolicy.RUNTIME)

public@interfacePoweredDevices{

ParameteredPoweredDevice[]value()default{};

}

Theuseofthisannotationcouldbeasfollows:

@PoweredDevices(

{@ParameteredPoweredDevice("1956"),@ParameteredPoweredDevice({"1968","2018"})}

)

NotethatthisisnotthesameashavingtheParameteredPoweredDeviceannotationwiththreeparameters.Thisisanannotationthathastwoparameters.Eachparameterisanannotation.Thefirsthasonestringparameterandthesecondhastwo.

Asyoucansee,annotationscanbefairlycomplex,andsomeoftheframeworks(orrathertheprogrammerswhocreatedthem)ranamokusingthem.Beforeyoustartwritingaframework,researchtoseewhetherthereisalreadyaframeworkthatyoucanuse.Also,checkwhetherthereissomeotherwaytosolveyourproblem.99%ofannotationhandlingcodecouldbeavoidedandmadesimpler.Thelesscodewewriteforthesamefunctionality,thehappierweare.Weprogrammersarethelazytypesandthisisthewayithastobe.

Thelastexample,wheretheparameteroftheannotationisanarrayofannotations,isimportanttounderstandhowwecancreaterepeatableannotations.

RepeatableannotationsAnnotatethedeclarationoftheannotationwith@Repeatabletodenotethattheannotationcanbeappliedmultipletimesatoneplace.Theparametertothisannotationisanannotationtypethatshouldhaveaparameteroftype,whichisanarrayofthisannotation.Don'ttrytounderstand!I'llgiveanexampleinstead.Ialreadyhave,infact:wehave@PoweredDevices.Ithasanargumentthatisanarrayof@ParameteredPoweredDevice.Considerthatwenowannotatethis@interfaceasthefollowing:

...

@Repeatable(PoweredDevices.class)

public@interfaceParameteredPoweredDevice{

...

Then,wecansimplifytheuseof@ParameteredPoweredDevice.WecanrepeattheannotationmultipletimesandtheJavaruntimewillautomaticallyencloseitinthewrappingclass,which,inthiscase,is@PoweredDevices.Inthiscase,thefollowingtwowillbeequivalent:

...

@ParameteredPoweredDevice("1956")

@ParameteredPoweredDevice({"1968","2018"})

publicclassNeedPowercordimplementsConsistencyChecker{

...

@PoweredDevices(

{@ParameteredPoweredDevice("1956"),@ParameteredPoweredDevice({"1968","2018"})}

)

publicclassNeedPowercordimplementsConsistencyChecker{

...

ThereasonforthiscomplexapproachisagainanexampleofbackwardcompatibilitythatJavastrictlyfollows.AnnotationswereintroducedinJava1.5andrepeatableannotationshavebeenavailableonlysinceversion1.8.WewillsoontalkaboutthereflectionAPIthatweusetohandletheannotationsduringruntime.ThisAPIinthejava.lang.reflect.AnnotatedElementinterfacehasagetAnnotation(annotationClass)method,whichreturnsanannotation.Ifasingleannotationcanappearmorethanonceonaclass,method,andsoon,thenthereisnowayofcallingthismethodtogetallthedifferentinstanceswithallthedifferentparameters.Backwardcompatibilitywasensuredbyintroducingthecontainingtypethatwrapsthemultipleannotations.

Annotationinheritance

Annotations,justlikemethodsorfields,canbeinheritedbetweenclasshierarchies.Ifanannotationdeclarationismarkedwith@Inherited,thenaclassthatextendsanotherclasswiththisannotationcaninheritit.Theannotationcanbeoverriddenincasethechildclasshastheannotation.BecausethereisnomultipleinheritanceinJava,annotationsoninterfacescannotbeinherited.Evenwhentheannotationisinherited,theapplicationcodethatretrievestheannotationofacertainelementcandistinguishbetweentheannotationsthatareinheritedandthosethataredeclaredontheentityitself.Therearemethodstogettheannotationsandseparatemethodstogetthedeclaredannotationsthataredeclaredontheactualelement,andnotinherited.

@Documentedannotations

The@Documentedannotationexpressestheintentthattheannotationispartofthecontractoftheentityand,thisway,ithastogetintothedocumentation.ThisisanannotationthattheJavaDocgeneratorlooksatwhencreatingthedocumentationforanelementthatreferencesthe@Documentedannotation.

JDKannotationsThereareotherannotationsdefinedintheJDKinadditiontothosethataretobeusedtodefineannotations.Wehavealreadyseensomeofthese.Themostfrequentlyusedisthe@Overrideannotation.Whenthecompilerseesthisannotation,itchecksthatthemethodreallyoverridessomeinheritedmethod.Failingtodosowillresultinanerror,savingusfrommiserableruntimedebugging.

The@Deprecatedannotationsignalsinthedocumentationofamethod,class,orsomeotherelementthattheelementisnottobeused.Itisstillthereinthecode,becausesomeusersmaystilluseit,butinthecaseofanewdevelopmentthatdependsonthelibrarycontainingtheelement,thenewlydevelopedcodeshouldnotuseit.Theannotationhastwoparameters.Oneparameterissince,whichcanhaveastringvalueandmaydeliverversioninformationabouthowlongorsincewhichversionofthemethod,orclassisdeprecated.TheotherparameterisforRemoval,whichshouldbetrueiftheelementwillnotappearinthefutureversionsofthelibrary.Somemethodsmaybedeprecatedbecausetherearebetteralternativesbutthedevelopersdonotintendtoremovethemethodfromthelibrary.Insuchacase,theforRemovalcanbesettofalse.

The@SuppressWarningannotationisalsoafrequentlyusedone,thoughitsuseisquestionable.Itcanbeusedtosuppresssomeofthewarningsofthecompiler.Itisrecommendedtowritecode,ifpossible,whichcanbecompiledwithoutanywarning.

The@FunctionalInterfaceannotationdeclaresthataninterfaceintendstohaveonlyonemethod.Suchinterfacescanbeimplementedaslambdaexpressions.Youwilllearnaboutlambdaexpressionslaterinthischapter.Whenthisannotationisappliedonaninterfaceandthereismorethanonemethoddeclaredintheinterface,thecompilerwillsignalcompilationerror.Thiswillpreventanydeveloperearlyonfromaddinganothermethodtoaninterfaceintendedtobeusedtogetherwithfunctionalprogrammingandlambdaexpressions.

Usingreflection

Nowthatyouhavelearnthowtodeclareannotationsandhowtoattachthemtoclassesandmethods,wecanreturntoourProductInformationclass.Recallthatwewantedtospecifythetypeofproductsinthisclassandthateachproducttypeisrepresentedbyan@interfaceannotation.Wehavealreadylisteditinthepreviousfewpages,theonewewillimplementinour@PoweredDeviceexample.Wewilldevelopthecodeassumingthatlatertherewillbemanysuchannotations,producttypes,andconsistencycheckersthatareannotatedwith@Componentandwithoneormoreofourannotations.

GettingannotationsWewillextendtheProductInformationclasswiththefollowingfield:

privateList<Class<?extendsAnnotation>>check;

SincethisisaDTO,andSpringneedsthesettersandgetters,wewillalsoaddanewgetterandsettertoit.Thisfieldwillcontainthelistofclassesthateachclassimplementoneofourannotationsandalsothebuilt-inJDKinterface,Annotation,becausethatisthewaytheJavacompilergeneratesthem.Atthispoint,thismaybeabitmurkybutIpromisethatthedawnwillbreakandtherewillbelightaswegoon.

Togettheproductinformation,wehavetolookitupbyID.Thisistheinterfaceandservicethatwedevelopedinthelastchapter,except,thistime,wehaveanothernewfield.Thisis,infact,asignificantdifferencealthoughtheProductLookupinterfacedidnotchangeatall.Inthelastchapter,wedevelopedtwoversions.Oneoftheversionswasreadingthedatafromapropertiesfile,theotheronewasconnectingtoaRESTservice.

PropertiesfilesareuglyandoldtechnologybutamustifeveryouintendtopassaJavaintervieworworkonenterpriseapplicationsdevelopedatthestartofthe21stcentury.Ihadtoincludeitinthelastchapter.Itwasmyownurgetoincludeitinthebook.Atthesametime,whilecodingforthischapter,Ididnothavethestomachtokeepusingit.IalsowantedtoshowyouthatthesamecontentcouldbemanagedinaJSONformat.

Now,wewillextendtheimplementationofResourceBasedProductLookuptoreadtheproductinformationfromJSONformattedresourcefiles.Mostofthecoderemainsthesameintheclass;therefore,weonlylistthedifferencehere:

packagepackt.java9.by.example.mybusiness.bulkorder.services;

import...

@Service

publicclassResourceBasedProductLookupimplementsProductLookup{

privatestaticfinalLoggerlog=LoggerFactory.getLogger(ResourceBasedProductLookup.class);

privateProductInformationfromJSON(InputStreamjsonStream)

throwsIOException{

ObjectMappermapper=newObjectMapper();

returnmapper.readValue(jsonStream,

ProductInformation.class);

}

...

privatevoidloadProducts(){

if(productsAreNotLoaded){

try{

Resource[]resources=

newPathMatchingResourcePatternResolver().

getResources("classpath:products/*.json");

for(Resourceresource:resources){

loadResource(resource);

}

productsAreNotLoaded=false;

}catch(IOExceptionex){

log.error("Testresourcescannotberead",ex);

}

}

}

privatevoidloadResource(Resourceresource)

throwsIOException{

finalintdotPos=

resource.getFilename().lastIndexOf('.');

finalStringid=

resource.getFilename().substring(0,dotPos);

finalProductInformationpi=

fromJSON(resource.getInputStream());

pi.setId(id);

products.put(id,pi);

}

...

Intheprojectresources/productsdirectorywehaveafewJSONfiles.Oneofthemcontainsthedesklampproductinformation:

{

"id":"124",

"title":"DeskLamp",

"check":[

"packt.java9.by.example.mybusiness.bulkorder.checkers.PoweredDevice"

],

"description":"thisisalampthatstandsonmydesk",

"weight":"600",

"size":["300","20","2"]

}

ThetypeofproductisspecifiedinaJSONarray.Inthisexample,thisarrayhasonlyoneelementandthatelementisthefullyqualifiednameoftheannotationinterfacethatrepresentsthetypeofproduct.WhentheJSONmarshallerconvertstheJSONtoaJavaobject,itrecognizesthatthefieldthatneedsthisinformationisaList,soitconvertsthearraytoalistand,also,theelementsfromStringtoClassobjectsrepresentingtheannotationinterface.

NowthatwehavetheresourcesloadedfromJSONformattedresourcesandwesawhoweasyitistoreadJSONdatawhenusingSpring,wecangetbacktotheorderconsistencycheck.TheCheckerclassimplementsthelogictocollectthepluggablecheckersandtoinvokethem.Italsoimplementstheannotation-basedscreeningsoasnottoinvokethecheckerswedon'treallyneedfortheactualproductsintheactualorder:

packagepackt.java9.by.example.mybusiness.bulkorder.services;

import...

@Component()

@RequestScope

publicclassChecker{

privatestaticfinalLoggerlog=

LoggerFactory.getLogger(Checker.class);

privatefinalCollection<ConsistencyChecker>checkers;

privatefinalProductInformationCollectorpiCollector;

privatefinalProductsCheckerCollectorpcCollector;

publicChecker(

@AutowiredCollection<ConsistencyChecker>checkers,

@AutowiredProductInformationCollectorpiCollector,

@AutowiredProductsCheckerCollectorpcCollector){

this.checkers=checkers;

this.piCollector=piCollector;

this.pcCollector=pcCollector;

}

publicbooleanisConsistent(Orderorder){

Map<OrderItem,ProductInformation>map=

piCollector.collectProductInformation(order);

if(map==null){

returnfalse;

}

Set<Class<?extendsAnnotation>>annotations=

pcCollector.getProductAnnotations(order);

for(ConsistencyCheckerchecker:

checkers){

for(Annotationannotation:

checker.getClass().getAnnotations()){

if(annotations.contains(

annotation.annotationType())){

if(checker.isInconsistent(order)){

returnfalse;

}

break;

}

}

}

returntrue;

}

}

OneoftheinterestingthingstomentionisthattheSpringauto-wiringisveryclever.WehaveafieldwiththeCollection<ConsistencyChecker>type.Usually,auto-wiringworksifthereisexactlyoneclassthathasthesametypeastheresourcestowire.Inourcase,wedonothaveanysuchcandidatesincethisisacollection,butwehavemanyConsistencyCheckerclasses.AllourcheckersimplementthisinterfaceandSpringrecognizesit,instantiatesthemall,magicallycreatesacollectionofthem,andinjectsthecollectionintothisfield.

Usuallyagoodframeworkworkslogically.IwasnotawareofthisfeatureofSpring,butIthoughtthatthiswouldbelogicaland,magically,itworked.Ifthingsarelogicalandjustwork,youdonotneedtoreadandrememberthedocumentation.Abitofcautiondoesnotharmhowever.AfterIexperiencedthatthisfunctionalityworksthisway,IlookeditupinthedocumentationtoseethatthisisreallyaguaranteedfeatureofSpringandnotsomethingthatjusthappenstoworkbutmaychangeinfutureversionswithoutnotice.Usingonlyguaranteedfeaturesisextremelyimportantbutisneglectedmanytimesinourindustry.

WhentheisConsistentmethodisinvoked,itfirstcollectstheproductinformationintoHashMap,assigningaProductInformationinstancetoeachOrderItem.Thisisdoneinaseparateclass.Afterthis,ProductsCheckerCollectorcollectstheConsistencyCheckerinstancesneededbyoneormoreproductitems.Whenwehavethisset,weneedtoinvokeonlythosecheckersthatareannotatedwithoneoftheannotationsthatareinthisset.Wedothatinaloop.

Inthiscode,weusereflection.Weloopovertheannotationsthateachcheckerhas.Togetthecollectionofannotations,weinvokechecker.getClass().getAnnotations().Thisinvocationreturnsacollectionofobjects.EachobjectisaninstanceofsomeJDKruntimegeneratedclassthatimplementstheinterfacewedeclaredasanannotationinitsownsourcefile.Thereisnoguarantee,though,thatthedynamicallycreatedclassimplementsonlyour@interfaceandnotsomeotherinterfaces.Therefore,togettheactualannotationclass,wehavetoinvoketheannotationTypemethod.

TheProductCheckerCollectorandProductInformationCollectorclassesareverysimple,andwewilldiscussthemlaterwhenwelearnaboutstreams.Theywillserveasagoodexampleatthatplace,whenweimplementthemusingloopsand,rightafterthat,usingstreams.

Havingthemall,wecanfinallycreateouractualcheckerclasses.Theonethathelpsusseethatthereisapowercordorderedforourlampisthefollowing:

packagepackt.java9.by.example.mybusiness.bulkorder.checkers;

import...

@Component

@PoweredDevice

publicclassNeedPowercordimplementsConsistencyChecker{

privatestaticfinalLoggerlog=

LoggerFactory.getLogger(NeedPowercord.class);

@Override

publicbooleanisInconsistent(Orderorder){

log.info("checkingorder{}",order);

CheckHelperhelper=newCheckHelper(order);

return!helper.containsOneOf("126","127","128");

}

}

Thehelperclasscontainssimplemethodsthatwillbeneededbymanyofthecheckers,forexample:

publicbooleancontainsOneOf(String...ids){

for(finalOrderItemitem:order.getItems()){

for(finalStringid:ids){

if(item.getProductId().equals(id)){

returntrue;

}

}

}

returnfalse;

}

InvokingmethodsInthisexample,weusedonlyonesinglereflectioncalltogettheannotationsattachedtoaclass.Reflectioncandomanymorethings.Handlingannotationsisthemostimportantuseforthesecallssinceannotationsdonothavetheirownfunctionalityandcannotbehandledinanyotherwayduringruntime.Reflection,however,doesnotstoptellinguswhatannotationsaclassoranyotherannotableelementhas.Reflectioncanbeusedtogetalistofthemethodsofaclass,thenameofthemethodsasstrings,theimplementedinterfacesofaclass,theparentclassitextends,thefields,thetypesoffields,andsoon.Reflectiongenerallyprovidesmethodsandclassestowalkthroughtheactualcodestructuredowntothemethodlevel,programmatically.

Thiswalkthroughdoesnotonlyallowreadingtypesandcodestructurebutalsomakesitpossibletosetfieldvaluesandcallmethodswithoutknowingthemethods'nameatcompiletime.Wecanevensetfieldsthatareprivateandarenotgenerallyaccessiblebytheoutsideworld.Itisalsotonotethataccessingthemethodsandfieldsthroughreflectionisusuallyslowerthanthroughcompiledcodebecauseitalwaysinvolveslookupbythenameoftheelementinthecode.

Theruleofthumbisthatifyouseethatyouhavetocreatecodeusingreflection,thenrealizethatyouareprobablycreatingaframework(orwritingabookaboutJavathatdetailsreflection).Doesitsoundfamiliar?

Springalsousesreflectiontodiscovertheclasses,methods,andfields,andalsotoinjectanobject.ItusestheURLclassloadertolistalltheJARfilesanddirectoriesthatareontheclasspath,loadsthem,andexaminestheclasses.

Foracontrivedexample,forthesakeofdemonstration,let'sassumethattheConsistencyCheckerimplementationswerewrittenbymanyexternalsoftwarevendors,andthearchitectwhooriginallydesignedtheprogramstructurejustforgottoincludetheisConsistentmethodintheinterface.(Atthesametime,tosaveourmentalhealth,wecanalsoimaginethatthispersonisnotworkinganymoreinthecompanyfordoingso.)Asaconsequence,thedifferentvendorsdeliveredJavaclassesthat"implement"thisinterfacebutwecannotinvokethemethod,notonlybecausewedonothaveacommonparentinterfacethathasthismethodbutalsobecausethevendorsjusthappenedtousedifferentnamesfortheirmethods.

Whatcanwedointhissituation?Business-wise,askingallthevendorstorewritetheircheckersisruledoutbecausethemknowingweareintroubleattachesaheftypricetagtothetask.Ourmanagerswanttoavoidthatcostandwedevelopersalsowanttoshowthatwecanmendthesituationanddomiracles.(Later,Iwillhaveacommentonthat.)

Wecouldjusthaveaclassthatknowseverycheckerandhowtoinvokeeachoftheminmanydifferentways.Thiswouldrequireustomaintainthesaidclasswheneveranewcheckerisintroducedtothesystem,andwewanttoavoidthat.Thewholepluginarchitectureweareusingwasinventedforthisverypurposeinthefirstplace.

Howcanweinvokeamethodonanobjectthatweknowhasonlyonedeclaredmethod,whichacceptsan

orderasaparameter?Thatiswherereflectioncomesintothepicture.Insteadofcallingchecker.isInconsistent(order),weimplementasmallprivatemethod,isInconsistent,whichcallsthemethod,whateveritsnameis,viareflection:

privatebooleanisInconsistent(ConsistencyCheckerchecker,Orderorder){

Method[]methods=checker.getClass().getDeclaredMethods();

if(methods.length!=1){

log.error(

"Thechecker{}haszeroormorethanonemethods",

checker.getClass());

returnfalse;

}

finalMethodmethod=methods[0];

finalbooleaninconsistent;

try{

inconsistent=(boolean)method.invoke(checker,order);

}catch(InvocationTargetException|

IllegalAccessException|

ClassCastExceptione){

log.error("Callingthemethod{}onclass{}threwexception",

method,checker.getClass());

log.error("Theexceptionis",e);

returnfalse;

}

returninconsistent;

}

WecangettheclassoftheobjectbycallingthegetClassmethod,andontheobjectthatrepresentstheclassitself,wecancallgetDeclaredMethods.Fortunately,thecheckerclassesarenotlitteredbymanymethods,sowecheckthatthereisonlyonemethoddeclaredinthecheckerclass.NotethatthereisalsoagetMethodsmethodinthereflectionlibrarybutitalwayswillreturnmorethanonemethod.Itreturnsthedeclaredandtheinheritedmethods.Becauseeachandeveryclassinheritsfromjava.lang.Object,atleastthemethodsoftheObjectclasswillbethere.

Afterthis,wetrytoinvoketheclassusingtheMethodobjectthatrepresentsthemethodinthereflectionclass.NotethatthisMethodobjectisnotdirectlyattachedtoaninstance.Weretrievedthemethodfromtheclass,andthus,whenweinvokeit,weshouldpasstheobjectitshouldworkonasafirstparameter.Thisway,x.y(z),becomesmethod.invoke(x,z).ThelastparameterofinvokeisavariablenumberofargumentsthatarepassedasanObjectarray.Inmostcases,whenweinvokeamethod,weknowtheargumentsinourcodeevenifwedonotknowthenameofthemethodandhavetousereflection.Wheneventheargumentsarenotknownbutareavailableasamatterofcalculation,thenwehavetopassthemasanObjectarray.

Invokingamethodviareflectionisariskycall.Ifwetrytocallamethodthenormalway,whichisprivate,thenthecompilerwillsignalanerror.Ifthenumberofargumentsortypesarenotappropriate,thecompilerwillagainwillgiveusanerror.Ifthereturnedvalueisnotboolean,orthereisnoreturnvalueatall,thenweagaingetacompilererror.Inthecaseofreflection,thecompilerisclueless.Itdoesnotknowwhatmethodwewillinvokewhenthecodeisexecuting.Theinvokemethod,ontheotherhand,canandwillnoticeallthesefailureswhenitisinvoked.Ifanyoftheaforementionedproblemsoccur,thenwewillgetexceptions.Iftheinvokemethoditselfseesthatitcannotperformwhatweaskofit,thenitwillthrowInvocationTargetExceptionorIllegalAccessException.Iftheconversionfromtheactualreturnvaluetobooleanisnotpossible,thenwewillgetClassCastException.

Aboutdoingmagic,itisanaturalurgethatwefeellikemakingsomethingextraordinary,somethingoutstanding.Thisisokaywhenweareexperimentingwithsomething,doingahobbyjob.Ontheotherhand,thisisstronglynotokaywhenweareworkingonaprofessionaljob.Averageprogrammers,whodo

notunderstandyourbrilliantsolution,willmaintainthecodeinanenterpriseenvironment.Theywillturnyournicelycombedcodeintohaystackwhilefixingsomebugsorimplementingsomeminornewfeatures.EvenifyouaretheMozartofprogramming,theywillbe,atbest,no-namesingers.Abrilliantcodeinanenterpriseenvironmentcanbearequiem,withalltheimplicationsofthatmetaphor.

Lastbutnotleast,thesadrealityisthatweareusuallynottheMozartsofprogramming.

Notethatincasethereturnvalueoftheoriginalvalueisprimitive,thenitwillbeconvertedtoanobjectbyreflection,andthenwewillconvertitbacktotheprimitivevalue.Ifthemethoddoesnothaveareturnvalue,inotherwords,ifitisvoid,thenthereflectionwillreturnajava.lang.Voidobject.TheVoidobjectisonlyaplaceholder.Wecannotconvertittoanyprimitivevalueoranyothertypeofobjects.ItisneededbecauseJavaisstrictandinvokehastoreturnanObject,sotheruntimeneedssomethingthatitcanreturn.AllwecandoischeckthatthereturnedvalueclassisreallyVoid.

Let'sgoonwiththestorylineandoursolution.Wesubmittedthecodeanditworksinproductionforawhiletillanewupdatefromasoftwarevendorbreaksit.Wedebugthecodeinthetestenvironmentandseethattheclassnowcontainsmorethanonemethod.Ourdocumentationclearlystatesthattheyshouldonlyhaveonepublicmethod,andtheyprovidedacodethathas...hmm...werealizethattheothermethodsareprivate.Theyareright;theycanhaveprivatemethodsaccordingtothecontract,sowehavetoamendthecode.Wereplacethelinesthatlookuptheoneandonlymethod:

Method[]methods=checker.getClass().getDeclaredMethods();

if(methods.length!=1){

...

}

finalMethodmethod=methods[0];

Thenewcodewillbeasfollows:

finalMethodmethod=getSingleDeclaredPublicMethod(checker);

if(method==null){

log.error(

"Thechecker{}haszeroormorethanonemethods",

checker.getClass());

returnfalse;

}

Thenewmethodwewritetolookuptheoneandonlypublicmethodisasfollows:

privateMethodgetSingleDeclaredPublicMethod(

ConsistencyCheckerchecker){

finalMethod[]methods=

checker.getClass().getDeclaredMethods();

MethodsingleMethod=null;

for(Methodmethod:methods){

if(Modifier.isPublic(method.getModifiers())){

if(singleMethod!=null){

returnnull;

}

singleMethod=method;

}

}

returnsingleMethod;

}

Tocheckwhetherthemethodispublicornot,weuseastaticmethodfromtheModifierclass.Therearemethodstocheckallpossiblemodifiers.ThevaluethatthegetModifiersmethodreturnsisanintbitfield.

Differentbitshavedifferentmodifiersandthereareconstantsthatdefinethese.Thissimplificationleadstoinconsistency,whichyoucancheckifamethodisaninterfaceorvolatile,thatis,actuallynonsense.Thefactisthatbitsthatcanonlybeusedforothertypesofreflectionobjectswillneverbeset.

Thereisoneexception,whichisvolatile.Thisbitisreusedtosignalbridgemethods.Bridgemethodsarecreatedbythecompilerautomaticallyandcanhavedeepandcomplexissuesthatwedonotdiscussinthisbook.Thereuseofthesamebitdoesnotcauseconfusionbecauseafieldcanbevolatile,butasafield,itcannotbeabridgemethod.Obviously,afieldisafieldandnotamethod.Inthesameway,amethodcannotbeavolatilefield.Thegeneralruleis:donotusemethodsonreflectionobjectswheretheydonothaveameaning;orelse,knowwhatyoudo.

Makingthestorylineevenmoreintricate,anewversionofacheckeraccidentallyimplementsthecheckingmethodasapackageprivate.Theprogrammersimplyforgottousethepublickeyword.Forthesakeofsimplicity,let'sassumethattheclassesdeclareonlyonemethodagain,butitisnotpublic.Howdowesolvethisproblemusingreflection?

Obviously,thesimplestsolutionistoaskthevendorstofixtheproblem:itistheirfault.Insomecases,however,wemustcreateaworkaroundoversomeproblems.Thereisanothersolution:creatingaclasswithapublicmethodinthesamepackage,invokingthepackageprivatemethodsfromtheotherclass,thusrelayingtheotherclass.Asamatteroffact,thissolution,asaworkaroundforsuchabug,seemstobemorelogicalandcleaner,butthistime,wewanttousereflection.

Toavoidjava.lang.IllegalAccessException,wehavetosetthemethodobjectasaccessible.Todoso,wehavetoinsertthefollowinglineinfrontoftheinvocation:

method.setAccessible(true);

Notethatthiswillnotchangethemethodtopublic.Itwillonlymakethemethodaccessibleforinvocationthroughtheveryinstanceofthemethodobjectthatwesetasaccessible.

IhaveseencodethatcheckswhetheramethodisaccessibleornotbycallingtheisAccessiblemethodandsavesthisinformation;itsetsthemethodasaccessibleifitwasnotaccessibleandrestorestheoriginalaccessibilityaftertheinvocation.Thisistotallyuseless.Assoonasthemethodvariablegoesoutofscope,andthereisnoreferencetotheobjectwesettheaccessibilityflagto,theeffectofthesettingwearsoff.Also,thereisnopenaltyforsettingtheaccessibilityofapublicoranotherwisecallablemethod.

SettingfieldsWecanalsocallsetAccessibleonFieldobjectsandthenwecanevensetthevalueofprivatefieldsusingreflection.Withoutfurtherfakestories,justforthesakeoftheexample,let'smakeaConsistencyCheckernamedSettableChecker:

@Component

@PoweredDevice

publicclassSettableCheckerimplementsConsistencyChecker{

privatestaticfinalLoggerlog=LoggerFactory.getLogger(SettableChecker.class);

privatebooleansetValue=false;

publicbooleanisInconsistent(Orderorder){

returnsetValue;

}

}

Thischeckerwillreturnfalse,unlesswesetthefieldtotrueusingreflection.Wedosetitassuch.WecreateamethodintheCheckerclassandinvokeitfromthecheckingprocessforeachchecker:

privatevoidsetValueInChecker(ConsistencyCheckerchecker){

Field[]fields=checker.getClass().getDeclaredFields();

for(finalFieldfield:fields){

if(field.getName().equals("setValue")&&

field.getType().equals(boolean.class)){

field.setAccessible(true);

try{

log.info("Settingfieldtotrue");

field.set(checker,true);

}catch(IllegalAccessExceptione){

log.error("SNAFU",e);

}

}

}

}

ThemethodgoesthroughallthedeclaredfieldsandifthenameissetValueandthetypeisboolean,thenitsetsittotrue.Thiswillessentiallyrenderallordersthatcontainapowereddeviceasrejected.

Notethatalthoughbooleanisabuilt-inlanguageprimitive,whichisnotaclassbyanymeans,itstillhasaclasssothatreflectioncancomparethetypeofthefieldgainstheclassthatbooleanartificiallyhas.Nowboolean.classisaclassliteralinthelanguage,andforeachprimitive,asimilarconstantcanbeused.Thecompileridentifiestheseasclassliteralsandcreatestheappropriatepseudoclassreferencesinthebytecodesothatprimitivescanalsobecheckedinthisway,asdemonstratedinthesamplecodeofthesetValueInCheckermethod.

Wecheckedthatthefieldhastheappropriatetype,andwealsocalledthesetAccessiblemethodonthefield.EventhoughthecompilerdoesnotknowthatwereallydideverythingtoavoidIllegalAccessException,itstillbelievesthatcallingsetonfieldcanthrowsuchanexception,asitisdeclared.However,weknowthatitshouldnothappen.(Famouslastwordsofaprogrammer?)Tohandlethissituation,wesurroundthemethodcallwithatryblock,andinthecatchbranch,welogtheexception.

FunctionalprogramminginJavaSincewehavecreatedalotofcodeinourexampleforthischapter,wewilllookatthefunctionalprogrammingfeaturesofJava,whichwillhelpusdeletemanylinesfromourcode.Thelesscodewehave,theeasieritistomaintaintheapplication;thus,programmerslovefunctionalprogramming.Butthisisnottheonlyreasonwhyfunctionalprogrammingissopopular.Itisalsoanexcellentwaytodescribecertainalgorithmsinamorereadableandlesserrorpronemannerthanconventionalloops.

Functionalprogrammingisnotanewthing.Themathematicalbackgroundwasdevelopedforitinthe1930s.Oneofthefirst(ifnotthefirst)functionalprogramminglanguagesisLISP.Itwasdevelopedinthe1950sanditisstillinuse,somuchthatthereisaversionofthelanguageimplementedontheJVM(Clojure).

Functionalprogramming,inshort,meansthatweexpresstheprogramstructureintermsoffunctions.Inthismeaning,weshouldthinkoffunctionsasinmathematicsandnotasthetermisusedinprogramminglanguagessuchasC.InJava,wehavemethods,andwhenwearefollowingthefunctionalprogrammingparadigm,wecreateandusemethodsthatbehavelikemathematicalfunctions.Amethodisfunctionalifitgivesthesameresultnomatterhowmanytimesweinvokeit,justassin(0)isalwayszero.Functionalprogrammingavoidschangingthestateofobjects,andbecausethestateisnotchanging,theresultsarealwaysthesame.Thisalsoeasesdebugging.

Ifafunctionhasoncereturnedacertainvalueforthegivenarguments,itwillalwaysreturnthesamevalue.Wecanalsoreadthecodeasadeclarationofthecalculationmorethanascommandsthatareexecutedoneaftertheother.Iftheexecutionorderisnotimportant,thenthereadabilityofthecodemayalsoincrease.

Javahelpsfunctionalprogrammingstylewithlambdaexpressionsandstreams.NotethatthesestreamsarenotI/Ostreamsanddonotreallyhaveanyrelationtothose.

Wewillfirsttakeashortlookatlambdaexpressionsandwhatstreamsare,andthen,wewillconvertsomepartsofourprogramtousetheseprogrammingconstructs.Wewillalsoseehowmuchmorereadablethesecodesbecome.

Readabilityisadebatabletopic.Acodemaybereadabletoonedeveloperandmaybelessreadabletoanother.Itverymuchdependsonwhattheygotusedto.Iexperiencemanytimesthatdevelopersgetdistractedwithstreams.Whendevelopersfirstmeetstreams,thewaytothinkaboutthemandhowtheylookisjuststrange.Butthisisthesameasstartingtolearnusingabicycle.Whileyouarestilllearningitsuseandyoufallmorethanyouroll,itisdefinitelyslowerthanwalking.Ontheotherhand,onceyouhavelearnthowtorideabike...

LambdaWehavealreadyusedlambdaexpressionsinChapter3,OptimizingtheSort-MakingCodeProfessionalwhenwewrotetheexception-throwingtest.Inthatcode,wesetthecomparatortoaspecialvaluethatwasthrowingRuntimeExceptionateachinvocation:

sort.setComparator((Stringa,Stringb)->{

thrownewRuntimeException();

});

TheargumenttypeisComparator;therefore,whatwehavetosetthereshouldbeaninstanceofaclassthatimplementsthejava.util.Comparatorinterface.Thatinterfacedefinesonlyonemethodthatimplementationshavetodefine:compare.Thus,wecandefineitasalambdaexpression.Withoutlambda,ifweneedaninstance,wehavetotypealot.Wehavetocreateaclass,nameit,declarethecomparemethodinit,andwritethebodyofthemethod,asshowninthefollowingcodesegment:

publicclassExceptionThrowingComparatorimplementsComparator{

publicintcompare(To1,To2){

thrownewRuntimeException();

}

}

Atthelocationofuse,weshouldinstantiatetheclassandpassitasanargument:

sort.setComparator(newExceptionThrowingComparator());

Wemaysaveafewcharactersifwedefinetheclassasananonymousclassbuttheoverheadisstillthere.Whatwereallyneedisthebodyoftheoneandsinglemethodthatwehavetodefine.Thisiswherelambdacomesintothepicture.

Wecanusealambdaexpressioninanyplacewherewewouldotherwiseneedaninstanceofaclassthathastodefineonlyonemethod.ThemethodsthataredefinedandinheritedfromObjectdonotcount,andwealsodonotcareaboutthemethodsthataredefinedasdefaultmethodsintheinterface.Theyarethere.Lambdadefinestheonethatisnotyetdefined.Inotherwords,lambdaclearlydepicts,withmuchlessoverheadasananonymousclass,thatthevalueisafunctionalitythatwepassasaparameter.

Thesimpleformofalambdaexpressionisasfollows:

parameters->body

Theparameterscanbeenclosedbetweenparenthesesorcanonlystandwithout.Thebodysimilarlycanbeenclosedbetweenthe{and}charactersoritcanbeasimpleexpression.Thiswayalambdaexpressioncanreducetheoverheadtoaminimum,usingtheparenthesesonlywheretheyarereallyneeded.

Itisalsoanextremelyusefulfeatureoflambdaexpressionsthatwedonotneedtospecifythetypesoftheparametersincaseitisobviousfromthecontextwhereweusetheexpression.Thus,theprecedingcodesegmentcanevenbeshorter,asfollows:

sort.setComparator((a,b)->{

thrownewRuntimeException();

});

Theparameters,aandb,willhavethetypeasneeded.Tomakeitevensimpler,wecanalsoomitthe(and)charactersaroundtheparametersincasethereisonlyone.

Theparenthesesarenotoptionalifthereismorethanoneparameter.Thisistoavoidambiguityinsomesituations.Forexample,themethodcall,f(x,y->x+y)couldhavebeenamethodwithtwoarguments:xandalambdaexpressionthathasoneparameter,y.Atthesametime,itcouldalsobeamethodcallwithalambdaexpressionthathastwoparameters,xandy.

Lambdaexpressionsareveryhandywhenwewanttopassfunctionalityasanargument.Thedeclarationofthetypeofargumentattheplaceofthemethoddeclarationshouldbeafunctionalinterfacetype.Theseinterfacescanoptionallybeannotatedusing@FunctionalInterface.TheJavaruntimehasmanysuchinterfacesdefinedinthejava.util.functionpackage.Wewilldiscusssomeoftheminthenextsectionalongwiththeiruseinstreams.Fortherest,thestandardJavadocumentationisavailablefromOracle.

StreamsStreamswerealsonewinJava8,justlikelambdaexpressions.Theyworktogetherverystrongly,sotheirappearanceatthesametimeisnotasurprise.Lambdaexpressionsaswellasstreamssupportthefunctionalprogrammingstyle.

Theveryfirstthingtoclarifyisthatstreamsdonothaveanythingtodowithinputandoutputstreams,exceptthename.Theyaretotallydifferentthings.Streamsaremorelikecollectionswithsomesignificantdifferences.(Iftherewerenodifferences,theywouldjusthavebeencollections.)Streamsareessentiallypipelinesofoperationsthatcanrunsequentiallyorinparallel.Theyobtaintheirdatafromcollectionsorothersources,includingdatathatismanufacturedon-the-fly.

Streamssupporttheexecutionofthesamecalculationonmultipledata.ThisstructureisreferredtoasSingleInstructionMultipleData(SIMD).Don'tbeafraidoftheexpression.Thisisaverysimplething.Wehavealreadydonethatmanytimesinthisbook.LoopsarealsokindofSIMDstructures.Whenweloopthroughthecheckerclassestoseewhetheranyofthoseopposestheorder,weperformthesameinstructionforeachandeverychecker.Multiplecheckersaremultipledata.

Oneproblemwithloopsisthatwedefinetheorderofexecutionwhenitisnotneeded.Inthecaseofcheckers,wedonotreallycarewhatorderthecheckersareexecutedin.Allwecareaboutisthatallareokaywiththeorder.Westillspecifysomeorderwhenweprogramtheloop.Thiscomesfromthenatureofloops,andthereisnowaywecouldchangethat.Thatishowtheywork.However,itwouldbeniceifwecouldjust,somehow,say"dothisandthatforeachandeverychecker".Thisisonepointwherestreamscomeintothepicture.

Anotherpointisthatcodethatusesloopsismoreimperativeratherthandescriptive.Whenwereadtheprogramofaloopconstruct,wefocusontheindividualsteps.Wefirstseewhatthecommandsintheloopdo.Thesecommandsworkontheindividualelementsofthedataandnotonthewholecollectionorarray.

Laterputtingtheindividualstepstogetherinourbrainwerealizewhatthebigpictureis,whattheloopisfor.Inthecaseofstreams,thedescriptionofoperationsisalevelhigher.Oncewelearnthestreammethods,itiseasiertoreadthem.Streammethodsworkonthewholestreamandnotontheindividualelements,andthusaremoredescriptive.

java.lang.Streamisaninterface.Anobjectwithatypeimplementingthisinterfacerepresentsmanyobjectsandprovidesmethodsthatcanbeusedtoperforminstructionsontheseobjects.Theobjectsmayormaynotbeavailablewhenwestarttheoperationononeofthem,ormayjustbecreatedwhenneeded.ThisisuptotheactualimplementationoftheStreaminterface.Forexample,supposewegenerateastreamthatcontainsintvaluesusingthefollowingcode:

IntStream.iterate(0,(s)->s+1)

Intheprecedingcodesnippet,alltheelementscannotbegeneratedbecausethestreamcontainsaninfinitenumberofelements.Thisexamplewillreturnthenumbers0,1,2,andsoonuntilfurtherstreamoperations,whicharenotlistedhere,terminatethecalculation.

WhenweprogramStream,weusuallycreateastreamfromaCollection—notalways,butmanytimes.TheCollectioninterfacewasextendedinJava8toprovidethestreamandparallelStreammethods.Bothofthemreturnstreamobjectsthatrepresenttheelementsofthecollection.Whilestreamreturnstheelementsinthesameorderastheyareinthecollectionincasethereisanaturalorder,theparallelStreamcreatesastreamthatmaybeworkedoninaparallelmanner.Inthiscase,ifsomeofthemethodsthatweuseonthestreamareimplementedinthatway,thecodecanusethemultipleprocessorsavailableinthecomputer.

Assoonaswehaveastream,wecanusethemethodsthattheStreaminterfacedefines.TheonetostartwithisforEach.Thismethodhasoneargument,whichisusuallyprovidedasalambdaexpressionandwillexecutethelambdaexpressionforeachelementofthestream.

IntheCheckerclass,wehavetheisConsistentmethod.Inthismethod,thereisaloopthatgoesthroughtheannotationsofthecheckerclass.Ifwewantedtologtheinterfacesthattheannotationintheloopimplements,wecouldaddthefollowing:

for(ConsistencyCheckerchecker:checkers){

for(Annotationannotation:

checker.getClass().getAnnotations()){

Arrays.stream(annotation.getClass().getInterfaces())

.forEach(

t->log.info("annotationimplementedinterfaces{}",t)

);

...

Inthisexample,wecreateastreamfromanarrayusingthefactorymethodfromtheArraysclass.Thearraycontainstheinterfacesreturnedbythereflectionmethod,getInterfaces.Thelambdaexpressionhasonlyoneparameter;thus,wedonotneedtouseparenthesesaroundit.Thebodyoftheexpressionisamethodcallthatreturnsnovalue;thus,wealsoomitthe{and}characters.

Whyallthishassle?Whatisthegain?Whycouldn'twejustwriteasimpleloopthatlogstheelementsofthearray?Thegainsarereadabilityandmaintainability.Whenwecreateaprogram,wehavetofocusonwhattheprogramshoulddoandnotonhowitshoulddoit.Inanidealworld,aspecificationwouldjustbeexecutable.Wemayactuallygetthereinthefuturewhenprogrammingworkwillbereplacedbyartificialintelligence.(Nottheprogrammers,though.)Wearenotthere,yet.Wehavetotellthecomputershowtodowhatwewanttoachieve.WeusedtohavetoenterbinarycodesontheconsoleofPDP-11togetmachinecodedeployedintothememorytohaveitexecuted.Later,wehadassemblers;stilllater,wehadFORTRANandotherhigh-levelprogramminglanguagesthathavereplacedmuchoftheprogrammingworkasitwas40yearsago.Allthesedevelopmentsinprogrammingshiftthedirectionfromhowtowardswhat.Today,weprograminJava9,andtheroadstillhasmilestogo.Themorewecanexpresswhattodoinsteadofhowtodo,theshorterandmoreunderstandableourprogramswillbe.Itwillcontaintheessenceandnotsomeartificiallitterthatisneededbythemachinestojustdowhatwewant.WhenIseealoopinacodeIhavetomaintain,Iassumethatthereissomeimportanceoftheorderinwhichtheloopisexecuted.Theremaybenoimportanceatall.Itmaybeobviousafterafewseconds.Itmayneedminutesormoretorealizethattheorderingisnotimportant.Thistimeiswastedandcanbesavedwithprogrammingconstructsthat

betterexpressthewhattodopartinsteadofthehowtodo.

FunctionalinterfacesTheargumenttothemethodshouldbejava.util.function.Consumer.Thisisaninterfacethatrequirestheacceptmethodtobedefined,andthismethodisvoid.Thelambdaexpressionoraclassthatimplementsthisinterfacewillconsumetheargumentoftheacceptmethodanddoesnotproduceanything.

Thereareseveralotherinterfacesdefinedinthatpackage,eachservingasafunctionalinterfaceusedtodescribesomemethodargumentsthatcanbegivenaslambdaexpressionsintheactualparameters.

Forexample,theoppositeofConsumerisSupplier.ThisinterfacehasamethodnamedgetthatdoesnotneedanyargumentbutgivessomeObjectasareturnvalue.

Ifthereisanargumentandalsoareturnedvalue,theinterfaceiscalledFunction.Ifthereturnedvaluehastobethesametypeastheargument,thentheUnaryOperatorinterfaceisourfriend.Similarly,thereisaBinaryOperatorinterface,whichreturnsanobjectofthesametypeasthearguments.JustaswegotfromFunctiontoUnaryOperator,wecanseethatintheotherdirection,thereisalsoBiFunctionincasetheargumentsandthereturnvaluesdonotsharethetype.

Theseinterfacesarenotdefinedindependentlyofeachother.IfamethodrequiresFunctionandwehaveUnaryOperatortopass,itshouldnotbeaproblem.UnaryOperatorisnothingelsebutFunctionthathasthesametypeofarguments.AmethodthatcanworkwithFunction,whichacceptsanobjectandreturnsanobject,shouldnothaveaproblemiftheyhavethesametype.Thosecanbe,butneednotbe,different.Toletthathappen,theUnaryOperatorinterfaceextendsFunctionandthuscanbeusedintheplaceofFunction.

Theinterfacesinthisclasswemetsofararedefinedusinggenerics.Becausegenerictypescannotbeprimitives,theinterfacesthatoperateonprimitivevaluesshouldbedefinedseparately.Predicate,forexample,isaninterfacethatdefinesbooleantest(Tt).Itisafunctionthatreturnsabooleanvalueandisusedmanytimesinstreammethods.

Therearealsointerfaces,suchasBooleanSupplier,DoubleConsumer,DoubleToIntFunction,andmore,thatworkwithprimitiveboolean,double,andint.Thenumberofpossiblecombinationsofthedifferentargumenttypesandreturnvaluesisinfinite...almost.

Funfact:Tobeveryprecise,itisnotinfinite.Amethodcanhaveatmost254arguments.ThislimitisspecifiedintheJVMandnotintheJavalanguagespecification.Ofcourse,oneisuselesswithouttheother.Thereare8primitivetypes(plusObject,plusthepossibilitythattherearelessthan254arguments),whichmeansthatthetotalnumberofpossiblefunctionalinterfacesis10254,giveortake,afewmagnitudes.Practically,infinite!

WeshouldnotexpecttohaveallthepossibleinterfacesdefinedintheJDKinthispackage.Theseareonlythoseinterfacesthatarethemostuseful.Thereisnointerface,forexample,thatusesshortorchar.Ifweneedanythinglikethat,thenwecandefinetheinterfaceinourcode.Orjustthinkhardandfindouthow

touseanalreadydefinedone.(Ihaveneverusedtheshorttypeduringmyprofessionalcarrier.Itwasneverneeded.)

Howarethesefunctionalinterfacesusedinstreams?TheStreaminterfacedefinesthemethodsthathavesomefunctionalinterfacetypesasarguments.Forexample,theallMatchmethodhasaPredicateargumentandreturnsaBooleanvalue,whichistrueifalltheelementsinthestreammatchPredicate.Inotherwords,thismethodreturnstrueifandonlyifPredicate,suppliedasanargument,returnstrueforeachandeveryelementofthestream.

Inthefollowingcode,wewillrewritesomeofthemethodsthatweimplementedinoursamplecodeusingloopstousestreams,andthroughtheseexamples,wewilldiscussthemostimportantmethodsthatstreamsprovide.Wesaveduptwoclasses,ProductsCheckerCollectorandProductInformationCollector,todemonstratethestreamusage.Wecanstartwiththese.ProductsCheckerCollectorgoesthroughalltheproductsthatarecontainedintheOrderandcollectstheannotationsthatarelistedintheproducts.Eachproductmaycontainzero,one,ormanyannotations.Theseareavailableinalist.Thesameannotationmaybereferencedmultipletimes.Toavoidduplicates,weuseHashSet,whichwillcontainonlyoneinstanceoftheelementseveniftherearemultipleinstancesintheproducts:

publicclassProductsCheckerCollector{

privatefinalProductInformationCollectorpic;

publicProductsCheckerCollector(@Autowired

ProductInformationCollectorpic){this.pic=pic;}

publicSet<Class<?extendsAnnotation>>

getProductAnnotations(Orderorder){

Map<OrderItem,ProductInformation>piMap=

pic.collectProductInformation(order);

finalSet<Class<?extendsAnnotation>>

annotations=newHashSet<>();

for(OrderItemitem:order.getItems()){

finalProductInformationpi=piMap.get(item);

if(pi!=null&&pi.getCheck()!=null){

for(Class<?extendsAnnotation>check:

pi.getCheck()){

annotations.addAll(pi.getCheck());

}

}

returnannotations;

}

}

Now,let'sseehowthismethodlookswhenwerecodeitusingstreams:

publicSet<Class<?extendsAnnotation>>

getProductAnnotations(Orderorder){

Map<OrderItem,ProductInformation>piMap=

pic.collectProductInformation(order);

returnorder.getItems().stream()

.map(piMap::get)

.filter(Objects::nonNull)

.peek(pi->{

if(pi.getCheck()==null){

log.info("Product{}hasnoannotation",

pi.getId());

}

})

.filter(pi->pi.getCheck()!=null)

.peek(pi->log.info("Product{}isannotatedwithclass{}",pi.getId(),pi.getCheck()))

.flatMap(pi->pi.getCheck().stream())

.collect(Collectors.toSet());

}

Themajorworkofthemethodgetsintoasingle,thoughhuge,streamexpression.Wewillcovertheelementsoftheexpressioninthecomingpages.Listreturnedbyorder.getItemsisconvertedcallingthestreammethod:

returnorder.getItems().stream()

Aswehavealreadymentioneditbriefly,thestreammethodispartoftheCollectioninterface.AnyclassthatimplementstheCollectioninterfacewillhavethismethod,eventhosethatwereimplementedbeforestreamswereintroducedinJava8.Thisisbecausethestreammethodisimplementedintheinterfaceasadefaultmethod.Thisway,ifwehappentoimplementaclassimplementingthisinterface,evenifwedonotneedstreams,wegetitforfreeasanextra.

ThedefaultmethodsinJava8wereintroducedtosupportbackwardcompatibilityofinterfaces.SomeoftheinterfacesoftheJDKweretobemodifiedtosupportlambdaandfunctionalprogramming.Oneexampleisthestreammethod.Withthepre-Java8featureset,theclassesimplementingsomeofthemodifiedinterfacesshouldhavebeenmodified.Theywouldhavebeenrequiredtoimplementthenewmethod.Suchachangeisnotbackwardcompatible,andJavaasalanguageandJDKwaspayingkeenattentiontobebackwardcompatible.Thus,defaultmethodswereintroduced.Theseletadeveloperextendaninterfaceandstillkeepitbackwardcompatible,providingadefaultimplementationforthemethods,whicharenew.Contrarytothisphilosophy,brandnewfunctionalinterfacesofJava8JDKalsohavedefaultmethods,though,havingnopriorversionintheJDK,theyhavenothingtobecompatiblewith.InJava9,interfaceswerealsoextendedandnowtheycancontainnotonlydefaultandstaticmethodsbutalsoprivatemethods.Thisway,interfacesbecamekindofequivalenttoabstractclasses,thoughtherearenofieldsinaninterfaceexceptconstantstaticfields.Thisinterfacefunctionalityopenupisamuchcriticizedfeature,whichjustposestheprogrammingstyleandstructuralissuesthatotherlanguagesallowingmultipleclassinheritanceface.JavawasavoidingthistillJava8andJava9.Whatisthetake-awayfromthis?Becarefulwithdefaultmethodsandalsowithprivatemethodsininterfaces.Usethemwiselyifatall.

TheelementsofthisstreamareOrderItemobjects.WeneedProductInformationforeachOrderItem.

MethodreferencesLuckythatwehaveMap,whichpairsorderitemswithproductinformation,sowecaninvokegetonMap:

.map(piMap::get)

ThemapmethodisagainsomethingthathasthesamenameassomethingelseinJavaandshouldnotbeconfused.WhiletheMapclassisadatastructure,themapmethodintheStreaminterfaceperformsmappingofthestreamelements.TheargumentofthemethodisaFunction(recallthatthisisafunctionalinterfacewerecentlydiscussed).Thisfunctionconvertsavalue,T,whichisavailableastheelementoftheoriginalstream(Stream<T>)toavalue,R,andthereturnvalueofthemapmethodisStream<R>.ThemapmethodconvertsStream<T>toStream<R>usingthegivenFunction<T,R>,callingitforeachelementoftheoriginalstreamandcreatinganewstreamfromtheconvertedelements.

WecansaythattheMapinterfacemapskeystovaluesinadatastructureinastaticway,andtheStreammethod,map,mapsonetypeofvaluestoanother(orthesame)typeofvaluesdynamically.

Wehavealreadyseenthatwecanprovideaninstanceofafunctionalinterfaceintheformofalambdaexpression.Thisargumentisnotalambdaexpression.Thisisamethodreference.ItsaysthatthemapmethodshouldinvokethegetmethodonMappiMapusingtheactualstreamelementasanargument.Weareluckythatgetalsoneedsoneargument,aren'twe?Wecouldalsowriteasfollows:

.map(orderItem->piMap.get(orderItem))

However,thiswouldhavebeenexactlythesameaspiMap::get.

Thisway,wecanreferenceaninstancemethodthatworksonacertaininstance.Inourexample,theinstanceistheonereferencedbythepiMapvariable.Itisalsopossibletoreferencestaticmethods.Inthiscase,thenameoftheclassshouldbewritteninfrontofthe::characters.Wewillsoonseeanexampleofthiswhenwewillusethestaticmethod,nonNull,fromtheObjectsclass(notethattheclassnameisinplural,anditisinthejava.utilpackageandnotjava.lang).

Itisalsopossibletoreferenceaninstancemethodwithoutgivingthereferenceonwhichitshouldbeinvoked.Thiscanbeusedinplaceswherethefunctionalinterfacemethodhasanextrafirstparameter,whichwillbeusedastheinstance.WehavealreadyusedthisinChapter3,OptimizingtheSort-MakingCodeProfessional,whenwepassedString::compareTo,whentheexpectedargumentwasaComparator.ThecompareTomethodexpectsoneargument,butthecomparemethodintheComparatorinterfaceneedstwo.Insuchasituation,thefirstargumentwillbeusedastheinstanceonwhichcomparehastobeinvokedandthesecondargumentispassedtocompare.Inthiscase,String::compareToisthesameaswritingthelambdaexpression(Stringa,Stringb)->a.compareTo(b).

Lastbutnotleast,wecanusemethodreferencestoconstructors.WhenweneedaSupplierof(let'sbesimple)Object,wecanwriteObject::new.

Thenextstepistofilteroutthenullelementsfromthestream.Notethat,atthispoint,thestreamhasProductInformationelements:

.filter(Objects::nonNull)

ThefiltermethodusesPredicateandcreatesastreamthatcontainsonlytheelementsthatmatchthepredicate.Inthiscase,weusedthereferencetoastaticmethod.Thefiltermethoddoesnotchangethetypeofstream.Itonlyfiltersouttheelements.

Thenextmethodweapplyisabitanti-functional.Purefunctionalstreammethodsdonotalterthestateofanyobject.Theycreatenewobjectsthattheyreturnbut,otherthanthat,thereisnosideeffect.peekitselfisnodifferentbecauseitonlyreturnsastreamofthesameelementsastheoneitisappliedon.However,thisno-operationfeatureluresthenoviceprogrammertodosomethingnon-functionalandwritecodewithside-effects.Afterall,whyuseitifthereisno(side)effectincallingit?

.peek(pi->{

if(pi.getCheck()==null){

log.info("Product{}hasnoannotation",pi.getId());

}

})

Whilethepeekmethoditselfdoesnothaveanysideeffects,theexecutionofthelambdaexpressionmayhave.However,thisisalsotrueforanyoftheothermethods.Itisjustthefactthat,inthiscase,itismoretemptingtodosomethinginadequate.Don't.Wearedisciplinedadults.Asthenameofthemethodsuggests,wemaypeekintothestreambutwearenotsupposedtodoanythingelse.Withprogrammingbeingaparticularactivity,inthiscase,peeking,isadequate.Andthatiswhatweactuallydoinourcode:welogsomething.

Afterthis,wegetridoftheelementsthathavenoProductInformation;wealsowanttogetridoftheelementsthathave,butthereisnocheckerdefined:

.filter(pi->pi.getCheck()!=null)

Inthiscase,wecannotusemethodreferences.Instead,weusealambdaexpression.Asanalternativesolution,wemaycreateabooleanhasCheckmethodinProductInformation,whichreturnstrueiftheprivatefieldcheckisnotnull.Thiswouldthenreadasfollows:

.filter(ProductInformation::hasCheck)

Thisistotallyvalidandworks,althoughtheclassdoesnotimplementanyfunctionalinterfaceandhasmanymethods,notonlythisone.However,themethodreferenceisexplicitandspecifieswhichmethodtoinvoke.

Afterthissecondfilter,welogtheelementsagain:

.peek(pi->log.info(

"Product{}isannotatedwithclass{}",pi.getId(),

pi.getCheck()))

ThenextmethodisflatMapandthisissomethingspecialandnoteasytocomprehend.Atleastforme,itwasabitmoredifficultthanunderstandingmapandfilterwhenIlearnedfunctionalprogramming:

.flatMap(pi->pi.getCheck().stream())

Thismethodexpectsthatthelambda,methodreference,orwhateverispassedtoitasanargument,

createsawholenewstreamofobjectsforeachelementoftheoriginalstreamthemethodisinvokedon.Theresultis,however,notastreamofstreams,whichalsocouldbepossible,butratherthereturnedstreamsareconcatenatedintoonehugestream.

Ifthestreamweapplyittoisastreamofintegernumbers,suchas1,2,3,...,andthefunctionforeachnumbernreturnsastreamofthreeelementsn,n+1,andn+2,thentheresultingstream,flatMap,producesastreamcontaining1,2,3,2,3,4,3,4,5,4,5,6,andsoon.

Finally,thestreamwehaveshouldbecollectedtoaSet.Thisisdonebycallingthecollectormethod:

.collect(Collectors.toSet());

Theargumenttothecollectormethodis(againanameoveruse)Collector.Itcanbeusedtocollecttheelementsofthestreamintosomecollection.NotethatCollectorisnotafunctionalinterface.Youcannotjustcollectsomethingusingalambdaorasimplemethod.Tocollecttheelements,wedefinitelyneedsomeplacewheretheelementsarecollectedastheever-newerelementscomefromthestream.TheCollectorinterfaceisnotsimple.Fortunately,thejava.util.streams.Collectorsclass(againnotetheplural)hasalotofstaticmethodsthatcreateandreturnObjectthatcreateandreturnCollectorobjects.

OneoftheseistoSet,whichreturnsaCollectorthathelpscollecttheelementsofthestreamintoaSet.ThecollectmethodwillreturntheSetwhenalltheelementsarethere.Thereareothermethodsthathelpcollectthestreamelementsbysumminguptheelements,calculatingtheaverage,ortoaList,Collection,ortoaMap.CollectingelementstoaMapisaspecialthing,sinceeachelementofaMapisactuallyakey-valuepair.WewillseetheexampleforthatwhenwelookatProductInformationCollector.

TheProductInformationCollectorclasscodecontainsthecollectProductInformationmethod,whichwewillusefromtheCheckerclassaswellasfromtheProductsCheckerCollectorclass:

privateMap<OrderItem,ProductInformation>map=null;

publicMap<OrderItem,ProductInformation>

collectProductInformation(Orderorder){

if(map==null){

map=newHashMap<>();

for(OrderItemitem:order.getItems()){

finalProductInformationpi=

lookup.byId(item.getProductId());

if(!pi.isValid()){

map=null;

returnnull;

}

map.put(item,pi);

}

}

returnmap;

}

ThesimpletrickistostorethecollectedvalueinMap,andifthatisnotnull,thenjustreturnthealreadycalculatedvalue,whichmaysavealotofservicecallsincasethismethodiscalledmorethanoncehandlingthesameHTTPrequest.

Therearetwowaysofcodingsuchastructure.Oneischeckingthenon-nullityoftheMapandreturningiftheMapisalreadythere.Thispatterniswidelyusedandhasaname.Thisiscalledguardingif.Inthiscase,thereismorethanonereturnstatementinthemethod,

whichmaybeseenasaweaknessoranti-pattern.Ontheotherhand,thetabulationofthemethodisonetabshallower.Itisamatteroftasteandincaseyoufindyourselfinthemiddleofadebateaboutoneortheothersolution,justdoyourselfafavorandletyourpeerwinonthistopicandsaveyourstaminaformoreimportantissues,forexample,whetheryoushouldusestreamsorjustplainoldloops.

Now,let'sseehowwecanconvertthissolutionintoafunctionalstyle:

publicMap<OrderItem,ProductInformation>collectProductInformation(Orderorder){

if(map==null){

map=

order.getItems()

.stream()

.map(item->tuple(item,item.getProductId()))

.map(t->tuple(t.r,lookup.byId((String)t.s)))

.filter(t->((ProductInformation)t.s).isValid())

.collect(

Collectors.toMap(t->(OrderItem)t.r,

t->(ProductInformation)t.s

)

);

if(map.keySet().size()!=order.getItems().size()){

log.error("Someoftheproductsintheorderdonothaveproductinformation,{}!={}",map.keySet().size(),order.getItems().size());

map=null;

}

}

returnmap;

}

Weuseahelperclass,Tuple,whichisnothingbuttwoObjectinstancesnamedrands.Wewilllistthecodeforthisclasslater.Itisverysimple.

Inthestreamsexpression,wefirstcreatethestreamfromthecollection,andthenwemaptheOrderItemelementstoastreamofOrderItemandproductIdtuples.ThenwemapthesetuplestotuplesthatnowcontainOrderItemandProductInformation.Thesetwomappingscouldbedoneinonemappingcall,whichwouldperformthetwostepsonlyinone.Idecidedtocreatethetwotohavesimplerstepsineachlineinavainhopethattheresultingcodewillbeeasiertocomprehend.

Thefilterstepisalsonothingnew.Itjustfiltersoutinvalidproductinformationelements.Thereshouldactuallybenone.IthappensiftheordercontainsanorderIDtoanon-existentproduct.Thisischeckedinthenextstatementwhenwelookatthenumberofcollectedproductinformationelementstoseethatalltheitemshaveproperinformation.

TheinterestingcodeishowwecollecttheelementsofthestreamintoaMap.Todoso,weagainusethecollectmethodandalsotheCollectorsclass.Thistime,thetoMapmethodcreatestheCollector.ThisneedstwoFunctionresultingexpressions.ThefirstoneshouldconverttheelementofthestreamtothekeyandthesecondshouldresultinthevaluetobeusedintheMap.Becausetheactualtypeofthekeyandthevalueiscalculatedfromtheresultofthepassedlambdaexpressions,weexplicitlyhavetocastthefieldsofthetupletotheneededtypes.

Finally,thesimpleTupleclassisasfollows:

publicclassTuple<R,S>{

finalpublicRr;

finalpublicSs;

privateTuple(Rr,Ss){

this.r=r;

this.s=s;

}

publicstatic<R,S>Tupletuple(Rr,Ss){

returnnewTuple<>(r,s);

}

}

Therearestillsomeclassesinourcodethatdeservetobeconvertedtofunctionalstyle.ThesearetheCheckerandCheckerHelperclasses.

IntheCheckerclass,wecanrewritetheisConsistentmethod:

publicbooleanisConsistent(Orderorder){

Map<OrderItem,ProductInformation>map=

piCollector.collectProductInformation(order);

if(map==null){returnfalse;}

finalSet<Class<?extendsAnnotation>>annotations=

pcCollector.getProductAnnotations(order);

return!checkers.stream().anyMatch(

checker->Arrays.stream(

checker.getClass().getAnnotations()

).filter(

annotation->

annotations.contains(

annotation.annotationType())

).anyMatch(

x->

checker.isInconsistent(order)

));

}

Sinceyouhavealreadylearntmostoftheimportantstreammethods,thereishardlyanynewissuehere.WecanmentiontheanyMatchmethod,whichwillreturntrueifthereisatleastoneelementsothatthePredicateparameterpassedtoanyMatchistrue.Itmayalsoneedsomeaccommodationsothatwecoulduseastreaminsideanotherstream.Itverywellmaybeanexamplewhenastreamexpressionisovercomplicatedandneedstosplitupintosmallerpiecesusinglocalvariables.

Finally,beforeweleavethefunctionalstyle,werewritethecontainsOneOfmethodintheCheckHelperclass.Thiscontainsnonewelementsandwillhelpyoucheckwhatyouhavelearnedaboutmap,filter,flatMap,andCollector.Notethatthismethod,aswediscussed,returnstrueifordercontainsatleastoneoftheorderIDsgivenasstrings:

publicbooleancontainsOneOf(String...ids){

returnorder.getItems().stream()

.map(OrderItem::getProductId)

.flatMap(itemId->Arrays.stream(ids)

.map(id->tuple(itemId,id)))

.filter(t->Objects.equals(t.s,t.r))

.collect(Collectors.counting())>0;

}

WecreatethestreamoftheOrderItemobjects,andthenwemapittoastreamoftheIDsoftheproductscontainedinthestream.ThenwecreateanotherstreamforeachoftheIDswiththeelementsoftheIDandoneofthestringIDsgivenastheargument.Then,weflattenthesesubstreamsintoonestream.Thisstreamwillcontainorder.getItems().size()timesids.lengthelements:allpossiblepairs.WewillfilteroutthosepairsthatcontainthesameIDtwice,andfinally,wewillcountthenumberofelementsinthestream.

ScriptinginJava9Wearealmostreadywithoursampleprogramforthischapter.Thereisoneissue,thoughitisnotprofessional.Whenwehaveanewproductthatneedsanewchecker,wehavetocreateanewreleaseofthecode.

Programsinprofessionalenvironmentshavereleases.Whenthecodeismodified,bugsarefixed,oranewfunctionisimplemented,therearenumerousstepsthattheorganizationrequiresbeforetheapplicationcangointoproduction.Thesestepscomposethereleaseprocess.Someenvironmentshavelightweightreleaseprocesses;othersrequirerigorousandexpensivechecks.Itisnotbecauseofthetasteofthepeopleintheorganization,though.Whenthecostofanon-workingproductioncodeislowanditdoesnotmatterifthereisanoutageorwrongfunctioningintheprogram,thenthereleaseprocesscanbesimple.Thisway,releasesgetoutfasterandcheaper.Anexamplecanbesomechatprogramthatisusedforfunbytheusers.Insuchasituation,itmaybemoreimportanttoreleasenewfancyfeaturesthanensuringbug-freeworking.Ontheotherendofthepalette,ifyoucreatecodethatcontrolsanatomicpowerplant,thecostoffailurecanbeprettyhigh.Serioustestingandcarefulcheckingofallthefeatures,evenafterthesmallestchange,canpayoff.

Inourexample,simplecheckersmaybeanareathatisnotlikelytoinduceseriousbugs.Itisnotimpossiblebutthecodeissosimple...Yes,Iknowthatsuchanargumentisabitfishy,butlet'sassumethatthesesmallroutinescouldbechangedwithlesstestingandinaneasierwaythantheotherpartsofthecode.Howtoseparatethecodefortheselittlescripts,then,sothattheydonotrequireatechnicalrelease,anewversionoftheapplication,andnotevenrestartingtheapplication?Wehaveanewproductthatneedsanewcheckandwewanttohavesomewaytoinjectthischeckintotheapplicationenvironmentwithoutanyservicedisruption.

Thesolutionwechooseisscripting.JavaprogramscanexecutescriptswritteninJavaScript,Groovy,Jython(whichistheJVMversionofthelanguagePython),andmanyotherlanguages.ExceptJavaScript,thelanguageinterpretersoftheselanguagesarenotapartoftheJDK,buttheyallprovideastandardinterface,whichisdefinedintheJDK.Theconsequenceisthatwecanimplementscriptexecutioninourcodeandthedevelopers,whoprovidethescripts,arefreetochooseanyoftheavailablelanguages;wedonotneedtocaretoexecuteaJavaScriptcode.WewillusethesameAPIastoexecuteGroovyorJython.Theonlythingweshouldknowiswhatlanguagethescriptisin.Thisisusuallysimple:wecanguessthatfromthefileextension,andifguessingisnotenough,wecandemandthatthescriptdevelopersputJavaScriptintofileswiththe.jsextension,Jythonintofileswith.jyor.py,Groovyintofileswith.groovy,andsoon.Itisalsoimportanttonotethatifwewantourprogramtoexecuteoneoftheselanguages,weshouldmakesurethattheinterpreterisontheclasspath.InthecaseofJavaScript,thisisgiven;therefore,asademonstrationinthischapter,wewillwriteourscriptsinJavaScript.Therewillnotbealot;thisisaJavabookandnotaJavaScriptbookafterall.

Scriptingisusuallyagoodchoicewhenwewanttopasstheabilityofprogrammaticallyconfiguringorextendingourapplication.Thisisourcasenow.

Thefirstthingwehavetodoistoextendtheproductioninformation.Incasethereisascriptthatcheckstheconsistencyofanorderthataproductisin,weneedafieldwherewecanspecifythenameofthe

script:

privateStringcheckScript;

publicStringgetCheckScript(){

returncheckScript;

}

publicvoidsetCheckScript(StringcheckScript){

this.checkScript=checkScript;

}

Wedonotwanttospecifymorethanonescriptperproduct;therefore,wedonotneedalistofscriptnames.Wehaveonlyonescriptspecifiedbythename.

Tobehonest,thedatastructureforthecheckerclassesandtheannotations,allowingmultipleannotationsperproductandalsopercheckerclass,wastoocomplicated.Wecouldnotavoidthat,though,tohaveacomplexenoughstructurethatcoulddemonstratethepowerandcapabilityofstreamexpressions.Nowthatweareoverthatsubject,wecangoonusingsimplerdatastructuresfocusingonscriptexecution.

WealsohavetomodifytheCheckerclasstonotonlyusethecheckerclassesbutalsothescripts.Wecannotthrowawaythecheckerclassesbecause,bythetimewerealizethatwebetterneedscriptsforthepurpose,wealreadyhavealotofcheckerclassesandwehavenofinancingtorewritethemtobescripts.Wellyes,weareinabookandnotinreallife,butinanenterprise,thatwouldbethecase.Thatiswhyyoushouldbeverycarefulwhiledesigningsolutionsforacorporate.Thestructuresandthesolutionswillbethereforalongtimeanditisnoteasytothrowapieceofcodeoutjustbecauseitistechnicallynotthebest.Ifitworksandisalreadythere,thebusinesswillbeextremelyreluctanttospendmoneyoncodemaintenanceandrefactoring.

Summary:wemodifytheCheckerclass.Weneedanewclassthatcanexecuteourscripts;thus,theconstructorismodified:

privatefinalCheckerScriptExecutorexecutor;

publicChecker(

@AutowiredCollection<ConsistencyChecker>checkers,

@AutowiredProductInformationCollectorpiCollector,

@AutowiredProductsCheckerCollectorpcCollector,

@AutowiredCheckerScriptExecutorexecutor){

this.checkers=checkers;

this.piCollector=piCollector;

this.pcCollector=pcCollector;

this.executor=executor;

}

WealsohavetousethisexecutorintheisConsistentmethod:

publicbooleanisConsistent(Orderorder){

finalMap<OrderItem,ProductInformation>map=

piCollector.collectProductInformation(order);

if(map==null){

returnfalse;

}

finalSet<Class<?extendsAnnotation>>annotations=

pcCollector.getProductAnnotations(order);

Predicate<Annotation>annotationIsNeeded=annotation->

annotations.contains(annotation.annotationType());

Predicate<ConsistencyChecker>productIsConsistent=

checker->

Arrays.stream(checker.getClass().getAnnotations())

.parallel().unordered()

.filter(annotationIsNeeded)

.anyMatch(

x->checker.isInconsistent(order));

finalbooleancheckersSayConsistent=!checkers.stream().

anyMatch(productIsConsistent);

finalbooleanscriptsSayConsistent=

!map.values().

parallelStream().

map(ProductInformation::getCheckScript).

filter(Objects::nonNull).

anyMatch(s->

executor.notConsistent(s,order));

returncheckersSayConsistent&&scriptsSayConsistent;

}

Notethatinthiscode,weuseparallelstreamsbecause,whynot?Wheneveritispossible,wecanuseparallelstreams,evenunordered,totelltheunderlyingsystemandalsototheprogrammerfellowsmaintainingthecodethatorderisnotimportant.

WealsomodifyoneofourproductJSONfilestoreferenceascriptinsteadofacheckerclassthroughsomeannotation:

{

"id":"124",

"title":"DeskLamp",

"checkScript":"powered_device",

"description":"thisisalampthatstandsonmydesk",

"weight":"600",

"size":["300","20","2"]

}

EvenJSONissimpler.NotethataswedecidedtouseJavaScript,wedonotneedtospecifythefilenameextensionwhenwenamethescript.

Wemaylaterconsiderfurtherdevelopmentwhenwewillallowtheproductcheckerscriptmaintainerstousedifferentscriptinglanguages.Insuchacase,wemaystillrequirethattheyspecifytheextension,andincasethereisnoextension,itwillbeaddedbyourprogramas.js.Inourcurrentsolution,wedonotcheckthat,butwemaydevoteafewsecondstothinkaboutittobesurethatthesolutioncanbefurtherdeveloped.Itisimportantthatwedonotdevelopextracodeforthesakeoffurtherdevelopment.Developersarenotfortunetellersandcannottellreliablywhatthefutureneedswillbe.Thatisthetaskofthebusinesspeople.

Weputthescriptintotheresourcedirectoryofourprojectunderthescriptsdirectory.Thenameofthefilehastobepowered_device.jsbecausethisisthenamewespecifiedintheJSONfile:

functionisInconsistent(order){

isConsistent=false

items=order.getItems()

for(iinitems){

item=items[i]

print(item)

if(item.getProductId()=="126"||

item.getProductId()=="127"||

item.getProductId()=="128"){

isConsistent=true

}

}

return!isConsistent

}

ThisisanextremelysimpleJavaScriptprogram.Asasidenote,whenyouiterateoveralistoranarrayinJavaScript,theloopvariablewilliterateovertheindexesofthecollectionorthearray.SinceIrarelyprograminJavaScript,IfellintothistrapandittookmemorethanhalfanhourtodebugtheerrorImade.

Wehavepreparedeverythingweneedtocallthescript.Westillhavetoinvokeit.Todoso,weusetheJDKscriptingAPI.First,weneedaScriptEngineManager.ThismanagerisusedtogetaccesstotheJavaScriptengine.AlthoughtheJavaScriptinterpreterhasbeenapartoftheJDKsinceJava7,itisstillmanagedinanabstractway.ItisoneofthemanypossibleinterpretersthataJavaprogramcanusetoexecutescript.ItjusthappenstobethereintheJDK,sowedonotneedtoaddtheinterpreterJARtotheclasspath.ScriptEngineManagerdiscoversalltheinterpretersthatareontheclasspathandregistersthem.

ItdoessousingtheServiceProviderspecification,whichhasbeenapartoftheJDKforalongtime,andbyJava9,italsogotextrasupportinmodulehandling.ThisrequiresthescriptinterpreterstoimplementtheScriptEngineFactoryinterfaceandalsotolisttheclassthatdoesitintheMETA-INF/services/javax.script.ScriptEngineFactoryfile.Thesefiles,fromalltheJARfilesthatarepartoftheclasspath,arereadasresourcesbyScriptEngineManager,andthroughthis,itknowswhichclassesimplementscriptinterpreters.TheScriptEngineFactoryinterfacerequiresthattheinterpretersprovidemethodssuchasgetNames,getExtensions,andgetMimeTypes.Themanagercallsthesemethodstocollecttheinformationabouttheinterpreters.WhenweaskaJavaScriptinterpreter,themanagerwillreturntheonecreatedbythefactorythatsaidthatoneofitsnamesisJavaScript.

Togetaccesstotheinterpretersthroughthename,filenameextensionormime-typeisonlyoneofthefunctionsofScriptEngineManager.TheotheroneistomanageBindings.

WhenweexecuteascriptfromwithintheJavacode,wedon'tdoitbecausewewanttoincreaseourdopaminelevels.Inthecaseofscripts,itdoesnothappen.Wewantsomeresults.Wewanttopassparametersandaftertheexecutionofthescript,wewantvaluesbackfromthescriptthatwecanuseintheJavacode.Thiscanhappenintwoways.Oneisbypassingparameterstoamethodorfunctionimplementedinthescriptandgettingthereturnvaluefromthescript.Thisusuallyworks,butitmayevenhappenthatsomescriptinglanguagedoesnotevenhavethenotionofthefunctionormethod.Insuchacase,itisnotapossibility.Whatispossibleistopasssomeenvironmenttothescriptandreadvaluesfromtheenvironmentafterthescriptisexecuted.ThisenvironmentisrepresentedbyBindings.

BindingsisamapthathasStringkeysandObjectvalues.

Inthecaseofmostscriptinglanguages,forexample,inJavaScript,Bindingsisconnectedtoglobalvariablesinthescriptweexecute.Inotherwords,ifweexecutethefollowingcommandinourJavaprogrambeforeinvokingthescript,thentheJavaScriptglobalvariable,globalVariable,willreferencethemyObjectobject:

myBindings.put("globalVariable",myObject)

WecancreateBindingsandpassittoScriptEngineManagerbutjustaswellwecanusetheonethatitcreatesautomatically,andwecancalltheputmethodontheengineobjectdirectly.

TherearetwoBindingswhenweexecutescripts.OneissetontheScriptEngineManagerlevel.Thisisnamedglobalbinding.ThereisalsoonemanagedbyScriptEngineitself.ThisisthelocalBindings.Fromthescript

pointofview,thereisnodifference.Fromtheembeddingside,thereissomedifference.IncaseweusethesameScriptEngineManagertocreatemultipleScriptEngineinstances,thentheglobalbindingsaresharedbythem.Ifonegetsavalue,allofthemseethesamevalue;ifonesetsavalue,allotherswilllaterseethatchangedvalue.Thelocalbindingisspecifictotheengineitismanagedby.SinceweonlyintroduceJavascriptingAPIinthisbook,wedonotgetintomoredetailsandwewillnotuseBindings.WearegoodwithinvokingaJavaScriptfunctionandtogettheresultfromit.

TheclassthatimplementsthescriptinvocationisCheckerScriptExecutor:

packagepackt.java9.by.example.mybusiness.bulkorder.services;

import...

@Component

publicclassCheckerScriptExecutor{

privatestaticfinalLoggerlog=...

privatefinalScriptEngineManagermanager=

newScriptEngineManager();

publicbooleannotConsistent(Stringscript,Orderorder){

try{

finalReaderscriptReader=getScriptReader(script);

finalObjectresult=

evalScript(script,order,scriptReader);

assertResultIsBoolean(script,result);

log.info("Script{}wasexecutedandreturned{}",

script,result);

return(boolean)result;

}catch(ExceptionwasAlreadyHandled){

returntrue;

}

}

Theonlypublicmethod,notConsistent,getsthenameofthescripttoexecuteandalsoorder.Thelatterhastobepassedtothescript.FirstitgetsReader,whichcanreadthescripttext,evaluatesit,andfinallyreturnstheresultincaseitisbooleanorcanatleastbeconvertedtoboolean.Ifanyofthemethodsinvokedfromherethatweimplementedinthisclassiserroneous,itwillthrowanexception,butonlyafterappropriatelyloggingit.Insuchcases,thesafewayistorefuseanorder.

Actually,thisissomethingthatthebusinessshoulddecide.Ifthereisacheckscriptthatcannotbeexecuted,itisclearlyanerroneoussituation.Inthiscase,acceptinganorderandlaterhandlingtheproblemsmanuallyhascertaincosts.Refusinganorderorconfirmationbecauseofsomeinternalbugisalsonotahappypathoftheorderprocess.Wehavetocheckwhichapproachcausestheleastdamagetothecompany.Itiscertainlynotthedutyoftheprogrammer.Inoursituation,weareinaneasysituation.

Weassumethatthebusinessrepresentativessaidthattheorderinsuchasituationshouldberefused.Inreallife,similardecisionsaremanytimesrefusedbythebusinessrepresentativessayingthatitjustshouldnothappenandtheITdepartmenthastoensurethattheprogramandthewholeoperationistotallybugfree.Thereisapsychologicalreasonforsucharesponse,butthisreallyleadsusextremelyfarfromJavaprogramming.

EnginescanexecuteascriptpassedthroughReaderorasString.Becausenowwehavethescriptcodeinaresourcefile,itseemstobeabetterideatolettheenginereadtheresourceinsteadofreadingittoaString:

privateReadergetScriptReader(Stringscript)

throwsIOException{

finalReaderscriptReader;

try{

finalInputStreamscriptIS=newClassPathResource(

"scripts/"+script+".js").getInputStream();

scriptReader=newInputStreamReader(scriptIS);

}catch(IOExceptionioe){

log.error("Thescript{}isnotreadable",script);

log.error("Scriptopeningexception",ioe);

throwioe;

}

returnscriptReader;

}

Toreadthescriptfromaresourcefile,weusetheSpringClassPathResourceclass.Thenameofthescriptisprependedwiththescriptsdirectoryandappendedbythe.jsextension.Therestisfairlystandardandnothingwehavenotseeninthisbook.Thenextmethodthatevaluatesthescriptismoreinteresting:

privateObjectevalScript(Stringscript,

Orderorder,

ReaderscriptReader)

throwsScriptException,NoSuchMethodException{

finalObjectresult;

finalScriptEngineengine=

manager.getEngineByName("JavaScript");

try{

engine.eval(scriptReader);

Invocableinv=(Invocable)engine;

result=inv.invokeFunction("isInconsistent",order);

}catch(ScriptException|NoSuchMethodExceptionse){

log.error("Thescript{}thruwup",script);

log.error("Scriptexecutingexception",se);

throwse;

}

returnresult;

}

Toexecutethemethodinthescript,firstofall,weneedascriptenginethatiscapableofhandlingJavaScript.Wegettheenginefromthemanagerbyitsname.IfitisnotJavaScript,weshouldcheckthatthereturnedengineisnotnull.InthecaseofJavaScript,theinterpreterispartoftheJDKandcheckingthattheJDKconformstothestandardwouldbeparanoid.

IfeverwewanttoextendthisclasstohandlenotonlyJavaScriptbutalsoothertypesofscripts,thischeckhastobedone,andalsothescriptengineshouldprobablyberequestedfromthemanagerbythefilenameextension,whichwedonothaveaccesstointhisprivatemethod.Butthatisfuturedevelopment,notinthisbook.

Whenwehavetheengine,wehavetoevaluatethescript.Thiswilldefinethefunctioninthescriptsothatwecaninvokeitafterwards.Toinvokeit,weneedsomeInvocableobject.InthecaseofJavaScript,theenginealsoimplementsanInvocableinterface.Notallscriptenginesimplementthisinterface.Somescriptsdonothavefunctionsormethods,andthereisnothingtoinvokeinthem.Again,thisissomethingtodolater,whenwewanttoallownotonlyJavaScriptscriptingbutalsoothertypesofscripting.

Toinvokethefunction,wepassitsnametotheinvokeFunctionmethodandalsotheargumentsthatwewanttopasson.Inthiscase,thisistheorder.InthecaseofJavaScript,theintegrationbetweenthetwolanguagesisfairlydeveloped.Asinourexample,wecanaccessthefieldandthemethodsoftheJavaobjectsthatarepassedasargumentsandthereturnedJavaScripttrueorfalsevalueisalsoconvertedtoBooleanmagically.Therearesomesituationswhentheaccessisnotthatsimplethough:

privatevoidassertResultIsBoolean(Stringscript,

Objectresult){

if(!(resultinstanceofBoolean)){

log.error("Thescript{}returnednonboolean",

script);

if(result==null){

log.error("returnedvalueisnull");

}else{

log.error("returnedtypeis{}",

result.getClass());

}

thrownewIllegalArgumentException();

}

}

}

Thelastmethodoftheclasschecksthatthereturnedvalue,whichcanbeanythingsincethisisascriptengine,isconvertibletoboolean.

Itisimportanttonotethatthefactthatsomeofthefunctionalityisimplementedinscriptdoesnotguaranteethattheapplicationworksseamlessly.Theremaybeseveralissuesandscriptsmayaffecttheinnerworkingoftheentireapplication.Somescriptingenginesprovidespecialwaystoprotecttheapplicationfrombadscripts,othersdonot.Thefactthatwedonotpassbutordertothescriptdoesnotguaranteethatascriptcannotaccessotherobjects.Usingreflection,staticmethods,andothertechniquestherecanbewaystoaccessjustanythinginsideourJavaprogram.Wemaybeabiteasierwiththetestingcyclewhenonlyascriptchangesinourcodebase,butitdoesnotmeanthatweshouldblindlytrustanyscript.

Inourexample,itprobablywouldbeaverybadideatolettheproducersoftheproductsuploadscriptstooursystem.Theymayprovidetheircheckscripts,butthesescriptshavetobereviewedfromthesecuritypointofviewbeforebeingdeployedintothesystem.Ifthisisproperlydone,thenscriptingisanextremelypowerfulextensiontotheJavaecosystem,givinggreatflexibilitytoourprograms.

Summary

Inthischapter,wehavedevelopedtheorderingsystemofourenterpriseapplication.Alongwiththedevelopmentofthecode,wemetmanynewthings.Youlearnedaboutannotationsandhowtheycanbehandledbyreflections.Althoughnotstronglyrelated,youlearnedhowtouselambdaexpressionsandstreamstoexpresssomeprogrammingconstructssimplerthanconventionalloops.Inthelastpartofthechapter,weextendedtheapplicationusingscripting,byinvokingJavaScriptfunctionsfromJavaandalsobyinvokingJavamethodsfromJavaScript.

Infact,withallthisknowledge,wematuredtoaJavalevelthatisneededforenterpriseprogramming.Therestofthetopicsthebookcoversarefortheaces.Butyouwanttobeone,don'tyou?ThisiswhyIwrotetherestofthechapters.Readon!

BuildinganAccountingApplicationUsingReactiveProgramming

Inthischapter,wewilldevelopasampleprogramthatdoestheinventorymanagementpartofthecompanywecreatedtheorderhandlingcodefor.Donotexpectafullydeveloped,ready-to-use,professionalapplication,andalso,donotexpectthatwewillgetintothedetailsofaccountingandbookkeeping.Thatisnotouraim.Wewillfocusmoreontheprogrammingtechniquethatisofourinterest—reactiveprogramming.Sorrypals,Iknowthatbookkeepingandaccountingisfun,butthisisnotthatbook.

Reactiveprogrammingisanold(well,whatisoldincomputerscience?)approachthathascomerecentlytoJava.Java9isthefirstreleasethatsupportssomeoftheaspectsofreactiveprogramminginthestandardJDK.Inonesentence,reactiveprogrammingisaboutfocusingmoreonhowthedataflowsandlessonhowtheimplementationhandlesthedataflow.Asyoumayrecall,thisisalsoasteptowardsdescribingwhatwewanttodofromthedescriptionofhowtodoit.

Aftergoingthroughthischapter,youwillunderstandwhatreactiveprogrammingisandwhattoolsthereareinJavathatyoucanutilize.Youwillalsounderstandwhatreactiveprogrammingisgoodforandwhenandhowyoucanutilizethisprincipleinthefuture,astherewillbemoreandmoreframeworkssupportingreactiveprogramminginJava.Inthischapter,youwilllearnthefollowingtopics:

ReactiveprogrammingingeneralReactivestreamsinJavaHowtoimplementoursamplecodeinareactiveway

Reactive...what?Therearereactiveprogramming,reactivesystems,andreactivestreams.Thesearethreedifferentthingsrelatedtoeachother.Itisnotwithoutreasonthatallthethreearecalledreactive.

Reactiveprogrammingisaprogrammingparadigmsimilartoobject-orientedprogrammingandfunctionalprogramming.Areactivesystemisasystemdesignthatsetscertainaimsandtechnologicalconstraintsonhowacertaintypeofinformationsystemsshouldbedesignedtobereactive.Therearealotofresemblancestoreactiveprogrammingprinciplesinthis.Areactivestreamisasetofinterfacedefinitionsthathelpstoachievesimilarcodingadvantagetoreactivesystemsandwhichcanbeusedtocreatereactivesystems.ReactivestreaminterfacesareapartofJDK9,butareavailablenotonlyinJava,butalsoinotherlanguages.

Wewilllookattheseinseparatesections,attheendofwhich,youwillpresumablyhaveabetterunderstandingofwhyeachofthemiscalledreactive.

ReactiveprogramminginanutshellReactiveprogrammingisaparadigmthatfocusesmoreonwherethedataflowsduringcomputationthanonhowtocomputetheresult.Iftheproblemisbestdescribedasseveralcomputationsthatdependontheoutputofeachotherbutseveralmaybeexecutedindependentoftheother,reactiveprogrammingmaycomeintothepicture.Asasimpleexample,wecanhavethefollowingcomputationthatcalculatesthevalueofhfromsomegivenb,c,e,andfvalues,usingf1,f2,f3,f4,andf5assimplecomputationalsteps:

a=f1(b,c)

d=f2(e,f)

k=f3(e,c)

g=f4(b,f,k)

h=f5(d,a,g)

IfwewritetheseinJavaintheconventionalway,themethodsf1tof5willbeinvokedoneaftertheother.Ifwehavemultipleprocessorsandareabletoparallelizetheexecution,wemayperformsomeofthemethodsinparallel.This,ofcourse,assumesthatthesemethodsarepurelycomputationalmethodsanddonotchangethestateoftheenvironment,and,inthisway,canbeexecutedindependentofeachother.Forexample,f1,f2,andf3canbeexecutedindependentofeachother.Theexecutionofthefunctionf4dependsontheoutputoff3,andtheexecutionoff5dependsontheoutputoff1,f2,andf4.

Ifwehavetwoprocessors,wecanexecutef1andf2together,followedbytheexecutionoff3,thenf4,andfinally,f5.Thesearefoursteps.Ifwelookattheprecedingcalculationnotascommandsbutratherasexpressionsandhowthecalculationsdependoneachother,thenwedonotdictatetheactualexecutionorderandtheenvironmentmaydecidetocalculatef1andf3together,thenf2andf4,andfinallyf5,savingonestep.Thisway,wecanconcentrateonthedataflowandletthereactiveenvironmentactuponitwithoutputtingextraconstraints.

Thisisaverysimpleapproachofreactiveprogramming.Thedescriptionofthecalculationintheformofexpressionsgivesthedataflow,butintheexplanation,westillassumedthatthecalculationisexecutedsynchronously.Ifthecalculationsareexecutedondifferentprocessorsthatareondifferentmachines

connectedtoanetwork,thenthecalculationmaynotanddoesnotneedtobesynchronous.Reactiveprogramscanbeasynchronouslyexecutediftheenvironmentisasynchronous.Itmayhappenthatthedifferentcalculations,f1tof4,areimplementedanddeployedondifferentmachines.Insuchacase,thevaluescalculatedaresentfromonetotheotheroverthenetworkandthenodesexecutethecalculationeverytimethereisachangeintheinputs.Thisisverysimilartogoodoldanalogcomputersthatwerecreatedusingsimplebuildingblocksandthecalculationsweredoneusinganaloguesignals.

Theprogramwasimplementedasanelectroniccircuit,andwhentheinputvoltageorcurrent(usuallyvoltage)changedintheinputs,theanalogcircuitsfolloweditinlight'sspeed,andtheresultappearedintheoutput.Insuchacase,thesignalpropagationwaslimitedbythespeedoflightonthewiresandanalogcircuitryspeedinthewiredmodules,whichwasextremelyfastandmaybeatdigitalcomputers.

Whenwetalkaboutdigitalcomputersthepropagationofthesignalisdigital,andthisway,itneedstobesentfromonecalculationnodetotheotherone,beitsomeobjectinJVMorsomeprogramonthenetwork.Anodehastoexecuteitscalculationif:

SomeofthevaluesintheinputhavechangedTheoutputofthecalculationisneeded

Iftheinputhasnotchanged,thentheresultshouldeventuallybethesameasthelasttime;thus,thecalculationdoesnotneedtobeexecutedagain—itwouldbeawasteofresources.Iftheresultofthecalculationisnotneeded,thenthereisnoneedtoperformthecalculationeveniftheresultwouldnotbethesameasthelastone.Noonecares.

Toaccommodatethis,reactiveenvironmentsimplementtwoapproachestopropagatethevalues.Thenodesmaypullthevaluesfromtheoutputofothermodules.Thiswillensurethatnocalculationthatisnotneededwillbeexecuted.Themodulesmaypushtheiroutputtothenextmodulethatdependsonthem.Thisapproachwillensurethatonlychangedvaluesignitecalculation.Someoftheenvironmentsmayimplementahybridsolution.

Whenvalueschangeinthesystem,thechangeispropagatedtowardstheothernodesthatagainpropagatethechangestoanothernodeandsoon.Ifweimaginethecalculationdependenciesasadirectedgraph,thenthechangestraveltowardsthetransitiveclosureofthechangedvaluesalongthenodesconnected.Thedatamaytravelwithallthevaluesfromonenodeoutputtotheothernodeinputoronlythechangemaytravel.Thesecondapproachismorecomplexbecauseitneedsthechangeddataandalsometainformationthatdescribeswhathaschanged.Ontheotherhand,thegainmaybesignificantwhentheoutputandinputsetofdataishugeandonlyasmallportionofitischanged.Itmayalsobeimportanttocalculateandpropagateonlytheactualdeltaofthechangewhenthereisahighprobabilitythatsomeofthenodesdonotchangetheoutputformanyofthedifferentinputs.Insuchacase,thechangepropagationmaystopatthenodewherethereisnorealchangeinspiteofthechangedinputvalues.Thiscansaveupalotofcalculationinsomeofthenetworks.

Intheconfigurationofthedatapropagation,thedirectedacyclicgraphcanbeexpressedinthecodeoftheprogram,itcanbeconfiguredoritcanevenbesetupandchangedduringtheexecutionofthecodedynamically.Whentheprogramcodecontainsthestructureofthegraph,theroutesandthedependenciesarefairlystatic.Tochangethedatapropagation,thecodeoftheprogramhastobechanged,recompiled,

anddeployed.Inthecaseofmultiplenetworknodeprograms,thismayevenneedmultipledeploymentsthatshouldbecarefullyfurnishedtoavoiddifferentincompatibleversionsrunningondifferentnodes.Thereshouldbesimilarconsiderationswhenthegraphisdescribedinsomeconfiguration.Insuchacase,thecompilationoftheprogram(s)maynotbeneededwhenonlythewiringofthegraphischanged,buttheburdentohavecompatibleconfigurationondifferentnodesinthecaseofanetworkexecutionisstillthere.

Lettingthegraphchangedynamicallyalsodoesnotsolvethisproblem.Thesetupandthestructurearemoreflexibleand,atthesametime,morecomplex.Thedatapropagatedalongtheedgesofthegraphmaycontainnotonlycomputationaldatabutalsodatathatdriveschangeinthegraph.Manytimes,thisleadstoaveryflexiblemodelcalledhigher-orderreactiveprogramming.

Reactiveprogramminghasalotofbenefitsbut,atthesametime,maybeverycomplex,sometimestoocomplex,forsimpleproblems.Itistobeconsideredwhentheproblemtobesolvedcaneasilybedescribedusingdatagraphandsimpledatapropagations.Wecanseparatethedescriptionoftheproblemandtheorderoftheexecutionofthedifferentblocks.Thisisthesameconsiderationthatwediscussedinthepreviouschapter.Wedescribemoreaboutthewhattodopartandlessaboutthehowtodopart.

Ontheotherhand,whenthereactivesystemdecidestheorderofexecution,whatischanged,andhowthatshouldbereflectedontheoutputofotherblocks,itshoulddosowithoutknowingthecoreoftheproblemthatitissolving.Insomesituations,codingtheexecutionordermanuallybasedontheoriginalproblemcouldperformbetter.

Thisissimilartothememorymanagementissue.Inmodernruntimeenvironments,suchastheJVM,Pythonruntime,Swiftprogramming,orevenGolang,thereissomeautomatedmemorymanagement.WhenprogramminginC,theprogrammerhasfullcontrolovermemoryallocationandmemoryrelease.Inthecaseofreal-timeapplications,wheretheperformanceandresponsetimeisoftheutmostimportance,thereisnowaytoletanautomatedgarbagecollectortaketimeanddelaytheexecutionfromtimetotime.Insuchacase,theCcodecanbeoptimizedtoallocatememorywhenneeded;thereisaresourcefortheallocationandreleaseofmemorywhenpossibleandthereistimetomanagememory.Theseprogramsarebetterperformingthantheonescreatedforthesamepurposeusingagarbagecollector.Still,wedonotuseCinmostoftheapplicationsbecausewecanaffordtheextraresourceneededforautomatedmemorycollection.Eventhoughitwouldbepossibletowriteafastercodemanagingthememorymanually,automatedcodeisfasterthanwhatanaverageprogrammerwouldhavecreatedusingC,andalso,thefrequencyofprogrammingerrorsismuchlower.

Justastherearesomeissuesthatwehavetopayattentiontowhenusingautomatedmemorymanagement,wehavetopayattentiontosomeissuesinareactiveenvironment,whichwouldnotexistinthecaseofmanualcoding.Still,weusethereactiveapproachforitsbenefits.

Themostimportantissueistoavoidloopsinthedependencygraph.Althoughitisabsolutelyperfecttowritethedefinitionofcalculations,areactivesystemwouldprobablynotbeabletocopewiththesedefinitions.Somereactivesystemsmayresolveinsomesimple-casecyclicredundancy,butthatissomeextrafeatureandwegenerallyjusthavetoavoidthat.Considerthefollowingcomputations:

a=b+3

b=4/a

Here,adependsonb,sowhenbchanges,aiscalculated.However,balsodependsona,whichisrecalculatedand,inthisway,thesystemgetsintoaninfiniteloop.Theprecedingexampleseemstobesimple,butthatisthefeatureofagoodexample.Real-lifeproblemsarenotsimple,andinadistributedenvironment,itisextremelyhardsometimestofindcyclicredundancy.

Anotherproblemiscalledglitch.Considerthefollowingdefinition:

a=b+3

q=b+a

Whentheparameterbischanged,forexample,from3to6,thevalueofawillchangefrom6to9,andthus,qwillchangefrom9to15.Thisisverysimple.However,theexecutionorderbasedontherecognitionofthechangesmayfirstalterthevalueofqfrom9to12beforemodifyingitto15inasecondstep.Thiscanhappenifthecalculatingnoderesponsibleforthecalculationofqrecognizesthechangeinbbeforethevalueofaasaconsequenceofthechangeinthevalueofb.Forashortperiodoftime,thevalueofqwillbe12,whichdoesnotmatchthepreviousandalsodoesnotthechangedstate.Thisvalueisonlyaglitchinthesystemthathappensafteraninputchangesandalsodisappearswithoutanyfurtherchangeintheinputinthesystem.

Ifyouhaveeverlearntthedesignoflogicalcircuits,thenstatichazardsmayringabell.Theyareexactlythesamephenomenon.

Reactiveprogrammingalsoassumesthatthecalculationsarestateless.Theindividualnodesthatperformthecalculationmayhaveastateinpracticeand,manytimes,theydo.Itisnotinherentlyeviltohaveastateinsomecalculation.However,debuggingsomethingthathasastateissignificantlymorecomplexthandebuggingsomethingthatisstateless,functional.

Itisalsoanimportantaidtothereactiveenvironment,lettingitperformdifferentoptimizationsbasedonthefactthatthecalculationsarefunctional.Ifthenodeshaveastate,thenthecalculationsmaynotbe

rearrangedfreelybecausetheoutcomemaydependontheactualevaluationorder.Thesesystemsmaynotreallybereactive,or,atleast,itmaybedebated.

Reactivesystems

Reactivesystemisdefinedinthereactivemanifestoathttp://www.reactivemanifesto.org/.Thecreatorsofthemanifestorealizedthatwiththechangeoftechnology,newsystempatternswillneedtobedevelopedinenterprisecomputingtoleveragethenewtechnologyandyieldbetteroutcomes.Themanifestoenvisionssystemsthatare:

ResponsiveResilientElasticMessage-driven.

Thefirstthreefeaturesareuservalues;thelastoneismoreofatechnologicalapproachtogetthevalues.

ResponsiveAsystemisresponsiveifitgivesresultsinareliablemanner.Ifyoutalktome,Iwillansweryourquestionor,atleast,tellyouthatIdonotknowtheanswerorthatIwasnotabletounderstandthequestion.Betterifyougettheanswer,butifasystemcannotgivethattoyouitisstillexpectedtogivesomethingback.Ifyouhavepastexperiencewithclientoperatingsystemsfromjusttenyearsagoandsomeoldcomputers,youcanunderstandthis.Gettingarotatinghourglassisfrustrating.Youjustdonotknowwhetherthesystemisworkingtogetyoutheansweroristotallyfrozen.

Areactivesystemhastoberesponsive.Theresponseshouldcomeinatimelymanner.Theactualtimingdependsontheactualsystem.Itmaybemilliseconds,seconds,orevenhoursincasethesystemisrunningonaspaceshiptravellingtowardstheothersideofJupiter.Theimportantthingisthatthesystemshouldguaranteesomesoftupperlimitfortheresponsetime.Thisdoesnotnecessarilymeanthatthesystemshouldbeareal-timesolution,whichisamuchstricterrequirement.

Theadvantageofresponsivenessisnotonlythattheuserdoesnotbecomenervousinfrontofthecomputer.Afterall,mostoftheseservicesareusedbyotherservicesthatmainlycommunicatewitheachother.Therealadvantageisthaterrordiscoveryismorereliable.Ifareactivesystemelementbecomesnonresponsive,itiscertainlyanerrorcondition,andsomethingshouldbedoneaboutit,outofthescopeofnormaloperations(replaceafaultycommunicationcard,restartasystem,andsoon).Thesoonerwecanidentifyanerrorstate,thecheaperitistofixit.Themorewecanidentifywheretheproblemis,thelesstimeandmoneywecouldspendlocalizingtheerror.Responsivenessisnotaboutspeed.Itisaboutbetteroperation,betterquality.

ResilientResilientsystemskeepworkingevenwhenthereissomeerror.Well,notanyerror.Thatwouldbeamiracle,orsimplynonsense!Anerrorgenerallyisanerror.IftheArmageddoncomesanditistheendoftheworldasweknowit,evenresilientsystemswillnotberesponsive.Forsmallerdisruptions,however,theremaybesomecuretomakethesystemsresilient.

Therearetechniquesthatmayhelpifonlyadiskfails,thereisapoweroutage,orthereisaprogrammingerror.Systemsmaybereplicated,sowhenoneoftheinstancesstopsresponding,someotherinstancemaytakeupthetaskofthefailingoneandcangoonworking.Systemspronetoerrorsmaybeisolatedfromeachotherintermsofspaceortime.Whenthereisanearthquakeorfloodatonelocation,theotherlocationmaystillgoonworking.Ifdifferentcomponentsdonotneedtocommunicateinrealtimeandmessagesarestoredandforwardedinareliablemanner,thenthisisnotaproblemevenifthetwosystemsareneveravailableatthesametime.Theycanstillcooperatetakingupthemessages,performingthetasktheyaresupposedto,andsendingouttheresultingmessageafterwards.

Errorsinthesystemhavetobeaddressedevenifthesystemremainsresponsive.Errorsdonotaffecttheresponsivenessofaresilientsystem,butthelevelofresiliencedecreasesandshouldberestored.

ElasticElasticitymeansthatthesystemisadaptingtotheload.Wecanhaveahugesystem,withlotsofprocessorscapableofservingthelargestanticipateddemand.Thatisnotelasticity.Sincethedemandisnotconstantand,mostofthetime,issmallerthanthemaximum,theresourcesofsuchasystemareidle.Thiscauseswasteoftime,CPUcycle,energy,and,thus,ecologicalfootprint.

Havingsystemsrunonthecloudcanavoidsuchlosses.Thecloudisnothingbutmanycomputersthatsomebodyoperatesformultipleapplications,formultiplecorporationseven,andeachrentsonlytheCPUcyclesthatitreallyneedsandonlywhenitneeds.Othertimes,whentheloadissmallertheCPUandtheelectricpowercanbeusedbysomeoneelse.Sincedifferentapplicationsanddifferentcorporationshavedifferentpeaktimes,thelossofresourcesislesswiththismodel.Therearemanyissuesthathavetobesolved,suchasdataisolationandprotectionofinformationfromeavesdropping,butthesearemainlysolved.Secretservicecorporationswillnotrentresourcesfromacloudservicetoruntheircomputations(perhaps,they'ddoforsomeotherpurpose)andsomeotherparanoidcompaniesmayalsorefrainfromdoingthat,butmostofthecompanieswilldo.Itismoreeffectiveandisthuscheaperevenafterconsideringallthesideeffectsonecanconsider.

Elasticitymeansthattheallocatedresourcesfollow,orratheranticipatethecomingneeds.Whenthesystemanticipateshighercapacityneeds,itallocatesmoreresourcesandatoff-peaktime,itreleasestheresourcessothatothercloudcustomerscanuseit.

Elasticityalsoassumesthatthesystemisscalable.Thetwothings,elasticityandscalability,arecloselyrelatedbutarenotthesame.Scalabilitymeansthattheapplicationcanaccommodatehigherload,allocatingmoreresources.Scalabilitydoesnotcarewhetherthisallocationisstaticbuyingandpoweringofhugecomputerboxesinacomputingcenterdedicatedtotheapplicationordynamicallocationofresourcesfromthecloudondemand.Scalabilitysimplymeansthatifthedemanddoubles,thentheresourcescanalsobemultipliedtomeetthedemand.Ifthemultiplicationfactorintheresourcesneededisthesameorisnotmorethanthefactorindemand,thentheapplicationisscalable.Ifweneedmoreresourcestomeetthedemand,orifwecannotmeetthedemandevenifthedemandincreasesonlymoderately,thentheapplicationisnotscalable.Elasticapplicationsarealwaysscalable;otherwise,theycannotbeelastic.

Message-drivenReactivesystemsaremessage-driven;notbecauseweneedmessage-drivensystemsbutmuchratherbecausemessage-drivensystemsarethosethatcandeliverresponsiveness,resilience,andelasticityatthesametime.

Message-drivenarchitecturemeansthattheinformationtravelsbetweenthecomponentsdisconnected.Onecomponentsendsamessageandthenforgetsit.Itdoesnotwaitfortheothercomponenttoactuponthemessage.Whenthemessageissent,allthetasksonbehalfofthesendingcomponentareperformedandalltheresourcesneededtohandlethetasksarereleased,resultinginthemessagebeingreleasedandreadytobeusedforthenexttask.

Message-drivendoesnotnecessarilymeannetworking.Messagescantravelbetweenobjects,threads,andprocessesinsidethesamemachine.Ontheotherhand,iftheinterfacestothemessagingarchitecturearewell-designed,thenthecomponentsdonotneedtobemodifiediftheinfrastructurechangesandthemessagesthatwerepreviouslypassingbetweenthreadswillnowhavetotravelthroughtheoceaninIPpackets.

Sendingmessagesmakesitpossibletoisolatethesenderandthereceiverinspaceandtime,justaswedescribed,asameansforelasticity.Thereceivermaypickupthemessagesometimeafteritarrived,whenithastheresourcestodoso.Responsiveness,though,requiresthatthistimeisnotintheunreachabledistantfuturebutinsomelimiteddistance.Ifthemessagecannotbeprocessedsuccessfully,anothermessagemaysignaltheerror.Anerrormessageisnottheresultweexpect,butitisstillsomeresponseandthesystemremainsresponsivewithallthebenefitsitmeans.

Back-pressureMessagehandling,withtheappropriatemessaginginterfacesandimplementation,supportsback-pressure.Back-pressureisameanstolessentheburdenonacomponentwhenitcannotorcanbarelyhandlemoremessages.Messagesmaybequeuingforprocessing,butnoreal-lifequeuehasunlimitedcapacityandreactivesystemsshouldnotloseamessageuncontrolled.Back-pressuresignalstheloadofthecomponenttothemessageproducers,askingthemtolessentheproduction.Itislikeawaterpipe.Ifyoustartclosingtheoutletofthepipe,thepressurestartstoincreaseinthepipebackward,thewatersourceforcingittodeliverlessandlesswater.

Back-pressureisaneffectivewayofhandlingloadbecauseitmovesloadhandlingtothecomponentthatcanreallydoit.Inold-fashionedqueuingsystems,thereisaqueuethatstorestheitemstillthecomponentreceivingthemcanconsumethem,doingitsjob.Aqueuedesigncanbegoodifthereisawell-definedlimitforthesizeoftheloadandforthemaximumsizeofthequeue.Ifeverthequeueisfull,theitemscannotbedeliveredandthesystemstalls.

Applyingback-pressureisabitdifferent.Aqueuemaystillbeusedinfrontofthecomponentsforperformanceoptimizationandtoensureresponsiveness.Theproduceroftheitemcanstillputtheproducediteminthequeueandreturntoattendingtoitsowndutiesanddoesnotneedtowaittilltheconsumercanattendtotheitem.Thisisdecoupling,aswementionedearlier.Seeingthatthequeueisfulloralmostfullcanalsoactasaverysimpleback-pressure.Itisnottrueifsomeonesaysthatqueuesaretotallymissingthisfeature.Attimes,itmaysimplybetotallysufficientjusttolookatthecapacityofaqueue,andalsotheitemsinit,toseeifthereissomeneedtolessentheloadonthereceiverthequeuebelongsto.Buttheproducerdoesthis,notthereceiver,andthatisanessentialproblem.

Theproducerseesthatthereceiverisnotkeepingpacewiththesupplybuttheproducerdoesnothaveanyinformationaboutthecause,andnotknowingthecausecannotpredictthefuturebehavior.Havingaback-pressureinformationchannelfromthereceivertotheproducermakesthestorymorefinegrained.

Theproducermayseethatthereare,say,10slotsinthequeueanditthinksthatthereisnoproblem;theproducerdecidestodelivereightmoreitemsinthenext150ms.Oneitemusuallytakes10mstoprocess,giveortake;thustheitemsareexpectedtobeprocessedinlessthan100ms,whichisjustbetterthantherequired200msmaximum.Theproduceronlyknowsthatanitemusuallytakes10mstoprocess.Thereceiver,ontheotherhand,seesthatthelastitemitgotintothequeuerequiressomuchprocessingthat,byitself,itwillrequire200ms.Tosignalthis,itcantelltheproducerovertheback-pressurenottodelivernewitemstillfurthernotice.Thereceiverknowsthattheitemswouldhavefitinthequeuefinebutwouldnotbeprocessedinatimelymanner.Usingthisinformation,theproducerwillissuesomecommandstothecloudcontroltoallocateanotherprocessingandsendsthenexteightitemstothenewreceiver,lettingtheoldonedoitscumbersomejobithastowiththatfarabovethanaverageitem.

Back-pressureletsyouaidthedataloadcontrol,withinformationcreatedbythereceiversthathavethemostinformationaboutprocessingtheitems.

ReactivestreamsReactivestreamsstartedasaninitiativetoprovideastandardofhandlingdatastreamsinanasynchronousmodebyregulatingthepushofthedatausingback-pressure.Theoriginalsiteoftheprojectishttp://www.reactive-streams.org/.

ReactivestreamsarenowimplementedinJDK9inthejava.util.concurrentpackage.

Theaimofthedefinitionofreactivestreamsistodefinetheinterfacethatcanhandlethepropagationofthegenerateddatainatotallyasynchronouswaywithouttheneedonthereceivingsidetobuffertheunlimitedcreateddata.Whendataiscreatedinastreamandismadeavailabletobeworkedontheworkerthatgetsthedata,hastobefastenoughtohandleallthedatathatisgenerated.Thecapacityshouldbehighenoughtohandlethehighestproduction.Someintermediatebuffersmayhandlepeaks,butifthereisnocontrolthatstopsordelaysproductionwhentheconsumerisatthetopofitscapacity,thesystemwillfail.Reactivesysteminterfacesaredesignedtoprovideawaytosupportback-pressure.Back-pressureisaprocesstosignaltheproducerofthedatatoslowdownoreventostoptheproductiontothelevelthatfitstheconsumer.Everycalltheinterfacesdefineisasynchronoussothattheperformanceofonepartisnotaffectedbythedelaysintheexecutionofotherparts.

Theinitiativedidnotaimtodefinethewayinwhichdataistransferredbetweenproductionandconsumption.ItfocusesontheinterfacestogiveaclearstructurefortheprogramsandalsotogiveanAPIthatwillworkwithalltheimplementations.

ReactiveprogramminginJavaJavaisnotareactivelanguage.ThisdoesnotmeanthatwecannotcreatereactiveprogramsinJava.Therearelibrariesthatsupportdifferentreactiveprogrammingapproaches.ItistomentiontheAkkaframeworkandtheReactiveXthatalsoexistforotherlanguagesaswell.WithJava9,theJDKstartstosupportreactiveprogramming,providingafewclassesandinterfacesforthepurpose.Wewillfocusonthesefeatures.

TheJDKcontainsthejava.util.concurrent.Flowclass,whichcontainsrelatedinterfacesandsomestaticmethodstosupportflowcontrolledprograms.ThemodelthatthisclasssupportsisbasedonPublisher,Subscriber,andSubscription.

Asaverysimpleexplanation,aPublisheracceptsasubscriptionfromaSubscriber.ASubscribergetsthedataitsubscribedtowhenthedataisavailable.Theinterfacesfocusontheverycoreofthedataflowcontrolofthecommunicationandareabitabstract.Nosurprise,theyareinterfaces.However,itmaynotbesimpletounderstandtheirworkingatfirst.

ThePublisherinterfacedefinesthesubscribemethod.Thisistheonlymethodthisinterfacedefinesandthatisbecausethisistheonlythingthatarealpublishercanbeasked.Youcansubscribetothepublications.TheargumentofthemethodisaSubscriberthatsubscribestothepublications:

voidsubscribe(Flow.Subscriber<?superT>subscriber)

ThereisareadilyavailablePublisherclassintheJDKthatwewilllookatlater.WhenthesubscribemethodofthePublisheriscalled,ithastodecideifthesubscribercangetthesubscriptionornot.Usually,thesubscriptionisacceptedbuttheimplementationhasthefreedomtorefuseasubscriptionattempt.Publishermayrefuseasubscriptionif,forexample,thesubscriptionfortheactualsubscriberwasalreadyperformedandthePublisherimplementationdoesnotallowmultiplesubscriptionsfromthesamesubscriber.

TheimplementationofthemethodisrequiredtocalltheonErrormethodofsubscriber,withThrowableastheargument.Inthecaseofmultiplesubscriptions,IllegalStateExceptionseemstobesuitable,astheJDKdocumentationdefinesatthemoment.

Ifthesubscriptionissuccessful,PublisherisexpectedtocalltheonSubscribemethodofsubscriber.TheargumenttothismethodisaSubscriptionobject(aninstanceofaclassthatimplementstheinterfaceSubscription).Thisway,thePublishernotifiestheSubscriberthatthesubscriptionrequestwasaccepted,andalsopassesanobjecttomanagethesubscription.

Managingthesubscriptionasanabstractioncouldbeimaginedasacomplextask,butinthecaseofreactivestreams,itisverysimple.Allthesubscribercanandshoulddoistosetthenumberofitemsitcanreceiveatthemoment,andthesubscriptioncanbecancelled.

WhyshouldthePublishercallbacktheonSubscribemethodofSubscriber?Whydoesn'titsimplyreturnthesubscriptionorthrowsomeerror?ThereasonforthiscomplexbehavioristhatitmaynotbetheSubscriberthatinvokesthesubscribemethod.Justasinreallife,I

cansubscribeandpayforayearofamagazinesubscriptionasaChristmasgift.(ThisistheseasonwhenIamwritingthispartofthebook.)Inourcode,somewiringcomponentresponsibleforwhoisnotifiedaboutacertaindatachangecallssubscribeandnotnecessarilythesubscriber.TheSubscriberisonlyresponsiblefortheminimalthingsthatasubscribershouldberesponsiblefor.Theotherreasonisthatthewholeapproachisasynchronous.Whenwesubscribetosomething,thesubscriptionmaynotbeavailableandreadyimmediately.Therecouldbesomelong-runningprocessesthatneedtofinishtillthesubscriptionwillbeavailableandthecallerthatiscallingsubscribedoesnotneedtowaitforthecompletionoftheprocess.Whenthesubscriptionisreadyitispassedtothesubscriber,totheveryentitythatreallyneedsit.

TheSubscriberinterfacedefinestheonSubscribe,onError(wehavealreadytalkedaboutthese),onCompleteandonNextmethods.

ItisimportantinthedefinitionoftheseinterfacesthatthesubscribergetstheitemsfromPublisherorfromsomeotherobjecttowhichthePublisherdelegatesthistaskviasomepush.Thesubscriberdoesnotneedtogotothenewsstandtogetthenextissue;someonecallingtheonNextmethoddeliverstheissuetoitdirectly.

ThisalsobearstheconsequencethatunlesstherearesomecontrolsinthehandsoftheSubscriber,itcouldhappenthatthePublisherfloodstheSubscriberwithitems.NoteverySubscriberiscapableofhandlingunlimiteditems.TheSubscribergetsaSubscriptionobjectuponperformingthesubscriptionandthisobjectcanbeusedtocontroltheflowoftheitemobjects.

ThePublishercreatestheSubscriptionobjectandtheinterfacedefinestwomethods:cancelandrequest.ThecancelmethodshouldbecalledbytheSubscribertonotifythePublisherthatitshouldnotdelivermoreitems.Thesubscriptioniscancelled.Therequest(longn)methodspecifiesthatthesubscriberispreparedtogetatmostnitemsviasubsequentcallstotheonNextmethod:

Ifthesubscriberhasalreadyinvokedtherequestmethod,thespecifiednumberisaddedtothesubscriptioncounter.Inotherwords,thespecifiedlongvaluedoesnotreflecttheactualstateofthesubscriber.Itisa

delta,increasingsomecountersmaintainedbythePublisherthatcountsthenumberofitemsthatcanbedeliveredaddingthevalueofthelongargumentanddecrementingbyoneoneachitemdeliveredtotheSubscriber.Themostusualapproachistocallrequest(1)eachtimetheSubscriberhasfinishedprocessingarequest.

IftherequestmethodisinvokedwiththeLong.MAX_VALUEargument,thePublishermayjustsendanyitemthatitcanwithoutcountingandwithoutlimit.Thisisessentiallyswitchingofftheback-pressuremechanism.

Thespecificationalsomentionsthatthecalltocanceldoesnotnecessarilymeanthattherewillbenomoreissuesdeliveredatall.Cancellationisdoneonbesteffort.Justasinreallife,whenyousendyourmailtothedailypaperwithyourintenttocancelthesubscription,thepublisherwillnotsendanagenttostopthepostmanbeforehedropstheissuetoyourmailbox.Ifsomethingwasalreadyonitswaywhenthecancellationarrivedtothepublisheritgoesitsway.IfthePublisherhasalreadystartedsomeasynchronousprocessthatcannotreasonablybestopped,thenthemethodonNextmethodwillbeinvokedwithsomeoftheelements.

ThePublisherandSubscriberinterfaceshaveagenericparameter,T.ThisisthetypeofitemsthatthePublisherinterfacepublishesandtheSubscriberinterfacegetsintheonNextmethod.Tobeabitmoreprecise,theSubscriberinterfacecanhaveanRtype,whichisasuperclassofT;thus,itiscompatiblewiththePublisherinterface.Forexample,ifPublisherpublishesLongvalues,thentheSubscriberinterfacecanacceptLong,Number,orObjectintheargumentoftheonNextmethod,dependingonthedeclarationoftheclassthatimplementsSubscriber.

TheFlowclassalsocontainsaProcessorinterfacethatextendsbothPublisherandSubscriber.Thisinterfaceistheretobeimplementedbyclassesthatalsoacceptdataandsenddatatoothercomponentsinthereactiveflow.Suchelementsareverycommoninreactivestreamprogramsbecausemanyelementsthatperformsometasksgettheitemstoworkonfromotherreactivestreamelements;thus,theyareSubscribersand,atthesametime,theysenditaftertheyhavefinishedtheirtasks;thus,theyarePublishers.

ImplementinginventoryNowthatwehavediscussedalotoftechnologiesandprogrammingapproach,itisverymuchthetimetoimplementsomesamplecode.Wewillimplementinventorykeepinginourapplicationusingreactivestreams.Fortheexample,theinventorywillbeverysimple.ItisaMap<Product,InventoryItem>thatholdsthenumberofitemsforeachproduct.TheactualmapisConcurrentHashMapandtheInventoryItemclassisabitmorecomplexthanaLongnumbertoproperlyhandleconcurrencyissues.Whenwedesignaprogramthatisbuiltonresponsivestreams,wedonotneedtodealwithmuchconcurrencylocking,butwestillshouldbeawarethatthecoderunsinamultithreadenvironmentandmayexhibitstrangebehaviorifwedonotfollowsomerules.

ThecodefortheInventoryclassisfairlysimplesinceithandlesonlyamap:

packagepackt.java9.by.example.mybusiness.inventory;

import...;

@Component

publicclassInventory{

privatefinalMap<Product,InventoryItem>inventory=

newConcurrentHashMap<>();

privateInventoryItemgetItem(Productproduct){

inventory.putIfAbsent(product,newInventoryItem());

returninventory.get(product);

}

publicvoidstore(Productproduct,longamount){

getItem(product).store(amount);

}

publicvoidremove(Productproduct,longamount)

throwsProductIsOutOfStock{

if(getItem(product).remove(amount)!=amount)

thrownewProductIsOutOfStock(product);

}

}

Theinventoryitemmaintainingtheclassisabitmorecomplexsincethisisthelevelwherewehandleabitofconcurrencyor,atleast,thisistheclasswherewehavetopaysomeattention:

packagepackt.java9.by.example.mybusiness.inventory;

importjava.util.concurrent.atomic.AtomicLong;

publicclassInventoryItem{

privatefinalAtomicLongamountOnStock=

newAtomicLong(0);

voidstore(longn){

amountOnStock.accumulateAndGet(n,

(stock,delta)->stock+delta);

}

longremove(longdelta){

classClosureData{

longactNr;

}

ClosureDatad=newClosureData();

amountOnStock.accumulateAndGet(delta,

(stock,n)->

stock>=n?

stock-(d.actNr=n)

:

stock-(d.actNr=0)

);

returnd.actNr;

}

}

Whenweaddproductstotheinventory,wehavenolimit.Thestorageshelvesareextremelyhugeandwedonotmodelthattheyoncemaygetfullandtheinventorymaynotbeabletoaccommodatemoreitems.Whenwewanttoremoveitemsfromtherepository,however,wehavetodealwiththefactthattheremaynotbeenoughitemsfromtheproduct.Insuchacase,wedonotremoveanyitemsfromtherepository.Weservethecustomertofullsatisfactionorwedonotserveatall.

Tomaintainthenumberoftheitemsintheinventory,weuseAtomicLong.ThisclasshastheaccumulateAndGetmethod.ThismethodgetsaLongparameterandaLongBinaryOperatorthatweprovideinourcodeasalambda.ThiscodeisinvokedbytheaccumulateAndGetmethodtocalculatethenewvalueofthestock.Ifthereareenoughitems,thenweremovetherequestednumberofitems.Iftherearenotenoughitemsonstock,thenweremovezero.Themethodreturnsthenumberofitemsthatweactuallyreturn.Sincethatnumberiscalculatedinsidethelambda,ithastoescapefromthere.Todoso,weuseClosureDatadefinedinsidethemethod.

Notethat,forexample,inGroovywecouldsimplyuseaLongdvariableandalterthevariableinsidetheclosure.Groovycallslambdatoclosures,sotosay.InJavawecannotdosobecausethevariablesthatwecanaccessfrominsidethemethodshouldbeeffectivelyfinal.However,thisisnothingmorethanabitmoreexplicitnotationthatbelongstotheclosureenvironment.TheClosureDatadobjectisfinalasopposedtothefieldtheclasshas,whichcanbemodifiedinsidethelambda.

ThemostinterestingclassthatwearereallyinterestedinthischapterisInventoryKeeper.ThisclassimplementstheSubscriberinterfaceandiscapableofconsumingorderstomaintaintheinventory:

packagepackt.java9.by.example.mybusiness.inventory;

import...

publicclassInventoryKeeperimplementsFlow.Subscriber<Order>{

privatestaticfinalLoggerlog=

LoggerFactory.getLogger(InventoryKeeper.class);

privatefinalInventoryinventory;

publicInventoryKeeper(@AutowiredInventoryinventory){

this.inventory=inventory;

}

privateFlow.Subscriptionsubscription=null;

privatestaticfinallongWORKERS=3;

@Override

publicvoidonSubscribe(Flow.Subscriptionsubscription){

log.info("onSubscribewascalled");

subscription.request(WORKERS);

this.subscription=subscription;

}

TheonSubscribemethodisinvokedaftertheobjectissubscribed.Thesubscriptionispassedtotheobjectandisalsostoredinafield.Sincethesubscriberneedsthissubscriptioninsubsequentcalls,whenanitempassedinonNextisprocessedandanewitemisacceptable,afieldisagoodplacetostorethisobjectin.Inthismethod,wealsosettheinitialrequesttothreeitems.Theactualvalueissimplydemonstrative.Enterpriseenvironmentsshouldbeabletoconfiguresuchparameters:

privateExecutorServiceservice=

Executors.newFixedThreadPool((int)WORKERS);

ThemostimportantpartofthecodeistheonNextmethod.Whatitdoesisactuallygoesthroughalltheitemsoftheorderandremovesthenumberofitemsfromtheinventory.Ifsomeoftheitemsareoutofstock,thenitlogsanerror.Thisistheboringpart.Theinterestingpartisthatitdoesthisthroughanexecutorservice.ThisisbecausethecalltoonNextshouldbeasynchronous.ThepublishercallsonNexttodelivertheitem,butweshouldnotmakeitwaitfortheactualprocessing.Whenthepostmanbringsyourfavoritemagazine,youdon'tstartreadingitimmediatelyandmakethepostmanwaitforyoursignatureapprovingacceptance.AllyouhavetodoinonNextisfetchthenextorderandmakesurethatthiswillbeprocessedinduetime:

@Override

publicvoidonNext(Orderorder){

service.submit(()->{

intc=counter.incrementAndGet();

for(OrderItemitem:order.getItems()){

try{

inventory.remove(item.getProduct(),

item.getAmount());

}catch(ProductIsOutOfStockexception){

log.error("Productoutofstock");

}

}

subscription.request(1);

counter.decrementAndGet();

}

);

}

@Override

publicvoidonError(Throwablethrowable){

log.info("onErrorwascalledfor{}",throwable);

}

@Override

publicvoidonComplete(){

log.info("onCompletewascalled");

}

}

TheactualimplementationinthiscodeusesThreadPoolwiththreethreadsinit.Also,thenumberofrequireditemsisthree.Thisisalogicalcoincidence:eachthreadworksonasingleitem.Itdoesnotneedtobelikethat,evenifinmostcasesitis.Nothingcanstopusfrommakingmorethreadsworkingonthesameitemifthatmakessense.Theoppositeisalsotrue.Onesinglethreadmaybecreatedtoworkonmultipleitems.Thesecodeswillprobablybemorecomplexandthewholeideaofthesecomplexexecutionmodelsistomakethecodingandthelogicsimpler,movethemultithreading,coding,andimplementationissuesintotheframework,andfocusonthebusinesslogicintheapplicationcode.ButIcannottellthattheremaynotbeanexampleforasubscriberworkingmultiplethreadsonmultipleitems,intermingled.

Thelastcodewehavetolookatinthischapteristheunittestthatdrivesthecodewithsomeexamples:

publicvoidtestInventoryRemoval(){

Inventoryinventory=newInventory();

SubmissionPublisher<Order>p=

newSubmissionPublisher<>();

WecreatePublisherusingtheJDKclass,SubmissionPublisher,whichneatlyimplementsthisinterfacedeliveringmultithreadfunctionalityforuswithoutmuchhassle:

p.subscribe(newInventoryKeeper(inventory));

Wecreateaninventorykeeperandwesubscribetothepublisher.Thisdoesnotstartdeliveringanythingbecausetherearenopublicationsyet,butitcreatesabondbetweenthesubscriberandthepublishertelling,themthatwheneverthereisaproductsubmitted,thesubscriberwantsit.

Afterthat,wecreatetheproductsandstorethemintheinventory,20piecesaltogether,andwealsocreateanorderthatwants10productstobedelivered.Wewillexecutethisordermanytimes.Thisisabitofsimplification,butforthetest,thereisnoreasontocreateseparateorderobjectsthathavethesameproductsandthesameamountsinthelistofitems:

Productproduct=newProduct();

inventory.store(product,20);

OrderItemitem=newOrderItem();

item.setProduct(product);

item.setAmount(10);

Orderorder=newOrder();

List<OrderItem>items=newLinkedList<>();

items.add(item);

order.setItems(items);

Afterallthishasbeendone,wesubmittheordertothePublisher10times.Itmeansthatthereare10ordersforthesameproduct,eachaskingfor10pieces,thatis,100piecestogether.Thoseare100piecesagainstthewarehousewherewehaveonly20ofit.Whatweshouldexpectisthatonlythefirsttwoorderswillbefulfilledandtherestwillberejectedandthatiswhatwillactuallyhappenwhenweexecutethiscode:

for(inti=0;i<10;i++)

p.submit(order);

log.info("Allordersweresubmitted");

Afteralltheordersarepublished,wewaitforhalfasecondsothattheotherthreadshavetimetoexecuteandthenwefinish:

for(intj=0;j<10;j++){

log.info("Sleepingabit...");

Thread.sleep(50);

}

p.close();

log.info("Publisherwasclosed");

}

Notethatthisisnotaregularunittestfile.Itissometestcodetoplayaround,whichIalsorecommendforyoutoexecute,debug,andlookatthedifferentlogoutputs.

SummaryInthisshortchapter,wehadalookatreactiveprogramming,reactivesystems,andreactivestreams.Wediscussedthesimilaritiesandthedifferencesbetweenthesethatmayleadtoconfusions.WepaidspecialattentiontoJava9reactivestreamsthathavepracticallynothingtodowithStreamclassesandmethods.

Inthesecondhalfofthechapter,wediscussedaverysimpleexamplethatusesreactivestreams.

Afterreadingthischapter,youhavelearnedalotabouttheJavalanguageandprogramming.WedidnotdetailallthesmallbitsofJava,butthatisnotpossibleinabook.Idaresaythatthereisnoman(orwomanforthatmatter)ontheEarthorarounditonanorbitalroute,whereverhumansare,whoknowseverythingaboutJava.We,however,knowenoughbynowtostartcodinginanenterpriseenvironmentandtolearnmoreandmoreonthegotillweretire,orevenafterthat.Whatisstillleftisalittlebitofprogramming.IntheprevioussentenceIsaidcodingtomakesomedistinction.Codingisnotthesameasprogramming.Codingisatechniqueusedintheprofessionofprogramming.Duringthenext,andlast,chapterwewillseetheaspectsofprogrammingandhowitcan,andshould,bedoneinprofessionalmanner.Thisisrarelyapartofanintroductorybook,butIamhappythatwecouldagreeonthistopicwiththepublisher.Thisway,youcanfinishthebooknotonlywiththeknowledgethatyoulearnfromthisbook,butalsowithavision,lookingaheadontheroadyouwillwalkupthehillsidetothetop.Youwillknowthetopics,areas,andsubjectsthatyoucangoonlearning.

FinalizingJavaKnowledgetoaProfessionalLevel

Bynow,youhavelearnedthemostimportantareasandtopicsneededforaprofessionalJavadeveloper.Whatwestillhaveaheadofusinthisbookistodiscusssometopicsthatwillleadyoufrombeingajuniordevelopertoaseniordeveloper.Readingthischapterwillnotmakeanyoneaseniordeveloper,though.Thepreviouschaptersweretheroadsthatwewalkedthrough.Thischapterisonlythemap.Ifeachofthepreviouschapterscoveredashortwalkofafewmilesinthejourneyofcodingtoreachtheharbor,thenthischapteristhenauticalmaptodiscoveranewcontinent.

Wewillbrieflybiteintosomeverydeepandhigh-levelprofessionalareas,suchascreatingaJavaagent,compile-timeannotationprocessing,polyglotprogramming,abitofarchitecturedesignandtools,andtechniquestoworkinteams.We'lldoitjustforthetaste.Now,youhaveenoughknowledgetounderstandtheimportanceofthesetopics,andgettingatastewillcreateanappetiteforthecomingyearsofself-development,or,atleast,thatismyintentiontomakeyou,thereader,addicted.

Javadeeptechnologies

Inthissection,wewilllistthreetechnologies:

JavaagentPolyglotprogrammingAnnotationprocessing

KnowingthemisnotamustforaJavaprofessional.Knowingaboutthemis.Javaagentsareusedmainlyindevelopmentenvironmentsandinoperation.TheyarecomplexruntimetechnologiesthatinteractwiththealreadyrunningJVM.Annotationprocessingistheotherend.AnnotationprocessorsarepluggedintotheJavacompiler.Polyglotprogrammingisinthemiddle.ItisJVMprogramming,justlikeprogramminginJava,butbyusingsomedifferentlanguageor,perhaps,somedifferentlanguageandJavatogether.Orevenmanylanguages,suchasJython,Groovy,Clojure,andJavatogether.

Wewilldiscussthesetechnologiessothatwewillgetsomeideaaboutwhattheyareandwheretolookforfurtherinformationincasewewanttolearnmoreaboutthem.

JavaagentAJavaagentisaJavaprogramthatisloadedbytheJavaruntimeinaspecialwayandcanbeusedtointerferewiththebytecodeoftheloadedclasses,alteringthem.Theycanbeusedto:

Listorlog,andreporttheloadedclassesduringruntime,astheyareloadedModifytheclassessothatthemethodswillcontainextracodetoreportruntimebehaviorSupportdebuggerstoalterthecontentofaclassasthedevelopermodifiesthesourcecode

Thistechnologyisusedin,forexample,theproductsJRebelandXRebelfromhttps://zeroturnaround.com/.

AlthoughJavaagentsworkinthedeepdetailsofJava,theyarenotmagic.TheyareabitcomplexandyouneedadeepunderstandingofJava,butanyonewhocanprograminJavacanwriteaJavaagent.Allthatisrequiredisthattheclass,whichistheagent,hassomepredefinedmethodspackagedintoaJARfilealongwiththeotherclassesoftheagentandhasaMETA-INF/MANIFEST.MFfilethatdefinesthenamesoftheclassesimplementingthepremainand/oragentmainmethods,andsomeotherfields.

ThedetailedandprecisereferencedocumentationispartoftheJDKJavaDocavailableathttp://download.java.net/java/jdk9/docs/api/inthedocumentationofthejava.lang.instrumentpackage.

WhenaJavaapplicationisstartedwithaJavaagent,thecommandlinehastocontainthefollowingoption:

-javaagent:jarpath[=options]

Here,jarpathpointstotheJARfilethatcontainstheagentclassandthemanifestfile.Theclassmusthaveamethodnamedpremainoragentmain.Itmayhaveoneortwoarguments.TheJVMtriestocallthetwo-argumentversionfirstaftertheJVMisinitialized:

publicstaticvoidpremain(StringagentArgs,Instrumentationinst);

Ifatwo-argumentversiondoesnotexist,thentheone-argumentversionisused,whichisessentiallythesameasthetwo-argumentversionbutmissestheinstrumentationargument,which,inmyopinion,doesnotmaketoomuchsensesinceaJavaagentcannotdomuchwithouttheInstrumentationobject:

publicstaticvoidpremain(StringagentArgs);

TheagentArgsparameteristhestringpassedasanoptiononthecommandline.Thesecondargument,Instrumentation,providesmethodstoregisterclasstransformersthatcanmodifyclassbytecodesandalsomethodsthatcanasktheJVMtoperformredefinitionorretransformationofclassesduringruntime.

Javaapplicationscanalsoloadanagentaftertheprogramhasalreadystarted.Insuchacase,theagentcannotbeinvokedbeforethemainmethodoftheJavaapplication,sinceithasalreadystartedbythattime.Toseparatethetwocases,JVMcallsagentmaininsuchascenario.Notethateitherpremainoragentmainisinvokedforanagentandneverboth.Asingleagentcanimplementbothsothatitiscapableofperformingitstaskloadedatthestartup,specifiedonthecommandlineoraftertheJVMstarted.

Ifagentmainisused,ithasthesameargumentsaspremain.

Thereisonemajorandimportantdifferencebetweentheinvocationofpremainandagentmain.Ifanagentcannotbeloadedduringstartup,forexample,ifitcannotbefound,iftheJARfiledoesnotexist,iftheclassdoesnothavethepremainmethod,orifitthrowsanexception,theJVMwillabort.IftheagentisloadedaftertheJVMisstarted(inthiscase,agentmainistobeused),theJVMwillnotabortifthereissomeerrorintheagent.

Thisapproachisfairlyreasonable.ImaginethatthereisaserverapplicationthatrunsontheTomcatservletcontainer.Whenanewversionisstarted,thesystemisdownforamaintenanceperiod.Ifthenewversioncannotbestartedbecausetheagentisnotbehavingwell,thenitisbetternotstarted.Thedamagetodebugthesituationandfixit,orrollbacktheapplicationtotheoldversionandcallforalongerfixingsessionmaybelessthanstartinguptheapplicationandnothavingtheproperagentfunctionality.Iftheapplicationstartsuponlywithouttheagent,thenthesuboptimaloperationmaynotimmediatelyberecognized.Ontheotherhand,whenanagentisattachedlater,theapplicationisalreadyrunning.Anagentisattachedtoanalreadyrunningapplicationtogetinformationfromanalreadyrunninginstance.Tostopthealreadyrunninginstanceandfailit,especiallyinanoperationalenvironment,ismoredamagingthanjustnotattachingtheagent.Itmaynotgounnoticedanywaybecausetheagentthatismostprobablyattachedisusedbyoperationalpersonnel.

ApremainoragentmainagentgetsanInstrumentationobjectasthesecondargument.Thisobjectimplementsseveralmethods.Oneofthemis:

voidaddTransformer(ClassFileTransformertransformer)

Theagentimplementsthetransformer,andithasthetransformmethodsignature:

byte[]transform(Modulemodule,ClassLoaderloader,

StringclassName,

Class<?>classBeingRedefined,

ProtectionDomainprotectionDomain,

byte[]classfileBuffer)

throwsIllegalClassFormatException

ThismethodiscalledbytheJVMwhenaclassisloadedorwhenitistobetransformed.Themethodgetstheclassobjectitself,but,moreimportantly,itgetsthebytearraycontainingthebytecodeoftheclass.Themethodisexpectedtoreturnthebytecodeofthetransformedclass.Modifyingthebytecodeneedssomeknowledgeofhowthebytecodeisbuiltandwhatthestructureofaclassfileis.Therearelibrariesthathelptodothat,suchasJavassist(http://www.javassist.org/)orASM(http://asm.ow2.org/).Nevertheless,Iwillnotstartcodingbeforegettingacquaintedwiththestructureofthebytecode.

Agents,runninginaseparatethreadandpresumablyinteractingwiththeuserorthefilesystemandbaseduponsomeexternalobservationatanytime,maycallthefollowingmethodtoperformtheretransformationoftheclassesusingtheregisteredtransformers:

voidretransformClasses(Class<?>...classes)

Theagentcanalsocallthefollowingmethod,whichwillredefinetheclassesgivenasarguments:

voidredefineClasses(ClassDefinition...definitions)

TheClassDefinitionclassissimplyaClassandabyte[]pair.ThiswillredefinetheclassesthroughtheclassmaintainingmechanismoftheJVM.

NotethatthesemethodsandJavaagentsinteractwiththedeep,low-levelpartoftheJVM.ThisalsobearstheconsequencethatitisveryeasytodestroythewholeJVM.Thebytecodeisnotchecked,unlikeduringtheloadingoftheclass,andthus,ifthereissomeerrorinit,theconsequencemaynotonlybeanexceptionbutalsothecrashingoftheJVM.Also,theredefinitionandthetransformationsshouldnotalterthestructureoftheclasses.Theyshouldnotchangetheirinheritancefootprint,add,rename,orremovemethods,orchangethesignatureofthemethods,andthisisalsotrueforfields.

Alsonotethatthealreadycreatedobjectswillnotbeaffectedbythechanges;theywillstillusetheolddefinitionoftheclassandonlynewinstanceswillbeaffected.

PolyglotprogrammingPolyglotprogrammingisthetechniquewhentherearedifferentprogramminglanguagesusedinthesameapplication.Suchanapproachisnotonlyappropriatewhenadifferentpartoftheapplicationrunsonadifferentenvironment.Forexample,theclientexecutesinthebrowserusingJavaScript,CSS,andHTMLwhiletheserverisprogrammedtoruninaTomcatenvironmentinJava.Thisisadifferentstory,and,usually,thisisnotthetypicalusewhensomeoneisspeakingaboutpolyglotprogramming.

WhentheapplicationthatrunsontheserverpartiallyrunsinJavaandalsoinsomeotherlanguage,thenwecanspeakaboutpolyglotprogramming.Forexample,wecreatetheorderhandlingapplicationinJavaandsomeofthecodethatchecksthecorrectnessoftheorderbasedontheproduct-specificcodesthattheordercontainsiswritteninJavaScript.Doesitringabell?WehavealreadydonethatinthisbooktodemonstratethescriptingAPIoftheJDK.Thatwasrealpolyglotprogramingevenifwedidnotmentionitthatway.

TheJVMthatrunsthecompiledJavacodeisaverygoodtargetfordifferentlanguagecompilers,andthus,therearemanylanguagesthatcompileforit.WhentheJVMrunsthebytecodeofaclass,itdoesnotknowwhatthesourcelanguagewas,anditdoesnotreallycare;somecompilercreatedthebytecodeanditjustexecutesthat.

Wecanusedifferentlanguages,suchasJython,Groovy,andScala,tonameafewpopularonesthatcompilefortheJVM.Wecanwriteoneclassusingonelanguageandtheotheroneusinganother.WhentheyareputtogetherintoaJAR,WAR,oranEARfile,theruntimesystemwilljustrunthem.

Whendoweusepolyglotprogramming?

PolyglotconfigurationUsually,weturntowardspolyglotprogrammingwhenwewanttocreateanapplicationthatismoreflexibleandmoreconfigurable.Applicationsthatgetinstalledinmanyinstances,usually,atdifferentcustomersiteshavesomeconfigurations.TheseconfigurationscanbeXMLfiles,propertiesfiles,andINIfiles(thosecomefromWindows).Astheprogramsdevelopsoonerorlater,thesestaticconfigurationpossibilitiesreachtheirlimits.Applicationdeveloperssoonseethattheyneedtoconfiguresomefunctionalitythatiscumbersometodescribeusingthesetechnologies.Configurationfilesstartbeinglargerand,also,thecodethatreadsandinterpretstheconfigurationfilesgrowlarge.Gooddevelopershavetorealizethatthisisthesituation,andbeforetheconfigurationfilesandthecodehandlingthembecomeunmanageable,somescriptingconfiguration,polyglotprogramminghastobeimplemented.

Decentdeveloperteamsmayreachapointwhentheydeveloptheirconfigurationlanguageandtheinterpreterofthatlanguage.ItcanbebasedonXML,oritcanjustbeanyotherlanguage.Afterall,writingalanguageisfun;Ihavedoneitafewtimesmyself.Mostofthesewere,however,hobbiesandnotprofessionalprojects.Usually,thereisnocustomervalueincraftinganotherlanguage.Wecanbetteruseanexistingone.

Inthecaseofconfiguration,Groovyisaveryhandylanguagethatsupportscomplexclosureandmeta-classsyntaxandimplementation.Thisway,thelanguageisextremelysuitabletocreateadomain-specificlanguage.SinceGroovyiscompiledtoJVM,GroovyclassescanbeinvokeddirectlyfromJava,andintheotherwayround,readingtheconfigurationisessentiallyinvokingtheclasscompiledfromtheconfigurationfile.Thecompilationcanbeduringapplicationbuildtime,butinthecaseofconfiguration,itmakesmoresensetodoitduringapplicationstartup.WehavealreadyseenthattheGroovyimplementationofthescriptingAPIorthespecialAPIthatGroovyprovidesisabsolutelycapableofdoingthat.

Haveweseenexamplesofthisinourbook?Itmaybeasurprisetoyou,butwehaveinfactusedGroovytodescribesomeconfigurationmanytimes.GradlebuildfilesarenothingmorethanGroovyDSLdevelopedmainlyinGroovytosupportprojectbuildconfiguration.

PolyglotscriptingConfigurationisnottheonlyapplicationofpolyglotprogramming.Configurationisexecutedattheprogramstartupandtheconfigurationdataisusedasstaticdataafterwards.Wecanexecutescriptsduringtheapplication'sexecutionanytimeandnotonlyduringitsstartup.Thiscanbeusedtoprovideextrafunctionalitytotheprogram'suserwithinstallationsthatusethesameapplicationbutarefurnishedwithdifferentscripts.

Oneofthefirstapplicationsthatprovidedsuchscriptingcapabilitywastheemacseditor.ThecoreoftheapplicationwaswritteninClanguageanditcontainedaLispinterpreterthatlettheusertowritescripts,whichwereexecutedintheeditorenvironment.Theengineeringprogram,AutoCAD,alsousedaLispinterpreterforsimilarpurposes.WhywasLispusedforthispurpose?Lisphasverysimplesyntax,andtherefore,itiseasytoparseLispcode.Atthesametime,thelanguageispowerful,andlastbutnotleast,therewereopensourceLispinterpreters(atleastone)availablebythetime.

Togetthiskindofflexibility,applications,manytimes,providepluginAPIs,whichadevelopercanusetoextendtheapplication.This,however,requiresthatthedevelopersetsupcodingtools,includingIDE,buildtool,continuousintegration,andsoon,thatis,aprofessionalprogrammingenvironment.Whenthetasktobesolvedbythepluginissimple,theoverheadissimplytoolarge.Insuchacase,ascriptingsolutionishandier.

Scriptingisnotasolutionforeverything.Whenthescriptsextendingtheapplicationtendtobecometoocomplex,itmeansthatthescriptingpossibilityisjusttoomuch.Itisdifficult,however,totakebackatoyfromachild.Ifusersgetusedtothescriptingpossibility,thentheywillnottakeiteasyifthenextversionoftheapplicationwereleasedoesnotprovidethatpossibility.Thus,itisextremelyimportanttoassessthepossibleuseofthescriptingcapabilityinourapplication.Scriptingand,moregenerally,anyfeatureofourprogramwillnotbeusedforwhatweintendedthemfor.Theywillbeusedforwhateveritispossibletousethemfor.Userscangobeyondallimaginationwhenitcomestoabusingsomefeature.Itmaybeagoodideatothinkaboutlimitingthescriptingpossibilitybeforehand,limitingtherunningtimeofthescriptsorthesizeofthescriptourprogramagreestoworkwith.Iftheselimitationsaresetreasonably,andtheusersunderstandandacceptthese,apluginstructureinadditiontothescriptingcapabilityhastobeconsidered.

Thesecurityofanapplication,includingpluginorscriptingextension,isalsoveryimportant.ThescriptsorpluginsrunonthesameJVMasthecoreapplication.Somescriptinglanguagesprovidesomefencearoundthescriptsthatlimitstheaccesstothecoreapplication'sobjectsandclasses,butthisisanexception.Usually,scriptsrunwiththesameprivilegeasthecoreapplicationandthatwaytheycandojustanything.Thus,scriptsshouldbetrustedthesamewayasthecoreapplication.Scriptinstallationormodificationshouldneverbepossibleforanunprivilegeduseroftheapplication.Suchanactionisalmostalwaysreservedforthesystemadministrator.

Ifanunprivilegedusercanuploadascripttotheserverandthenhaveitexecuted,wejustopenedasecurityholeinourapplication.Sinceaccessrestrictionsareenforcedbytheapplication,itiseasyto

overridetheselimitationsusinganuncontrolledscript.Thehackercanjustaccessotherusers'dataeasily,whichheisnotentitledto,andreadandmodifyourdatabase.

BusinessDSLPolyglotprogrammingmayalsocomeintothepicturewhentheapplication'scodecanbeseparatedintobusinesscodeandtechnologycode.Thebusinesscodecontainsthetop-levelbusinesslogicthatweactuallywritetheapplicationfor,andthisisthecodethatcontainsthelogicthatthecustomerpaysfor.ThetechnologycodeistosupportthealgorithmscodedinthebusinessDSL.

Mostoftheenterpriseapplicationscontainthesetwotypesofcodebutmanydonotseparatethem.Thisleadstoamonolithicapplicationthatcontainsrepetitivecode.Whenyoufeelthatyouarewritingthesametypeofcodewhenyouneedpersistenceornetworking,andagainthesametypeofcodewhilecodingsomebusinessrules,thenthisisthecodesmellthatsuggeststhatthetwocodetypesarenotseparated.DSLandscriptingarenotamagicwandanddonotsolvealltheproblemsthatstemfromawrongapplicationstructure.Insuchasituation,thecodehastoberefactoredfirsttoseparatethebusinesslogicandtheinfrastructurecode,anditisonlythesecondsteptoimplementaDSLandabusinessAPIsupportingitandtorewritethebusinesscodeintotheDSL.EverystepofsuchaprojectdeliversvaluefortheapplicationandevenifitnevergetstoDSLandscripting,theeffortinvestedisnotwasted.

ThebusinessDSLscriptingisverysimilartopluggablescripts,exceptthatthistimeitisnottheapplicationthatcallsthescriptsfromtimetotimetoexecutesomespecialextensionfunctionality.Instead,theDSLcodecallstheapplicationthroughthebusinessAPIthatitprovides.TheadvantageofprovidingtheAPIandusingaDSListhatthecodethatimplementsthebusinesslogicgetsridofthetechnicaldetails,canbeveryabstract,and,thisway,bemuchclosertoabusiness-leveldescriptionoftheproblemratherthanjustprogramcode.EvensomebusinesspersoncanunderstandabusinessDSL,andthoughitisnotagoalinreal-lifeexamples,theycouldevenwritecode.

AtTUVienna,wealsousedasimilarapproachtomakesemiconductorsimulationmoreusableforthesemiconductordesignengineer.ThecorecalculatingcodewaswritteninFortran.AClanguageframeworkthathandledthemassivesimulationdatainputandoutputandthatembeddedtheXLISPinterpreterexecutedtheseprograms.TheLispcodecontainedthesimulationconfigurationdataandcouldalsocontainsimpleloopswhenthesimulationwastobeexecutedformanyconfigurationpoints.Itwaspolyglotprogramming,exceptthatwedidnotknowthatthisisgoingtobethenameyearsafterthisapplicationcodingstyle.

ProblemswithpolyglotPolyglotprogrammingisnotonlyallaboutadvantages.Beforejumpingintothisdirection,developersmakingthedecisionhavetoconsideralotofthings.

Usinganotherlanguagefortheapplicationneedsknowledge.FindingpeoplewhocancodeinthelanguagesthatareusediseventuallymoredifficultthanfindingdeveloperswhoonlyknowJava.(ThisisalsotrueifthekernelapplicationlanguageisnotJava.)Differentlanguagesrequiredifferentmindsetsand,manytimes,differentpeople.Theteamshouldalsohavesomememberswhoareproficientinbothlanguages,anditisalsoanadvantageifmostofthepeopleknowatleastabitabouttheotherlanguage.

ThetoolsetsupportingJavaisoutstanding.Thebuildtools,integrateddevelopmentenvironment,libraries,debuggingpossibilities,andloggingframeworks,tonameafew,areallextremelygoodcomparedwithotherlanguages.Polyglotdevelopmentneedssupportfortheotherlanguageaswell,whichmaynotbeasadvancedasthesupportforJava.Manytimes,itisreallyanissuetodebugDSLsolutionsandIDEsupportmayalsobelagging.

WhenweprograminJava,manytimes,wetakeforgrantedthattheIDEreadsthemeta-dataofthelibrariesandwheneverweneedtocallamethod,orreferenceaclass,theIDEsuggeststhebestpossibility.XMLandpropertiesfilesmayalsobesupportedandtheIDEmayknowsomeofthemostusedframeworks,suchasSpring,andunderstandtheXMLconfigurationhandlingthenamesoftheclassesashyperlinks,evenwhentheclassnamesareinsidesomeattributestrings.

Thisisfarfrombeingthiseasyinthecaseofotherlanguages.Forthelanguagesthathaveawideuserbase,thetoolingsupportmaybegood,butifyoupicksomeexoticlanguage,youareonyourown.Themoreexoticthelanguagethelesssupportyoumayhave.

YoucancreatesometooltosupportyourDSLthatyoudevelop.Itisnothardtodosousingtoolssuchashttp://www.eclipse.org/Xtext/.Insuchacase,youaretiedtoEclipse,whichmayormaynotbeaproblem.Youcanpickaspeciallanguage,forexample,Kotlin,whichisextensivelysupportedbyIntelliJ,becausethesamecompanysupportsthelanguageandtheIDE,butagain,youbuyintoaspecialtechnologythatcanbeexpensivetoreplaceincaseyouhaveto.Itisgenerallytruenotonlyforlanguagesbutalsoforanytechnologyyouincludeintoyourdevelopment.Whenyouselectone,youshouldconsiderthesupportandthecostofgettingoffthehorseiforwhenitstartsdying.

AnnotationprocessingWehavealreadydiscussedannotationsingreatdetail.Youmayrecallthatwedefinedourannotationinterfacesusingthefollowingannotation:

@Retention(RetentionPolicy.RUNTIME)

ThistoldtheJavacompilertokeeptheannotationandputitintotheJVMcodesothatthecodecanaccessitduringruntimeusingreflection.ThedefaultvalueisRetentionPolicy.CLASS,whichmeansthattheannotationgetsintothebytecode,buttheJVMdoesnotmakeitavailablefortheruntimesystem.IfweuseRetentionPolicy.SOURCE,theannotationdoesnotevengetintotheclassfile.Inthiscase,thereisonlyonepossibilitytodoanythingwiththeannotation:compiletime.

Howcanwewritecodethatrunsduringcompiletime?Javasupportsthenotionofannotationprocessors.Ifthereisaclassontheclasspathofthecompilerthatimplementsthejavax.annotation.processing.Processorinterface,thenthecompilerwillinvoketheimplementedmethodsoneormoretimes,passinginformationaboutthesourcefilethatthecompilerisactuallyprocessing.Themethodswillbeabletoaccessthecompiledmethods,classes,orwhateverisannotated,andalsotheannotationthattriggeredtheprocessorinvocation.Itisimportant,however,thatthisaccessisnotthesameasinruntime.Whattheannotationprocessoraccessesisneitheracompilednoraloadedclass,thatis,itisavailablewhenthecodeusesreflection.Thesourcefileatthistimeisundercompilation;thus,thedatastructuresthatdescribethecodeareactuallystructuresofthecompiler,aswewillseeinournextexample.

Theannotationprocessoriscalledoneormoretimes.Thereasonitisinvokedmanytimesisthatthecompilermakesitpossiblefortheannotationprocessorstogeneratesourcecodebasedonwhatitseesinthepartiallycompiledsourcecode.IftheannotationprocessorgeneratesanyJavasourcefile,thecompilerhastocompilethenewsourcecodeandperhapscompilesomeofthealreadycompiledfilesagain.Thisnewcompilationphaseneedsannotationprocessorsupportuntiltherearenomoreroundstoexecute.

Annotationprocessorsareexecutedoneaftertheother,andtheyworkonthesamesetofsourcefiles.Thereisnowaytospecifytheorderoftheannotationprocessorexecutions;thus,twoprocessorsworkingtogethershouldperformtheirtasks,nomatterinwhatordertheyareinvoked.Also,notethatthesecodesruninsidethecompiler.Ifanannotationprocessorthrowsanexception,thenthecompilationprocesswillmostprobablyfail.Thus,throwinganexceptionoutoftheannotationprocessorshouldonlybedoneifthereisanerrorthatcannotberecoveredandtheannotationprocessordecidesthatthecompilationafterthaterrorcannotbecomplete.

Whenthecompilergetstothephasetoexecutetheannotationprocessors,itlooksattheclassesthatimplementthejavax.annotation.processing.Processorinterfaceandcreatesinstancesoftheseclasses.Theseclasseshavetohaveapublicno-argumentconstructor.Tostreamlinetheexecutionoftheprocessorsandtoinvokeaprocessoronlyfortheannotationsthatitcanhandle,theinterfacecontainstwomethods:

getSupportedSourceVersiontoreturnthelatestversiontheannotationprocessorcansupportgetSupportedAnnotationTypestoreturnasetofStringobjectscontainingthefullyqualifiedclassnameof

theannotationsthatthisprocessorcanhandle

IfanannotationprocessorwascreatedforJava1.8,itmayworkwithJava9,butitmayalsonotwork.Ifitdeclaresthatthelatestsupportedversionis1.8,thenthecompilerinaJava9environmentwillnotinvokeit.Itisbetternottoinvokeanannotationprocessorthancallingitandmessingupthecompilationprocess,whichmayevencreatecompiledbuterroneouscode.

Thevaluesreturnedbythesemethodsarefairlyconstantforanannotationprocessor.Anannotationprocessorwillreturnthesamesourceversionitcanhandleandwillreturnthesamesetofannotations.Therefore,itwouldbeclevertohavesomewaytodefinethesevaluesinthesourcecodeinadeclarativemanner.

Itcanbedonewhenweextendthejavax.annotation.processing.AbstractProcessorclassinsteadofdirectlyimplementingtheProcessorinterface.Thisabstractclassimplementsthesemethods.Bothofthemgettheinformationfromtheannotationsothatwecandecoratetheclassthatextendstheabstractclass.Forexample,thegetSupportedAnnotationTypesmethodlooksattheSupportedAnnotationTypesannotationandreturnsanarrayofannotationtypestringsthatarelistedintheannotation.

Now,thisisabitbraintwistingandcanalsobeconfusingatfirst.Weareexecutingourannotationprocessorduringcompiletime.ButthecompileritselfisaJavaapplication,andinthisway,thetimeisruntimeforthecodethatrunsinsidethecompiler.ThecodeofAbstractProcessoraccessestheSupportedAnnotationTypesannotationasaruntimeannotationusingreflectionmethods.Thereisnomagicinit.ThemethodintheJDK9isasfollows:

publicSet<String>getSupportedAnnotationTypes(){

SupportedAnnotationTypessat=this.getClass().getAnnotation

(SupportedAnnotationTypes.class);

if(sat==null){

...errormessageissenttocompileroutput...

returnCollections.emptySet();

}

else

returnarrayToSet(sat.value());

}

(Thecodehasbeeneditedforbrevity.)

Tohaveanexample,wewillsortoflookatthecodeofapolyglotannotationprocessor.Ourverysimpleannotationprocessorwillprocessonesimpleannotation:com.javax0.scriapt.CompileScript,whichcanspecifyascriptfile.TheannotationprocessorwillloadthescriptfileandexecuteitusingthescriptinginterfaceofJava9.

ThiscodewasdevelopedasademonstrationcodebytheauthorofthisbookafewyearsagoandisavailablewiththeApachelicensefromGitHub.Thus,thepackageoftheclassesisretained.

Theannotationprocessorcontainstwocodefiles.Oneoftheannotationitselfthattheprocessorwillworkon:

@Retention(RetentionPolicy.SOURCE)

@Target(ElementType.TYPE)

public@interfaceCompileScript{

Stringvalue();

Stringengine()default"";

}

Asyoucansee,thisannotationwillnotgetintotheclassfileaftercompilation;thus,therewillbenotraceduringruntimesothatanyclasssourcemayoccasionallyusethisannotation.TargetoftheannotationisElementType.TYPE,meaningthatthisannotationcanonlybeappliedtothoseJava9languageconstructsthataresomekindoftypes:class,interface,andenum.

Theannotationhastwoparameters.Thevalueshouldspecifythenameofthescriptfile,andtheenginemayoptionallydefinethetypeofthescriptthatisinthatfile.Theimplementationwe'llcreatewilltrytoidentifythetypeofthescriptfromthefilenameextension,butifsomebodywouldliketoburysomeGroovycodeintoafilethathasthe.jyextension(whichisusuallyforJython),sobeit.

TheprocessorextendsAbstractProcessorand,inthisway,someofthemethodsareinheritedattheexpenseofsomeannotationsusedintheclass:

packagecom.javax0.scriapt;

import...

@SupportedAnnotationTypes("com.javax0.scriapt.CompileScript")

@SupportedSourceVersion(SourceVersion.RELEASE_9)

publicclassProcessorextendsAbstractProcessor{

ThereisnoneedtoimplementthegetSupportedAnnotationTypesandgetSupportedSourceVersionmethods.Thesearereplacedbytheuseoftheannotationsontheclass.Wesupportonlyoneannotationinthisprocessor,theonethatwedefinedinthepreviouslylistedsourcefile,andwearepreparedtomanagethesourcecodeuptoJavaversion9.Theonlymethodwehavetooverrideisprocess:

@Override

publicbooleanprocess(

finalSet<?extendsTypeElement>annotations,

finalRoundEnvironmentroundEnv){

for(finalElementrootElement:

roundEnv.getRootElements()){

try{

processClass(rootElement);

}

catch(Exceptione){

thrownewRuntimeException(e);

}

}

returnfalse;

}

Thismethodgetstwoarguments.Thefirstisthesetofannotationsthatitwasinvokedfor.Thesecondistheroundenvironment.Becausetheprocessorcanbeinvokedmanytimes,thedifferentinvocationsmayhavedifferentenvironments.EachinvocationisinaroundandtheRoundEnvironmentargumentisanobjectthatcanbeusedtogetinformationaboutthegivenround.Itcanbeusedtogettherootelementsoftheroundforwhichthisannotationisinvoked.Inourcase,thiswillbeasetofclasselementsthathavetheCompileScriptannotation.Weiterateoverthisset,andforeachclass,weinvoketheprocessClassmethod(seethenextcodesnippet).Themethodmaythrowsomecheckedexceptionandthemethodprocesscannotbecauseitshouldmatchthesamemethodoftheinterface.Thus,wecatchanyexceptionthatmaybethrownandwere-throwtheseencapsulatedinRunTimeException.Ifanyoftheseexceptionsarethrownbythecalledmethod,thenthecompilationcouldnotrunthescriptsanditshouldbetreatedasfailed.Thecompilationshouldnotsucceedinsuchacase:

privatevoidprocessClass(finalElementelement)

throwsScriptException,FileNotFoundException{

for(finalAnnotationMirrorannotationMirror:

element.getAnnotationMirrors()){

processAnnotation(annotationMirror);

}

}

Theactualannotationisnotavailableduringcompiletimeaswealreadymentioned.Hence,whatwehaveavailableisonlyacompiletimemirrorimageoftheannotation.IthastheAnnotationMirrortype,whichcanbeusedtogettheactualtypeoftheannotationand,also,thevaluesoftheannotation.Thetypeoftheannotationisavailableduringcompiletime.Thecompilerneedsit;otherwise,itcouldnotcompiletheannotation.Thevaluesareavailablefromtheannotationitself.OurprocessAnnotationmethodhandleseachannotationitgetsasanargument:

privatevoidprocessAnnotation(

finalAnnotationMirrorannotationMirror)

throwsScriptException,FileNotFoundException{

finalStringscript=

FromThe.annotation(annotationMirror).

getStringValue();

finalStringengine=

FromThe.annotation(annotationMirror).

getStringValue("engine");

execute(script,engine);

}

Our@CompileScriptannotationdefinestwoparameters.Thefirstvalueisthescriptfilenameandthesecondoneisthescriptingenginename.Ifthesecondoneisnotspecified,thenanemptystringissetasthedefaultvalue.Theexecutemethodiscalledforeachandeveryoccasionoftheannotation:

privatevoidexecute(finalStringscriptFileName,

finalStringengineName)

throwsScriptException,FileNotFoundException{

finalScriptEngineManagerfactory=

newScriptEngineManager();

finalScriptEngineengine;

if(engineName!=null&&engineName.length()>0){

engine=factory.getEngineByName(engineName);

}

else{

engine=

factory.getEngineByExtension

(getExtensionFrom(scriptFileName));

}

ReaderscriptFileReader=newFileReader

(newFile(scriptFileName));

engine.eval(scriptFileReader);

}

Themethodtriestoloadthescript,basedonthefilename,andtriestoinstantiatethescriptengine,basedonthegivenname.Ifthereisnonamegiven,thenthefilenameextensionisusedtoidentifythescriptingengine.Bydefault,theJavaScriptengineisontheclasspathasitispartoftheJDK.IfanyotherJVM-basedscriptingengineisinuse,thenithastobemadeavailableontheclasspathoronthemodulepath.

Thelastmethodoftheclassisasimplescriptmanipulationmethod,nothingspecial.Itjustchopsoffthefilenameextensionsothattheenginecanbeidentifiedbasedontheextensionstring:

privateStringgetExtensionFrom(finalStringscriptFileName){

finalintindexOfExtension=scriptFileName.lastIndexOf('.');

if(indexOfExtension==-1){

return"";

}

else{

returnscriptFileName.substring(indexOfExtension+1);

}

}

Andjustforthesakeofcompleteness,wehavetheclosingbraceoftheclass:

}

ProgrammingintheenterpriseWhenaprofessionalworksforanenterprise,shedoesnotworkalone.Therearealotofpeople,developersaswellasothercoworkers,wehavetocooperatewith.TheoldertheITdepartmentoftheenterpriseis,andthelargertheenterpriseis,themorespecializedrolespeoplearein.Youwillcertainlymeetbusinessanalysts,projectmanagers,testengineers,buildengineers,subject-matterexperts,testers,architects,scrummasters,andautomationengineers,tonameafewroles.Someoftheserolesmayoverlap,nopersonmayhavemorethanoneresponsibility,andwhileinothercases,somerolescouldevenbemorespecialized.Someoftherolesareverytechnicalandrequirelessbusiness-relatedknowledge;othersaremorebusinessoriented.

Workingtogetherasateamwithsomanypeopleandwithsomanydifferentrolesisnotsimple.Thecomplexityofthetaskmaybeoverwhelmingforanovicedeveloperandcannotbedonewithoutdefinitepoliciesthatallmembersoftheoperationfollow,moreorless.Perhapsyourexperiencewillshowthatitismoretimeslessthanmore,butthatisadifferentstory.

Forthewaydevelopersworktogether,therearewell-establishedindustrypractices.ThesesupporttheSoftwareDevelopmentLifecycle(SDLC)usingwaterfall,agile,oramixofthetwomodelsinsomeway.Inthefollowingsections,wewilllookattoolsandtechniquesthatare,oratleastshouldhavebeen,usedineverysoftwaredevelopmentorganization.Theseare:

StaticcodeanalysistoolsthatcontrolthequalityofthecodeexaminingthesourcecodeSourcecodeversioncontrolthatstoresalltheversionsofthesourcecodeandhelpgetthesourcecodeforanyoldversionofthedevelopmentSoftwareversioningtokeepsomeorderofhowweidentifythedifferentversionsanddonotgetlostamongthedifferentversionsCodereviewandtoolsthathelpinpin-pointingbugsthatarenotrevealedbytestsandaidknowledgesharingKnowledgebasetoolstorecordanddocumentthefindingsIssuetrackingtoolsthatrecordbugs,customerissues,andothertasksthatsomebodyhastoattendtoSelectionprocessandconsiderationsforexternalproductsandlibrariesContinuousintegrationthatkeepsthesoftwareinaconsistentstateandreportsimmediatelyifthereissomeerrorinitbeforetheerrorpropagatestootherversionsorothercode,dependingonhowtheerroneouscodegetsdevelopedReleasemanagement,whichkeepstrackofthedifferentreleaseversionsofthesoftwareCoderepository,whichstoresthecompiledandpackedartifacts

Thefollowingdiagramshowsthemostwidelyusedtoolsforthesetasks:

StaticcodeanalysisStaticcodeanalysistoolsreadthecodejustlikethecompilerandanalyzeit,butinsteadofcompilation,theytrytofinderrorsormistakesinit.Notthesyntaxerrors.Forthat,wealreadyhavetheJavacompiler.Mistakes,suchasusingaloopvariableoutsidealoop,whichmaybeabsolutelyvalidbutisusuallybadstyleand,manytimes,suchusagecomesfromsomesimplemistakes.Theyalsocheckthatthecodefollowsthestylingrulesthatweset.

Staticcodeanalyzershelpidentifymanysmallandobviouserrorsinthecode.Sometimes,theyareannoying,warningaboutsomethingthatmaynotbereallyaproblem.Insuchacase,itisbettertocodetheprogramabitdifferently,notbecausewewantthestaticcodeanalysistorunwithoutwarning.Weshouldnevermodifythecodebecauseofatool.Ifwecodesomethinginsuchawaythatitpassessomequalitychecktoolandnotbecauseitisbetterthatway,thenweareservingthetoolsinsteadofthetoolsservingus.

Thereasontochangethecodetopassthecodeanalysisisthatitisveryprobablethatthecodeismorereadabletoanaverageprogrammerifitdoesnotviolatethecodingstyle.Youortheotherteammemberscanbeexcellentprogrammerswhounderstandthecodeveryeasilyevenifitusessomespecialconstruct.However,youcannotsaythataboutalltheprogrammerswhowillmaintainyourcodeinthefuture.Thecodelivesalonglife.Iworkwithsomeprogramsthathavebeenwritten50yearsago.Theyarestillrunningandmaintainedbyyoungprofessionalsaroundtheageof30.Itmeansthattheywerenotevenbornwhenthecodewasdeveloped.Itcaneasilyhappenthatthepersonmaintainingyourcodeisnotevenbornbythetimeyouwritethecode.Youcannottellanythingabouttheirclevernessandcodingpractices.Thebestwecandoistopreparefortheaverageandthatisexactlywhatstaticcodeanalysistoolsaresetfor.

Thechecksthatthesetoolsperformarenothardwiredintothetools.Somespeciallanguageinsidethetoolsdescribestherulesandtheycanbedeleted,otherrulescanbeadded,andrulescanbemodified.Thisway,youcanaccommodatethecodingstandardsoftheenterpriseyouworkfor.Thedifferentrulescanbecategorizedascosmetic,minor,major,andcritical.Cosmeticthingsaremainlywarningsandwedonotreallycareaboutthem,eventhoughitisnicetofixeventheseissues.Sometimes,thesesmallthingsmaysignalsomereallybigissue.Wecansetlimitsforthenumberofminorandmajorbugsbeforethecheckisdeclaredasfailingandalsoforthecriticalerrors.Inthelastcase,thislimitisusuallyzero.Ifacodingerrorseemstobecritical,thenbetternothaveanyinthecode.

ThemostfrequentlyusedtoolsareCheckstyle,FindBugs,andPMD.Theexecutionofthesetoolsisusuallyautomated,andthoughtheycanbeexecutedfromtheIDEorfromthedeveloper'scommandline,theirmainuseisonthecontinuousintegration(CI)server.Duringthebuild,thesetoolsareconfiguredontheCIservertorun,anditcanbeconfiguredsuchthatthebuildshouldbebrokenifthestaticcodeanalysisfailswithsomelimit.Executingthestaticcodeanalysisisusuallythenextstepaftercompilationandunittestexecution,andbeforetheactualpackaging.

TheSonarQubetool(https://www.sonarqube.org/)isaspecialtoolinadditiontobeingastaticcodeanalysistool.SonarQubemaintainsthehistoryofthepreviouschecksaswellassupportsunittestcodecoverageandcanreportthechangeofthequalityovertime.Thisway,youcanseehowthequality,coveragepercentage,andnumberofdifferentqualificationsofcodestyleerrorshavechanged.Manytimes,youcan

seethatwhenapproachingthereleasedate,thecodequalitydecreasesaspeopleareinarush.Thisisverybadbecausethisisthetimewhenmostofthebugsshouldbeeliminated.Havingastatisticaboutthequalitymayhelpchangethepracticebyseeingthetrendsbeforethequality,andthusthemaintainabilityofthecodegetsoutofhand.

SourcecodeversioncontrolSourcecodeversioncontrolsystemsstoredifferentversionsofthesourcecode.Thesedays,wecannotimagineprofessionalsoftwaredevelopmentwithoutit.Thiswasnotalwaysthecase,buttheavailabilityoffreeonlinerepositoriesencouragedhobbydeveloperstousesomeversioncontrol,andwhenthesedevelopersworkedforenterpriseslater,itwasevidentthattheuseofthesesystemsiskindofamust.

Therearemanydifferentrevisioncontrolsystems.ThemostwidelyusedoneisGit.TheversioncontrolthatwaspreviouslywidelyusedwasSVNand,evenbeforethat,CVS.Thesearelessandlessusedthesedays.WecanseeSVNasasuccessorofCVSandGitasasuccessorofSVN.Inadditiontothese,thereareotherversioncontrolsystemssuchasMercurial,Bazaar,orVisualStudioTeamServices.Foracomprehensivelistoftheavailabletools,visittheWikipediapageathttps://en.wikipedia.org/wiki/List_of_version_control_software.

MybetisthatyouwillmeetGitinthefirstplaceandthereisahighprobabilityofyoucomingacrossSVNwhenprogrammingforanenterprise.Mercurymayappearinyourpracticebutanyoftheothersthatcurrentlyexistareveryrare,areusedforaspecificarea,oraresimplyextinct.

Versioncontrolsystemsallowthedevelopmentteamtostorethedifferentversionsofthesoftwareinanorganizedmanneronastoragethatismaintained(backedupregularlyinareliablemanner).Thisisimportantfordifferentpurposes.

Thefirstthingisthatdifferentversionsofthesoftwaremaybedeployedtodifferentinstances.Ifwedevelopsoftwareforclientsandwehavemanyclientswithwhomwehopetohavetomakeaterrificbusiness,thendifferentclientsmayhavedifferentversions.Thisisnotonlybecausesomeclientsarereluctanttopayfortheupgrade,andwejustdonotwanttogivethenewversionforfree.Manytimes,thecoststhatriseonthesideofthecustomerpreventtheupgradeforalongtime.Softwareproductsdonotworkontheirowninanisolatedenvironment.Differentclientshavedifferentintegratedenvironments;thesoftwarecommunicateswithdifferentotherapplications.Whenanewversionistobeintroducedinanenterpriseenvironment,ithastobetestedforwhetheritworkswithallthesystemsithastocooperatewith.Thistestingtakesalotofeffortandmoney.Ifthenewfeaturesorothervaluesthatthenewversiondeliversovertheoldonedonotjustifythecost,thenitwouldbewasteofmoneytodeploythenewversion.Thefactthatthereisanewversionofoursoftwaredoesnotmeanthattheoldversionsarenotusable.

Ifthereissomebugatthecustomer'send,thenitisvitalthatwefixthebuginthatversion.Todoso,thebughastobereproducedinthedevelopmentenvironment,whicheventuallymeansthatthesourcecodeforthatversionhastobeavailableforthedevelopers.

Thisdoesrequirethecustomerdatabasetocontainreferencestothedifferentversionsofoursoftwareproductsthatareinstalledatthecustomersite.Tomakeitmorecomplicated,acustomermayhavemorethanoneversionatatimeindifferentsystemsandmayalsohavedifferentlicenses,sotheissueismorecomplexthanitfirstseems.Ifwedonotknowwhichversiontheclienthas,thenweareintrouble.Sincethedatabaseregisteringtheversionsforthecustomersandreallifemayget

unsynchronized,softwareproductslogtheirversionatstartup.Wehaveaseparatesectionaboutversioninginthischapter.

Ifthebugisfixedintheversionthattheclienthas,theincidentatthecustomer'sendmaybesolvedafterdeployment.Theproblem,though,stillremainsiftheversionisnotthepreviousversionofthesoftware.Thebugfixintroducedtoanoldversionofthesoftwaremaystillbelurkingaroundinthelateror,forthatmatter,earlierversions.Thedevelopmentteamhastoidentifywhichversionsarerelevanttoclients.Forexample,anoldversionthatisnotinstalledanymoreatanyoftheclients'sitesdoesnotdeservetheinvestigation.Afterthat,therelevantversionshavetobeinvestigatedtocheckwhethertheyexhibitthebug.Thiscanonlybedoneifwehavethesourceversion.Someoldversionsmaynothavethebugifthecodecausingthebugisintroducedinlaterversions.Somenewversionsmayalsobeimmunetothebugbecausethebugwasalreadyfixedinthepreviousversion,orsimplybecausethepieceofcodethatcausedthebugwasrefactoredevenbeforethebugmanifested.Somebugsmayevenaffectaspecificversioninsteadofarangeofproducts.Bigfixingmaybeappliedtodifferentversionsandtheymayneedslightlydifferentfixes.Allthisneedsamaintainedsourceversionrepository.

Evenwhenwedonothavedifferentcustomerswithdifferentversions,itismorethanlikelythatwehavemorethanoneversionofoursoftwareindevelopment.Thedevelopmentofamajorreleaseiscomingtoanend,andtherefore,onepartoftheteamresponsiblefortestingandbugfixingfocusesonthoseactivities.Atthesametime,thedevelopmentoffeaturesforthenextversionstillgoeson.Thecodeimplementingthefunctionalitiesforthenextversionshouldnotgetintotheversionthatisabouttobereleased.Thenewcodemaybeveryfresh,untested,andmayintroducenewbugs.Itisverycommontointroducefreezetimesduringthereleaseprocess.Forexample,itmaybeforbiddentoimplementanynewfeatureoftheupcomingrelease.Thisiscalledfeaturefreeze.

Revisioncontrolsystemsdealwiththesefreezeperiods,maintainingdifferentbranchesofthecode.Thereleasewillbemaintainedinonebranchandtheversionforlaterreleasesinadifferentone.Whenthereleasegoesout,thebugfixesthatwereappliedtoitshouldalsobepropagatedtothenewerversion;otherwise,itmightsohappenthatthenextversionwillcontainbugsthatwerealreadyfixedinthepreviousversion.Todoso,thereleasebranchismergedwiththeongoingone.Thus,versioncontrolsystemsmaintainagraphoftheversions,whereeachversionofthecodeisanodeinthegraphandthechangesarevertices.

Gitgoesveryfarinthisdirection.Itsupportsbranchcreationandmergingsowellthatdeveloperscreateseparatebranchesforeachchangethattheycreateandthentheymergeitbackwiththemasterbranchwhenthefeaturedevelopmentisdone.Thisalsomakesforagoodopportunityforcodereview.ThedevelopermakingthefeaturedevelopmentorbugfixcreatesapullrequestintheGitHubapplication,andanotherdeveloperisrequestedtoreviewthechangeandperformthepull.Thisisakindoffour-eyedprincipleappliedtocodedevelopment.

Someoftherevisioncontrolsystemskeeptherepositoryonaserverandanychangegetstotheserver.Theadvantageofthisisthatanychangecommittedgetstoaserverdiskthatisregularlybackedupandisthussafe.Sincetheserver-sideaccessiscontrolled,anycodesenttotheservercannotberolledbackwithouttrace.Allversions,eventhewrongversions,arestoredontheserver.Thismayberequiredbysomelegalcontrol.Ontheotherhand,ifcommitrequiresnetworkaccessandserverinteraction,itmaybeslowandthiswill,inthelongrun,motivatedevelopersnottocommittheirchangesfrequently.Thelonger

achangeremainsonthelocalmachine,themoreriskwehaveoflosingsomeofthecode,andmergingbecomesmoreandmoredifficultwithtime.Tohealthissituation,Gitdistributestherepositoryandthecommithappenstothelocalrepository,whichisexactlythesameastheremoteoneonsomeserver.Therepositoriesaresynchronizedwhenonerepositorypushesthechangestoanotherone.Thisencouragesthedeveloperstomakefrequentcommitstotherepository,givingshortcommitmessages,whichhelpsintrackingthechangemadetothecode.

Someolderversioncontrolsystemssupportfilelocking.Thisway,whenadeveloperchecksoutacodefile,otherscannotworkonthesamepieceofcode.Thisessentiallyavoidsthecollisionsduringcodemerging.Overtheyears,thisapproachdidnotseemtofitthedevelopmentmethodologies.Mergeissuesarelessofaproblemthanfilesthatarecheckedoutandforgotten.SVNsupportsfilelockingbutthisisnotreallyseriousanddoesnotpreventonedevelopertocommitchangestoafilethatsomebodyelselocked.Itismoreofonlyasuggestionthanreallocking.

Sourcecoderepositoriesareveryimportantbutshouldnotbeconfusedwithreleaserepositories,whichstorethecompiledreleasedversionofthecodeinbinary.Sourceandreleaserepositoriesworktogether.

SoftwareversioningSoftwareversioningismagic.ThinkaboutthedifferentversionsofWindowsorStarWarsmovies.Well,thelatterisnotreallysoftwareversioningbutitshowsthattheissueisverygeneral.InthecaseofJava,versioningisnotthatcomplex.Firstofall,theversionofJavaweusenowis9.Thepreviousversionwas1.8,beforethat1.7,andsoon,downto1.0.EarlierversionsofJavawerecalledOakbutthatishistory.Afterall,thatis,whocantellwhatJava2was?

Fortunately,whenwecreateaJavaapplication,thesituationissimpler.TherehasbeenasuggestionfromOracle,fromthetimeofJava1.3,abouthowtoversionJARs:

http://docs.oracle.com/javase/7/docs/technotes/guides/extensions/versioning.html

Thisdocumentdistinguishesbetweenspecificationversionandimplementationversion.IfthespecificationofaJARcontentchanges,thecodehastobehavedifferentlyfromhowitwasbehavingtillthen;thespecificationversionshouldchange.Ifthespecificationisnotchangedbuttheimplementationdoes--forexample,whenwefixabug--thentheimplementationversionchanges.

Inpractice,nobodyhasusedthisscheme,althoughitisabrilliantideatoseparatetheimplementationandspecificationversions,atleast,intheory.Ievenbetthatmostofyourcolleagueshavenoteveneverheardaboutthisversioning.Whatweuseinpracticeissemanticversioning.

Semanticversioning(http://semver.org/)mixesthespecificationandimplementationversionsintoonesingleversionnumbertriplet.Thistriplethastheformatofmmp,thatis:

m:majorversionnumberm:minorversionnumberp:patchnumber

Thespecificationsaysthatthesenumbersstartwithzeroandincreasebyone.Ifthemajornumberiszero,itmeansthatthesoftwareisstillindevelopment.Inthisstate,theAPIisnotstableandmaychangewithoutanewmajorversionnumber.Themajorversionnumbergetsto1whenthesoftwareisreleased.LaterithastobeincreasedwhentheAPIoftheapplication(library)haschangedfromthepreviousversionandtheapplicationisnotbackwardcompatiblewiththepreviousversion.Theminorversionnumberisincreasedwhenthechangeeffectsonlytheimplementationbutthechangeissignificant,perhaps,eventheAPIisalsochangingbutinabackward-compatiblemanner.Thepatchversionisincreasedwhensomebugisfixed,butthechangeisnotmajorandtheAPIdoesnotchange.Theminorandthepatchlevelshavetoberesettozeroifanyversionnumberinthetripletinfrontofanyofthemisincreased:majorversionnumberincreaseresetsbothminorandpatchversion;minorversionnumberincreaseresetspatchnumber.

Thisway,semanticversioningkeepsthefirstelementofthetripletforthespecificationversion.Theminorisamixofthespecificationandimplementationversions.Apatchversionchangeisclearlyanimplementationversionchange.

Inadditiontothese,semanticversioningallowsappendingapre-releasestring,suchas-RC1and-RC2.Italsoallowstheappendingofmetadata,suchasadateafteraplussign,forexample,+20160120asadate.

Theuseofsemanticversioninghelpsthosethatusethesoftwaretoeasilyspotcompatibleversionsandtoseewhichversionisolderandwhichisnewer.

CodereviewWhenwecreateprogramsinaprofessionalway,itisdoneinteams.Thereisnoone-manshowprogrammingotherthaninahobbyorgoingalongwiththetutorials.Itisnotonlybecauseitismoreeffectivetoworkinteamsbutalsobecauseonepersonisvulnerable.Ifyouworkaloneandgethitbythebusoryouhitthelotteryandloseyourabilityormotivationtoworkontheproject,yourcustomerisintrouble.Thatisnotprofessional.Professionalprojectsshouldberesilienttoanymemberfallingoff.

Teamworkneedscooperationandoneformofcooperationiscodereview.Thisistheprocesswhenadeveloperoragroupofdevelopersreadsapartofthecodethatsomeotherteammembershavewritten.Therearedirectgainsfromthisactivity;

Thedevelopersreadingthecodegetmoreknowledgeaboutthecode;theylearnthecode.Thisway,ifthedevelopercreatingthecodegetsoutoftheprocessforanyreason,theotherscancontinuetheworkwithminimalbump.Codingstylescanbealigned.Developers,evenseniors,payingcarefulattentionmakecodingmistakes.Thismaybeabugoritmaybeacodingstyleviolation.Codingstyleisimportantbecausethemorereadablethecodeis,thelesspossibilityofithavingunnoticedbugs.(Alsoseethenextbulletpoint.)Itisalsoimportantthatthecodingstyleisthesamefortheteam.Allteammembersshouldusethesamestyle.LookingatacodethathasadifferentstylefromtheoneIwroteisabithardertofollowandunderstand.Thedifferencesmaydistractthereaderandtheteammembershavetobeabletoreadthecode.Thecodebelongstotheteamandnotasingledeveloper.Anyteammembershouldknowthecodeandbeabletomodifyit.Duringcodereview,alotofbugscanbediscovered.Thepartieslookingatthecodeandtryingtounderstandtheworkingofitmayoccasionallydiscoverbugsfromthestructureofthecode,whichareotherwisehardtodiscoverusingtests.Ifyouwant,codereviewisthewhitestwhiteboxtest.Peoplethinkdifferentlyanddifferentmindsetscatchdifferentbugs.

Codereviewcanbedoneonlineandoffline.Itcanbedoneinteamsorpeer-to-peer.

MostteamsfollowthecodereviewprocessthatGitHubsupports,whichisthesimplest.Changestothecodearecommittedtoabranchandarenotmergedwiththecodedirectlybut,rather,apullrequestiscreatedonthewebinterface.Thelocalpolicymayrequirethatadifferentdeveloperperformthepull.Thewebinterfacewillhighlightthechangesandwecanaddcommentstothechangedcode.Ifthecommentsaresignificant,thentheoriginaldeveloperrequestingthepullshouldmodifythecodetoanswerthecommentsandrequestthepullagain.Thisensuresthatatleasttwodevelopersseeanychange;theknowledgeisshared.

Feedbackispeer-to-peer.Itisnotaseniorteachingajunior.Thatneedsadifferentchannel.CommentsinGitHubarenotgoodforthispurpose;atleast,therearebetterchannels.Perhapstalkingfacetoface.Commentsmaycomefromaseniortoajuniororfromajuniortoasenior.Inthiswork,givingfeedbackonthequalityofthecode,seniorsandjuniors,areequal.

Thesimplestandperhapsthemostfrequentcommentisthefollowing:IcanseethatXyz.javawaschangedinthemodificationbutIseenochangemadeto

XyzTest.java.Thisisalmostaninstantrefusalforthemerge.Ifanewfeatureisdeveloped,unittestshavetobecreatedtotestthatfeature.Ifabugisfixed,thenunittestshavetobecreatedtopreventthebugfromcomingback.Ipersonallygotthiscommentmanytimes,evenfromjuniors.Oneofthemtoldme,"Weknowthatyouweretestingusifwedaredtogivefeedback."GodknowsIwasnot.Theydidnotbelieve.

WhilechangereviewandGitHubisagoodtoolduringdevelopment,itmaynotbeappropriatewhenalargerchunkofcodehastobereviewed.Insuchacase,othertools,suchasFishEye,havetobeused.Inthistool,wecanselectthesourcefilesforrevieweveniftheywerenotrecentlychanged.Wecanalsoselectreviewersanddeadlines.CommentingissimilartoGitHub.Finally,thistypeofcodereviewfinisheswithacodereviewsession,wherethedevelopersgatheranddiscussthecodeinperson.

Whileorganizingsuchasession,itisimportantthatapersonwhohasexperiencemanagingotherpeoplemediatesthesesessions.Codeanddiscussiononstylescangetverypersonal.Atthesametime,whenattendingsuchameeting,youshouldalsopayattentionsoasnottogetpersonal.Therewillbeenoughparticipantswhomaynotknowthisorarelessdisciplined.

Neverattendareviewsessionwithoutreviewingthecodefirstusingtheonlinetools.Whenyoumakecomments,thelanguageshouldbeverypoliteforthereasonIhavealreadymentioned.Finally,themediatorofthemeetingshouldbeabletoseparateimportantandnotsoimportantissuesandtostopdebateonbagatellethings.Somehow,thelessimportantissuesaremoresensitive.Ipersonallydonotcareaboutformattingthetabsizeifitistwoorfourspacesandifthefileshouldcontainonlyspacesoriftabcharactersareallowed,butpeopletendtoliketowastetimeonsuchissues.

ThemostimportantissueduringcodereviewsessionsisthatweareprofessionalanditmayhappenthatIreviewandcommentyourcodetoday,buttomorrow,itwillbejusttheopposite,andweworktogetherandwehavetoworktogetherasateam.

KnowledgebaseKnowledgebasewasabuzzwordafewyearsago.Fewcompanieswereevangelizingtheideaofwikitechnologyandnobodywasusingit.Today,thelandscapeofknowledgebaseistotallydifferent.Allenterprisesusesomekindofwikiimplementationthatistheretoshareknowledge.TheymostlyuseConfluence,buttherearealsootherwikisolutionsavailable,commercialandfreeaswell.

Knowledgebasesstoreinformationthatyou,asadeveloper,wouldwritedowninapapernotebookforyourlaterreference,forexample,theIPaddressofthedevelopmentserver,directorieswheretoinstalltheJARfiles,whatcommandstouse,whatlibrariesyouhavecollected,andwhyyouusethem.Themajordifferenceisthatyouwriteitinaformattedwayintoawikianditisavailableimmediatelyforotherdevelopers.Itisabitofaburdenonthedevelopertowritethesepages,anditneedssomeself-disciplinefirst.StickingtotheexampleoftheIPaddressofthedevelopmentserverandtheinstalldirectories,youhavetowritenotonlytheIPaddressoftheserverbutalsosometextexplainingwhattheinformationis,becausetheothersmaynotunderstanditotherwise.Itisalsoabitofworktoplacethepagewiththeinformationinthewikisystemwithagoodname,linkingittootherpages,orfindingtheappropriatepositionofthepageinthetreeofpages.Ifyouwereusingthepapernotebook,youcouldjustwritedowntheIPaddressandthedirectoriesonthefirstfreepageofthebookandyouwouldjustrememberallothers.

Thewikiapproachwillpaybackwhencoworkersdonotneedtofindtheinformationthemselves;youcanfindtheinformationinaneasierwaybecauseothercoworkershavealsorecordedtheirfindingsintheknowledgebaseand,lastbutnotleast,afewmonthslater,youfindtheinformationyourecordedyourself.Inthecaseofapapernotebook,youwouldturnthepagestofindtheIPaddressandyoumayormaynotrememberwhichoneistheprimaryandwhichisthesecondaryserver.Youmayevenforgetbythenthattherearetwoservers(orwasitadoublecluster?).

Tohavealonglistofavailablewikisoftware,visithttps://en.wikipedia.org/wiki/Comparison_of_wiki_software.

IssuetrackingIssuetrackingsystemskeeptrackofissues,bugs,andothertasks.Thefirstissuetrackingsystemswerecreatedtomaintainthelistofbugsandalsothestateofthebugfixingprocesstoensurethatabug,identifiedandrecorded,willnotgetforgotten.Later,thesesoftwaresolutionsdevelopedandbecamefull-fledgedissuetrackersandareunavoidableprojectmanagementtoolsineveryenterprise.

ThemostwidelyusedissuetrackingapplicationusedinmanyenterprisesisJira,butonthehttps://en.wikipedia.org/wiki/Comparison_of_issue-tracking_systemspage,youcanfindmanyotherapplicationslisted.

Themostimportantfeatureofanissuetrackerapplicationisthatithastorecordanissueindetailinaneditablemanner.Ithastorecordthepersonwhorecordedtheissueincasemoreinformationisneededduringissuehandling.Thesourceoftheissueisimportant.Similarly,issueshavetobeassignedtosomeresponsibleperson,whoisaccountablefortheprogressofissuehandling.

Modernissuetrackingsystemsprovidecomplexaccesscontrol,workflowmanagement,relationmanagement,andintegrationwithothersystems.

Accesscontrolwillonlyallowthepersonwhohassomethingtodowithanissueaccesstoit,sootherscannotalterthestateofanissueorevenreadtheinformationattachedtoit.

Anissuemaygothroughdifferentworkflowstepsdependingonthetypeofissue:abugmaybereportedorreproduced,arootcauseanalyzed,afixdevelopedortested,apatchcreated,afixmergedwiththenextreleaseversionorpublishedintherelease.Thisisasimpleworkflowwithafewstates.

Relationmanagementallowssettingdifferentrelationsbetweenissuesandallowingtheusertonavigatefromissuetoissuealongtheserelations.Forexample,aclientreportsabug,andthebugisidentifiedasbeingthesameasanotheralreadyfixed.Insuchacase,itwouldbeinsanetogothroughtheoriginalworkflowandcreatinganewpatchforthesamebug.Instead,theissuegetsarelationpointingtotheoriginalissueandsetsthestatetobeclosed.

Integrationwithothersystemsisalsousefultokeepaconsistentdevelopmentstate.Versioncontrolmayrequirethat,foreverycommit,thecommitmessagecontainsareferencetotheissuethatdescribestherequirement,bug,orchangethatthecodemodificationsupports.Issuesmaybelinkedtoknowledgebasearticlesoragileprojectmanagementsoftwaretoolsusingweblinks.

TestingWehavealreadydiscussedtestingwhenwetalkedaboutunittesting.Unittestingisextremelyimportantinagiledevelopmentandithelpskeepthecodecleanandreducethenumberoferrors.Butthisisnottheonlytypeoftestingthatyouwillseeinenterprisedevelopment.

TypesoftestsTestingisperformedformanyreasonsbutthereareatleasttworeasonsthatwehavetomention.Oneistohuntthebugsandcreateerror-freecodeasmuchaspossible.Theotheristoprovethattheapplicationisusableandcanbeutilizedforthepurposeitwasmeantfor.Itisimportantfromtheenterprisepointofviewandconsidersalotofaspectsthatunittestdoesnot.Whileunittestfocusesononeunitand,thus,isanextremelygoodtooltopointoutwheretheerroris,itistotallyunusablewhenitcomestodiscoveringbugsthatcomefromerroneousinterfacesbetweenmodules.Theunittestsmockexternalmodulesand,thus,testthattheunitworksasexpected.However,ifthereisanerrorinthisexpectationandtheothermodulesdonotbehaveinthesamewayastheunittestmock,theerrorwillnotbediscovered.

Todiscovertheerrorsonthislevel,whichisthenextlevelaboveunittest,wehavetouseintegrationtests.Duringintegrationtests,wetesthowindividualunitscanworktogether.WhenweprograminJava,theunitsareusuallyclasses;thus,theintegrationtestwilltesthowthedifferentclassesworktogether.Whilethereisaconsensus(moreorless)aboutwhataunittestisinJavaprogramming,thisislesssointhecaseofintegrationtests.

Inthisregard,theexternaldependencies,suchasothermodulesreachableviathenetworkordatabaselayersmaybemocked,ormaybesetupusingsometestinstanceduringintegrationtesting.Theargumentisnotaboutwhetherthesepartsshouldbemockedornot,onlytheterminology.Mockingsomecomponentssuchasthedatabasehasadvantagesaswellasdrawbacks.Asinthecaseofanymock,thedrawbackisthecostofsettingupthemockaswellasthefactthatthemockbehavesdifferentlyfromtherealsystem.Suchadifferencemayresultinsomebugsstillremaininginthesystemandlurkingthereuntilalatercaseoftestingor,Godforgive,productionisused.

Integrationtestsareusuallyautomatedinawaysimilartounittests.However,theyusuallyrequiremoretimetoexecute.Thisisthereasonwhythesetestsarenotexecutedateachsourcecodechange.Usually,aseparatemavenorGradleprojectiscreatedthathasadependencyontheapplicationJARandcontainsonlyintegrationtestcode.Thisprojectisusuallycompiledandexecuteddaily.

Itmayhappenthatdailyexecutionisnotfrequentenoughtodiscovertheintegrationissuesinatimelymanner,butamorefrequentexecutionoftheintegrationtestsisstillnotfeasible.Insuchacase,asubsetoftheintegrationtestcasesisexecutedmorefrequently,forexample,everyhour.Thistypeoftestingiscalledsmoketesting.Thefollowingdiagramshowsthepositionofthedifferenttestingtypes:

Whentheapplicationistestedinafullysetupenvironment,thetestingiscalledsystemtesting.Suchtestingshoulddiscoveralltheintegrationbugsthatmayhavebeenlurkingandcoveredduringtheprevioustestingphases.Thedifferenttypeofsystemtestscanalsodiscovernon-functionalissues.Bothfunctionaltestingandperformancetestingaredoneonthislevel.

Functionaltestingchecksthefunctionsoftheapplication.Itensuresthattheapplicationfunctionsasexpectedoratleasthasfunctionsthatareworthinstallingintheproductionenvironmentandcanleadtocostsavingorprofitincrease.Inreallife,programsalmostneverdeliverallthefunctionsthatwereenvisionedinanyrequirementdocumentation,butiftheprogramisusableinasanemanner,itisworthinstallingit,assumingthattherearenosecurityissuesorotherissues.

Incasetherearealotoffunctionsintheapplication,functionaltestingmaycostalot.Insuchacase,somecompaniesperformasanitytest.Thistestdoesnotcheckthefullfunctionalityoftheapplication,onlyasubsettoensurethattheapplicationreachesaminimalqualityrequirementanditisworthspendingthemoneyonthefunctionaltesting.

Theremaybesometestcasesthatarenotenvisionedwhentheapplicationwasdesignedandthusthereisnotestcaseinthefunctionaltestplan.Itmaybesomeweirduseraction,auserpressingabuttononthescreenwhennobodythoughtitwaspossible.Users,evenifbenevolent,happentopressortouchanythingandenterallpossibleunrealisticinputsintoasystem.Ad-hoctestingtriestoamendthisshortage.Atesterduringad-hoctestingtriesallthepossiblewaysofuseoftheapplicationthatheorshecanimagineatthemomentthetestisexecuted.

Thisisalsorelatedtosecuritytesting,alsocalledpenetrationtestingwhenthevulnerabilitiesofthesystemarediscovered.Thesearespecialtypesofteststhatareperformedbyprofessionalswhohavetheircoreareaofexpertiseinsecurity.Developersusuallydonothavethatexpertise,butatleast,thedevelopersshouldbeabletodiscussissuesthatarediscoveredduringsuchatestandamendtheprogramtofixthesecurityholes.ThisisextremelyimportantinthecaseofInternetapplications.

Performancetestingchecksthattheapplication,inareasonableenvironment,canhandletheexpectedloadthattheuserputsonthesystem.Aloadtestemulatestheuserswhoattackthesystemandmeasurestheresponsetimes.Iftheresponsetimeisappropriate,thatis,lowerthantherequiredmaximumunderthemaximumload,thenthetestpasses;otherwise,itfails.Ifaloadtestfails,itisnotnecessarilyasoftwareerror.Itmaysohappenthattheapplicationneedsmoreorfasterhardware.Loadtestsusuallytestthefunctionalityoftheapplicationinonlyalimitedwayandonlytestusescenariosthatposereadloadontheapplication.

Manyyearsago,weweretestingawebapplicationthathadtohavearesponsetimeof2seconds.Theloadtestwasverysimple:issueGETrequestssothatthereareamaximumof10,000requestsactiveatthesametime.Westartedwith10clients,andthenascriptwasincreasingtheconcurrentusersto100,then1,000,andthensteppingupbythousandeveryminute.Thisway,theloadtestwas12minuteslong.Thescriptprintedtheaverageresponsetime,andwewerereadytoexecutetheloadtestat4:40pmonaFriday.Theaverageresponsetimestartedfromafewmillisecondsandwentupto1.9secondsastheloadwasincreasedto5,000concurrentusers,andfromthere,itwasdescendingdownto1secondastheloadwasincreasedto10,000users.YoucanunderstandtheattitudeofthepeopleonaFridayafternoon,beinghappythatwemettherequirements.Mycolleaguesleftfortheweekendhappily.IremainedtestingabitmorebecauseIwasbotheredbythephenomenonthattheresponsetimedecreasesasweincreasetheloadabove5,000.First,Ireproducedthemeasurementandthenstartedlookingatthelogfiles.At7pm,Ialreadyknewwhatthereasonwas.Whentheloadwentabove5,000,theconnectionstheApacheserverwasmanagingstartedtoexhaustandthewebserverstartedtosendback500internalerrorcodes.ThatissomethingthatApachecanveryeffectivelydo.Itisveryfastintellingyouthatyoucannotbeserved.Whentheloadwasaround10,000concurrentusers,70%oftheresponsesalreadyhad500errors.Theaveragewentdown,buttheuserswereactuallynotserved.IreconfiguredtheApacheserversothatitcouldservealltherequestsandforwardeachtoourapplicationjusttolearnthattheresponsetimeofourapplicationwasaround10secondsatthemaximumload.Around10pm,whenmywifewascallingmymobilethethirdtime,IalsoknewhowlargeamemoryIshouldsetintheTomcatstartupfileintheoptionsfortheJVMtogetthedesired2-secondresponsetimeincaseof10,000concurrentusers.

Stresstestisalsoatypeofperformancetestthatyoumayalsoface.Thistypeoftestincreasestheloadonthesystemuntilitcannothandletheload.Thattestshouldensurethatthesystemcanrecoverfromtheextremeloadautomaticallyormanuallybut,innocase,willdosomethingthatitshouldn'tatall.Forexample,abakingsystemshouldnotevercommitanunconfirmedtransaction,nomatterhowbigtheloadthereis.Iftheloadistoohigh,thenitshouldleavethedoughrawbutshouldnotbakeextrabread.

Themostimportanttestatthetopofthehierarchyistheuseracceptancetest.Thisisusuallyanofficialtestthatthecustomer,whobuysthesoftware,executesandinthecaseofsuccessfulexecution,paysthepriceforthesoftware.Thus,thisisextremelyimportantinprofessionaldevelopment.

TestautomationTestscanbeautomated.Itisnotaquestionofwhetheritispossibletoautomatizeatest,onlywhetheritisworthdoingso.Unittestsandintegrationtestsareautomated,andastimeadvances,moreandmoretestsgetautomatedaswemovealongtohigherstepstowardstheuseracceptancetest(UAT).UATisnotlikelytobeautomated.Afterall,thistestcheckstheintegrationbetweentheapplicationandtheuser.Whiletheuser,asanexternalmodule,canbemockedusingautomationinlowerlevels,weshouldreachthelevelwhentheintegrationtesthappenswithoutmocks.

Therearemanytoolsthathelptheautomationoftests.Theblockerfortestautomation,thesedays,isthecostofthetoolstodoso,thecostoflearninganddevelopingthetests,andthefearthattheautomatedtestsarenotdiscoveringsomeoftheerrors.

Itistruethatitiseasiertodosomethingwrongwithaprogramthanwithout.Thisissotrueforalmostanythingnotonlyfortesting.Andstillwedouseprograms;whyelsewouldyoureadthisbook?Someoftheerrorsmaynotbediscoveredduringautomatedfunctionaltesting,whichwouldotherwisehavebeendiscoveredusingmanualtests.Atthesametime,whenthesametestisexecutedthehundredthtimebythesamedeveloper,itisextremelyeasytoskipanerror.Anautomatedtestwillnoteverdothat.Andmostimportantly,thecostoftheautomatedtestisnot100timesthecostofrunningitonce.

Wehaveusedtestautomationtoolsinthisbook.SoapUIisatoolthathelpsyoucreateteststhatcanbeexecutedautomatically.OthertestingtoolsthatareworthlookingatareCucumber,Concordion,Fintnesse,andJBehave.Thereisagoodcomparisonoftoolsathttps://www.qatestingtools.com/.

BlackboxversuswhiteboxYoumayhaveheardmanytimesthatatestisablackboxtest.Thissimplymeansthatthetestdoesnotknowanythingabouthowthesystemundertest(SUT)isimplemented.ThetestreliesonlyontheinterfaceoftheSUTthatisexportedfortheoutsideworld.Awhiteboxtest,ontheotherendofthescale,teststheinternalworkingoftheSUTandverymuchreliesontheimplementation:

Boththeapproacheshaveadvantagesanddisadvantages.Weshoulduseone,orthemixtureofthetwoapproaches,awaythatfitsthepurposeoftheactualtestingneedsthemost.Ablackboxtestnotrelyingontheimplementationdoesnotneedtochangeiftheimplementationchanges.Iftheinterfaceofthetestedsystemchanges,thenthetestshouldalsobechanged.Awhiteboxtestmayneedchangesiftheimplementationchanges,eveniftheinterfaceremainsthesame.Theadvantageofthewhiteboxtestisthat,manytimes,itiseasiertocreatesuchatestandthetestingcanbemoreeffective.

Togetthebestofbothworlds,systemsaredesignedtobetestable.Becareful,though.Itmeansmanytimesthatthefunctionalityinternaltothetestedsystemispropagatedtotheinterface.Thatway,thetestwilluseonlytheinterfaceand,thus,canbedeclaredtobeablackbox,butitdoesnothelp.Ifsomethingchangesintheinternalworkingofthetestedsystem,thetesthastofollowit.Theonlydifferenceisthatyoumaycallitablackboxtestiftheinterfacealsochanges.Thatdoesnotsaveanywork.Rather,itincreasesit:wehavetocheckallthemodulesthatrelyontheinterfaceiftheyalsoneedanychange.

Idonotsaythatweshouldnotpayattentiontocreatingtestablesystems.Manytimesmakingasystemtestableresultsincleanerandsimplercode.Ifthecode,however,getsmessierandmuchlongerbecausewewanttomakeittestable,thenweareprobablynotgoingintherightway.

SelectinglibrariesProgrammingfortheenterpriseorevenprogrammingamoderatelysizedprojectcannotbedonewithouttheuseofexternallibraries.IntheJavaworld,mostofthelibrariesthatweuseareopensourceand,moreorless,freetouse.Whenwebuyalibrarythatissoldformoney,thereisusuallyastandardprocessenforcedbythepurchasingdepartment.Insuchacase,thereisawrittenpolicyabouthowtoselectthevendorandthelibrary.Inthecaseof"free"software,theydonotusuallycare,thoughtheyshould.Insuchacase,theselectionprocessmainlylieswiththeITdepartmentanditisthereforeimportanttoknowthemajorpointstobeconsideredbeforeselectingalibraryevenifforfree.

Inthepreviousparagraph,Iputthewordfreebetweenquotes.Thatisbecausethereisnosoftware,whichisfree.Thereisnosuchthingasafreelunch,astheysay.Youhaveheardthismanytimesbutitmaynotbeobviousinthecaseofanopensourcecodelibraryorframeworkyouaregoingtoselect.Themajorselectionfactorforanypurchaseorimplementationisthecost,theprice.Ifthesoftwareisfree,itmeansthatyoudonotneedtopayanupfrontfeeforthesoftware.However,thereisacostinintegratingitandusingit.Supportcostsmoney.Somebodymaysaythatthesupportiscommunitysupportandalsoavailablefreeofcharge.Thethingisthatthetimeyouspendhuntingforaworkaroundthathelpsyoutogetoverabugisstillmoney.Itisyourtime,orincaseyouareamanager,itisthetimeoftheprofessionalinyourdepartmentwhosetimeyoupayfor,or,asamatteroffact,itcanbeanexternalcontractorwhowillhandyouahugebillincaseyoudonothavetheexpertisein-housetosolvetheissue.

Sincefreesoftwaredoesnothaveapricetagattached,wehavetolookattheotherfactorsthatareimportantintheselectionprocess.Attheendoftheday,theyallwillaffectthecostinsomeway.Sometimes,thewayacriterionaltersthecostisnotobviousoreasilycalculable.However,foreachone,wecansetno-golevelsthatarebasedontechnologydecisions,andwecancomparelibrariesforbeingbetterorworsealongwitheachofthecriteria.

Fitforthepurpose

Perhaps,thisisthemostimportantfactor.Otherfactorsmaybearguedaboutintermsofthescaleofimportance,butifalibraryisnotappropriateforthepurposewewanttouse,thenthisiscertainlynotsomethingtoselect,nomatterwhat.Itmaybeobviousinmanycases,butyoumaybesurprisedhowmanytimesIhaveseenaproductselectedbecauseitwasthefavoriteofapersoninsomeotherprojectandthelibrarywasforcedforuseinthenewprojecteventhoughtherequirementsweretotallydifferent.

LicenseThelicenseisanimportantquestionasnotallfreesoftwareisfreeforalluses.Someofthelicensesallowfreeuseforhobbyprojectsandeducationbutrequireyoutopurchasethesoftwareforprofessional,profit-orienteduse.

Themostwidelyusedlicensesandtheirexplanation(andthewholetextofthelicense)isavailableonthewebpageoftheOpenSourceInitiative(https://opensource.org/licenses).Itlistsninedifferentlicenses,andtomakethesituationabitmorecomplex,theselicenseshaveversions.

OneoftheoldestlicensesistheGeneralPublicLicense(GPL)standingforGNU.Thislicensecontainsthefollowingsentences:

Forexample,ifyoudistributecopiesofsuchaprogram,whethergratisorforafee,youmustpassontotherecipientsthesamefreedomsthatyoureceived.Youmustmakesurethatthey,too,receiveorcangetthesourcecode.

Ifyoucreatesoftwareforafor-profitenterpriseandthecompanyintendstosellsoftware,youprobablycannotuseanylineofcodethatisfromaGPL-licensedsoftware.Itwouldimplythatyouarerequiredtopassonyourownsourcecode,whichmaynotbethebestsalesstrategy.Apachelicense,ontheotherhand,maybeokayforyourcompany.Thisissomethingthatthelawyersshoulddecide.

Eventhoughthisisthelawyers'work,thereisoneimportantpointthatwedevelopersmustbeawareofandpaycloseattentionto.Sometimes,thelibrariescontaincodefromotherprojectsandtheirlicense,asadvertised,maynotbetherealone.AlibrarymaybedistributedundertheApachelicensebutcontainscodethatisGPL-licensed.ThisisobviouslyaviolationoftheGPLlicense,whichwascommittedbysomeopensourcedevelopers.Whywouldyoucare?Herecomestheexplanationviaanimaginedsituation.

Youdevelopsoftwareforanenterprise.Let'ssaythatthiscompanyisoneofthelargestcarmanufacturersoftheworld,oritisoneofthelargestbanks,pharma,whatever.TheowneroftheGPLsoftwareseeksremediesforthemisuseofhersoftware.Willshesuethesoftwaredeveloper,JohnDoe,whohasatotalwealthof200K,oryourcompany,claimingthatyoudidnotdulycheckthelicenseofthecode?Shecertainlywillnotdigforgoldwherethereisnone.Suingthecompanyyouworkformaynotbesuccessful,butcertainlynotagoodprocessyouoranyoneatthecompanywants.

Whatcanweassoftwareprofessionalsdo?

Wehavetouselibrariesthatarewellknown,usedwidely.Wecancheckthesourcecodeofthelibrarytoseewhetherthereissomecopiedcode.Somepackagenamesmaypresentsomeclue.YoucanGooglesomepartofthesourcecodetofindmatches.Lastbutnotleast,thecompanycansubscribetoservicesthatprovidesimilarresearchforthelibraries.

Documentation

Documentationisanimportantaspect.Ifthedocumentationisnotappropriate,itwillbehardtolearnhowtousethelibrary.Someoftheteammembersmayhavealreadyknownthelibrary,but,again,thismaynotbethecaseforlaterteammembers.Weshouldconsiderourcolleagues,whoareexpectedtobeaverageprogrammers,andtheywillhavetolearntheuseofthelibrary.Thusdocumentationisimportant.

Whenwespeakaboutdocumentation,weshouldnotonlythinkabouttheJavaDocreferencedocumentationbutalsotutorialsandbooksiftheyareavailable.

ProjectaliveItisimportantnottoselectalibraryforusethatisnotalive.Havealookattheroadmapofthelibrary,thelasttimeareleasewasshipped,andthefrequencyofthecommits.Ifthelibraryisnotalive,weshouldconsidernotusingit.Librariesworkinanenvironmentandtheenvironmentchanges.Thelibrarymayconnecttoadatabase.Thenewversionofthedatabasemayprovidenewfeaturesthatgiveusbetterperformanceonlyifthelibraryismodifiedtoaccommodatethesenewfeatures.ThelibrarycommunicatesoverHTTP;willitsupportthenew2.0versionoftheprotocol?Ifnothingelse,theversionoftheJavaenvironmentwillchangeovertheyearsandthelibraryweuseshouldsoonerorlaterfollowittoleveragethenewfeatures.

Thereisnoguaranteethatanalivelibrarywillalwaysstayalive.However,alibrarythatisalreadydeadwillcertainlynotresurrect.

Eveniftheprojectisaliveatthemoment,therearesomepointsthatmaygivesomehintsaboutthefutureofthelibrary.Ifthecompanydevelopingitiswell-establishedandfinanciallystable,andthelibraryisdevelopedwithareasonablebusinessmodel,thenthereisalowriskthattheprojectdies.Iftherearealotofcompanieswhousethelibrary,thenitislikelythattheprojectwillstayaliveeveniftheoriginalteamstopsworkingonitortheoriginalfinancingstructurechanges.However,theseareonlysmallfactorsandnotwell-establishedfacts.Thereisnoguarantee,andtellingthefutureismoreanartthanascience.

MaturityMaturityissimilartothepreviouscriterion.Aprojectmayverywellbealivejuststartingup,butifitisinitsinfancy,webetternotusethelibraryforalargeproject.Whenaprojectisinitsearlyphase,alotofbugscanbeinthecode,theAPImaychangeradically,andpresumably,theremayonlybeasmallnumberofcompaniesrelyingonthecode.Thisalsomeansthatthecommunitysupportislower.

Ofcourse,ifalltheprojectsselectonlymatureopensourcecode,thennoopensourceprojectwouldevergettothematurestate.Weshouldassesstheimportanceoftheproject.Istheprojectbusiness-critical?Willtheprojectbecomebusiness-critical?

Iftheprojectisnotbusiness-critical,thecompanymayaffordtoinventafreshlibrarythatisnotthatmature.Itmaybereasonableiftherearenomaturelibrariesforthepurposebecausethetechnologyyouaregoingtouseisrelativelynew.Insuchacase,theprojectinthecompanyisprobablyalsonewandnotbusiness-criticalyet.Itwillbebusiness-critical,wehope,aftersometime,butbythattime,thelibrarywillbemature,ormayjustdieandwecanselectacompetingsolutionbeforetheprojectbecomestooexpensivetoswitch.

Judgingthematurityofalibraryisalwaysdifficultandhastobealignedwiththematurityandimportanceoftheprojectthatwewanttousethelibraryfor.

Numberofusers

Ifthelibraryisaliveandmaturebuttherearenotmanyusers,thensomethingissmelly.Whydon'tpeopleusethelibraryifitisgood?Ifthenumberofusersforalibraryorframeworkislowandtherearenolargecorporationsamongtheusers,thenitisprobablynotagoodone.Nobodyusingitmaysignalthatourassessmentoftheothercriteriamaynotbeappropriate.

Alsonotethatifthereareonlyafewusersofthelibrary,thentheknowledgeinthecommunityisalsoscarceandwemaynotbeabletogetcommunitysupport.

The"Ilikeit"factor

Lastbutnotleast,theIlikeitfactorisextremelyimportant.Thequestionisnotwhetheryoulikethelibrarybutratherhowmuchthedeveloperslikeit.Developerswilllikealibrarythatiseasytouseandfuntoworkwith,andthiswillresultinlowcost.Ifthelibraryishardtouseanddevelopersdonotlikeit,thentheywillnotlearntouseittothelevelofprofessionrequiredforgoodquality,onlytothelevelthatisjustneeded.Theendresultwillbesuboptimalsoftware.

ContinuousintegrationanddeploymentContinuousintegrationmeansthatwheneveranewversionispushedtothesourcecoderepository,thecontinuousintegrationserverkicksin,pullsthecodetoitsdisk,andstartsthebuild.Itcompilesthecodefirst,thenrunstheunittests,firesthestaticcodeanalysistools,and,ifallgoesright,itpackagesasnapshotreleaseanddeploysitonadevelopmentserver.

CIservershavewebinterfacesthatcanbeusedtocreatearelease.Insuchacase,thedeploymentcanevengotothetestserversoreventoproductiondependingonlocalbusinessneedsandonthepolicythatwascreatedaccordingly.

Automatingthebuildanddeploymentprocesshasthesameadvantagesasanyotherautomation:repeatedtaskscanbeperformedwithoutmanualintervention,whichistedious,boring,and,thus,error-proneifdonebyahuman.Theoutstandingadvantageisthatifthereissomeerrorinthesourcecodethatcanbediscoveredbytheautomatedbuildprocess,itwillbediscovered.Novicedeveloperssaythatitischeaperandeasiertobuildthecodelocally,whichthedevelopersdoanyway,andthenpushthecodetotheserverifthebuildprocessisalreadychecked.Itispartlytrue.Developershavetocheckthatthecodeisofgoodqualityandbuildswell,beforesendingittothecentralrepo.However,thiscannotalwaysbeachieved.Someerrorsmaynotmanifestonlocalenvironments.

ItmaysohappenthatonedeveloperaccidentallyusesanewerversionofJavathantheonesupportedandusesanewfeatureofthenewversion.Enterprisesdonotgenerallyusethelatesttechnology.Theytendtouseversionsthatareproven,havemanyusers,andaremature.Thisyear,in2017,whenJava9isgoingtobereleasedinJuly,hugeenterprisesstilluseJava1.6and1.7.SinceJava9hasmanynewfeaturesthatarenottrivialtoimplement,IexpectthattheadoptionofthetechnologymaytakeevenlongerthantheadoptionofJava1.8,whichgaveusfunctionalprogrammingandlambda.

Itmayalsohappenthatanewlibraryisaddedtothedependenciesofthebuildandthedeveloperwhoaddedittothebuildfile(pom.xml,orbuild.gradle)coulduseitwithoutanyproblemonherlocalmachine.Itdoesnotmeanthatthelibraryisofficiallyaddedtotheproject,anditmaynotbeavailableinthecentralcoderepository(Artifactory,Nexus,orotherimplementationsofthecoderepository).Thelibrarymayhaveonlybeenonthelocalrepositoryofthedeveloper,andshemayhaveassumedthatsincethecodecompiles,thebuildisOK.

Somelargeorganizationsusedifferentcoderepositoriesfordifferentprojects.Thelibrariesgetintotheserepositoriesfollowingmeticulousexaminationanddecisions.Somelibrariesmaygetthere,whileothersmaynot.Thereasontohavedifferentrepositoriescouldbenumerous.Someprojectisdevelopedforonecustomerwhohasadifferentpolicyaboutanopensourceprojectthantheother.Iftheenterprisedevelopscodeforitself,itmaysohappenthatsomelibraryisphasedoutornotsupportedanymore,andcanonlybeusedforprojectsthatareold.Amaintenancereleasemaynotneedtoreplacealibrary,butnewprojectsmaybenotbeallowedtouseadyingsoftwarelibrary.

TheCIservercanrunonasinglemachineoritcanrunonseveralmachines.Incaseitservesmany

projects,itmaybesetupasacentralserverwithmanyagentsrunningondifferentmachines.Whensomebuildprocesshastobestarted,thecentralserverdelegatesthistasktooneoftheagents.Theagentsmayhavedifferentloads,runningseveraldifferentbuildprocesses,andmayhavedifferenthardwareconfiguration.Thebuildprocessmayhaverequirementsregardingthespeedoftheprocessororabouttheavailablememory.Someagentmayrunsimplerbuildsforsmallerprojectsbutwouldfailtoexecutethebuildofalargeprojectorofsomesmallprojectthatstillhasahugememoryrequirementtoexecutesometests.

Whenabuildfails,thebuildserversendsoute-mailstothedevelopers,andthepersonwhosentthelastupdatetothecoderepositoryisobligatedtofixthebugwithoutdelay.Thisencouragesthedeveloperstocommitfrequently.Thesmallerthechange,thefewerchancesthereareofabuildproblem.Thebuildserverwebinterfacecanbeusedtoseetheactualstateoftheprojects,whichprojectisfailingtobuild,andwhichisjustfine.Ifabuildfails,thereisaredsigninthelineofthebuild,andifthebuildisOK,thesignisgreen.

Manytimes,thesereportsarecontinuallydisplayedonsomeoldmachineusingahugedisplaysothateverydeveloperorjustanybodywhoenterstheroomcanseetheactualstateofthebuilds.Thereisevenspecialhardwarethatyoucanbuythathasred,yellow,andgreenlampstofollowthestateofthebuildandringabellwhenthebuildfails.

ReleasemanagementDevelopingsoftwaremeansacontinuouslychangingcodebase.Noteveryversionofthesoftwareissupposedtobeinstalledinproduction.Mostoftheversionsarepushedtotherepositoryonabranchhalfcomplete.Someversionsaremeantonlyfortestingandafewaremeanttobeinstalledinproductionevenifonlysomeofthosewillfinallygettoproduction.

Almostallthetime,thereleasesfollowthesemanticversioningthatwediscussedinanearliersection.Theversionsthataremeantonlytobetestedusuallyhavethe-SNAPSHOTmodifierattheendoftheversionnumber.Forexample,the1.3.12-SNAPSHOTversionistheversionthatwasoncedebugged,andisgoingtobecomethe1.3.12version.Thesnapshotversionsarenotdefiniteversions.Theyarethecodeasitisbythen.Becauseasnapshotreleasenevergetsinstalledinproduction,itisnotneededtoreproduceasnapshotversionformaintenance.Thus,thesnapshotversionsarenotincreasedcontinually.Sometimes,theymaybechanged,butthatisarareexception.

Itmaysohappenthatweworkonabugfix,1.3.12-SNAPSHOT,andduringthedevelopment,wechangesomuchcodethatwedecidethatithastobe1.4.0whenitisreleased,andwerenamethesnapshotas1.4.0-SNAPSHOT.Thisisararecase.Manytimes,thereleasecreationcreatesa1.4.0versionfrom1.3.12-SNAPSHOTasthedecisionaboutthenewreleasenumberistakenbythetimethereleaseiscreated.

Whenthereleaseprocessisstarted,usuallyfromthewebinterfaceoftheCIserver,thedevelopercreatingthereleasehastospecifythereleaseversion.Thisisusuallythesameasthesnapshotversionwithoutthe-SNAPSHOTpostfix.Thebuildprocessnotonlycreatesthebuildinthiscasebutalsotagsthesourcecoderepositoryversionitwasusingandloadsthepackagedprogram(artifact)tothecoderepository.Thetagcanbeusedlatertoaccesstheexactversionofthesourcecodethatwasusedtocreatetherelease.Ifthereisabuginaspecificversion,thenthisversionhastobecheckedoutonadevelopermachinetoreproducethebugandfindtherootcause.

Ifthebuildofareleasefails,itcanberolledback,oryoubetterjustskipthatreleasenumberandnoteitasafailedreleasebuild.Anexistingreleasecanneverhavetwoversions.Thesourcecodeistheonlyonethatisforthatreleaseandthegeneratedcodehastobeexactlytheoneinanystorage.Subsequentcompilationofthesamesourcemayresultinslightlydifferentcode,forexample,ifadifferentversionofJavaisusedtocreatethelatterone.Eveninsuchacase,theonethatwascreatedbythebuildserverinthefirstplaceistheversionthatbelongstotherelease.Whenabugisreproducedandthecodeisrecompiledfromtheexactsamesource,itisalreadyasnapshotversion.Multiplereleasesmaybepossiblefromthesamesourceversion,forexample,compiledwithJavaversionsfrom1.5to1.8andversion9butasinglereleasealwaysbelongstotheexactsamesourcecode.

IfareleasethatwassupposedtobeareleaseversionfailsduringQAchecks,thenanewreleasehastobecreatedandthefailedreleasehastobenotedassuch.Theversionthatmarketingusestonamethedifferentversionsshouldnothavearelationtothetechnicalversionnumbersweworkwith.Manytimes,itis,anditcausesmuchheadache.Ifyourealizethatthetwoaretotallydifferentthingsandonedoesnothavetodoanythingwiththeother,lifegetssimpler.LookatthedifferentversioningoftheWindowsoperatingsystemorJava.Asmarketing,Javaused1.0then1.1,butJava1.2wasadvertisedasJava2andstillthecodecontained1.2(whichnowsevenmajorreleaseslateralsobecomes9insteadof1.9)

Thelastpartofreleasemanagementisthatdeploymentsshouldregistertheversionnumbers.Thecompanyhastoknowwhichreleaseisinstalledonwhichserver,andofwhichclient.

CoderepositoryCoderepositorystoresthelibrariesandhelpsmanagethedependenciesofthedifferentlibraries.Intheoldtimes,whenJavaprojectsusedANTasabuildtoolandwithoutthelateraddedIvydependencymanagement,thelibrariesthatwereneededbyaprojectweredownloadedtothesourcecode,usuallytotheliblibrary.Ifalibraryneededanotherlibrary,thenthosewerealsodownloadedandstoredmanually,andthiscontinueduntilallthelibrariesthatoneofthealreadydownloadedlibrariesneededwerecopiedtothesourcecodetree.

Thiswasalotofmanualworkand,also,thelibrarycodewasstoredinthesourcecoderepositoryinmanycopies.Acompiledlibraryisnotsourcecodeandhasnothingtodointhesourcecoderepository.Manualworkthatcanbeautomatedhastobeautomated.Notbecausedevelopersarelazy(yes,weareandwehavetobe)butbecausemanualworkiserrorproneand,thus,expensive.

ThiswaswhenApacheIvywasinventedandMaven,followingANT,alreadysupportedrepositorymanagementbuiltin.Theyallstoredthelibrariesstructuredindirectoriesandsupportedmetadatathatdescribedthedependenciestootherlibraries.LuckythatGradledidnotinventitsowncoderepository.Instead,itsupportsbothMavenandIvyrepositories.

Usingtherepository,thebuildtoolsautomaticallydownloadthelibrariesthatareneeded.Incasealibraryhasanewversion,thenthedeveloperonlyhastoupdatetheversionoftheneededlibraryinthebuildconfigurationandalltasks,includingdownloadingallthenewversionsoftheotherlibrariesthatareneededbythatversion,aredoneautomatically.

WalkinguptheladderAtthispoint,youhavegotalotofinformationthatwillrocketyourstartasanenterpriseJavadeveloper.Youhavegotabaseknowledgethatyoucanbuildon.ThereisalongwaytobecomeaprofessionalJavadeveloper.Thereisalotofdocumentationtoread,alotofcodetoscanandunderstand,andalsoalotofcodetowritetillyoucanclaimtobeaprofessionalJavadeveloper.Youmayprobablyfacemanyyearsofcontinuouseducation.Thegoodthingisthatevenafterthat,youcancontinueyourjourneyandyoucaneducateyourself,asbeingaprofessionalJavadeveloperisrarelyajobpeopleretirefrom.No,no!Notbecausetheydiewhileatit!Rather,professionalsoftwaredevelopersgainingexperiencestarttocodelessandlessandsupportthedevelopmentprocessindifferentways,whichleveragesmoreoftheirexperience.Theycanbecomebusinessanalysts,projectmanagers,testengineers,subject-matterexperts,architects,scrummasters,automationengineers,andsoon.Isitafamiliarlist?Yes,thesearethepeopleyouwillworkwithasadeveloper.Manyofthemmayhavestartedasadeveloperthemselves.Thefollowingdiagramshowstherelativepositionoftheseroles:

Let'stakeabitmoredetailedlookintowhattheserolesperforminenterprisedevelopment:

Businessanalystsworkwiththeclientandcreatethedocuments,specifications,usecases,anduserstoriesneededbythedeveloperstodevelopthecode.Projectmanagersadministertheprojectsandhelptheteamingettingthingsdoneincooperationwithotherteams,caringforalltheprojectmattersthatdeveloperscannotattendtoorwouldunnecessarilyburntheirtimethattheyshouldhavedevotedtocoding.Subject-matterexpertsaremoreadvancedinknowingthebusinessneeds,soitisabitrareforadevelopertobecomeone,butincasetheindustryyouworkinistechnologyoriented,itmaynotbeincredibletobecomeone.TestengineerscontroltheQAprocessandunderstandnotonlythetestmethodologiesandrequirementsoftestingbutalsothedevelopmentprocesssothattheycansupportbugfixesandnot

onlyidentifythem,whichwouldbepoor.ArchitectsworkwithBAsanddesignahigh-levelstructureoftheapplicationsandcode,anddocumentitinawaythathelpsthedeveloperstofocusontheactualtaskstheyhavetoperform.Architectsarealsoresponsibleforthesolutiontousetechnologies,solutions,andstructureswhichfitthepurpose,arefutureproof,affordable,andsoon.Scrummateshelpthedevelopmentteamtofollowtheagilemethodologyandhelptheteamincontrollingtheadministrationandresolvingproblems.

TherearemanywaystogoasasoftwaredeveloperandIonlylistedsomeofthepositionsthatyoucanfindinanenterprisetoday.Astechnologydevelops,Icanimaginethatin20yearsfromtoday,softwaredeveloperswillteachandcurateartificialintelligencesystemsandthatwillbewhatwerefertoasprogrammingtoday.Whocantell?

Summary

Goinginthisdirectionisagoodchoice.BeingaJavadeveloperandbecomingaprofessionalinitisaprofessionthatwillpaywellinthecoming10to20yearsforsureandperhapsevenlater.Atthesametime,Ipersonallyfindthistechnologyfascinatingandinteresting,andaftermorethan10yearsofJavaprogrammingandmorethan35yearsofprogramming,Istilllearnsomethingnewiniteveryday.

Inthisbook,youlearnedthebasicsofJavaprogramming.Fromtimetotime,Ialsomentionedissues,suggesteddirections,andwarnedyouaboutpitfallsthatarenotJava-specific.However,wealsodidthehomeworkoflearningtheJavalanguage,theinfrastructure,thelibraries,developmenttools,andnetworkinginJava.YoualsolearnedthemostmodernapproachesthatcameonlywithJava8and9,suchasfunctionalprogramminginJava,streams,andreactiveprogramming.IfyouknowallthatIhavewritteninthisbook,youcanstartworkingasaJavadeveloper.What'snext?Go,andfindyourtreasureinprogrammingandinJava!