Introduction to Java 120 February 2004Sifei HE © 2004 Introduction to Java for CS381, EE4.Web By...

20 February 2004 Sifei HE © 2004

Introduction to JavaIntroduction to Java

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Introduction to Java for Introduction to Java for CS381, EE4.WebCS381, EE4.Web

By Sifei [email protected] of ComputingSchool of EPSFebruary 2004



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BeansBeans

• Baked haricot beans, typically cooked in tomato sauce and tinned?

• An edible seed growing in long pods on certain plants.

• The hard seed of a coffee or cocoa plant.



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Java BeanJava Bean

• “A Java Bean is a reusable software component that can be manipulated visually in a builder tool”

Sun Microsystems

• Sun’s tutorial on Java Beanshttp://java.sun.com/docs/books/tutorial/javabeans

• Package: java.beans



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ComponentsComponents

• Components are self-contained, reusable software units that can be visually composed into composite components, applets, applications, and servlets using visual application builder tools.

• JavaBean components are known as Beans.



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JavaBeans, a component modelJavaBeans, a component model

• JavaBeans is Java’s component model• The model is made up of an architecture

and an API• The API makes it possible to write

component software in Java• The architecture provides the framework

(services and rules) that allows components to participate properly



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Some DistinctionsSome Distinctions

• Features that distinguish a Java Bean from Java objects are:– Support for introspection– Support for customization– Support for events– Support for properties– Support for persistence

• Bean requires a zero-argument constructor.• Bean does not have public member variables.

– Access through pre-defined methods



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Sources of Builder ToolsSources of Builder Tools

• Bean Builder from Sun:– http://java.sun.com/products/javabeans/beanbuilder/

• NetBeans:– http://www.netbeans.org

• JBuilder:– http://www.borland.com/jbuilder/

• Note, you also need to have the Java SDK installed.



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Bean BooksBean Books

Developing Java Beans

Robert Englander

O’Reilly Recommended

JavaBeans Programming

J. O’Neil and

H. Schildt

McGraw Hill Good. Contains many examples.

JavaBeans by Example

Henri Jubin Prentice Hall Simpler than the above, but with useful examples

NetBeans – the Definitive Guide

Boudreau, Glick, Greene, Spurlin, Woehr

O’Reilly Not so good on Beans. More about the IDE.



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The Component ModelThe Component Model

• Discovery and Registration

• Raising and Handling of Events

• Persistence

• Visual Presentation

• Support for Visual Programming



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• Raising and Handling of Events

– Beans (or JavaBeans components) use events to communicate with other Beans

– A Bean that wants to receive events (a listener Bean) registers its interest with the Bean that triggers the event (a source Bean)



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• Persistence

– Persistence enables Beans to save and restore their state

– JavaBeans uses Java Object Serialization to support persistence

– implements java.io.Serializable



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• Visual Presentation

– The Bean is free to choose its own visual presentation (fonts, colours, shape, etc)

– Many of these characteristics will be properties of the Bean (some might be persistent too)

• Note: Not all beans are visual beans. Beans need not to be visual !



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• Support of Visual Programming

– User can select a component from the toolbox and place it into a container

– Properties of the component can then be edited to create the desired behaviour



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Properties, Methods & EventsProperties, Methods & Events

• A property is a subset of a component’s state.– Allow the user to control the behavior of that

component so it operates as desired.

• Methods can be invoked to execute code in a component.

• An event is a notification that is generated by a component when there is some change in its state.



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PropertiesProperties

• Named attributes that can be read or set by other beans

• Accessibility of a bean’s properties is only via its get and set methods

• In the Java Beans model, properties will only be recognised if appropriate get and (or) set methods are defined– not necessarily both



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A Pepper BeanA Pepper Beanpublic class Pepper { private String iKind; private int iHeatLevel;

public Pepper( ) { }

public String getKind( ) { return iKind; } public void setKind(String aKind) { iKind = aKind; }}



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Introspection and PropertiesIntrospection and Properties• The process by which builder tools discover a Bean’s

features.• By following specific naming conventions, the properties

of a Bean that are “revealed” to the world can be identified:

public class SignalGenerator {private double frequency;// … constructor methods herepublic double getFrequency( ) { return frequency; }

public void setFrequency(double frequency) { this.frequency = frequency;}

}



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Conventions for Access MethodsConventions for Access Methods• Simple Properties:

– For a property of type Type and name Name:– public Type getName( );– public void setName(Type value);

• Boolean Properties:– public boolean isName( );– public void setName(boolean value);– public boolean getName();

• Indexed Properties:– public Type getName(int index);– public Type[] getName();– public void setName(int index, Type value);– public void setName(Type values[]);



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MethodsMethods

• A Bean may be implemented by a Java Class

• That Class contains a number of methods that may be used to access and control the component

• These are generally all the public methods of the Class that implements the Bean



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EventsEvents

• JavaBeans components interact by generating “Events”• Several components may register an interest in an Event that

is generated by a specific component• Occurrence of the Event triggers methods to be called in all

the components that are “listening” for it• public void add<ListenerType>(<ListenerType> listener);• public void remove<ListenerType>(<ListenerType>

listener);



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QuoteLog

Graph



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Reacting to ChangeReacting to Change

• In general, a change in the value of a property of a bean will trigger some activity

• Could code this directly into the get and set methods, but this “hard-wires” application specific dependencies into the methods

• More flexible (improving reusability) if events are generated when the properties are accessed



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e.g. setting Kinde.g. setting Kindpublic void setKind(String aKind)

throws java.lang.Exception {if( aKind == “Strawberry” ) {

Exception ex = new Exception(“Not a chili pepper!”);

throw ex;}else

iKind = aKind;}



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Application CodeApplication Code

Pepper pepper = new Pepper( );try {

pepper.setKind(“Strawberry”);}catch (Exception ex) {

System.out.println(“Exception - message is: ”+

ex.getMessage( ) };}



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IntrospectionIntrospection

• The process by which builder tools discover a Bean’s features.

Beans support introspection in two ways:Beans support introspection in two ways: By adhering to specific rules (By adhering to specific rules (design patternsdesign patterns) )

when naming properties, methods and eventswhen naming properties, methods and events By explicitly providing property, method and By explicitly providing property, method and

event info within a event info within a Bean InformationBean Information class class



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• Low-level reflection:– Follow Bean coding style (we have seen)– Analysis of the Bean’s class can then

reveal properties and methods

• Revealing complex properties:– Implement a “BeanInfo” class



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• But the used within a visual development tool, a Bean must expose its:– properties,– methods– events

• Java reflection mechanism may not be sufficient



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The BeanInfo InterfaceThe BeanInfo Interface

• Sometimes use of the design patterns:– reveals insufficient information– reveals to much information– forces inappropriate design choices

• Sometimes the information we wish to expose cannot be represented by a design pattern– e.g. choice of a graphic icon to represent the

Bean on a toolbar in a visual development tool



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java.beans.BeanInfojava.beans.BeanInfoMethod Description

getAdditionalBeanInfo( ) Returns any additional BeanInfo objects that are relevant

getBeanDescriptor( ) Returns the Bean descriptor object

getDefaultEventIndex( ) Returns the default event index

getDefaultPropertyIndex( ) Returns the default property index

getEventSetDescriptors( ) Returns the event set descriptors

getIcon( ) Returns the specified icon for the Bean

getMethodDescriptors( ) Returns the method descriptors

getPropertyDescriptors( ) Returns the property descriptors



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Implementing Implementing BeanInfoBeanInfo

• You do not have to provide a detailed implementation

• If any of the methods in the interface returns null, then low-level reflection will be used to gather all the information that is available

• If a non-null value is returned, then that is all the information on that aspect that is made available



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Hiding InformationHiding Informationpublic class Pepper implements java.io.Serializable { private String iKind; private int iHeatLevel; public Pepper() {} public String getKind() { return iKind; } public void setKind(String aKind) { iKind = aKind; } public int getHeatLevel() { return iHeatLevel; } public void setHeatLevel(int aHeatLevel) { iHeatLevel = aHeatLevel; }}



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PepperBeanInfo.javaPepperBeanInfo.java

import java.beans.*;import java.beans.SimpleBeanInfo; public class PepperBeanInfo extends SimpleBeanInfo { public PropertyDescriptor[] getPropertyDescriptors() { PropertyDescriptor[] pdArray = new PropertyDescriptor[1]; try { pdArray[0] = new PropertyDescriptor("heatLevel", Pepper.class); } catch (IntrospectionException e) { System.out.println("Error creating a PropertyDescriptor"); } return pdArray; }}



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ExplanationExplanation

• SimpleBeanInfo implements BeanInfo• It contains default implementations that

return null• Inheriting the default implementation tells the

caller to use reflection• Overriding the default implementation

enables the developer to hide information by only providing descriptors for those features they wish to expose



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Reflection vs. IntrospectionReflection vs. Introspection

• Reflection analyses the Bean itself to discover properties, methods and events

• Introspection relies (mostly) on the presence of a “meta-object” to provide information about the Bean (an instance of a BeanInfo class)

• What are the benefits of Introspection?

• What are the weaknesses?



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IntrospectorIntrospector

• java.beans.Introspector– seeks a BeanInfo for a Bean– if any descriptors in BeanInfo return null, then use

reflection– if no BeanInfo, then use reflection

• The application always receives a BeanInfo object from the Introspector– (unless an exception occurs)

• The Introspector also “introspects” superclasses



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CustomisationCustomisation

• Simple properties– Development tool will build property sheets

dynamically– User may then edit the properties to customise

the Bean

• For the Advanced User– Create a specific customiser for a Bean– This is kept separate to the Bean Class, as with

a BeanInfo Class



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Further FeaturesFurther Features

• Visibility

– It is not necessary for a Bean to be visible at run-time (e.g. Bean controlling access to a device or data feed)

– It is necessary however for a Bean to support the visual builder tool. Even an ‘invisible’ run-time Bean shall be shown on the builder tool



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• Multithreading

– Always assume your code will be used in a multithreaded environment

– Make sure that your Beans are thread-safe– Multithreading in JavaBeans is no different

than multithreading in Java



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• Security

– By default assume that your Beans are running in a non-trusted applet

– Apply security restrictions such as• Allow no access to the local file system

• Limit socket connections to the host system

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