Hibernate One

8/13/2019 Hibernate One

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Difference between JDBC and Hibernate?

Hibernate is database independent, the code will work for all kinds of database like derby,

MySQL, oracle, SQLServer etc. Where as in JDBC query must be database specific.

As hibernate is set of objects, no need of SQL knowledge. In jdbc sql knowledge is must.

In hibernate, criteria query take care of query tuning. In case of JDBC you need to tune your

query.

As hibernate supports two level of cache, you can store data into catch for better performance

while in JDBC you need to implement your java code.

Hibernate supports query cache and it will provide the statistics about your query and database

status. JDBC doesnt provides any statistics.

Development fast in case of Hibernate because you dont need to write queries.

No need to create any connection pool in case of hibernate. You can use c3p0. In case of JDBC

we need to create our own connection pool.

In the xml file you can see all the relation between tables in case of hibernate, Easy readability.

You can load your objects on start up using lazy=false in case of hibernate. But JDBC dont have

such support.

Hibernate supports automatic versioning of rows but JDBC not.

Hibernate supports inheritance, associations, collections.

In hibernate if we save the derived class object, then its base class object will also be stored into

database, it means hibernate supporting inheritance.

Hibernate supports relationships like one-to-many, one-to-one, many-to-many, many-to-one.

Hibernate also supports collections like List, Set, Map(Only new collections).

In JDBC all exceptions are checked exceptions, so we must write code in try, catch and throws

but in hibernate we only have unchecked type exceptions, so no need to write try, catch or

throws. Actually in hibernate we have the translator which converts checked to unchecked.

Hibernate has capability to generate primary keys automatically while we are storing therecords in database.

Hibernate has its own query language, i.e. HQL (hibernate query language) which is database

independent.

While we are inserting any record, if we dont have any particular table in the database, JDBC

will rises an error like VIEW no exist, and throws exception, but in case of hibernate, if it not

found any table in the database this will create table for us.

Hibernate supports caching mechanism by this, the number of round trips between an

applications and the database will be reduced, by using this caching technique an applications

performance will be increased automatically.

Hibernate supports annotations, apart from XML. Hibernate provided Dialect classes, so we no need to write sql queries in hibernate, instead we

use the methods provided by that API.

Getting pagination in hibernate is quite simple.

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What is hibernate template?

Org.springframework.orm.

different methods for quer

HibernateException into un

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What are the benefits does Hibern

The benefits of HibernateTemplate

HibernateTemplate, a sprin

Common functions are sim

Sessions are automatically

Commit or rollback transac

executed.

Exceptions are automatical

HibernateTemplate class g

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What is difference between sorted

Sorted collection:

A sorting collection is sorti

collections framework. The

the data being read from d

If your collection is not larg

Order collection:

Order collection is sorting

this when retrieval.

If your collection is very lar

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What is ORDER BY clause?

The ORDER BY keyword is

The ORDER BY keyword sor

descending order, you can

ibernate.HibernateTemplate is a helper class whic

ying/retrieving data from the database. It also conv

checked DataAccessExceptions.

=========================================

teTemplate provide?

are:

g template class simplifies interactions with hibern

plified to single method calls.

opened, closed after your code executed.

tions are automatically done by template class, aft

ly caught and converted to runtime exceptions.

t deprecated from hibernate 4.x.

=========================================

and ordered collection in hibernate?

g a collection by utilizing the sorting features provi

sorting occurs in the memory of JVM which runnin

atabase using java comparator.

e, it will be more efficient way to sort it.

collection by specifying the order-by clause for sor

ge, it will be more efficient way to sort it.

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sed to sort the resultset by one or more columns.

ts the records in ascending order by default. To sor

use the DESC keyword.

provides

erts checked

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ate session.

r your code

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ded by the java

g hibernate, after

ting this collection

==============

the records in a


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=====================================================================================

What the collections types are in hibernate?

Bag

Set

List

Array

Map

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What is lazy fetching in hibernate?

Lazy fetching is associated with child objects loading for its parents. While loading the parent,

the selection of loading a child object is to be specified/mentioned in the hbm.xml file.

Hibernate does not load the whole child objects by default. Lazy=true means not to load the

child objects.

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Lifecycle of pojo class objects?

Pojo class object having 3 states:

Transient state

Persistent state

Detached state

Transient and Persistent states:

Whenever an object of a pojo class is created then it will be in the transient state.

When the object is in a transient state it doesnt represent any row of the database, I mean not

associated with any session object, if we speak more we can say no relation with the database

its just an normal object.

If we modify the data of a pojo class object, when it is in transient state then it doesnt effect on

the database table.

When the object is in persistent state, then it represent one row of the database, if the object is

in persistent state then it is associated with the unique session.

If you want to move an object from persistent to detached state, we need to do either closing

that session or need to clear the cache of the session.

If we want to move an object from persistent state into transient state then we need to deletethat object permanently from the database.


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Note:

See the above client program, line numbers 16 to 19 we just loaded the object and called the

corresponding setters methods, its not related to the database row.

If you see, line number 24 we called save method in the session interface, means the object is

now having the relation with the database.

If we want to convert the object from transient state to persistent state we can do in 2 ways

o By saving that object like above.o By loading object from database.

If we do any modifications all the changes will first applied to the object in session cache only (let we do

the modifications 5 times, then 5 times we need to save the changes into the database right, which

means number of round trips from our application to database will be increased, Actually if we load an

object from database, first it will saves in the cache-memory so if we do any number of changes all will


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be affected at cache level only and finally we can call save or update method so with the single call of

save or update method the data will be saved into the database.)

If we want to save an object into database then we need to call any one of the following methods:

Save()

Persist() SaveOrUpdate()

Transient state:

One newly created object, without having any relation with the database, means never persisted, not

associated with any session object.

Persistent state:

Having the relation with the database, associated with a unique session object.

Detached state and Detached state to Persistent state conversion:


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When close the session object, the persistent instance will become a detached instance.

Notes:

We have opened the session1 at line number 14 and closed at line number 20, see have been

loaded the product class object by using get() method.

Remember get() method always returns the super class object. See line number 22, iam trying to change the price, but it wont effect the database because its

not in the session cache so I need to take one more session to update this value in the database,

so for that reason I took one more session from line numbers 24 30.

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What the core interfaces are of hibernate framework?

Session interface.

SessionFactory interface.

Transaction interface.

Query and Criteria interface.

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What the different types of caches are in hibernate?

Two types of caches provided by hibernate:

First-level cache:

First level cache is associated with the session object.

By default hibernate uses first-level cache on a per-transaction basis.

Hibernate mainly uses this cache mainly to reduce the number of SQL queries it needs togenerate within a given transaction.

Second-level cache:

Second level cache is associated with the SessionFactory object.


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============================

How does hibernate second-level c

Hibernate always tries to fi

from the second-level cach

If this fails again, the object Hibernates static initialize(

second-level cache before

for manipulation proxies.

============================

What is a query cache in hibernate

The query cache is responsi

objects returned by querie

Query cache does not cach

identifier values and result

Hibernate stores timestam

new record then will fire q

This will always return youquery.

Query cache is an optional

the cached query results a

queries that are run freque

============================

What is hibernate caching?

Caching is all about application per

database to avoid the number of d

performance critical applications.

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What is callback interface in hibern

Callbacks interfaces allow the appli

to an object. Example: when an obj

implement these callbacks, but the

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ache work?

rst retrieve objects from the session and if this fails

e.

s are directly loaded from the database. ) method, which populates a proxy object, will atte

oing to the database. The hibernate class provides

=========================================

ible for caching the results and to be more precise t

.

e the state of the actual entities in the resultset; it

of value type.

s along with query to track new records and if hib

ery again evicting old data.

updated record because actual state object was ne

feature and requires two additional physical cache

d the timestamps when a table was last updated. T

ntly with the same parameters.

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ormance optimization and it sits between your app

tabase hits as many as possible to give a better pe

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ate?

cation to receive a notification when something int

ect is loaded, saved, or deleted. Hibernate applicati

y are useful for implementing certain kinds of gene

==============

it tries to retrieve

pt to hit the

static methods

==============

he keys of the

ache only

rnate found a

er cached with

regions that hold

his is useful for

==============

lication and the

formance for

==============

resting happens

ons dont need to

ic functionality.


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=====================================================================================

What is difference between update() and merge() method?

Both methods in hibernate are used to convert the object which is in detached state into persistence

state.

Note:

See from line numbers 6-9, we just loaded one object s1 into session1 cache and closed session1 at line

number 9, so now object s1 in the session1 cache will be destroyed as session1 cache will expireswhenever we say session1.close.

Now s1 object will be in some RAM location, not in the session1 cache.

Here s1 is in detached state, and at line number 11 we modified that detached object s1, now if we call

update() method then hibernate will an error, because we can update the object in the session only.

So we opened another session[session2] at line number 13, and again loaded the same student object

from the database, but with name s2.

So in this session2, we called session2.merge(s1); now into s2 object s1 changes will be merged and

saved into the database.

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What difference between load and get method?

Load method:

It throws ObjectNotFoundException if supplied id is not found.

Load just returns a proxy by default and database wont be hit until proxy is first invoked.

Get method:

It returns null, if supplied id is not found.

Get will hit the database immediately.

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What are the different caching strategies?

Read-only:

This strategy is useful for data that is read frequently but never updated. This is by far the

simplest and best-performing cache strategy.

Read/write:

Read/write caches may be appropriate if your data needs to be updated. They carry more

overhead than read-only caches. In non-JTA environments, each transaction should be

completed when session.close() or session.disconnect() is called.

Nonstrict read/write:

This strategy does not guarantee that two transactions wont simultaneously modify the same

data. Therefore, it may be most appropriate for data that is read often but only occasionally

modified.

Transactional:

This is fully transactional cache that may be used only in a JTA environment.

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What is version checking in hibernate?

Version checking used in hibernate when more than one thread trying to access same data.

Example:

User-A edit the row of the TABLE for update (in the user interface changing data this is user

thinking time) and in the same time User-B edit the same record for update and click the

update. Then User-A click the update and update done. Change made by User-B gone. In

hibernate you can prevent slate object updation using version checking.


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Check the version of the ro

you are fetching the row of

number and match with yo

============================

Difference between getCurrentSes

getCurrentSession () :

A session is opened when

when the transaction ends

Every time it returns same

openSession ():

If you decide to use manag

have to flush() and close() i

Every time it returns differ

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What does session.refresh() do?

It is possible to reload an o

This is useful when databa

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How to get JDBC connections in hib

User session.connection()

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Whats the use of session.lock() in

Session.lock() method of se

This method of reattaching

reattaching the object and

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What is the difference between th

Both of these methods and

object.

w when you are updating the row. Get the version

the TABLE for update. On the time of updation just

ur version number (on the time of fetching).

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ion () and openSession () in hibernate?

etCurrentSession() method is called for the first tim

automatically.

session object.

e the session yourself then go for SessionFactory.o

t. It does not flush and close() automatically.

nt session object.

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bject and all its collections at any time, using the re

e triggers are used to initialize some of the propert

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ernate?

ethod to get JDBC connection.

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hibernate?

ssion is used to reattach an object which has been

doesnt check for any data synchronization in data

hence may lead to lack of synchronization in data.

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session.update() method and the session.lock() m

saveOrUpdate() method are intended for reattachi

f the row when

fetch the version

==============

e and closed

enSession(), you

==============

esh() method.

ies of the object.

==============

==============

detached earlier.

ase while

==============

ethod?

ng a detached


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The lock() method simply reattaches the object to the session without checking or database on

the assumption that the database in sync with the detached object.

It is best practice to use either update() or saveOrUpdate() method.

Use lock() method only if you are absolutely sure that the detached object is in sync with your

detached object or if it does not matter because you will be overwriting all the columns that

would have changed later on within the same transaction.

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What is sessionFactory interface?

The sessionFactory is the concept that is a single data store and thread safe.

It is thread safe.

The cache that is immutable of compiled mappings for a specific database.

A sessionFactory will be built only at the time of its startup.

In order to access it in the application code, it should be wrapped in singleton.

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What is session interface?

This represents hibernate session which perform the manipulation on the database entities.

Session manages persistence state, fetching persisted ones and management of the transaction

demarcation.

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Explain about the dirty checking feature of hibernate?

Dirty checking is a feature of hibernate that saves time and effort to update the database when

states of objects are modified inside a transaction. All persistent objects are monitored by

hibernate. It detects which objects have been modified and then calls update statements on all

updated objects.

Hibernate session contains a PersistenceContext object that maintains a cache of all the objects

read from the database as a Map. So, when you modify an object within the same session.

Hibernate compares the objects and triggers the updates when the session is flushed. The

objects that are in the PersistenceContext are persistent objects.

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Explain about mapping description file?

This mapping file has an extension .hbm.xml which instructs mapping between java class and

database tables.

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What is addClass function?

This function translates a j

as an input stream from th

efficient usage of classes in

============================

How can a whole class be mapped

Mark the class as mutable=

By default it is true.

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What is hibernate tuning?

The key to obtain better pe

Optimization, Session Man

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How to execute stored procedure i

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Difference between SQL and HQL?

HQL uses pojo-class-name i

column-name for retrievin

SQL is based on a relationalprogramming with relation

SQL manipulates data stor

about objects and its prop

SQL concerned about the r

relation between two obje

HQL is case sensitive while

va class name into file name. This translate file na

java class loader. This addClass function is import

your code.

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s immutable?

false.

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rformance in any hibernate application is to emplo

gement, and Data Caching.

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hibernate?

=========================================

nstead of using table-name and property of pojo-cl

object from DB while SQL use table-name and col

l database model whereas HQL is a combination ofal database concept.

d in tables and modifies its rows and columns. HQL

rties.

lationship that exists between two tables while H

ts.

SQL is not case sensitive.

e is then loaded

nt if you want

==============

==============

SQL

==============

==============

ass instead of

mn-name.

object-oriented

is concerned

L considers the


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HQL and SQL both fire que

objects that are translated

SQL works with tables and

HQL works with classes an

database.

HQL supports concepts likeeasy-to-learn language tha

You write database type i

runtime.

HQL contains concepts like

developers.

HQL result is not plain data

HQL even returns the child

In case of HQL, hibernate a

underlying database.

============================

Difference between HQL and criteri

HQL is to perform both sel

selecting the data, we cann

HQL is suitable for executin

queries.

HQL doesnt support pagin

Criteria used to take more

With criteria we are safe wi

as your queries are either fi

============================

What is pagination in hibernate?

Pagination is used when yo

record into a single page.

For convenient and better

result set is divide into sma

setFirstResult(int arg0): sets the sta

setMaxResults(): sets the maximum

============================

ies in a database. In the case of HQL, the queries ar

to SQL queries in the target database.

columns to manipulate the data stored in it.

their properties to finally be mapped to a table str

polymorphism, inheritance, association, etc. It is amakes SQL object oriented.

dependent queries in HQL that are converted into

pagination, dynamic profiling etc. all that are unkn

but a combination of objects that can be modified

objects as part of the query result.

utomatically generates the SQL query and executes

=========================================

ia API?

ct and non-select operations on the data, but crite

ot perform non-select operations using criteria.

g static queries, whereas Criteria is suitable for exe

tion concept, but we can achieve pagination with

ime to execute then HQL.

ith SQL injection because of its dynamic query gene

ixed or parameterized, there is no safe from SQL inj

=========================================

u have to display large amount of result. It is tough

isplay hibernate provides concept of pagination. In

ll number of pages and at a time you can fetch one

rting point of your table record.

result into your single page.

=========================================

e in the form of

ucture in a

powerful and

SQL queries at

wn to SQL

programmatically.

it against

==============

ia is only for

cuting dynamic

riteria.

ration but in HQL

ection.

==============

to display all the

pagination large

page.

==============


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Types of inheritance provided by hibernate?

There are three types of inheritance mapping in hibernate?

Table per class with unions

Table per class hierarchy

Table per subclass

Example:

Class Manager and worker are inherited from Employee abstract class.

Table per concrete class with unions

In this case there will be 2 tables: Manager and worker [all common attributes will be

duplicated].

Table per class hierarchy

Single table can be mapped to a class hierarchy

There will be only one table in database called Employee that will represent all the attributes

required for all 3 classes.

But it needs some discriminating column to differentiate between Manage and Worker.

Table per subclass

In this case there will be 3 tables represent Employee, Manager and Worker. Common attributes

will not duplicated.

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Types of mapping in hibernate?

Collection mapping

Association mapping

Component mapping

Collection mapping

Collection mapping refers to a one-to-many or many-to-mapping relationship which will be

mapped by using an implementation of java.util.collection.

Association mapping

Association mapping refers to a many-to-one or one-to-one relationship which will be mapped

by using another class which you have mapped in hibernate (also called an entity). The

association object has its own lifecycle and is simply related to the first object.

Component mapping

Component mapping refers to mapping a class (or collection of classes) whole lifecycle is bound

tightly to the parent. This is also called composition in the strict definition of the word in


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object-oriented programming. Basically if you delete the parent object the child object should

also be deleted; it also cannot exist on its own without a parent.

=====================================================================================

Difference between NamedNativeQuery and NamedQuery?

NamedNativeQuery lets you write a named SQL query, while NamedQuery lets you write a

named HQL query.

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Difference between save and persist method in hibernate?

Return type of persist method is void while return type of save method is serializable object.

Persist method and save method both make a transient instance persistent. However, persist

method doesnt guarantee that the identifier value will be assigned to the persistent instance

immediately, the assignment might happen at flush time.

Persist method guarantees that will not execute an INSERT statement if it is called outside oftransaction boundaries. Save method not guarantee the same, it returns an identifier, and if an

INSERT has to be executed to get the identifier (e.g. identity generator), this INSERT happens

immediately, no matter if you are inside or outside of a transaction.

Fourth difference between save and persist method in hibernate is related to previous

difference on save vs persist. Because of its behavior of persist method outside transaction

boundary, its useful in long-running conversations with an extended session context. On the

other hand save method is not good in a long-running conversation with an extended session

context.

Save, saveOrUpdate and persist methods are used to save data into database. But save can only

INSERT records but saveOrUpdate can either INSERT or UPDATE records.

=====================================================================================

What is named SQL query in hibernate?

Named queries are SQL queries which are defined in mapping document using tag

and called session.getNamedQuery(nameofquery) method.

Named query allows you to refer a particular query by the name you provided, by the way you

can define named query in hibernate either by using annotations or xml mapping file, as I said

above.

@NameQuery is used to defined single name query and @NameQueries is used to define

multiple name query hibernate.

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Why its important to provide no argument constructor in hibernate entities?

Every hibernate entity class must contain a no argument constructor, because hibernate

framework creates instance of them using reflection API, by calling Class.newInstance() method.


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This method will throw InstantiationException if it doesnt found no argument constructor inside

entity class.

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Can we make an entity class final in hibernate?

Yes, you can make hibernates entity class final, but thats not a good practice. Since hibernate

uses proxy pattern for performance improvement in case of lazy association, by making an

entity final, hibernate will no longer be able to use proxy, because java doesnt allow extension

of final class, thus limiting your performance improvement options. Though, you can avoid this

penalty, if your persistent class is an implementation of interface, which declares all public

methods defined in entity class.

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What are extension interfaces?

When the built-in functionalities provided by hibernate is not sufficient enough, it provides away so that user can include other interfaces and implement those interfaces for user desire

functionality. These interfaces are called as Extension interfaces.

=====================================================================================

What are the extension interfaces that are there in hibernate?

ProxyFactory interface

Used to create proxies.

ConnectionProvider interface

Used for JDBC connection management.

TransactionFactory interface

Used for transaction management.

Transaction interface

Used for transaction management.

TransactionManagementLookup interface

Used in transaction management.

Cache interface

Provides caching techniques and strategies.

CacheProvider interface

Same as cache interface.


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ClassPersister interface

Provides ORM strategies.

IdentifierGenerator interface

Used for primary key gener

Dialect abstract class

Provides SQL support

============================

What is meant by method chaining

Method chaining is a progr

This is less readable when

format. Look how a session

============================

What are POJOs?

POJO stands for plain old java obje

methods for all the properties thatlogic related to that property.

============================

How does hibernate distinguish bet

Hibernate uses the version

If not uses the identifier val

hibernate managed surrog

managed by hibernate) sur

Write your own strategy wi

============================

What is Bag in hibernate?

A bag is a java collection th

It allow duplicate values.

We cannot define primary

ation.

=========================================

?

amming technique that is supported by many hiber

ompared to actual java code. And it is not mandat

Factory is created when we use method chaining.

=========================================

ts. These are just basic JavaBeans that have define

are there in that bean. Besides they can also have s

=========================================

ween transient (i.e. newly created) and detached o

property, if there is one.

lue. No identifier value means a new object. This do

te keys. Does not work for natural keys and assign

rogate keys.

th interceptor.isUnsaved().

=========================================

at stores elements without caring about the indexi

eys in bag.

==============

nate interfaces.

ry to use this

==============

setter and getter

ome business

==============

bject?

es work only for

d (i.e. not

==============

g.


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=====================================================================================

Generators in hibernate?

The following are the list of main generators we are using in the hibernate:

1. Assigned

2. Increment

3. Sequence

4. Identity

5. Hilo

6. Native

7. Foreign

8. Uuid.hex

9. Uuid.string

In the above generators list, the first 7 are used fro int, long, short types of primary keys, and last 2 are

used when the primary key column type is string type (varchar2).

Assigned

Supported by all database.

It is default generator which is used by hibernate.

If generator class is assigned, then the programmer is responsible for assigning the primary key

value to object which is going to save into the database.

Increment

Supported by all databases.

This generator is used for generating the id value for the new record by using the formula Max

of id value in Database + 1.

If we manually assigned the value for primary key for an object, then hibernate doesnt consider

that value and used max value of id in database + 1 concept only.

If there is no record initially in the database, then for the first time this will saves primary key

value as 1.

Sequence

Not supported by MySql.

While inserting a new record in a database, hibernate gets next value from the sequence under

assigns that value for the new record.

If programmer has created a sequence in the database then that sequence name should be

passed as the generator.

If the programmer has not passed any sequence name, then hibernate creates its own sequence

with name hibernate-sequence and gets next value from that sequence, and then assigns that

id value for new record.

But remember, if hibernate wants to create its own sequence, in hibernate configuration file,

hbm2ddl.auto property must be set enabled.


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Example:

Create sequence MySequence incremented by 5:

First it will starts with 1 by default.

Though you send the primary key value.., hibernate uses this sequence concept only.

But if we not create any sequence, then first 1 and increments by 1 ..bla bla. In this case

hibernate creating right..? so ensure we have hbm2ddl.auto enabled in the configuration file.

Identity

Not supported by oracle.

In this case the id value is generated by the database, but not by the hibernate, but in case of

increment hibernate will take over this.

This identity generator doesnt needs any parameters to pass.

This identity generator is similar to increment generator, but the difference was increment

generator is database independent and hibernate uses a select operation for selecting max id

before inserting new record.

But in case of identity, no select operation will be generated in order to insert an id value for

new record by hibernate.

Hilo

This generator is database independent.

For the first record, the id value will be inserted as 1.

For the second record the id value will be inserted as 2343.

For the next records the id value will be incremented by 2343 and will stores into the database (I

mean adds to the previous).

Actually this hibernate stores the count of id values generated in a column of separated table,

with name hibernate_unique_key by default with the column name next_hi.

If we want to modify the table and column names then we need to pass 2 parameters for the

hilo generators.

Native

When we use this generator class, it first checks whether the database supports identity or not,

if not checks for sequence and if not, then hilo will be used finally the order will be..

o Identity

o Sequence

o hilo

Example:

If we are connecting with oracle, if we use generator class as native then it is equal to the generator

class sequence.

=====================================================================================


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Hibernate One

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