Hibernate Interview Questions

Hibernate Interview Questions

1.      What is ORM?         

ORM is mediator software between a java application and a database to transfer to and fro in the form of objects

2. What is mapping?

Mapping is a mechanism of providing mappings b/w a java class to database table and a java class

Properties to a table coloumns.mapping can be done in two ways

             1. Using xml

             2. Using annotations

3. What is framework?

A framework is a an abstraction layer on top of existing technologies. Frameworks are introduced by

Third party vendors to reduce the burden of java developers

4. What is Hibernate?

Hibernate is a persistent layer framework, which is built on top of jdbc technology it transfers objects b/w java applications to database

5. Feature of hibernate?

Hibernate is suitable for all java types of java projects because hibernate doesn’t require any

Server to run its code

1.Hibernate supports timestamp and  versioning

2.Hibernate is database independent. it uses own query  language called Hibernate Query Language(HQL)

3.In hibernate all exceptions are unchecked exceptions  so a programmer is no need to handle the exceptions

4.Hibernate supports associations(relationships) like one to one, one to many, many to many, many to one

5.Hibernate supports lazy loading it improves the performance of an application

Hibernate supports caching mechanism it reduces the rounds b/w java application

 And database.

6. What is pojo class?

A pojo stands for plain old java object. it is an java ordinary java class it is not extending or implementing any class or interface or a framework or any technology

Ex: public class Demo----------> pojo class

{

…………

}

Ex2: public class Demo extends Exception {……..>pojo class

………

}

3. Public class Test extends GenericServlet{……………>not a pojo class becoz extended some technology

4. p c Demo{……….>pojo class

Void main () {

……logic of some technology……….

}

Void m2 () {

………….logic of servlet technology……

}

7. What is java Bean?

A java bean is like ordinary java class with the following conditions

1. Class must be public

2. Class must have public default constructer

Ex.1.public class Demo {…………..>java bean

}

2.      Class Demo implements sertilizable {………..>not a java bean

8. What is DAO? Or DAO class

Stands for data access object. It is a design pattern suggested that to separate business logic and persistent logic

DAO is implemented in java in the form of class that class is called DAO class. The DAO class like

a pojo class which purely contains persistent logic of an application

9. In how many ways mapping can be done in Hibernate?

Two ways,    1.by constructing xml file

3.      by using annotations

9.  What configuration file contains in hibernate?

          1. Connection properties

          2. Hibernate properties

          3 mapping properties

10. What jdbc statement used by Hibernate internally?

            PreparedStatement

11. What is the need of dialect in configuration file?

To generate database related sql queries internally

12. What is the benefit of configuration file name as hibernate.cfg.xml?

àWe named configuration file as hibernate.cfg.xml.we no need to pass file name as

Parameter to configure () method

àIf file name is some other like satya.cfg.xml.we need pass the configuration file name

To configure () method as mandatory

Ex: 1 )If file name is hibernate.cfg.xml

                       Conf.configure ();-------------->correct

                       Conf.configure (“hibernate.cfg.xml”);--------->correct

2) If file name is satya.cfg.xml

                         Conf.configure ();----------->.incorrect

                      Conf.configure(“satya.cfg.xml”);-------àcorrect

16. Can we develop an hibernate application without creating hibernate.cfg.xml OR not?

Yes. Instead of hibernate.cfg.xml we can create hibernate. Properties file

We can also create an hibernate application by without hiberbate.cfg.xml and hibernate. Properties

17. What is the diff b/w hibernate.cfg.xml and hibernate. properties?

Hibernate.cfg.xml allows connection, hibernate and mapping properties to configure. But in hibernate. Properties file only connection and hibernate properties are allowed mapping properties are not allowed.

Instead of these we can directly also we can write a client application

EX.          Configuration conf=new Configuration ();

                  //connection properties

Conf.setProperty (“hibernate.connection.driver_class”,”pass driver “);

Conf.setProperty(“hibernate.connection.url”,”pass url “);

Conf.setProperty(“hibernate.connection.username”,”username “);

Conf.setProperty(“hibernate.connection.password”,”pass pwd “);

                //hibernate properties

Conf.setProperty (“hibernate.dialect”,”pass class name with pakg  “);

Conf.setProperty(“hibernate.hbm2dll.auto”,”pass any like create,update “);

Conf.setProperty(“hibernate.show-sql”,”pass true “);

Mapping properties

Conf.addFile(“product.hbm.xml”);

18.What is purpose of generator class?

In hibernate the primary key (Id) value of an object will be created by either programmer or hibernate.the purpose of this is to generate an id in a table

There are different generator class in hibernate.

 --------àassigned

--------àincrement

-------àsequence

-------àhilo

------àidentity

-----ànative

----àforeign

-------------------àthe default generator class in hibernate is “assigned”

-----------------àthe <generator > must be offter id tag  cfg.xml file

-------------àassigned ,increment and Hilo generator classess are database independent

-----------àsequence identity ,native and foreign are database dependent

19.How to generate a user defined generator class in hibernate?

If we want to create an user defined generator then follow the below steps

1.create a public class and implement from it org .hibernate.id.IdentifierGenerator

2.override the only abstract method of IdentifierGenerator interface called generate() method

3.define the id generation logic in generate() method

            EX.import org.hibernate.id.*;

            Import java.io.*;

            Import org.hibernate.engine.*;

            Public class MyIdGenerator implements IdentifierGenerator

            {

            public Serializable generate(SessionImplementaor     implementor,Object o)throws Exception

            {

                                    //logic

}

20. Why Composite id in hibernate?

----à A database table contains multiple primary keys then we use composite id  in mapping file

----àIf a database table contains single primary key we go for <id> tag in mapping file

Composite id can be used in two ways

1. We can create both primary and non primary keys  properties in a pojo class

2. We can separate primary keys in a separate java class  and we can create a reference of the class in pojo class

21. What are the different state of pojo object in hibernate or object life cycle states?

1. Transient state

2. Persistent state

3. Detached state       

--------àwhen a pojo class object is newly created or it is assigned with null then the object will be transient state.to convert transient state to persistent state if object is new then we call following one of the methods

            1. Save ()

            2.  Persist ()

            3. SaveOrUpdate ()

If object is null then to convert this object from transient to persistent state we call the following methods

            1load()

            2.get()

---------àwhen a pojo class object is entered in to session cache then that object is in persistent state.To convert  an object from persistent to detached state by calling one of the following methods   

            1.close()

            2.clear()

            3evict()

---------à when an pojo class  object come out of a session cache then the object is entered into detached state..To convert detached to persistent state by calling one of the following methods

            1.update()

            2.merge()

            3.saveOrUpdate()

--------àwe can also convert an object from detached state to transient or from persistent to transient by calling  following method

            Delete()

22.What is the difference b/w save() and persist() methods?

------àwhen generator class is assigned then a programmer explicitly assigns an id of an object. so programmer is no need to get the id of saved object so in this case persist() method is performed

------àwhen a programmer is other then assigned then id of an object will be assigned by hibernate. so a programmer need to get the  id of saved object in this case save() method is performed

23.What is difference b/w save() and saveOrUpdate() methods?

Save() method can perform save() operation but saveOrUpdate() method performs save() and Update() operations

saveOrUpdate () method performs save() operation ,if the id is new if the id already exist then performs update() operations

24. For properties of a pojo class primitives or wrappers is better type?

--àWrapper types is better then primitive types,bcoz if we don’t assign a primitive propertiey value then while saving its default value will be stored in database.This default value leads to misunderstanding of the data

-àTo overcome the above problem ,if we take wrapper type then instead of default null value will be stored in the database so there  will be no misunderstanding of data

25. What is the difference b/w merge() and update() methods?

-----àWhile converting an object from a detached state to persistent state ,if a session cache already contains the sane object and if we call update() then hibernate throws “non uniqueObjectException”bcoz  a session cache of an hibernate  can’nt maintain two objects of same pojo class with same id

----------àby overcoming the above exception instead of calling update() we can call merge() method .It will find changes b/w detached object.

24) How many ways objects can be identified in hibernate?

           

1. using object identity: using == operator

2. using object equavality:using equals()method

3.using database identity:Relational database objects can identified if they present in same row

25)How mapping of java objects is  done with database tables?

To map java objects to database tables. We need to have java bean properties names same as columns names of database tables.

In database

Java class name=Database table name

Java class properties names=columns of database

Java class one object=one row of database

25)what is an HQl?

An object oriented format of an sql is called Hql. hql is simple,flexiable,and efficient query language to do various types of relational database operations without writing any complex queries

àby using this we can perfoerm multi row operations

-àBy using hql we can perform both select and non-select operations

-àAn hql is database independent

26) What is Criteria API?   

Criteria is multi row mechanisms is only for select operation on a database. We cant perform non select operations on database

è By using Criteria API we can load full object and partial object with multiple properties, adding conditions to criteria, finding aggregate results, pagination of objects and sorting order of objects

27) Pagination methods in criteria?

We have two pagination methods in criteria

            1. SetFirstResults (3);

            2. setMaxResults (5)

           

28)What is Native SQL?

àNative sql is used for executing sql queries directly on a database from an hibernate application

àIt is useful when we are migrating with jdbc to hibernate app .

àit is for both select and non-select operations

29) Relationships/Associations in hibernate?

            1. One to many

            2. Many to one

            3. one to one

            4 many to many

30) What are the collections supported by hibernate I relationships?

Hibernate supported by 3 collections

            1. Set  

            2. List

            3. Map

31) What are the different values of cascade attribute?

            We have six values of cascade attribute

            1. none (default)

            2. save-update

            3. save

            4. all

            5. delete

            6. all-save-delete

32) What is <bag> tag?

àwhen a collection type is List,we use <list> tag in mapping file---but lis for list  tag hibernate uses additional column in child for storing list object

àThe list index in child table is only for representation but there will be no use with it in operations

àTo avoid index column from child table we use <bag> tag in hbm file instead of <list> tag

32) What are the values of lazy attribute values in one-to many relationship?

We have two values for lazy attribute in one-to-many relationship

1.lazy=”true”------àlazy loading

2.lazy=”false”------àearly loading

Here default value  true

33) What are the values of lazy attribute values in many-to-one relationship?

We have two values for lazy attribute in one-to-many relationship

1.lazy=”proxy”------àproxy loading. It is equal to lazy loading

2.lazy=”false”---------àearly loading

Here default  value  proxy

34)What is inverse attribute in one-to-many relationship?

When one-to-many uni or bi direction relationship is applied then hibernate uses update operation to assign foreign for each child record separately

àTo reduce the update operation we can use inverse=”true” attribute in collection tag(set/map/list)

-àthe default value of inverse attribute is false

35.What is <idbag> tag  in many-to-many relationship in mapping file?

àIn many-to-many relationship ,joint table holds forgin keys of the relationship

àIn joint table contains multiple forgin key columns here. So there is no uniqueness column (like primary key) in that table.we have to provide uniqueness to that we use <idbag> tag in hbm file

àTo configure the <idbag> tag in pojo class the collection type must be List

à<idbag> tag uses <collection-id>tag to inform the uniqueness of the column joint table to hibernate.<collection-id> tag uses generator tag.the generator class other then assgined

36)Where foreign generator class used and what is the purpose?

àThis generator class only used in one-to-one relationship with primary key

àThe use of foreign generator class to inform the that primary keys of child table  also acting as a foreign key

37. What is “1+n” problem in hibernate?  

While working with one-to-many relationship ,while reading the parent objects with respective child objects of each parent ,hibernate uses one select operation for loading all parent objects and select operations for loading child objects of parent objects This problem is called “1+n” problem

For example:

We have three parent objects with some number of Childs for each parent then hibernate uses “1+3” select operations to read three parent objects with their child objects

38)How to solve “1+n” problem in hibernate?

Solving “1+n” problem is nothing but reducing “1+n” selects operations into one select operation

àTo solve this problem we have two options

1.use join fetch in hql query

2.use setFetchMode() method of criteria

To load  parents with their childs using a single query  then we use the following

-> Query qry=session.createQuery(“from  Parent p join fetch p.childs”);

List list=qry.list();

à Criteria crit=session.createCriteria(Parent.class);

Crit.setFetchMode(“childs”,FetchMode.JOIN());

           

38) How many ways can u apply one-to-one relationship?

In two ways one-to one can be applied

1. one-to-one with foreign key

2. one-to-one with primary key

39) Why relationships in hibernate?

à In hibernate if all the data of an entity is stored in a single object. then redundancy occurs like database tables

To reduce the redundancy of data in objects ,we dived the data into multiple objects. then we assign relation b/w the objects

àIn hibernate; to divide data of one object into two objects we use pojo classes

40) Do you have any Entity beans in hibernate?

Yes we have,In hibernate Pojo classes are called Entity beans

41)How can u say a relationship need collection type property or reference type property?

àIf the relationship ends with” to-many” then collection type is need in pojo parent class

àIf the relationship is ends with” to-one” then a reference id need in child pojo class

42) Do you have any joins in hibernate?

            OR

What are the types joins in hibernate?

Yes .we have four types of jons  in hibernate.

            1.left join

            2.right join

            3.full join

            4.inner join

The default join is inner join

42) What is the use of joins in hibernate?

Joins are used to select the data from multiple objects using a single query with relationship

43)What is inheritance mapping?

While constructing a pojo classes if some properties are repeated in more then one pojo classless then inheritance is applied b/w the pojo classless to reduce the repeated properties(redundancy)

It applicable when primary key is in base class. If primary key is derived class then is not possible

We have three inheritance mapping strategies

            1.table per class hierchiey

            2.table per concrete class hierarchy

            3.table for sub class hieachiey

àin first hiearchiey,hibernate stores all class hierarchies data into a single table.for this we have to inform to hibernate in mapping file we use tag <subclass>

àIn second, hibernate stores each concrete class data of inheritance hierarchies into a separate table of database. we have to inform to hibernate  that use <union-subclass> tag in hbm file

àIn third, hibernate stores each concrete class hiearachiey in a separate table of database. for this to inform to hibernate use<joined-subclass> in hbm file

44) What is component mapping in hibernate?

While designing a pojo class in hibernate if you have any common properties, we can go to inheritance mapping or component mapping.

 àInheritance mapping is called “is-a “relationship

àComponent mapping is called “has-a” relationship

45) What is the diff b/w component mapping and Inherttance mapping?

à The main diff is, If primary key (id)is belongs to parent class then Inheritance mapping is applicable. If primary key (id) belongs to child class then Component mapping is applicable

46) What are the different types of mappings in hibernate?

We have six types of mappings in hibernate

            1. Partial object mapping

            2. Full object mapping

            3. Component mapping

            4. Inheritance mapping

            5. Association mapping

            6. Collection mapping

47) What is the diff b/w partial and full object mapping?

In hibernate its not mandatory to map all properties of pojo class to  column of table if one or more properties are not mapped then it is called partial object mapping

If all properties are mapped to column of table then it is called full object mapping

48.In how many ways objects can be fetched  from database in hibernate?

In hibernate objects can be fetched from database in four ways

                        1.Using HQL

                        2.Using Criteria API

                        3. Using Standard SQl(Native SQL)

                        4.Using Identifier

49) How primary key is created by using hibernate?

With the help of generator tag in mapping file of hibernate the primary key is created

50) What are the different fetching strategies’ of hibernate?

The following fetching strategies available in hibernate

            1. Join fetching

            2. Batch fetching

            3. Select fetching

            4. Sub-select fetching

51. What are the inheritance models of hibernate?

 We have three inheritance models of hibernate

1.table per class hierarchy

2.table per concrete class hierarchy

3.table per subclass hierarchy

52.What is Named SQL query in hibernate?

While working with HQl,Native SQl, If we want to reuse the same query for multiple times  in multiple client classes of an application we use Named SQl query

53. How can you see hibernate generated SQl on console?

By writing hibernate properties in configuration file. That properties is show_sql

54.) What is annotation?

Internally every annotation is an interface and its having implementation class.

Every annotation starts with @ symbol

To avoid mapping files in hibernate we use annotations to add metadata in a pojo class of hibernate

The implementation classes  of annotation interfaces of JPA are given by

Hibernate-jpa-version.jar

Common annotations added to pojo class are

1.@Entity--->applicable for class

2.@Table---->applicable for class

3.@Id---->applicable for  a field or a method

4.@Column--->applicable for a field or method

55) What is default level caching in hibernate?

In hibernate we have to levels of caching

            1. First level or session level or local caching

            2. Second level or factory level global caching

The default caching level is first level

56) What is first level or session level or local caching?

This level of caching cannot be shared by more than one session

57) What is second level or factory level caching?

If we want to share the more then one session we need second level caching

The second level caching is associated with Session Factory objects all the sessions of their factory share the cache

58) Different cache techniques provided by third party cache providers? 

            1. EH Cache (Easy hibernate)

            2. OS Cache (open symphony)

            3. Swarn Cache (Apache)

            4. Tree Cache (JBoss)

59) What are the different ways to disable the second level caching?

Hibernate second level caching can disabled any one of following way

1. by setting <use-second-level-cache>as false

2. by using CACHEMODE.IGNORE

3. Using cache provider as “org.hibernate.cache.NoCacheProvider”

60) How to enable the Second level caching hibernate?

By default second level caching is disabled. If we want to enable we use the following in cfg file

By setting <use-second-level-cache>as true

<property name=”hibernate. Cache. use-second-level-cache”>true</property>

 cache provider as “org.hibernate.cache.provider-class”

<property name=”hibernate.Cache.provider-class”>fully qualified provider class name</property>

61) What are the caching strategies in hibernate?

We have four caching strategies in hibernate

            1. Read-onlyàread only but never for update

            2. Read/Writeàupdated

            3. NonStrict read/writeàtwo transactions wont simultaneously modified the same data

            4. Transactionalàit is only used in JTA environments

1&2 is mostly used strategies

1.what is the advantage of Hibernate over jdbc?

There are so many
1)    Hibernate is data base independent, your code will work for all ORACLE,MySQL ,SQLServer etc.
In case of JDBC query must be data base specific. So hibernate based persistance logic is database independent persistance logic and JDBC based persistance logic is database dependent logic.
2)    As Hibernate is set of Objects ,
3) No need to learn SQL language.You can treat TABLE as a Object . Only Java knowledge is need.
In case of JDBC you need to learn SQL.
3)    Dont need Query tuning in case of Hibernate. If you use Criteria Quires in Hibernate then hibernate automatically tuned your query and return best result with performance.
In case of JDBC you need to tune your queries.
4)     You will get benefit of Cache. Hibernate support two level of cache. First level and 2nd level. So you can store your data into Cache for better performance.
In case of JDBC you need to implement your java cache .
5)    Hibernate supports Query cache and It will provide the statistics about your query and database status.
JDBC Not provides any statistics.
6)    Development fast in case of Hibernate because you dont need to write queries
7)    No need to create any connection pool in case of Hibernate. You can use c3p0.
In case of JDBC you need to write your own connection pool
8)    In the xml file you can see all the relations between tables in case of Hibernate. Easy readability.
9)    You can load your objects on start up using lazy=false in case of Hibernate.
JDBC Dont have such support.
10 ) Hibernate Supports automatic versioning of rows but JDBC Not.

2.What is Hibernate? 
Hibernate is an open source, light weight Object Relational Mapping tool to develop the database independent persistence login in java and j2ee based applications.
Hibernate is a pure Java object-relational mapping (ORM) and persistence framework that allows you to map plain old Java objects to relational database tables using (XML) configuration and mapping files. Its purpose is to relieve the developer from a significant amount of relational data persistence-related programming tasks
3.What is ORM ? 
ORM stands for object/relational mapping, means providing the mapping between class with table and member variables with columns is called ORM. ORM is the automated persistence of objects in a Java application to the tables in a relational database.

4.hat does ORM consists of ? 
An ORM solution consists of the following four pieces:

• API for performing basic CRUD operations
• API to express queries referring to classes
• Facilities to specify metadata
• Optimization facilities : dirty checking,lazy associations fetching

5.What are the ORM levels ? 
The ORM levels are:

• Pure relational (stored procedure.)
• Light objects mapping (JDBC)
• Medium object mapping
• Full object Mapping (composition,inheritance, polymorphism, persistence by reachability)

.
6.Why do you need ORM tools like hibernate? 
The main advantage of ORM like hibernate is that it can develop the database independent persistence logic. Apart from this, ORM provides following benefits:

• Improved productivity
• High-level object-oriented API
• Less Java code to write
• No SQL to write
• Improved performance
• Sophisticated caching
• Lazy loading
• Eager loading
• Improved maintainability
• A lot less code to write
• Improved portability
• ORM framework generates database-specific SQL for you

7.What Does Hibernate Simplify? 
Hibernate simplifies:

• Saving and retrieving your domain objects
• Making database column and table name changes
• Centralizing pre save and post retrieve logic
• Complex joins for retrieving related items
• Schema creation from object model

 8.What is the main difference between Entity Beans and Hibernate ?

1)In Entity Bean at a time we can interact with only one data Base. Where as in Hibernate we can able to establishes the connections to more than One Data Base. Only thing we need to write one more configuration file.

2) EJB need container like Weblogic, WebSphare but hibernate don't nned. It can be run on tomcat.


3) Entity Beans does not support OOPS concepts where as Hibernate does.

4) Hibernate supports multi level cacheing, where as Entity Beans doesn't.

5) In Hibernate C3P0 can be used as a connection pool.

6) Hibernate is container independent. EJB not.


9.What are the Core interfaces and classes of Hibernate framework? 
The five core interfaces are used in just about every Hibernate application. Using these interfaces, you can store and retrieve persistent objects and control transactions.

• Configuration class (org.hibernate.cfg package)
• Session interface (org.hibernate package)
• SessionFactory interface  (org.hibernate package)
• Transaction interface (org.hibernate package)
• Query and Criteria interfaces (org.hibernate package)

10.What is the general flow of Hibernate communication with RDBMS? 
The general flow of Hibernate communication with RDBMS is :

• Load the Hibernate configuration file and create configuration object. It will automatically load all hbm mapping files because mapping file can be configured in configuration file.
• Create session factory from configuration object
• Get one session from this session factory
• Create HQL Query
• Execute query to get list containing Java objects.

11.What is the need for Hibernate mapping file? 
Hibernate mapping file is used to provides the mapping between java class with table member variables with column names of the table. And also we can configure primary key generation algorithm, relations and so on. Typical mapping file look as follows:

12.What are the important tags of hibernate.cfg.xml?

This file can be used to provide the database information like driverclass name, url, database usename, database password, dialect, connection pooling mapping file and so on.
Following are the important tags of hibernate.cfg.xml: 

13.What role does the Session interface play in Hibernate? 
The main runtime interface between a Java application and Hibernate The Session interface is the primary interface used by Hibernate applications. It is a single-threaded, short-lived object representing a conversation between the application and the persistent store. It allows you to create query objects to retrieve persistent objects.
The main function of the Session is to offer create, read and delete operations for instances of mapped entity classes. Instances may exist in one of three states:

transient: never persistent, not associated with any Session
persistent: associated with a unique Session
detached: previously persistent, not associated with any Session

Session session = sessionFactory.openSession();
Session interface role:

• Wraps a JDBC connection
• Factory for Transaction
• Holds a mandatory (first-level) cache of persistent objects, used when navigating the object graph or looking up objects by identifier

14.What role does the SessionFactory interface play in Hibernate? 
SessionFactorys are immutable. The behaviour of a SessionFactory is controlled by properties supplied at configuration time. These properties are defined on Environment.
The application obtains Session instances from a SessionFactory. There is typically a single SessionFactory for the whole application—created during application initialization. The SessionFactory caches generate SQL statements and other mapping metadata that Hibernate uses at runtime. It also holds cached data that has been read in one unit of work and may be reused in a future unit of work
Implementors must be threadsafe.
SessionFactory sessionFactory = configuration.buildSessionFactory();

15.What are the most common ways to specify the  Hibernate configuration properties?
The most common methods of Hibernate configuration are:

• Programmatic configuration

        By using setProperty(-) method of org.hibernate.cfg.Configuration.

• XML configuration (hibernate.cfg.xml)
• By using .properties file
• By Using annotaions.(from Hibernate 3.3 on words)

16.How do you map Java Objects with Database tables?

• First we need to write Java domain objects (beans with setter and getter).
• Write hbm.xml, where we map java class to table and database columns to Java class variables.

Example :
<hibernate-mapping>
  <class name="com.durgasoft.EmployeeBean"  table="EMPLOYEE">
     <id name=”eid” colume=”id”/>
   <property name="ename" column="NAME" length="255"
     not-null="true"  type="java.lang.String"/>
   <property name="address" column="ADDR" length="255"
     not-null="true"  type="java.lang.String"/>
 </class>
</hibernate-mapping>

17.How do you define sequence generated primary key algorithm in hibernate? 
By using <id>, <generator> tags we can configure the primary key and primary key generation algorithm.
Example:-
<id name="userid" column="USER_ID" type="java.lang.Long">
   <generator class="sequence">
     <param name="table">SEQ_NAME</param>
   <generator>
</id>

18.What is component mapping in Hibernate?

• A component is an object saved as a value, not as a reference
• A component can be saved directly without needing to declare interfaces or identifier properties
• Required to define an empty constructor
• Shared references not supported

19 . Dif

ference between getCurrentSession() and openSession() in Hibernate ?

getCurrentSession() :
Obtains the current session. The "current session" refers to a Hibernate Session bound by Hibernate behind the scenes, to the transaction scope.
A Session is opened when getCurrentSession() is called for the first time and closed when the transaction ends. It is also flushed automatically before the transaction commits. You can call getCurrentSession() as often and anywhere you want as long as the transaction runs. Only the Session that you obtained with sf.getCurrentSession() is flushed and closed automatically.

openSession() :
If you decide to use manage the Session yourself the go for sf.openSession() , you have to flush() and close() it.
It does not flush and close() automatically.
Example :
Transaction tx =session.berginTransaction();

Session session = factory.openSession();

try {
tx.begin();

// Do some work
session.createQuery(...);
session.persist(...);

session.flush(); // Extra work you need to do

tx.commit();
}
catch (RuntimeException e) {
tx.rollback();
throw e; // or display error message
}
finally {
session.close(); // Extra work you need to do
}

20.What are the types of Hibernate instance states ? 
Three types of instance states:

• Transient -The instance is not associated with any persistence context
• Persistent -The instance is associated with a persistence context
• Detached -The instance was associated with a persistence context which has been closed – currently not associated

21.What are the types of inheritance models in Hibernate? 
There are three types of inheritance models in Hibernate:

• Table per class hierarchy
• Table per subclass
•  Table per concrete class

22.What is Hibernate Query Language (HQL)?
Hibernate Query Language is query language which is used to develop the data independent query language in the application. This HQL queries are not related to any database. Hibernate offers a query language that embodies a very powerful and flexible mechanism to query, store, update, and retrieve objects from a database. This language, the Hibernate query Language (HQL), is an object-oriented extension to SQL.

23.What are the ways to express joins in HQL? 
HQL provides four ways of expressing (inner and outer) joins:-

• An implicit association join
• An ordinary join in the FROM clause
• A fetch join in the FROM clause.
• A theta-style join in the WHERE clause.

24 . Transaction with plain JDBC in Hibernate ?

If you don't have JTA and don't want to deploy it along with your application, you will usually have to fall back to JDBC transaction demarcation. Instead of calling the JDBC API you better use Hibernate's Transaction and the built-in session-per-request functionality:

To enable the thread-bound strategy in your Hibernate configuration:

set hibernate.transaction.factory_class to org.hibernate.transaction.JDBCTransactionFactory
set hibernate.current_session_context_class to thread

Session session = factory.openSession();
Transaction tx = null;
try {
tx = session.beginTransaction();

// Do some work
session.load(...);
session.persist(...);

tx.commit(); // Flush happens automatically
}
catch (RuntimeException e) {
tx.rollback();
throw e; // or display error message
}
finally {
session.close();
}

25 . What are the general considerations or best practices for defining your Hibernate persistent classes?

1.You must have a default no-argument constructor for your persistent classes and there should be getXXX()and setXXX() methods for all your persistable instance variables.

2.You should implement the equals() and hashCode() methods based on your business key and it is important not to use the id field in your equals() and hashCode() definition if the id field is a surrogate key (i.e. Hibernate managed identifier). This is because the Hibernate only generates and sets the field when saving the object.

3. It is recommended to implement the Serializable interface. This is potentially useful if you want to migrate around a multi-processor cluster.

4.The persistent class should not be final because if it is final then lazy loading cannot be used by creating proxy objects.

26 . Difference between session.update() and session.lock() in Hibernate ?

The session.update method is used to update the persistence object in the in the database.
 The session.lock() method simply reattaches the object to the session without checking or updating the database on the assumption that the database in sync with the detached object. It is the best practice to use either session.update(..) or session.saveOrUpdate(). Use session.lock() 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.

27.What are the Collection types in Hibernate ?

• Set
• List
• Array
• Map
• Bag

28.What is the difference between sorted and ordered collection in hibernate? 
sorted collection vs. order collection :-

sorted collection	order collection
A sorted collection is sorting a collection by utilizing the sorting features provided by the Java collections framework. The sorting occurs in the memory of JVM which running Hibernate, after the data being read from database using java comparator.	Order collection is sorting a collection by specifying the order-by clause for sorting this collection when retrieval.
If your collection is not large, it will be more efficient way to sort it.	If your collection is very large, it will be more efficient way to sort it .

29.What are the ways to express joins in HQL? 
HQL provides four ways of expressing (inner and outer) joins:-

• An implicit association join
• An ordinary join in the FROM clause
• A fetch join in the FROM clause.
• A theta-style join in the WHERE clause.

30.What do you mean by Named – SQL query? 
Named SQL queries are defined in the mapping xml document and called wherever required.
Example:
<sql-query name = "empdetails">
   <return alias="emp" class="com.durgasoft.Employee"/>
      SELECT emp.EMP_ID AS {emp.empid},
                 emp.EMP_ADDRESS AS {emp.address},
                 emp.EMP_NAME AS {emp.name}
      FROM Employee EMP WHERE emp.NAME LIKE :name
</sql-query>

Invoke Named Query :
List people = session.getNamedQuery("empdetails")
                     .setString("TomBrady", name)
                     .setMaxResults(50)
                     .list();

31.How do you invoke Stored Procedures?

<sql-query name="selectAllEmployees_SP" callable="true">
 <return alias="emp" class="employee">
   <return-property name="empid" column="EMP_ID"/>      
   <return-property name="name" column="EMP_NAME"/>      
   <return-property name="address" column="EMP_ADDRESS"/>
    { ? = call selectAllEmployees() }
 </return>
</sql-query>

32.Explain Criteria API 
The interface org.hibernate.Criteria represents a query against a particular persistent class. The Session is a factory for Criteria instances. Criteria is a simplified API for retrieving entities by composing Criterion objects. This is a very convenient approach for functionality like "search" screens where there is a variable number of conditions to be placed upon the result set.
Example :
List employees = session.createCriteria(Employee.class)
                         .add(Restrictions.like("name", "a%") )
                         .add(Restrictions.like("address", "Boston"))
                         .addOrder(Order.asc("name") )
                         .list();

33.What’s the difference between load() and get()?

load()	get()
Only use the load() method if you are sure that the object exists.	If you are not sure that the object exists, then use one of the get() methods.
load() method will throw an exception if the unique id is not found in the database.	get() method will return null if the unique id is not found in the database.
load() just returns a proxy by default and database won’t be hit until the proxy is first invoked.	get() will hit the database immediately.

34.What is the difference between and merge and update ? 
Use update() if you are sure that the session does not contain an already persistent instance with the same identifier, and merge() if you want to merge your modifications at any time without consideration of the state of the session.
35.Define cascade and inverse option in one-many mapping? 
cascade - enable operations to cascade to child entities.
cascade="all|none|save-update|delete|all-delete-orphan"

inverse - mark this collection as the "inverse" end of a bidirectional association.
inverse="true|false"
Essentially "inverse" indicates which end of a relationship should be ignored, so when persisting a parent who has a collection of children, should you ask the parent for its list of children, or ask the children who the parents are?
36.Define HibernateTemplate? 
org.springframework.orm.hibernate.HibernateTemplate is a helper class which provides different methods for querying/retrieving data from the database. It also converts checked HibernateExceptions into unchecked DataAccessExceptions.
37.What are the benefits does HibernateTemplate provide? 
The benefits of HibernateTemplate are :

• HibernateTemplate, a Spring Template class simplifies interactions with Hibernate Session.
• Common functions are simplified to single method calls.
• Sessions are automatically closed.
• Exceptions are automatically caught and converted to runtime exceptions.

38. How do you switch between relational databases without code changes?
Using Hibernate SQL Dialects , we can switch databases. Hibernate will generate appropriate hql queries based on the dialect defined.

39.If you want to see the Hibernate generated SQL statements on console, what should we do? 
By using “show_sql” property of the hibernate configuration file
In Hibernate configuration file set as follows:
<property name="show_sql">true</property>

40.What are derived properties? 
The properties that are not mapped to a column, but calculated at runtime by evaluation of an expression are called derived properties. The expression can be defined using the formula attribute of the element. 

41.Define cascade and inverse option in one-many mapping? 
cascade - enable operations to cascade to child entities.
cascade="all|none|save-update|delete|all-delete-orphan"

inverse - mark this collection as the "inverse" end of a bidirectional association.
inverse="true|false"
Essentially "inverse" indicates which end of a relationship should be ignored, so when persisting a parent who has a collection of children, should you ask the parent for its list of children, or ask the children who the parents are?

42 . Explain about transaction file?
Transactions denote a work file which can save changes made or revert back the changes. A transaction can be started by session.beginTransaction() and it uses JDBC connection, CORBA or JTA. When this session starts several transactions may occur.

43 . Difference between session.save() , session.saveOrUpdate() and session.persist()? 
All methods are used to store the data in to database
            session.save() : save() method uSave does an insert and will fail if the primary key is already persistent.

session.saveOrUpdate() : saveOrUpdate() insert the data in the database if that primary key data not available and it update the data if primary key data not availabt
session.persist() :it is the same like session.save(). But session.save() return Serializable object but session.persist() return void.
For Example :
         if you do :-
         System.out.println(session.save(question));
         This will print the generated primary key.
         if you do :-
         System.out.println(session.persist(question));
         Compile time error because session.persist() return void.

44 . Explain about the id field?
This id field is used to configure the primary key in the mapping file, and also we can configure primary key generation algorithm.

45.What is the use of dynamic-insert and dynamic-update attributes in a class mapping? 
Criteria is a simplified API for retrieving entities by composing Criterion objects. This is a very convenient approach for functionality like "search" screens where there is a variable number of conditions to be placed upon the result set.

• dynamic-update (defaults to false): Specifies that UPDATE SQL should be generated at runtime and contain only those columns whose values have changed
• dynamic-insert (defaults to false): Specifies that INSERT SQL should be generated at runtime and contain only the columns whose values are not null.

46.What is automatic dirty checking? 
Automatic dirty checking is a feature that saves us the effort of explicitly asking Hibernate to update the database when we modify the state of an object inside a transaction. 

47.What are Callback interfaces? 
Callback interfaces allow the application to receive a notification when something interesting happens to an object—for example, when an object is loaded, saved, or deleted. Hibernate applications don't need to implement these callbacks, but they're useful for implementing certain kinds of generic functionality.

48.What is Hibernate proxy? 
The proxy attribute enables lazy initialization of persistent instances of the class. Hibernate will initially return CGLIB proxies which implement the named interface. The actual persistent object will be loaded when a method of the proxy is invoked.

49.How can Hibernate be configured to access an instance variable directly and not through a setter method ? 
By mapping the property with access="field" in Hibernate metadata. This forces hibernate to bypass the setter method and access the instance variable directly while initializing a newly loaded object.

50.How can a whole class be mapped as immutable?
Mark the class as mutable="false" (Default is true),. This specifies that instances of the class are (not) mutable. Immutable classes, may not be updated or deleted by the application.

51 .  Explain about transparent persistence of Hibernate?
Transparent persistence is provided for Plain old Java objects or POJOs. For proper functioning of the applications importance should be given to the methods equals () and hash Code methods (). It has a requirement which should be strictly followed in the applications which is a no-argument constructor.

52 .  Explain about the dirty checking feature of Hibernate?
Dirty checking feature of the Hibernate allows users or developers to avoid time consuming data base write actions. This feature makes necessary updations and changes to the fields which require a change, remaining fields are left unchanged or untouched.

53 .  What is the effect when a transient mapped object is passed onto a Sessions save?
When a Sessions save () is passed to a transient mapped object it makes the method to become more persistent. Garbage collection and termination of the Java virtual machine stays as long as it is deleted explicitly. It may head back to its transient state.

54 .  Explain about addClass function?
This function translates a Java class name into file name. This translated file name is then loaded as an input stream from the Java class loader. This addClass function is important if you want efficient usage of classes in your code.