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Hibernate FAQs

1.What is ORM ?
ORM stands for object/relational mapping. ORM is the automated persistence of objects in a Java application to the tables in a relational database.

2.What does ORM consists of ?
An ORM solution consists of the followig four pieces:

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

3.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)

4.What is Hibernate?
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 files.Its purpose is to relieve the developer from a significant amount of relational data persistence-related programming tasks.

5.Why do you need ORM tools like hibernate?
The main advantage of ORM like hibernate is that it shields developers from messy SQL. 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

6.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

7.What is the need for Hibernate xml mapping file?
Hibernate mapping file tells Hibernate which tables and columns to use to load and store objects. Typical mapping file look as follows:


8.What are the most common methods of Hibernate configuration?
The most common methods of Hibernate configuration are:

  • Programmatic configuration
  • XML configuration (hibernate.cfg.xml)


9.What are the important tags of hibernate.cfg.xml?
Following are the important tags of hibernate.cfg.xml:


10.What are the Core interfaces are 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.

  • Session interface
  • SessionFactory interface
  • Configuration interface
  • Transaction interface
  • Query and Criteria interfaces

11.What role does the Session interface play in 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.

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

12.What role does the SessionFactory interface play in Hibernate?
The application obtains Session instances from a SessionFactory. There is typically a single SessionFactory for the whole applicationå¹¼reated 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

SessionFactory sessionFactory = configuration.buildSessionFactory();

13.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
  • Create session factory from configuration object
  • Get one session from this session factory
  • Create HQL Query
  • Execute query to get list containing Java objects

14.What is Hibernate Query Language (HQL)?
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.

15.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.test.User"  table="user">
   <property  column="USER_NAME" length="255"
      name="userName" not-null="true"  type="java.lang.String"/>
   <property  column="USER_PASSWORD" length="255"
     name="userPassword" not-null="true"  type="java.lang.String"/>
 </class>
</hibernate-mapping>
16.What’s the difference between load() and get()?
load() vs. 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. 

17.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.

18.How do you define sequence generated primary key in hibernate?
Using <generator> tag.
Example:-
<id column="USER_ID" name="id" type="java.lang.Long">
   <generator class="sequence">
     <param name="table">SEQUENCE_NAME</param>
   <generator>
</id>

19.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?

20.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.test.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();

21.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>

22.Explain Criteria API
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();

23.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.

24.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.

25.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.

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

27.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.

28.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

Example:


29.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 . 

 

31.What is the advantage of Hibernate over jdbc?
Hibernate Vs. JDBC :-


JDBC 

Hibernate 

With JDBC, developer has to write code to map an object model's data representation to a relational data model and its corresponding database schema.  

Hibernate is flexible and powerful ORM solution to map Java classes to database tables. Hibernate itself takes care of this mapping using XML files so developer does not need to write code for this. 

With JDBC, the automatic mapping of Java objects with database tables and vice versa conversion is to be taken care of by the developer manually with lines of code.  

Hibernate provides transparent persistence and developer does not need to write code explicitly to map database tables tuples to application objects during interaction with RDBMS.  

JDBC supports only native Structured Query Language (SQL). Developer has to find out the efficient way to access database, i.e. to select effective query from a number of queries to perform same task.  

Hibernate provides a powerful query language Hibernate Query Language (independent from type of database) that is expressed in a familiar SQL like syntax and includes full support for polymorphic queries. Hibernate also supports native SQL statements. It also selects an effective way to perform a database manipulation task for an application.  

Application using JDBC to handle persistent data (database tables) having database specific code in large amount. The code written to map table data to application objects and vice versa is actually to map table fields to object properties. As table changed or database changed then it’s essential to change object structure as well as to change code written to map table-to-object/object-to-table. 

Hibernate provides this mapping itself. The actual mapping between tables and application objects is done in XML files. If there is change in Database or in any table then the only need to change XML file properties.  

With JDBC, it is developer’s responsibility to handle JDBC result set and convert it to Java objects through code to use this persistent data in application. So with JDBC, mapping between Java objects and database tables is done manually.  

Hibernate reduces lines of code by maintaining object-table mapping itself and returns result to application in form of Java objects. It relieves programmer from manual handling of persistent data, hence reducing the development time and maintenance cost.  

With JDBC, caching is maintained by hand-coding.  

Hibernate, with Transparent Persistence, cache is set to application work space. Relational tuples are moved to this cache as a result of query. It improves performance if client application reads same data many times for same write. Automatic Transparent Persistence allows the developer to concentrate more on business logic rather than this application code.  

In JDBC there is no check that always every user has updated data. This check has to be added by the developer.  

Hibernate enables developer to define version type field to application, due to this defined field Hibernate updates version field of database table every time relational tuple is updated in form of Java class object to that table. So if two users retrieve same tuple and then modify it and one user save this modified tuple to database, version is automatically updated for this tuple by Hibernate. When other user tries to save updated tuple to database then it does not allow saving it because this user does not have updated data.  

32.What are the Collection types in Hibernate ?

  • Bag
  • Set
  • List
  • Array
  • Map


33.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.


34.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?

35.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.

36.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.

37.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.

38.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.


39.What do you mean by fetching strategy ?
A fetching strategy is the strategy Hibernate will use for retrieving associated objects if the application needs to navigate the association. Fetch strategies may be declared in the O/R mapping metadata, or over-ridden by a particular HQL or Criteria query.

40.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.

41.What is transactional write-behind?
Hibernate uses a sophisticated algorithm to determine an efficient ordering that avoids database foreign key constraint violations but is still sufficiently predictable to the user. This feature is called transactional write-behind.

42.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.

43.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


44.What are the differences between EJB 3.0 & Hibernate
Hibernate Vs EJB 3.0 :-


Hibernate 

EJB 3.0 

Session–Cache or collection of loaded objects relating to a single unit of work 

Persistence Context-Set of entities that can be managed by a given EntityManager is defined by a persistence unit 

XDoclet Annotations used to support Attribute Oriented Programming 

Java 5.0 Annotations used to support Attribute Oriented Programming 

Defines HQL for expressing queries to the database 

Defines EJB QL for expressing queries 

Supports Entity Relationships through mapping files and annotations in JavaDoc 

Support Entity Relationships through Java 5.0 annotations 

Provides a Persistence Manager API exposed via the Session, Query, Criteria, and Transaction API 

Provides and Entity Manager Interface for managing CRUD operations for an Entity 

Provides callback support through lifecycle, interceptor, and validatable interfaces 

Provides callback support through Entity Listener and Callback methods 

Entity Relationships are unidirectional. Bidirectional relationships are implemented by two unidirectional relationships 

Entity Relationships are bidirectional or unidirectional 


45.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

46. How will you configure Hibernate?
Answer:
The configuration files hibernate.cfg.xml (or hibernate.properties) and mapping files *.hbm.xml are used by the Configuration class to create (i.e. configure and bootstrap hibernate) the SessionFactory, which in turn creates the Session instances. Session instances are the primary interface for the persistence service.

" hibernate.cfg.xml (alternatively can use hibernate.properties): These two files are used to configure the hibernate sevice (connection driver class, connection URL, connection username, connection password, dialect etc). If both files are present in the classpath then hibernate.cfg.xml file overrides the settings found in the hibernate.properties file.

" Mapping files (*.hbm.xml): These files are used to map persistent objects to a relational database. It is the best practice to store each object in an individual mapping file (i.e mapping file per class) because storing large number of persistent classes into one mapping file can be difficult to manage and maintain. The naming convention is to use the same name as the persistent (POJO) class name. For example Account.class will have a mapping file named Account.hbm.xml. Alternatively hibernate annotations can be used as part of your persistent class code instead of the *.hbm.xml files.


47. What is a SessionFactory? Is it a thread-safe object?
Answer:
SessionFactory is Hibernates concept of a single datastore and is threadsafe so that many threads can access it concurrently and request for sessions and immutable cache of compiled mappings for a single database. A SessionFactory is usually only built once at startup. SessionFactory should be wrapped in some kind of singleton so that it can be easily accessed in an application code.

SessionFactory sessionFactory = new Configuration().configure().buildSessionfactory();


48. What is a Session? Can you share a session object between different theads?
Answer:
Session is a light weight and a non-threadsafe object (No, you cannot share it between threads) that represents a single unit-of-work with the database. Sessions are opened by a SessionFactory and then are closed when all work is complete. Session is the primary interface for the persistence service. A session obtains a database connection lazily (i.e. only when required). To avoid creating too many sessions ThreadLocal class can be used as shown below to get the current session no matter how many times you make call to the currentSession() method.

&
public class HibernateUtil {
&
public static final ThreadLocal local = new ThreadLocal();

public static Session currentSession() throws HibernateException {
Session session = (Session) local.get();
//open a new session if this thread has no session
if(session == null) {
session = sessionFactory.openSession();
local.set(session);
}
return session;
}
}

It is also vital that you close your session after your unit of work completes. Note: Keep your Hibernate Session API handy.


49. What are the benefits of detached objects?
Answer:
Detached objects can be passed across layers all the way up to the presentation layer without having to use any DTOs (Data Transfer Objects). You can later on re-attach the detached objects to another session.

50. What are the pros and cons of detached objects?
Answer:
Pros:
" When long transactions are required due to user think-time, it is the best practice to break the long transaction up into two or more transactions. You can use detached objects from the first transaction to carry data all the way up to the presentation layer. These detached objects get modified outside a transaction and later on re-attached to a new transaction via another session.
Cons
" In general, working with detached objects is quite cumbersome, and better to not clutter up the session with them if possible. It is better to discard them and re-fetch them on subsequent requests. This approach is not only more portable but also more efficient because - the objects hang around in Hibernate's cache anyway.

" Also from pure rich domain driven design perspective it is recommended to use DTOs (DataTransferObjects) and DOs (DomainObjects) to maintain the separation between Service and UI tiers.


51. How does Hibernate distinguish between transient (i.e. newly instantiated) and detached objects?
Answer

" Hibernate uses the version property, if there is one.
" If not uses the identifier value. No identifier value means a new object. This does work only for Hibernate managed surrogate keys. Does not work for natural keys and assigned (i.e. not managed by Hibernate) surrogate keys.
" Write your own strategy with Interceptor.isUnsaved().

52. What is the difference between the session.get() method and the session.load() method?
Both the session.get(..) and session.load() methods create a persistent object by loading the required object from the database. But if there was not such object in the database then the method session.load(..) throws an exception whereas session.get(&) returns null.


53. What is the difference between the session.update() method and the session.lock() method?
Both of these methods and saveOrUpdate() method are intended for reattaching a detached object. 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.

Note: When you reattach detached objects you need to make sure that the dependent objects are reatched as well.

54. How would you reatach detached objects to a session when the same object has already been loaded into the session?

You can use the session.merge() method call.


56. 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() (i.e accessor/getter) and setXXX( i.e. mutator/setter) 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.

5.Use XDoclet tags for generating your *.hbm.xml files or Annotations (JDK 1.5 onwards), which are less verbose than *.hbm.xml files.

57. Difference between session.save() , session.saveOrUpdate() and session.persist()?
session.save() : Save does an insert and will fail if the primary key is already persistent.
session.saveOrUpdate() : saveOrUpdate does a select first to determine if it needs to do an insert or an update.
Insert data if primary key not exist otherwise update data.
session.persist() : Does the same like session.save().
But session.save() return Serializable object but session.persist() return void.
         session.save() returns the generated identifier (Serializable object) and session.persist() doesn't.
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.

58. What is lazy fetching in Hibernate? With Example .
Lazy fetching decides whether to load child objects while loading the Parent Object.
You need to do this setting respective hibernate mapping file of the parent class.
Lazy = true (means not to load child)
By default the lazy loading of the child objects is true.
This make sure that the child objects are not loaded unless they are explicitly invoked in the application by calling getChild() method on parent.In this case hibernate issues a fresh database call to load the child when getChild() is actully called on the Parent object
.But in some cases you do need to load the child objects when parent is loaded.
Just make the lazy=false and hibernate will load the child when parent is loaded from the database.
Example :
If you have a TABLE ? EMPLOYEE mapped to Employee object and contains set of Address objects.
Parent Class : Employee class
Child class : Address Class
public class Employee {
private Set address = new HashSet(); // contains set of child Address objects
public Set getAddress () {
return address;
}
public void setAddresss(Set address) {
this. address = address;
}
}
In the Employee.hbm.xml file
<set name="address" inverse="true" cascade="delete" lazy="false">
<key column="a_id" />
<one-to-many class="beans Address"/>
</set>
In the above configuration.
If lazy="false" : - when you load the Employee object that time child object Adress is also loaded and set to setAddresss() method.
If you call employee.getAdress() then loaded data returns.No fresh database call.
If lazy="true" :- This the default configuration. If you don?t mention then hibernate consider lazy=true.
when you load the Employee object that time child object Adress is not loaded. You need extra call to data base to get address objects.
If you call employee.getAdress() then that time database query fires and return results. Fresh database call.
59. How to prevent concurrent update in Hibernate?
version checking used in hibernate when more then one thread trying to access same data.
For 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. Chnage made by user B is gone.
In hibernate you can perevent slate object updatation using version checking.
Check the version of the row when you are upding the row.
Get the version of the row when you are fetching the row of the TABLE for update.
On the time of updation just fetch the version number and match with your version number ( on the time of fetching).
This way you can prevent slate object updatation.
Steps 1:
Declare a variable "versionId" in your Class with setter and getter.
public class Campign {
private Long versionId;
private Long campignId;
private String name;
public Long getVersionId() {
return versionId;
}
public void setVersionId(Long versionId) {
this.versionId = versionId;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Long getCampignId() {
        return campignId;
    }
private void setCampignId(Long campignId) {
        this.campignId = campignId;
    }
}
Step 2.
In the .hbm.xml file
<class name="beans.Campign" table="CAMPIGN" optimistic-lock="version">
<id name="campignId" type="long" column="cid">
<generator class="sequence">
<param name="sequence">CAMPIGN_ID_SEQ</param>
</generator>
     </id>
    <version name="versionId" type="long" column="version" />
<property name="name" column="c_name"/>
</class>
Step 3.
Create a coulmn name "version" in the CAMPIGN table.
Step 4.
In the code
// foo is an instance loaded by a previous Session
session = sf.openSession();
int oldVersion = foo.getVersion();
session.load( foo, foo.getKey() );
if ( oldVersion!=foo.getVersion ) throw new StaleObjectStateException();
foo.setProperty("bar");
session.flush();
session.connection().commit();
session.close();
You can handle StaleObjectStateException() and do what ever you want.
You can display error message.
Hibernate autumatically create/update the version number when you update/insert any row in the table.

60. Difference between getCurrentSession() and openSession() in Hibernate ?
getCurrentSession() :
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.
To enable this strategy in your Hibernate configuration:
set hibernate.transaction.manager_lookup_class to a lookup strategy for your JEE container
set hibernate.transaction.factory_class to org.hibernate.transaction.JTATransactionFactory
Only the Session that you obtained with sf.getCurrentSession() is flushed and closed automatically.
Example :
try {
UserTransaction tx = (UserTransaction)new InitialContext() .lookup("java:comp/UserTransaction");
tx.begin();
// Do some work
sf.getCurrentSession().createQuery(...);
sf.getCurrentSession().persist(...);
tx.commit();
}
catch (RuntimeException e) {
tx.rollback();
throw e; // or display error message
}
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 :
UserTransaction tx = (UserTransaction)new InitialContext()
.lookup("java:comp/UserTransaction");
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
}

61. Filter in Hibernate with Example?
Filter in Hibernate ------
USER ( ID INT, USERNAME VARCHAR, ACTIVATED BOOLEAN) - TABLE
public class User
{
private int id;
private String username;
private boolean activated;
public boolean isActivated()
{
return activated;
}
public void setActivated(boolean activated)
{
this.activated = activated;
}
public int getId()
{
return id;
}
public void setId(int id)
{
this.id = id;
}

public String getUsername()
{
return username;
}
public void setUsername(String username)
{
this.username = username;
}
}
-----------------------------------------------------------------
<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-mapping
PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="User">
<id name="id" type="int">
<generator class="native"/>
</id>
<property name="username" type="string" length="32"/>
<property name="activated" type="boolean"/>
<filter name="activatedFilter" condition=":activatedParam = activated"/>
</class>
<filter-def name="activatedFilter">
<filter-param name="activatedParam" type="boolean"/>
</filter-def>
</hibernate-mapping>
--------------------------------------------------------------------
Save and Fetch using filter example
User user1 = new User();
user1.setUsername("name1");
user1.setActivated(false);
session.save(user1);
User user2 = new User();
user2.setUsername("name2");
user2.setActivated(true);
session.save(user2);
User user3 = new User();
user3.setUsername("name3");
user3.setActivated(true);
session.save(user3);
User user4 = new User();
user4.setUsername("name4");
user4.setActivated(false);
session.save(user4);
All the four user saved to Data Base User Table.
Now Fetch the User using Filter..
Filter filter = session.enableFilter("activatedFilter");
filter.setParameter("activatedParam",new Boolean(true));
Query query = session.createQuery("from User");
Iterator results = query.iterate();
while (results.hasNext())
{
User user = (User) results.next();
System.out.print(user.getUsername() + " is ");
}
Guess the Result :
name2 name3
Because Filer is filtering ( only true value) data before query execute
62.Modifying persistent objects?
DomesticCat cat = (DomesticCat) sess.load( Cat.class, new Long(69) );
cat.setName("PK");
sess.flush(); // changes to cat are automatically detected and persisted To Data Base.
No need any session.update() call.
63. Difference between list() and iterate() i9n Hibernate?
If instances are already be in the session or second-level cache iterate() will give better performance.
If they are not already cached, iterate() will be slower
than list() and might require many database hits for a simple query.
64. Cascade Save or Update in Hibernate ?
Cascade Save or Update - In one to Many- EXAMPLE
PROCESS_TYPE_LOV (PROCESS_TYPE_ID number, PROCESS_TYPE_NAME varchar) - TABLE
PROCESS (PROCESS_ID number,PROCESS_NAME varchar,PROCESS_TYPE_ID number)- TABLE
public class ProcessTypeBean {
  private Long processTypeId;
   private String processTypeName;
    public Long getProcessTypeId() {
        return processTypeId;
    }
    public void setProcessTypeId(Long processTypeId) {
        this.processTypeId = processTypeId;
    }
    public String getProcessTypeName() {
        return processTypeName;
    }
    public void setProcessTypeName(String processTypeName) {
        this.processTypeName = processTypeName;
    }     
}
public class ProcessBean {
   private Long processId;
    private String processName = "";
    private ProcessTypeBean processType;

    public Long getProcessId() {
        return processId;
    }
  public void setProcessId(Long processId) {
        this.processId = processId;
    }
 public String getProcessName() {
        return processName;
    }
  public void setProcessName(String processName) {
        this.processName = processName;
    }
 public ProcessTypeBean getProcessType() {
        return processType;
    }
  public void setProcessType(ProcessTypeBean processType) {
        this.processType = processType;
    }
}
<class name="com.bean.ProcessBean"
        table="PROCESS">
        <id name="processId" type="long" column="PROCESS_ID" />
        <property name="processName" column="PROCESS_NAME" type="string"
            length="50" />
        <many-to-one name="processType" column="PROCESS_TYPE_ID" class="ProcessTypeBean" cascade="save-update" />        
</class>
<class name="com.bean.ProcessTypeBean"
        table="PROCESS_TYPE_LOV">
        <id name="processTypeId" type="long" column="PROCESS_TYPE_ID" />
        <property name="processTypeName" column="PROCESS_TYPE_NAME"
            type="string" length="50" />        
    </class>
---------------------------------------------------------------------------------
Save Example Code -
ProcessTypeBean pstype = new ProcessTypeBean();
pstype.setProcessTypeName("Java Process");
ProcessBean process = new ProcessBean();
process.setProcessName("Production")
ProcessBean.setProcessType(pstype);
// session.save(pstype); -- This save not required because of in the mapping file cascade="save-update"
session.save(process); - This will insert both ProcessBean and ProcessTypeBean;
65. One To Many Mapping Using List ?
WRITER (ID INT,NAME VARCHAR) - TABLE
STORY (ID INT,INFO VARCHAR,PARENT_ID INT) - TABLE
One writer can have multiple stories..
-------------------------------------------------------------
Mapping File...
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping
PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">
<hibernate-mapping>
<class name="Writer" table="WRITER">
<id name="id" unsaved-value="0">
<generator class="increment"/>
</id>
<list name="stories" cascade="all">
<key column="parent_id"/>
<one-to-many class="Story"/>
</list>
<property name="name" type="string"/>
</class>
<class name="Story"
table="story">
<id name="id" unsaved-value="0">
<generator class="increment"/>
</id>
<property name="info"/>
</class>
</hibernate-mapping>
-------------------------------------------------------
public class Writer {
private int id;
private String name;
private List stories;
public void setId(int i) {
id = i;
}
public int getId() {
return id;
}
public void setName(String n) {
name = n;
}
public String getName() {
return name;
}
public void setStories(List l) {
stories = l;
}
public List getStories() {
return stories;
}
}
---------------------------------------------------
public class Story {
private int id;
private String info;
public Story(){
}
public Story(String info) {
this.info = info;
}
public void setId(int i) {
id = i;
}
public int getId() {
return id;
}
public void setInfo(String n) {
info = n;
}
public String getInfo() {
return info;
}
}
----------------------------------------------------
Save Example ..
Writer wr = new Writer();
wr.setName("Das");
ArrayList list = new ArrayList();
list.add(new Story("Story Name 1"));
list.add(new Story("Story Name 2"));
wr.setStories(list);
Transaction transaction = null;
try {
transaction = session.beginTransaction();
session.save(sp);
transaction.commit();
} catch (Exception e) {
if (transaction != null) {
transaction.rollback();
throw e;
}
} finally {
session.close();
}

66. Many To Many Relation In Hibernate ?

 

Best Example..for Many to Many in Hibernate ..
EVENTS ( uid int, name VARCHAR) Table
SPEAKERS ( uid int, firstName VARCHAR) Table
EVENT_SPEAKERS (elt int, event_id int, speaker_id int) Table
-----------------------------------------------------------
import java.util.Set;
import java.util.HashSet;
public class Speaker{
private Long id;
private String firstName;
private Set events;
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public Set getEvents() {
return this.events;
}
public void setEvents(Set events) {
this.events = events;
}
private void addEvent(Event event) {
if (events == null) {
events = new HashSet();
}
events.add(event);
}
}
--------------------------------------------------------
import java.util.Date;
import java.util.Set;
public class Event{
private Long id;
private String name;
private Set speakers;
public void setId(Long id) {
this.id = id;
}
public Long getId() {
return id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public void setSpeakers(Set speakers) {
this.speakers = speakers;
}
public Set getSpeakers() {
return speakers;
}
}
--------------------------------------------------------------
Event.hbm.xml
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 2.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">
<hibernate-mapping>
<class name="Event" table="events">
<id name="id" column="uid" type="long" unsaved-value="null">
<generator class="increment"/>
</id>
<property name="name" type="string" length="100"/>
<set name="speakers" table="event_speakers" cascade="all">
<key column="event_id"/>
<many-to-many class="Speaker"/>
</set>
</class>
</hibernate-mapping>
------------------------------------------------------------------
Speaker.hbm.xml
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 2.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">
<hibernate-mapping>
<class name="Speaker" table="speakers">
<id name="id" column="uid" type="long">
<generator class="increment"/>
</id>
<property name="firstName" type="string" length="20"/>
<set name="events" table="event_speakers" cascade="all">
<key column="speaker_id"/>
<many-to-many class="Event"/>
</set>
</class>
</hibernate-mapping>
----------------------------------------------------------------------
Save and Fetch Example
Event event = new Event();
event.setName("Inverse test");
event.setSpeakers(new HashSet());
event.getSpeakers().add(new Speaker("Ram", event));
event.getSpeakers().add(new SpeakerManyToMany("Syam", event));
event.getSpeakers().add(new SpeakerManyToMany("Jadu", event));
session.save(event); /// Save All the Data

event = (Event) session.load(Event.class, event.getId());
Set speakers = event.getSpeakers();
for (Iterator i = speakers.iterator(); i.hasNext();) {
Speaker speaker = (Speaker) i.next();
System.out.println(speaker.getFirstName());
System.out.println(speaker.getId());
}
67. What does session.refresh() do ?
It is possible to re-load an object and all its collections at any time, using the refresh() method. This is useful
when database triggers are used to initialize some of the properties of the object.
For Example - Triger on cat_name coulmn. Trigger is updating hit_count coulmn in the same Cat Table. When Insert data into Cat TABLE trigger update hit_count coulmn to 1. sess.refresh() reload all the data. No nned again to select call.
sess.save(cat);
sess.flush(); //force the SQL INSERT
sess.refresh(cat); //re-read the state (after the trigger executes)
68. How to get Hibernate statistics ?
SessionFactory.getStatistics() is give you all the statistics.
69. How to set 2nd level cache in hibernate with EHCache?
When you are creating SessionFactory just add the below steps
String ecache = appHome+File.separatorChar+"ehcache.xml";
try {
CacheManager.create(ecache);
} catch (CacheException e) {
// logger.logError(e);
}*/
Then
sessionFactory = configuration.buildSessionFactory();
ECache.xml is like
<ehcache>
<diskStore path="java.io.tmpdir"/>
<defaultCache
maxElementsInMemory="10000"
eternal="false"
timeToIdleSeconds="120"
timeToLiveSeconds="120"
overflowToDisk="true"
diskPersistent="false"
diskExpiryThreadIntervalSeconds="120"
/>
<cache name="bean.ApplicationBean"
maxElementsInMemory="300"
eternal="false"
overflowToDisk="false"
/>
</ehcache>
ApplicationBean will be avilable in 2nd level cache.
70. What are the core components in Hibernate?
SessionFactory (org.hibernate.SessionFactory)
A threadsafe (immutable) cache of compiled mappings for a single database. A factory for Session and a
client of ConnectionProvider. Might hold an optional (second-level) cache of data that is reusable
between transactions, at a process- or cluster-level.
Session (org.hibernate.Session)
A single-threaded, short-lived object representing a conversation between the application and the persistent
store. 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.
Persistent objects and collections
Short-lived, single threaded objects containing persistent state and business function. These might be ordinary
JavaBeans/POJOs, the only special thing about them is that they are currently associated with (exactly
one) Session. As soon as the Session is closed, they will be detached and free to use in any application
layer (e.g. directly as data transfer objects to and from presentation).
Transient and detached objects and collections
Instances of persistent classes that are not currently associated with a Session. They may have been instantiated
by the application and not (yet) persisted or they may have been instantiated by a closed Session.
Transaction (org.hibernate.Transaction)
(Optional) A single-threaded, short-lived object used by the application to specify atomic units of work.
Abstracts application from underlying JDBC, JTA or CORBA transaction. A Session might span several
Transactions in some cases. However, transaction demarcation, either using the underlying API or Transaction,
is never optional!

Architecture
Hibernate 3.0.2 9
ConnectionProvider (org.hibernate.connection.ConnectionProvider)
(Optional) A factory for (and pool of) JDBC connections. Abstracts application from underlying Datasource
or DriverManager. Not exposed to application, but can be extended/implemented by the developer.
TransactionFactory (org.hibernate.TransactionFactory)
(Optional) A factory for Transaction instances. Not exposed to the application, but can be extended/
implemented by the developer.
Extension Interfaces
Hibernate offers many optional extension interfaces you can implement to customize the behavior of your
persistence layer. See the API documentation for details.

71. What is a Hibernate Session? Can you share a session object between different theads?
Session is a light weight and a non-threadsafe object (No, you cannot share it between threads) that represents a single unit-of-work with the database. Sessions are opened by a SessionFactory and then are closed when all work is complete. Session is the primary interface for the persistence service. A session obtains a database connection lazily (i.e. only when required). To avoid creating too many sessions ThreadLocal class can be used as shown below to get the current session no matter how many times you make call to the currentSession() method.
public class HibernateUtil {
public static final ThreadLocal local = new ThreadLocal();
public static Session currentSession() throws HibernateException {
Session session = (Session) local.get();
//open a new session if this thread has no session
if(session == null) {
session = sessionFactory.openSession();
local.set(session);
}
return session;
}
}
72. Hibernate session.close does _not_ call session.flush ?
session.close() don't call session.flush() before closing the session.
This is the session.close() code in hibernate.jar
public Connection close() throws HibernateException {
      if ( isClosed() ) {
            throw new SessionException( "Session was already closed" );
        }
       if ( factory.getStatistics().isStatisticsEnabled() ) {
            factory.getStatisticsImplementor().closeSession();
        }
        try {
            try {
                if ( childSessionsByEntityMode != null ) {
                    Iterator childSessions = childSessionsByEntityMode.values().iterator();
                    while ( childSessions.hasNext() ) {
                        final SessionImpl child = ( SessionImpl ) childSessions.next();
                        child.close();
                    }
                }
            }
            catch( Throwable t ) {
                // just ignore
            }
            if ( rootSession == null ) {
                return jdbcContext.getConnectionManager().close();
            }
            else {
                return null;
            }
        }
        finally {
            setClosed();
            cleanup();
        }
    }
73. What is Hibernate proxy?
By default Hibernate creates a proxy for each of the class you map in mapping file. This class contain the code to invoke JDBC. This class is created by hibernate using CGLIB.
Proxies are created dynamically by subclassing your object at runtime.
The subclass has all the methods of the parent, and when any of the methods are accessed, the proxy loads up the real object from the DB and calls the method for you. Very nice in simple cases with no object hierarchy.
 Typecasting and instanceof work perfectly on the proxy in this case since it is a direct subclass.
74. what is lazy fetching in hibernate?
Lazy setting decides whether to load child objects while loading the Parent Object.You need to do this setting respective hibernate mapping file of the parent class.Lazy = true (means not to load child)By default the lazy loading of the child objects is true. This make sure that the child objects are not loaded unless they are explicitly invoked in the application by calling getChild() method on parent.In this case hibernate issues a fresh database call to load the child when getChild() is actully called on the Parent object.But in some cases you do need to load the child objects when parent is loaded. Just make the lazy=false and hibernate will load the child when parent is loaded from the database.Exampleslazy=true (default)Address child of User class can be made lazy if it is not required frequently.lazy=falseBut you may need to load the Author object for Book parent whenever you deal with the book for online bookshop
75. How to Execute Stored procedure in Hibernate?
Option 1:
Connection con = null;
try {
con = session.connection();
CallableStatement st = con
.prepareCall("{call your_sp(?,?)}");
st.registerOutParameter(2, Types.INTEGER);
st.setString(1, "some_Seq");
st.executeUpdate();
Option 2:
<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>
code :
SQLQuery sq = (SQLQuery) session.getNamedQuery("selectAllEmployees_SP");
List results = sq.list();

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