Hibernate Interview Questions Part I

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.


In the Employee.hbm.xml file

In the above configuration.
If lazy="false" :-

    When you load the Employee object that time child object Address 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 Address 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.

Learn more about Hibernate in our Video Tutorial

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.save() returns the generated identifier (Serializable object) and session.persist() doesn't.

  • session.persist() :

      Does 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.

How to Integrate Struts Spring Hibernate ?

Step 1. In the struts-config.xml, add plugin.


Step 2. In the applicationContext.xml file, Configure datasourse.


Step 3. Configure SessionFactory


Step 4. Configure User.hbm.xml


Step 5. In the applicationContext.xml, configure for DAO


Step 6. DAO Class

What is the difference between Hibernate and 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.

  2. As Hibernate is set of Objects,you don't 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. Don't 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 don't 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 don't have such support.

  10. Hibernate Supports automatic versioning of rows but JDBC Not.

How to prevent slate object updation 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. Change 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.

Step 1 :
Declare a variable "versionId" in your Class with setter and getter.


Step 2.
In the hbm.xml file


Step 3.
Create a coulmn name "version" in the CAMPIGN table.

Step 4.
In the code


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.

What is version checking 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. Change 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.

Step 1 :
Declare a variable "versionId" in your Class with setter and getter.


Step 2.
In the hbm.xml file


Step 3.
Create a coulmn name "version" in the CAMPIGN table.

Step 4.
In the code


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.

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 actually 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.

  • Examples,

    • Lazy=true, (default) Address child of User class can be made lazy if it is not required frequently.

    • Lazy=false, But you may need to load the Author object for Book parent whenever you deal with the book for online bookshop.

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. Change 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.

Step 1 :
Declare a variable "versionId" in your Class with setter and getter.


Step 2.
In the hbm.xml file


Step 3.
Create a coulmn name "version" in the CAMPIGN table.

Step 4.
In the code


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.

Difference between session.saveOrUpdate() and session.merge();

saveOrUpdate() does the following :

  • if the object is already persistent in this session, do nothing

  • if another object associated with the session has the same identifier, throw an exception

  • if the object has no identifier property, save() it

  • if the object's identifier has the value assigned to a newly instantiated object, save() it

  • if the object is versioned (by a <version> or <timestamp>), and the version property value is the same value assigned to a newly instantiated object, save() it

  • otherwise update() the object

merge() is very different :

  • if there is a persistent instance with the same identifier currently associated with the session, copy the state of the given object onto the persistent instance

  • if there is no persistent instance currently associated with the session, try to load it from the database, or
    create a new persistent instance

  • the persistent instance is returned

  • the given instance does not become associated with the session, it remains detached.

Difference between session.load() and session.get() ?

load() will throw an unrecoverable exception if there is no matching database row. get() will return null if there is no matching database row.

    Cat fritz = (Cat) session.load(Cat.class, "1");

Return the Cat Object with key 1. If there is no Cat Object with key 1 then throw will throw an unrecoverable exception.

If the class is mapped with a proxy, load() just returns an uninitialized proxy and does not actually hit the database until you invoke a method of the proxy. This behaviour is very useful if you wish to create an association to an object without actually loading it from the database. It also allows multiple instances to be loaded as a batch if batchsize is defined for the class mapping.

    Cat fritz = (Cat) session.get(Cat.class, "1");

If you are not certain that a matching row exists, you should use the get() method, which hits the database immediately and returns null if there is no matching row.

Equal and Not Equal criteria query.

Equal and Not Equal criteria query - Example


    List of organisation where town equals to pune.

    List organizationList = session.createCriteria(Organization.class).add(Restrictions.eq("town","pune")).list();

    List of organisation where town not equals pune.

    List organizationList = session.createCriteria(Organization.class).add(Restrictions.ne("town","pune")).list();

How does Value replacement in Message Resource Bundle work?

In the resource bundle file, you can define a template like :

Then the Error message is : First Name is required.

Other constructors are :

public ActionError(String key, Object value0, Object value1)

. . . public ActionError(String key, Object[] values);

Deleting persistent objects?

Session.delete() will remove an object's state from the database. Of course, your application might still hold a reference to a deleted object. It's best to think of delete() as making a persistent instance transient.sess.delete(cat);

How to set 2nd level cache in hibernate with EHCache?

When you are creating SessionFactory just add the below steps :


ECache.xml is like:


ApplicationBean will be avilable in 2nd level cache.

How to get Hibernate statistics ?

SessionFactory.getStatistics() gives you all the Hibernate statistics.

Criteria Query Two Condition.

Criteria Query Two Condition - Example


List of organisation where town equals to pune and status = "A".

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.

SQL statements execution order.
  1. All entity insertions, in the same order the corresponding objects were saved using Session.save().

  2. All entity updates.

  3. All collection deletions.

  4. All collection element deletions, updates and insertions.

  5. All collection insertions.

  6. All entity deletions, in the same order the corresponding objects were deleted using Session.delete().

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.

How to handle user think time using 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. Change 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.

Step 1 :
Declare a variable "versionId" in your Class with setter and getter.


Step 2.
In the hbm.xml file


Step 3.
Create a coulmn name "version" in the CAMPIGN table.

Step 4.
In the code


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.

SQL Queries In Hibernate.

You may express a query in SQL, using createSQLQuery() and let Hibernate take care of the mapping from result sets to objects. Note that you may at any time call session.connection() and use the JDBC Connection directly. If you chose to use the Hibernate API, you must enclose SQL aliases in braces :


SQL queries may contain named and positional parameters, just like Hibernate queries.

addScalar() method in hibernate.

  • addScalar() method confirms that maxWeight is always double type.

  • This way you don't need to check for it is double or not.

Modifying persistent objects?

No need of session.update() call.

How to create primary key using hibernate?

Increment generator class automatically generate the primary key for you.