Hibernate Interview Questions and Answers - 21-30

21) What role does the Session interface play in Hibernate?

A) 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

22) What role does the SessionFactory interface play in Hibernate?

A) 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

SessionFactory sessionFactory = configuration.buildSessionFactory();

23) What is the general flow of Hibernate communication with RDBMS?

A) 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.

24) What is Hibernate Query Language (HQL)?

A) 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.

25) How do you map Java Objects with Database tables?

A)
* First we need to write Java domain objects (beans with setter and getter). The variables should be same as database columns.
* Write hbm.xml, where we map java class to table and database columns to Java class variables.


26) What Does Hibernate Simplify?

A) 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

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

A) load( ) vs. get( )
load( ) :-
Only use the load( ) method if you are sure that the object exists.
load( ) method will throw an exception 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( ):-
If you are not sure that the object exists, then use one of the get( ) methods.
get( ) method will return null if the unique id is not found in the database.
get( ) will hit the database immediately.

28) What is the difference between and merge and update ?

A)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.

29) How do you define sequence generated primary key in hibernate?

A) Using tag.
Example:-
SEQUENCE_NAME

30) Define cascade and inverse option in one-many mapping?

A) 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?