ORM/JPA/Hibernate

Hibernate Interview Questions - PART-I

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

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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

14.What is Hibernate Query Language (HQL)?

Hibernate uses a powerful query language (HQL)  ( to query, store, update, and retrieve objects from a database ) that is similar in appearance to SQL. Compared with SQL, however, HQL is fully object-oriented and understands notions like inheritance, polymorphism and association.

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>

To be continued...