What is Hibernate?

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

o    High-level object-oriented API

o    Less Java code to write

o    No SQL to write

       Improved performance

o    Sophisticated caching

o    Lazy loading

o    Eager loading

       Improved maintainability

o    A lot less code to write

       Improved portability

o    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 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 workSessionFactory 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>