Hibernate

The application is designed around an object model. The SQL code is generated at build time. And the associations between objects are supported by the persistence mechanism, and queries are specified using an object-oriented expression language. This is best suited for medium-sized applications with some complex transactions. Used when the mapping exceeds 25 different database products at a time.

Full object mapping supports sophisticated object modeling: composition, inheritance, polymorphism and persistence. The persistence layer implements transparent persistence; persistent classes do not inherit any special base class or have to implement a special interface. Efficient fetching strategies and caching strategies are implemented transparently to the application.

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

* Eager loading

► Improved maintainability

* A lot less code to write

► Improved portability

* ORM framework generates database-specific SQL for you

The entities are represented as classes that are mapped manually to the relational tables. The code is hidden from the business logic using specific design patterns. This approach is successful for applications with a less number of entities, or applications with common, metadata-driven data models. This approach is most known to all.

The entire application, including the user interface, is designed around the relational model and SQL-based relational operations.

There are four levels defined for ORM quality.

Pure relational

object mapping

Medium object mapping

Full object mapping

It should have an API for performing basic CRUD (Create, Read, Update, Delete) operations on objects of persistent classes Should have a language or an API for specifying queries that refer to the classes and the properties of classes An ability for specifying mapping metadata It should have a technique for ORM implementation to interact with transactional objects to perform dirty checking, lazy association fetching, and other optimization functions

ORM stands for Object/Relational mapping. It is the programmed and translucent perseverance of objects in a Java application in to the tables of a relational database using the metadata that describes the mapping between the objects and the database. It works by transforming the data from one representation to another.

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.

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

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();

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

An Action Class is an adapter between the contents of an incoming HTTP rest and the corresponding business logic that should be executed to process this rest.

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.

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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

The most common methods of Hibernate configuration are:

* Programmatic configuration

* XML configuration (hibernate.cfg.xml)

HQL stands for Hibernate Query Language. Hibernate allows the user to express queries in its own portable SQL extension and this is called as HQL. It also allows the user to express in native SQL.

ORM tools require a metadata format for the application to specify the mapping between classes and tables, properties and columns, associations and foreign keys, Java types and SQL types. This information is called the object/relational mapping metadata. It defines the transformation between the different data type systems and relationship representations.

* 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.

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>