Introducing Hibernate In Java Using NetBeans

In one of my recent posts, I introduced the theoretical topic of Object-Relational Mapping (ORM) - http://gautam-m.blogspot.com/2010/11/object-relation-mapping.html. In this post I am going to take a step forward and introduce Hibernate – an open source Java persistence framework from JBoss.

This post covers four basic principles of Hibernate –

• Configuring a Java project for Hibernate
• Inserting data using Object Persistence
• Retrieving data using Hibernate Query Language (HQL)
• Changing the database configuration to connect to another DBMS

Though the post and screen-shots emphasize on NetBeans, the concept is the same for all IDEs. Hibernate configuration files can definitely be written without an IDE but make sure all the required class libraries are properly referenced.

Requirements to run the example in the article – NetBeans, MySQL, JavaDB, Java and Hibernate. Java can be downloaded here, installing the All NetBeans package will cover JavaDB and Hibernate and MySQL can be downloaded here.

Configuring a Java project for Hibernate

The crux of Hibernate is the creation and usage of configuration files. There are three types of configuration files which are to be setup for Hibernate –

1. The .cfg.xml file – this is the main configuration file which contains information about the database like the URL, the driver, the username and password, etc. Hibernate can optimize it’s behavior depending on the DBMS being used. To facilitate this, a property called Dialect is specified. However this is an optional property as Hibernate can deduce this depending on the JDBC metadata returned by the driver
2. The .reveng.xml file – this file holds the data corresponding to the schemas and tables being utilized by Hibernate in the application
3. The .hbm.xml – these files maps POJOs (Plain Old Java Objects) to the table schemas of the database

Typically one .cfg.xml and one .reveng.xml exist for a project and one .hbm.xml exists for each table (mapped to a class). The .hbm.xml maps the object properties to the table columns. It is possible to add new properties to the class which have no effect on the backend tables.

To create these files in a NetBeans project, select New File and select the following File Types from the Hibernate category –

1. Hibernate Configuration Wizard
2. Hibernate Reverse Engineering Wizard
3. Hibernate Mapping Files and POJOs from Database

Follow the wizards to complete the configuration setup. I used MySQL and JavaDB as my DBMS to create a sample application to insert and retrieve data. You can get the MySQL and JavaDB scripts along with the NetBeans project here.

Inserting data using Object Persistence

Inserting data is a cake-walk in Hibernate. All that is there to do is to create the object and store the object data in the database tables using the save method of a SessionFactory object. The following piece of code persists the object data –

        SessionFactory sessionFactory =
                new Configuration().configure().buildSessionFactory();
        Session session = sessionFactory.openSession();

        // Create the object
        session.save(object);

Retrieving data using Hibernate Query Language (HQL)

Retrieving data is done through a query language designed for Hibernate called the Hibernate Query Language. HQL is a Object-Oriented Query Language and is very much similar to the traditional SQL we use. The beauty of HQL is that the result of the query is returned as a list of objects rather than as a ResultSet. These objects can be used directly in the code without any overheads. HQL is very wide topic, so I am going to skip the details here but there are several tutorials available for HQL on the Internet. The createQuery method of the above Session object is used along with the list method of the Query object to get the objects –

        Query query = session.createQuery(queryString);
        for (Object object : query.list()) {
                // cast and use the object appropriately
        }

Changing the database configuration to connect to another DBMS

The best feature of Hibernate according to me is it’s ability to change a DBMS without any change to the application code. To change the DBMS, open the Hibernate Configuration File (typically hibernate.cfg.xml) and change the dialect, driver class, connection URL, username and password to the values corresponding to the new DBMS.

These changes can be done either through the design view or directly on the XML.