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Get started with Hibernate

Introducing and configuring Hibernate

By Christian Bauer and Gavin King, JavaWorld.com, 10/18/04

Page 3 of 5

Understanding the architecture

The programming interfaces are the first thing you have to learn about Hibernate in order to use it in the persistence layer of your application. A major objective of API design is to keep the interfaces between software components as narrow as possible. In practice, however, ORM APIs aren't especially small. Don't worry, though; you don't have to understand all the Hibernate interfaces at once. The figure below illustrates the roles of the most important Hibernate interfaces in the business and persistence layers.

We show the business layer above the persistence layer, since the business layer acts as a client of the persistence layer in a traditionally layered application. Note that some simple applications might not cleanly separate business logic from persistence logic; that's okay—it merely simplifies the diagram.

The Hibernate interfaces shown in the figure above may be approximately classified as follows:

• Interfaces called by applications to perform basic CRUD (create/read/update/delete) and querying operations. These interfaces are the main point of dependency of application business/control logic on Hibernate. They include Session, Transaction, and Query.
• Interfaces called by application infrastructure code to configure Hibernate, most importantly, the Configuration class.
• Callback interfaces that allow the application to react to events occurring inside Hibernate, such as Interceptor, Lifecycle, and Validatable.
• Interfaces that allow extension of Hibernate's powerful mapping functionality, such as UserType, CompositeUserType, and IdentifierGenerator. These interfaces are implemented by application infrastructure code (if necessary).

Hibernate makes use of existing Java APIs, including JDBC (Java Database Connectivity), Java Transaction API (JTA), and Java Naming and Directory Interface (JNDI). JDBC provides a rudimentary level of abstraction of functionality common to relational databases, allowing almost any database with a JDBC driver to be supported by Hibernate. JNDI and JTA allow Hibernate to be integrated with J2EE application servers.

In this section, we don't cover the detailed semantics of Hibernate API methods, just the role of each of the primary interfaces. You can find most of these interfaces in the package net.sf.hibernate. Let's take a brief look at each interface in turn.

The core interfaces

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
The Session interface is the primary interface used by Hibernate applications. An instance of Session is lightweight and is inexpensive to create and destroy. This is important because your application will need to create and destroy sessions all the time, perhaps on every request. Hibernate sessions are not thread-safe and should by design be used by only one thread at a time.