Enterprise JavaBeans, Fourth Edition

Chapter 2. Architectural Overview

In order to use Enterprise JavaBeans effectively, you need to understand the EJB architecture. This chapter explores the core of the EJB architecture: how enterprise beans are distributed as business objects. Chapter 3 explores the services and resource-management techniques supported by EJB.

In order to be truly versatile, the EJB component design had to be smart. For application developers, assembling enterprise beans requires little or no expertise in the complex system-level issues that often plague three-tier development efforts. While EJB makes the process easier for application developers, it also provides EJB server developers with a great deal of flexibility in how they support the EJB specification.

The Enterprise Bean Component

Enterprise JavaBeans server-side components come in three fundamentally different types: entity, session, and message-driven beans. Both session and entity beans are RMI-based server-side components that are accessed using distributed object protocols. Message-driven beans process messages from non-RMI systems like Java Message Service, legacy systems, and web services. All EJB servers must at least support a JMS-based message driven bean, but they may also support other types of message-driven bean.

A good rule of thumb is that entity beans model business concepts that can be expressed as nouns. For example, an entity bean might represent a customer, a piece of equipment, an item in inventory, or even a place. ...