Entity Bean Overview

With EJB 2.0 and the local interface support, most developers agree that entity beans should be paired with a session bean, servlet, or JSP that acts as the client interface. The entity bean is a coarse-grain bean that encapsulates functionality and represents data and dependent objects. Thus, you decouple the client from the data so that if the data changes, the client is not affected. For efficiency, the session bean, servlet, or JSP can be collocated with entity beans and can coordinate between multiple entity beans through their local interfaces. This is known as a session facade design. See the http://java.sun.com/ Web site for more information on session facade design.

An entity bean can aggregate objects together and effectively persist data and related objects under the umbrella of transactional, security, and concurrency support through the container. This and the following chapters focus on how to use the persistence functionality of the entity bean.

An entity bean manages persistent data in one of two ways: container-managed persistence (CMP) and bean-managed persistence (BMP). The primary difference between the two is as follows:

• Container-managed persistence—The EJB container manages data by saving it to a designated resource, which is normally a database. For this to occur, you must define the data that the container is to manage within the deployment descriptors. The container manages the data by saving it to the database. For details on container-managed persistence and how to use it in your entity bean, see Chapter 5, "CMP Entity Beans".

• Bean-managed persistence—The bean implementation manages the data within callback methods. All the logic for storing data to your persistent storage must be included in the ejbStore method and reloaded from your storage in the ejbLoad method. The container invokes these methods when necessary. For details on bean-managed persistence and how to implement it within your entity bean, see Chapter 8, "BMP Entity Beans".