Enterprise JavaBeans, Third Edition

EJB 2.0: Describing Relationships

CMP 2.0 entity bean classes are defined using abstract accessor methods that represent virtual persistence and relationship fields. As you learned in Chapter 6, Chapter 7, and Chapter 8, the actual fields themselves are not declared in the entity classes. Instead, the characteristics of these fields are described in detail in the XML deployment descriptor used by the entity bean. The abstract persistence schema is the set of XML elements in the deployment descriptor that describe the relationship and persistence fields. Together with the abstract programming model (i.e., the abstract accessor methods) and some help from the deployer, the container tool has enough information to map the entity and its relationships with other entity beans.

The relationships between entity beans are described in the <relationships> section of the XML deployment descriptor. The <relationships> section falls between the <enterprise-beans> and <assembly-descriptor> sections. Within the <relationships> element, each entity-to-entity relationship is defined in a separate <ejb-relation> element:

<ejb-jar>
    <enterprise-beans>
    ...
    </enterprise-beans>
    <relationships>
        <ejb-relation>
        ...
        </ejb-relation>
        <ejb-relation>
        ...
        </ejb-relation>
    </relationships>
    <assembly-descriptor>
    ...
    </assembly-descriptor>
</ejb-jar>

Defining relationship fields requires that an <ejb-relation> element be added to the XML deployment descriptor for each entity-to-entity relationship. These <ejb-relation> ...