WebLogic: The Definitive Guide

Container-Managed Relationships

Container-managed EJB persistence lets you separate the entity beans from the actual data stored in the underlying database. The XML deployment descriptors allow you to map the abstract persistence model to the underlying database schema. In CMP 2.0 persistence, you also can let the EJB container handle relationships between entity beans. WebLogic’s EJB container supports associations that can be navigated in either direction (bidirectional), or perhaps restricted to one direction (unidirectional).

A CMR field (or a relationship field) is defined between local interfaces of entity beans. If a bean is involved in a relationship, the bean may be aware of EJB instances at the other end. Thus, if you have defined CMR fields for EJBs at both “ends” of the relationship, the association between the two entity beans can be navigated in either direction. Typically, unidirectional associations are modeled with remote entity beans — i.e., when the entity bean does not reside in the same EJB JAR as the entity beans related to it. However, a bidirectional association may be defined only when both entity beans are packaged in the same EJB JAR. This means that you must use the same XML deployment descriptors to define their abstract persistence schema.

WebLogic’s persistence framework supports one-to-one, one-to-many, and many-to-many associations. Suppose you’ve defined an association between two entity beans, A and B. The ejb-jar.xml descriptor file then ...