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Professional EJB : The EJB 2.0 Entity Model

The Structure of the Deployment Descriptor

The deployment descriptor in EJB 2.0 has a major new section to support the new model of entity persistence. It provides for the description of the relationships between entity objects. Here is a table providing short descriptions of the major "top-level" elements of the EJB 2.0 deployment descriptor, and what has changed that is relevant to EJB 2.0 persistence:

Element	Description
<enterprise-beans>	This is the section where EJB components (session beans, message- driven beans, and entity beans) are described. The description for container-managed entity beans will include their container- managed fields. If the entity uses the EJB 2.0 model of container- managed persistence, it will also provide a name for the entity's schema and the EJB QL queries for its finder and ejbSelect() methods.
<relationships>	The relationships section of the deployment descriptor describes the relationships between entity beans using EJB 2.0, that are described in this deployment descriptor. Each relationship has two relationship roles, specifying the multiplicity, role source, cmr field if necessary, type of role collection if necessary, and cascade-delete if necessary.
<assembly-descriptor>	The application assembly information describes how the EJB component or components are composed into a larger application unit. The EJB 2.0 specification introduces the concept of unchecked methods, which helps to make fine- grained entities practical for your object model.
<ejb-client-jar>	The ejb-client-jar specifies an archive that contains the Java class files necessary for a client program to use the EJB components in this JAR. This section does not introduce anything relevant to EJB 2.0 persistence.

Of course, the entity element of the deployment descriptor changed to support the changes in EJB 2.0. The following diagram, produced using XMLSpy, shows the sub-elements of the deployment descriptor's entity element:

The <local> and <local-home> elements have been added to support local interfaces, and the <home> and <remote> elements have been made optional to handle the (likely) case that only local interfaces are specified. The <abstract-schema-name> element has been added to support abstract schemas in queries, and the <query> element has been added to specify EJB-QL syntax for ejbSelect() and finder methods.

Likewise, here is the visualization for the <relationships> element:

We'll be looking at the deployment descriptor for relationships in great detail in the next chapter.

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