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Professional EJB : The EJB 2.0 Entity Model

Section Summary

This section provided an overview of the new model for container-managed persistence. It introduced a lot of new material that will be further described in the next chapter. After reading this section, you should understand the following:

• The types and role of abstract methods in EJB 2.0 persistence
  Abstract methods play a key role in the EJB 2.0 persistence model. Abstract accessors are used to access and modify persistent state and relationship information for entity objects. Abstract ejbSelect() methods give the component developer access to the new EJB query language. Abstract methods are key to such optimizations as dirty checking and lazy loading.
• The separation of the client view from the object model
  You cannot pass a local interface or a relationship collection class across a remote interface. The client never sees the structure of an entity. Instead, the entity provides a view through its remote interface, possibly by using view objects.
• The representation of relationships
  Relationships are accessed through get and set accessors, and through collection classes if they have a multiplicity of many. They are also used in the EJB query language, and to manage the lifespan of entity objects (through cascade-delete). The set accessors of relationships that are represented by a collection class are only used in special circumstances. Relationships are defined in the deployment descriptor as having two roles each. A role consists of a multiplicity, a role source, a <cmr-field> if necessary, a collection type if necessary, and a cascade-delete element if necessary.
• The structure of the EJB query language (EJB QL)
  The new query language is based on SQL-92, and is enhanced by path expressions that allow the component developer to navigate over the relationships defined between entity objects. Rather than working with relational database tables, EJB QL works with the definition of entities in the deployment descriptor (known as their schema). There are three parts to an EJB QL query: the SELECT clause, the FROM clause, and the WHERE clause. These queries are used to define the behavior of finder methods (which are exposed to the client), and ejbSelect() methods (which are strictly used in the implementation) for entity beans with EJB 2.0 container-managed persistence.
• The structure of the deployment descriptor
  The deployment descriptor has a major new element to support the EJB 2.0 persistence model: the relationships section. In addition, there are several additions to the entity element of the enterprise-beans section to support EJB 2.0 persistence, such as an element for an abstract-schema-name and elements to specify queries. Finally, there is a new security permission type for a method, called Unchecked, which helps to make the use of fine-grained entities practical to build your object model.

The remaining part of this chapter will talk about how some of the changes in EJB 2.0 can affect your beans with bean-managed persistence.

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