Java Enterprise Best Practices

Create Domain Object Factories

If you need to support different persistence media such as the filesystem or database, there are alternatives to writing different BMP implementations for each persistence type. The problem with writing different BMP implementations is that you have to replicate code that deals with EJB semantics in each different implementation. Also, switching implementations involves either changing deployment descriptors for many components, or switching JAR files.

A good solution is to separate the details of persisting data from entity bean implementations. This is especially useful when you have to deal with persisting domain objects because a natural solution is to create abstract factories for domain objects. With or without domain objects, it’s easy to see that this additional implementation layer would have to allow entity beans to load, save, and find data. To illustrate this implementation model, consider a User domain object that is handled by an entity bean. In Example 2-11, the entity bean will expose fine-grained get/set methods, but will use a UserFactory to persist the User domain object.

Example 2-11. Using domain object factories

public class UserBean implements EntityBean { private EntityContext ctx; private transient UserFactory userFactory; private UserDomainObject user; public void setEntityContext (EntityContext ctx) { this.ctx = ctx; // Get the factory object for: userFactory = UserFactory.getInstance( ); } public void unsetEntityContext( ...