Business web hosting - unless you have a legacy EJB 1.1 system
unless you have a legacy EJB 1.1 system that you maintain. EJB 1.1 container- managed persistence is covered in Chapter 9. The next three chapters focus on developing entity beans that use EJB 2.0 container-managed persistence. In EJB 2.0, the data associated with an entity bean can be much more complex than was possible in EJB 1.1 or EJB 1.0. In EJB 2.0, container-managed persistence entity beans can have relationships with other entity beans, which wasn t well supported in the older version. In addition, container-managed persistence entity beans can be finer in granularity so that they can easily model things like Address, LineItem, or Cabin. This chapter develops two very simple entity beans, the Customer and Address EJBs, which will be used to explain how Enterprise JavaBeans 2.0 container- managed persistence entity beans are defined and operate at runtime. The Customer EJB has relationships with other several entities including address, phone, credit card, cruise, ship, cabin, and reservation EJBs. In the next few chapters, you ll learn how to leverage EJB 2.0 s powerful support for entity bean- to-bean relationships as well as understanding their limitations. In addition, you will learn about the Enterprise JavaBeans Query Language (EJB QL) in Chapter 8, which is used to define how the find methods and the new select methods should behave at runtime. It is common to refer to Enterprise JavaBeans 2.0 container-managed persistence as simply CMP 2.0. In the chapters that follow, we will use this abbreviation to distinguish between CMP 2.0 and CMP 1.1 (Enterprise JavaBeans 1.1 container- managed persistence). The abstract programming model In CMP 2.0, entity beans have their state managed automatically by the container. The container will take care of enrolling the entity bean in transactions and persisting its state to the database. The enterprise bean developer describes the attributes and relationships of an entity bean using virtual persistent fields and relationship fields. They are called virtual fields because the bean developer does not declare these fields explicitly; instead, abstract assessor (get and set) methods are declared in the entity bean class. The implementations of these methods are generated at deployment time by EJB vendor s container tools. So it s important to remember that the terms relationship field and persistent field are referring to the abstract accessor methods and not to actual fields declared in the classes. This use of terminology is a convention in EJB 2.0 that you should become confortable with. In Figure 6-1, the Customer EJB has four sets of accessor methods. The first two read and update the last and first names of the customer. These are examples of persistent fields; simple direct attributes of the entity bean. The other accessor methods obtain and set references to the Address EJB through its local interface, Address. This is an example of a relationship field called the addressfield. Copyright (c) 2001 O’Reilly & Associates
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