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Title: Professional EJB
ISBN: 1861005083
US Price: $ 59.99
Canadian Price:
C$ 89.95
UK Price: £ 47.99
© Wrox Press Limited, US and UK.

Reviews : Java Books :
Professional EJB : The EJB 2.0 Entity Model

EJB 2.0 Solutions

The Enterprise JavaBeans 2.0 specification provided comprehensive solutions to the problems of EJB 1.1 container-managed persistence by introducing a new model of container-managed persistence and adding the concept of local interfaces. Here is how some of its features match up to the problems of EJB 1.1 container-managed persistence. This chapter will provide explanations for all of these features:

EJB 1.1 Problem EJB 2.0 Solution
No appropriate way to represent dependent objects of entity beans. Introduction of local interfaces, which mitigates some of the performance issues of entity beans and also allows you to hide your object model from a remote client.
No foolproof way to implement load-on- demand.

No foolproof way to implement dirty checking.

 EJB 2.0 CMP entities use abstract accessor methods to set and retrieve data. The EJB container provides an implementation of these accessor methods, where load-on- demand and dirty checking logic can be located. Furthermore, relationships are also represented by abstract accessor methods and collection classes maintained by the EJB container. This allows relationships to be intelligently managed.
No portable way to ensure that the contents of a collection are suitable for the container's persistence strategy. Collection classes maintained by the EJB container are subject to usage rules that simplify the object-relational mapping. For instance, only a single type of entity may be contained in a persistence-manager collection class.
Potential data-aliasing problems with dependent objects and collections used by two or more entities in the same transaction. Dependent objects are represented by entity components, whose state is managed by the EJB container. Relationships are represented by abstract accessors and special collection classes, which are also managed by the EJB container. This allows for synchronization within a transaction and solves data aliasing problems.
No portable query language for finders. A portable query language based on a subset of SQL-92 is provided in the EJB 2.0 specification.

EJB 2.0 Container-Managed Persistence

The EJB 2.0 model of container-managed persistence did not modify the EJB 1.1 model of container- managed persistence. The specification writers did not want to break existing code. Instead, the two models for CMP exist side-by-side. The old EJB 1.1 model of CMP has not been deprecated; a compliant EJB 2.0 container must provide support for both EJB 1.1 CMP and EJB 2.0 CMP. This decision to introduce an entirely new model allowed for extensive changes to the way EJB 2.0 CMP works. The following section provides for a tour of some of these changes, before we go on to consider them in more detail. We'll look quickly at:

  • The role of abstract methods in EJB 2.0 CMP
  • Local and remote client views
  • How relationships are represented
  • The EJB 2.0 query language
  • The new structure of the EJB 2.0 deployment descriptor

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