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pluggable resource adapters and may be packaged as J2EE Connector resource

adapters. 

6.2.1.2

EJB Container Managed Persistence

Container managed persistence (CMP), which is a resource manager independent

data access API for entity beans, has been expanded and enhanced in the J2EE 1.4

platform. CMP technology enables applications to be easily integrated with various

databases or resource managers, plus it enhances portability. 

CMP shields the developer from the details of the underlying data store. The

developer does not need to know how to persist or retrieve data to or from a par 

ticular data store, since the EJB container handles these tasks. Instead, the devel 

oper need only indicate what data or state needs to be stored persistently. 

In addition, a developer uses the same API that is, the EJB CMP methods 

regardless of the underlying type of resource manager. The same entity bean can

thus be used with any type of resource manager or database schema. The technol 

ogy makes it possible to develop enterprise beans that can be customized at

deployment to work with existing data. That is, the same bean implementation can

be deployed to work with many different customer data schemes. The mapping

done at deployment may vary for each customer set up, but the bean itself is the

same. Since the EJB container generates suitable data access code for each situa 

tion, the bean developer does not have to know or care about the underlying

resource manager specific code. Furthermore, since it has complete control over

managing persistence, the container can optimize database access for better per 

formance.

The J2EE 1.4 platform includes the most up to date EJB specification and

CMP technology. Rather than declaring persistent variables in a bean's implemen 

tation, developers include abstract get and set accessor methods for persistent

variables. Persistent variables are thus treated similarly to JavaBeans properties.

Developers do not provide an implementation for these accessor methods since

they are abstract methods; instead, the EJB container provides the method imple 

mentations.

The CMP architecture also includes container managed relationships, which

allows multiple entity beans to have relationships among themselves. Container 

managed relationships are handled in much the same way as container managed

persistence. The bean implementation merely provides get and set accessor

methods for these fields, and the container provides the method implementations.

Similarly, the developer specifies the relationships in the deployment descriptor.