Chapter 6 

255

6.2.1.3

Java Data Objects

Java Data Objects (JDO) is an API that provides a standard, interface based Java

model abstraction of persistence. Application developers can use the JDO API to

directly store Java domain model instances into a persistent store, such as a data 

base. You may consider JDO as one alternative to using JDBC or enterprise beans

with container managed persistence. Keep in mind, however, that JDO is not stan 

dard in the J2EE 1.4 platform. 

There are some benefits to using JDO. Since JDO keeps applications indepen 

dent or insulated from the underlying database, application developers can focus

on their domain object model and not have to be concerned with the persistence

details, such as the field by field storage of objects. JDO also ensures that the

application uses the optimal data access strategies for best performance.

It is not unusual to compare JDO to enterprise beans with container managed

persistence, since both provide object relational mapping capabilities. The princi 

pal difference is that JDO maps database relationships to plain Java objects, while

EJB CMP maps relationships to transactional components managed by a con 

tainer. EJB CMP essentially provides a higher layer of service than JDO. Some

J2EE application servers, such as the J2EE 1.4 platform SDK, internally use JDO

to implement enterprise bean container managed persistence.

6.2.2 Messaging Technologies

Messaging systems allow unrelated applications to communicate asynchronously

and reliably. Not only do the communicating parties not have to be closely tied to

each other, they can also remain relatively anonymous. 

The J2EE platform provides the Java Message Service (JMS) API, which is a

standard Java API defined for enterprise messaging systems. Along with JMS, the

J2EE platform also provides message driven beans. Message driven beans are

EJB components that consume and process asynchronous messages delivered via

JMS or some other messaging system.

Let's take a look at messaging technologies in general, then examine two

J2EE specific technologies: Java Message Service and message driven beans.

6.2.2.1

Overview of Messaging Technologies

Prior to the advent of Web services, developers often chose messaging systems

(called MOM for Message Oriented Middleware) to create an integration architec 

ture. With a messaging system, two systems can communicate with each other by