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Enterprise Java

A beginner's guide to Enterprise JavaBeans

An introductory overview of the Java server-side application component standard

By Mark Johnson, JavaWorld.com, 10/01/98

Page 4 of 7

Enterprise JavaBeans goals
The EJB Spec tries to meet several goals at once:

• EJB is designed to make it easy for developers to create applications, freeing them from low-level system details of managing transactions, threads, load balancing, and so on. Application developers can concentrate on business logic and leave the details of managing the data processing to the framework. For specialized applications, though, it's always possible to get "under the hood" and customize these lower-level services.
  
  
• The EJB Spec defines the major structures of the EJB framework, and then specifically defines the contracts between them. The responsibilities of the client, the server, and the individual components are all clearly spelled out. (We'll go over what these structures are in a moment.) A developer creating an Enterprise JavaBean component has a very different role from someone creating an EJB-compliant server, and the specification describes the responsibilities of each.
  
  
• EJB aims to be the standard way for client/server applications to be built in the Java language. Just as the original JavaBeans (or Delphi components, or whatever) from different vendors can be combined to produce a custom client, EJB server components from different vendors can be combined to produce a custom server. EJB components, being Java classes, will of course run in any EJB-compliant server without recompilation. This is a benefit that platform-specific solutions can't hope to offer.
  
  
• Finally, the EJB is compatible with and uses other Java APIs, can interoperate with non-Java apps, and is compatible with CORBA.

How an EJB client/server system works

To understand how an EJB client/server system operates, we need to understand the basic parts of an EJB system: the EJB component, the EJB container, and the EJB object.

The Enterprise JavaBeans component
An Enterprise JavaBean is a component, just like a traditional JavaBean. Enterprise JavaBeans execute within an EJB container, which in turn executes within an EJB server. Any server that can host an EJB container and provide it with the necessary services can be an EJB server. (This means that many existing servers may be extended to be EJB servers, and in fact many vendors have achieved this, or have announced the intention to do so.)

An EJB component is the type of EJB class most likely to be considered an "Enterprise JavaBean." It's a Java class, written by an EJB developer, that implements business logic. All the other classes in the EJB system either support client access to or provide services (like persistence, and so on) to EJB component classes.

The Enterprise JavaBeans container
The EJB container is where the EJB component "lives." The EJB container provides services such as transaction and resource management, versioning, scalability, mobility, persistence, and security to the EJB components it contains. Since the EJB container handles all of these functions, the EJB component developer can concentrate on business rules, and leave database manipulation and other such fine details to the container. For example, if a single EJB component decides that the current transaction should be aborted, it simply tells its container (in a manner defined by the EJB Spec, and the container is responsible for performing all rollbacks, or doing whatever is necessary to cancel a transaction in progress. Multiple EJB component instances typically exist inside a single EJB container.

 

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