The J2EE platform includes a specification for a distributed component model known as Enterprise JavaBeans (EJB). If you've worked with Java for a while, you probably have a basic understanding of EJB and its relationship with business logic. In this article, we'll dig a little deeper and see how EJB can be used to encapsulate business logic while interacting with other resources. You can download the sample code for this article here. The sphere of tiers Our investigation into middleware concepts begins by proposing an architecture consisting of one client tier and three server tiers. Our three server tiers are broken down as follows:

• Presentation tier -- Responsible for fielding client requests, distributing those requests to business logic components, and then formatting a response to be passed back to the client
• Business tier -- Responsible for accepting service requests from the presentation tier, accessing data resources and other enterprise systems, and returning result data to the presentation tier
• Data tier -- Point of access for data resources and external enterprise systems

We use JavaBeans as data objects to pass back and forth between the server tiers. Figure A illustrates these concepts and components.

Figure A

Component model and EJB A software component has a number of identifying characteristics:

• Autonomous -- A software component should be able to perform its work independent of other components or applications.
• Dynamically discoverable -- Although a software component may not need to be discovered dynamically in a desktop environment, it is almost always preferable in a distributed system environment.
• Business process -- A software component should define a logical business process.
• Consisting of externally published interfaces -- A software component should expose one or more interfaces, providing external access to the component's functionality.
• Usable for building other components or applications -- A software component's primary job is to enable developers to build other components or applications by combining one or more components with custom application or system code.

To illustrate how components are used to build a more complex entity, we can compare a software component to a car part, where each car part (component) has a specific purpose (process). As long as a part has the correct wires and bolt holes (interfaces), it can be used in the assembly of a complete car (other components or applications).