Enterprise JavaBeans, Third Edition

Distributed Object Architectures

EJB is a component model for component transaction monitors, which are based on distributed object technologies. Therefore, to understand EJB you need to understand how distributed objects work. Distributed object systems are the foundation for modern three-tier architectures. In a three-tier architecture, as shown in Figure 1-1, the presentation logic resides on the client (first tier), the business logic resides on the middle tier (second tier), and other resources, such as the database, reside on the backend (third tier).

Figure 1-1. Three-tier architecture

All distributed object protocols are built on the same basic architecture, which is designed to make an object on one computer look like it’s residing on a different computer. Distributed object architectures are based on a network communication layer that is really very simple. Essentially, there are three parts to this architecture: the business object, the skeleton, and the stub.

The business object resides on the middle tier. It’s an instance of an object that models the state and business logic of some real-world concept, such as a person, order, or account. Every business object class has matching stub and skeleton classes built specifically for that type of business object. So, for example, a distributed business object called Person would have matching Person_Stub and Person_Skeleton ...