2.1. The J2EE Architecture
The J2EE platform gives you a multitiered application model to develop distributed components. Although any number of tiers is possible, a three-tier architecture is typical. Figure 2-1 shows the approach.
Figure 2-1. Three-Tier J2EE Architecture
The client machine supports web browsers, Java applets, and stand-alone applications. A client application may be as simple as a command line program running as an administrator client or a graphical user interface created from Java Swing or AWT (Abstract Window Toolkit) components. Regardless, J2EE applications encourage thin clients in the presentation tier. A thin client is a lightweight interface that does not perform database queries, implement business logic, or connect to legacy code. These types of “heavyweight” operations preferably belong to other tiers.
The J2EE server machine is the center of the architecture. This middle tier contains web components and business objects managed by the application server. The web components dynamically process user requests and construct responses to client applications. The business objects implement the logic of a business domain. Both components are managed by a J2EE application server that provides important system services for these components, such as security, transaction management, naming and directory lookups, and remote connectivity. By placing these services under control of the J2EE application server, client components focus only on presentation logic. And, business objects are easier for developers to write. Furthermore, the architecture encourages the separation of business logic from presentation logic (or model from view).
The database server machine handles the database back end. This includes mainframe transactions, databases, Enterprise Resource Planning (ERP) systems, and legacy code. Another advantage of the three-tier architecture is that older systems can take on a whole new “look” using the J2EE platform. This is the approach many businesses are taking as they migrate their legacy systems to the web in a modern distributed computing environment.