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Develop n-tier applications using J2EE

An introduction to the Java 2 Platform, Enterprise Edition specification by way of BEA's WebLogic Server

By Steven Gould, JavaWorld.com, 12/01/00

The J2EE platform consists of a set of services, application programming interfaces (APIs), and protocols that provide the functionality for developing multitiered Web-based applications.

In this article, we will examine the 13 core technologies that make up J2EE: JDBC, JNDI, EJBs, RMI, JSP, Java servlets, XML, JMS, Java IDL, JTS, JTA, JavaMail, and JAF. We will describe where and when it is appropriate to use each technology; we will also describe how the different technologies interact with each other.

Moreover, to give J2EE a real-world feel, we'll look at its main technologies in the context of WebLogic Server, a widely used J2EE implementation from BEA Systems. With that in mind, this introductory article will be of interest to developers new to WebLogic Server and J2EE, as well as project managers and business analysts with an interest in understanding what J2EE has to offer.

The big picture: Distributed architectures and J2EE

In the past, two-tier applications -- also known as client/server applications -- were commonplace. Figure 1 illustrates the typical two-tier architecture. In some cases, the only service provided by the server was that of a database server. In those situations, the client was then responsible for data access, applying business logic, converting the results into a format suitable for display, displaying the intended interface to the user, and accepting user input. The client/server architecture is generally easy to deploy at first, but is difficult to upgrade or enhance, and is usually based on proprietary protocols -- typically proprietary database protocols. It also makes reuse of business and presentation logic difficult, if not impossible. Finally, and perhaps most important in the era of the Web, two-tier applications typically do not prove very scalable and are therefore not well suited to the Internet.

Figure 1. Two-tier application architecture

Sun designed J2EE in part to address the deficiencies of two-tier architectures. As such, J2EE defines a set of standards to ease the development of n-tier enterprise applications. It defines a set of standardized, modular components; provides a complete set of services to those components; and handles many details of application behavior -- such as security and multithreading -- automatically.

Using J2EE to develop n-tier applications involves breaking apart the different layers in the two-tier architecture into multiple tiers. An n-tier application could provide separate layers for each of the following services:

• Presentation: In a typical Web application, a browser running on the client machine handles presentation.
• Dynamically generated presentation: Although a browser could handle some dynamically generated presentation, for the widest support of different browsers much of the action should be done on the Web server using JSPs, servlets, or XML (Extensible Markup Language) and XSL (Extensible Stylesheet Language).
• Business logic: Business logic is best implemented in Session EJBs (described later).
• Data access: Data access is best implemented in Entity EJBs (described later) and using JDBC.
• Backend system integration: Integration with backend systems may use a variety of technologies. The best choice will depend upon the exact nature of the backend system.

You may begin to wonder: why have so many layers? Well, the layered approach makes for a more scalable enterprise application. It allows each layer to focus on a specific role -- for example, allowing a Web server to serve Webpages, an application server to serve applications, and a database server to serve databases.