The J2EE Specification in Java

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Sun Microsystem's Java 2 Platform, Enterprise Edition (J2EE) specification is the proposed Javabased solution to the problem of building n-tier applications J2EE focuses on defining client- and server-side technologies that make these applications not only easier to build but also easier to integrate The J2EE spec encompasses all kinds of client/server types and interactions: It deals with Web clients, Web-based information servers, pure application servers, applets, and both synchronous and asynchronous solutions Although J2EE specifies many complex and powerful technologies, it is still just a specification It requires vendors to actually develop what it proposes Several have: BEA offers its WebLogic suite of products and IBM offers WebSphere, just to name the two most well-known Sun has also released a reference implementation, which is a very useful and cost-effective way to get started with J2EE Vendors differ on some important details, as we'll see later, but they must all implement the specification correctly or they won't be certified In this chapter, I provide a brief overview of J2EE and highlight the key component- and platform-level technologies it defines
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Overview of the Specification
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The J2EE specification (version 13) describes a set of technologies designed to address the presentation, business logic, and persistent storage needs of n-tier applications Generally speaking, a J2EE environment consists of several types of components that communicate with each other and a persistent storage device (a database) There are four categories of components:
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Java applets, which typically execute in a Web browser on a client machine Java applications, which execute on a local or remote client machine Java servlets, JavaServer Pages (JSPs), filters, and Web event listeners, which execute on a server machine Enterprise JavaBeans (EJBs), which are application objects that execute on a server machine
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Each of these component categories is associated with a specific type of J2EE container:
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Applet containers, which host applet components Application containers, which host application components Web containers, which host servlets and JSPs EJB containers, which host EJBs
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Containers manage their components Component developers can assume that the containers exist and manage the components per a specified contract (agreement) In plain terms, this means that components must obey certain interface guidelines and that containers rely on interfaceguaranteed functionality in the course of their management duties From the component perspective, a container is used to access the rest of the J2EE services Thus, the container acts as the layer (or API) that makes the rest of the system visible to the component being contained Through this layer, components can access other components or resources, such as an external database Each container type is targeted toward a particular style of deployment:
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Applet containers are the most limited They simply act as proxies to the other types of container Applets run in a Web browser and can contact Web containers in order to access the underlying business logic Application containers run as standalone programs and can access Web containers, EJB containers, or the database
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Web containers receive requests and format replies for Web clients They enable a J2EE system to be accessed via HTTP EJB containers represent the heart of the J2EE architecture: They manage the underlying business logic of the application This is where operations like order processing occur or new accounts are created EJBs are constructed based on the language of your business They consist of a number of built-in features to enable reliability, availability, security, scalability, and robustness
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Figure 3-1 shows the general relationships of the different types of containers The purpose of each of four J2EE containers is to permit access to underlying J2EE services (local or remote) through a set of common APIs These APIs allow the containers to do things such as conduct transactions, manage security, and pool resources The APIs and their basic purposes are listed in Table 3-1 Figure 3-2, a more detailed version of Figure 3-1, indicates where the various APIs and services fit into the different J2EE containers Figure 3-1 Relationships among J2EE containers
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