9.4 Java 2 Enterprise Edition

The Java 2 Enterprise Edition (J2EE) defines a standard for developing multitier enterprise services that are highly available, secure, reliable, and scalable. With the J2EE platform, it is possible to develop a large class of enterprise applications using only the J2EE APIs, ranging from a Java client accessing a database via JDBC to Web-based, multitier distributed enterprise systems.

The J2EE architecture includes EJBs and adds Web components and applet clients at the front end. At the back end, version 1.3 of the specification and later include the connector architecture to define transactional resource adapters. These allow plug-ins of legacy enterprise information systems (EISs) to any J2EE product. Figure 9-7 illustrates the architecture of a J2EE application [Sun 99a].

Figure 9-7. J2EE platform architecture

The various containers form runtime environments for their respective components. All components are built using the Java programming language. The arrows indicate access paths to other J2EE components.

Remote clients are implemented as stand-alone Java programs, as HTML pages, as applets, or as a combination of applets and HTML pages. The middle tier is split into an EJB tier containing the transactional business logic and a Web tier containing the software components that respond to HTTP requests . The persistence tier can be implemented in any database connected via JDBC or in a legacy EIS via connectors.

The deployed J2EE application components never interact directly. Instead, access is intercepted by the respective container. This allows the container to transparently provide services, such as declarative transaction management, access control, and others.

In addition, the containers manage security aspects, such as authorization (see Section 9.2), user authentication, and confidentiality. Users can authenticate by using HTTP basic authentication, HTTPS (HTTP, secure) client authentication, or form-based authentication. The Web container must support single sign-on by maintaining login sessions. Application client containers must also provide means to provide user authentication. The details are left to the J2EE product vendor.

J2EE Services

The J2EE platform requires the containers to provide a set of standard services to be used by application components, including communication protocols, Java enterprise APIs, and Java runtime support. The complete list of services is enumerated in the Java 2 Platform Enterprise Edition specifications for each release. ^[4]

^[4] For more information, see http://java.sun.com/j2ee/j2ee-1_4-pfd-spec.pdf.

Deployment

J2EE applications are packaged as enterprise archive files (.ear). These archives contain the components and other files. All references to other components and external resources are specified in deployment descriptors as XML documents. These references are bound at deployment time, using vendor-specific tools.