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The Enterprise Java BluePrints for the J2EE platform describe the J2EE application model and best practices for using the J2EE platform. Building on the J2SE platform, the J2EE application model provides a simplified approach to developing highly scalable and highly available internet or intranet based applications.
Thanks to the J2EE application model, maybe the most interesting thing about J2EE applications is what they don't do. That is, various complexities inherent in enterprise applications -- transaction management, life-cycle management, resource pooling -- are built into the platform and provided automatically to the components it supports. Component and application developers are free to focus on specifics such as business logic and user interfaces.
Another advantage of the J2EE platform is that the application model encapsulates the layers of functionality in specific types of components. Business logic is encapsulated in Enterprise JavaBeans (EJB) components. Client interaction can be presented through plain HTML web pages, through web pages powered by applets, Java Servlets, or JavaServer Pages technology, or through stand-alone Java applications. Components communicate transparently using various standards: HTML, XML, HTTP, SSL, RMI, IIOP, and others.
Reusable J2EE components mean competitive choices for enterprise developers and IT organizations. The J2EE platform enables them to assemble applications from a combination of standard, commercially available components and their own custom components. From general business application components to vertical market solutions, a range of standardized J2EE functionality is available off the shelf.
This means that an e-commerce site could be built using a combination of off-the-shelf EJB components for shopping cart behaviors, modified EJB components for specialized customer services, and completely customized layouts using JavaServer Pages technology that bring a unique look and feel to the site.
This approach means faster development time, better quality, maintainability and portability, and Web services interoperability across a range of enterprise platforms. The bottom line benefits are increased programmer productivity, better strategic use of computing resources, and greater return on an organization's technology investments.
The J2EE application model divides enterprise applications into three fundamental parts: components, containers, and connectors. Components are the key focus of application developers, while system vendors implement containers and connectors to conceal complexity and promote portability.
Containers intercede between clients and components, providing services transparently to both, including transaction support and resource pooling. Container mediation allows many component behaviors to be specified at deployment time, rather than in program code.
Connectors sit beneath the J2EE platform, defining a portable service API that communicates with existing enterprise vendor offerings. Connectors promote flexibility by enabling a variety of implementations of specific services. In particular, connectors implementing pluggable messaging contracts enable bidirectional communication between J2EE components and enterprise systems.
The J2EE platform provides choices for graphical user interfaces across a company's intranet or on the World Wide Web. Clients can run on desktops, laptops, PDAs, cell phones, and other devices. Pure client-side user interfaces can use standard HTML and Java applets. Support for simple HTML means quicker prototypes, and support for a broader range of clients. Additionally, the J2EE platform supports automatic download of the Java Plug-in to add applet support where it's lacking. The J2EE platform also supports stand-alone Java application clients.
For server-side generation of dynamic content, the J2EE platform supports two types of web component technologies: Java Servlets and JavaServer Pages (JSP). Java Servlets enable developers to easily implement server-side behaviors that take full advantage of the power of the rich Java API. JavaServer Pages technology combines the ubiquity of HTML with the power of server-side dynamic content generation. The JSP 2.0 specification supports static templates, simplified access to Java objects, and easy extensibility.
Enterprise JavaBeans (EJB) technology enables a simplified approach to multitier application development, concealing application complexity and enabling the component developer to focus on business logic.
EJB technology gives developers the ability to model the full range of objects useful in the enterprise by defining several types of EJB components: session beans, entity beans, message-driven beans. Session beans represent behaviors associated with client sessions -- for example, a user purchase transaction on an e-commerce site. Session beans can serve as Web service endpoints. Entity beans represent collections of data -- such as rows in a relational database -- and encapsulate operations on the data they represent. Entity beans are intended to be persistent, surviving as long as the data they're associated with remains viable. Message-driven beans allow J2EE applications to process messages asynchronously. A message-driven bean normally acts as a JMS message listener, which is similar to an event listener except that it receives JMS messages instead of events. The messages may be sent by any J2EE component--an application client, another enterprise bean, or a Web component--or by a JMS application or system that does not use J2EE technology.
The Java 2 Platform, Enterprise Edition version 1.4 is the most complete Web services platform ever. The platform features Web services support through the new JAX-RPC 1.1 API, which provides service endpoints based on servlets and enterprise beans. JAX-RPC 1.1 provides interoperability with Web services based on the WSDL and SOAP protocols. The J2EE 1.4 platform also supports the Web Services for J2EE specification, which defines deployment requirements for Web services and utilizes the JAX-RPC programming model. In addition to numerous Web services APIs, the J2EE 1.4 platform also features support for the WS-I Basic Profile 1.0. This means that in addition to platform independence and complete Web services support, the J2EE 1.4 platform offers platform Web services interoperability.
Based on these flexible component configurations, the J2EE application model means quicker development, easier customization and greater ability to deploy powerful enterprise applications. And, because it's based on the Java programming language, this model enables all J2EE applications to achieve all the benefits of Java technology: scalability, portability, and programming ease.