Chapter 2.
Overview Of J2EE Technology and Concepts
The Java 2 Enterprise Edition (J2EE) is a multitiered architecture for implementing enterprise-class applications and web based applications. This technology supports a variety of application types from large scale web applications to small client server applications. The main aim of J2EE technology is to create a simple development model for enterprise applications using component based application model. In this model such components use services provided by the container, which would otherwise typically need to be incorporated in application code. Note this may not be ideal in all scenarios: for example, a small scale application might be a better fit for a light-weight Java technology solution (e.g. Servlets, JSPs, etc.).
J2EE Components :
J2EE applications are made up of different components. A J2EE component is a self-contained functional software unit that is assembled into a J2EE application with its helper classes and files and that communicates with other components in the application. The J2EE specification defines the following J2EE components:
Application clients and applets are components that run on the client.
Java Servlet and JavaServer Pages technology components are Web components that run on the web server.
Enterprise JavaBeans components (enterprise beans) are business components that run on the application server.
All these J2EE components are assembled into a J2EE application, verified to be well formed and in compliance with the J2EE specification, and deployed to production, where they are run and managed by the J2EE application server.
In addition to these primary components, it includes standard services and supporting technologies which are :
Java Database Connectivity (JDBC) technology provides access to relational database systems.
Java Transaction API (JTA) or Java Transaction Service (JTS) provides transaction support for J2EE components.
Java Messaging Service (JMS) for asynchronous communication between J2EE components.
Java Naming and Directory Interface (JNDI) provides naming and directory access.
Note : All J2EE components are written in the Java programming language
J2EE Clients :
There are normally two types of J2EE client: a Web client and an Application client as shown above in figure.
A Web client consists of two parts, dynamic Web pages containing various types of markup language (HTML, XML and others), which are generated by Web components running in the Web tier, and a Web browser, which draws the pages received from the server. Another category of web clients are sometimes called as thin client. Thin clients usually do not do things like query databases, execute complex business rules, or connect to any legacy applications. When we use a thin client, heavyweight operations like these are handled by enterprise beans executing on the J2EE server where they can leverage the security, speed, services, and reliability of J2EE server-side technologies.
