Packaging of Java Platform
We talked about Java scaling from tiny cell phone to heavy-duty multi-CPU machines. This is made possible by the packaging of the Java platform in three different editions:
J2EE (Java 2 Platform, Enterprise Edition): Java platform for developing and deploying always running, backend, business applications. Here the focus is on scalability, high availability, robustness, ease of deployment, and integration with other applications. In terms of packaged components, J2EE is a superset of J2SE.
As we noted earlier, each edition is essentially a collection of API specifications. These specifications, and their reference implementations on certain platforms, are available from http://java.sun.com. However, read the license agreement to determine whether you can use the downloaded specifications and software for your specific purpose.
Other implementations, both commercial and open source, are also available. For example, IBM and BEA Systems, two well-known Java vendors, supply their own implementations of J2SE and J2EE under the brand name of WebSphere and WebLogic, respectively. It is important to keep in mind that even these different implementations may have common code licensed from Sun Microsystems. The practice among Java vendors is to develop code that differentiates their offering and includes licensed code from Sun Microsystems where they don't add significant value.
It is quite common, especially for J2EE specifications, to have implementation of a specific API or collection of related APIs. For example, Apache Tomcat is an open source implementation of the following J2EE specifications: Servlets and JSP (Java ServerPages). You can download and work with these without having a complete J2EE platform.