Any set of packages or classes can easily be made to play the role of an extension. The first step in turning a set of classes into an extension is to bundle them in a JAR file. Once that's done, you can turn the software into an extension in two ways:

• by placing the JAR file in a special location in the directory structure of the Java Runtime Environment, in which case it's called an installed extension.
• by referencing the JAR file in a specified way from the manifest of the another JAR file, in which case it's called a download extension.

This lesson shows you how the extension mechanism works by using a simple "toy" extension as an example.

Installed Extensions

In this section, you'll create a simple installed extension and see how extension software is treated as part of the platform by the runtime environment.

Download Extensions

This section will show you how modify a JAR file's manifest so that the JAR-bundled software can make use of download extensions.

Understanding Extension Class Loading

This section is a short detour that summarizes the Java platform's delegation model for loading classes, and shows how it relates to loading classes in extensions.

The next lesson, Making Extensions Secure uses the same extension to show how the Java platform controls the security permissions that are granted to extensions.