# The Java Extension Mechanism

### for Support of Optional Packages

Documentation Contents

> Note: Optional packages are the new name for what used to be known as standard extensions. The "extension mechanism" is that functionality of the Java 2 SDK and Java 2 Runtime Environment that supports the use of optional packages.

> Optional packages are packages of Java classes (and any associated native code) that application developers can use to extend the functionality of the core platform. The extension mechanism allows the Java virtual machine (VM) to use the classes of the optional extension in much the same way as the VM uses classes in the Java 2 Platform. The extension mechanism also provides a way for needed optional packages to be retrieved from specified URLs when they are not already installed in the Java 2 SDK or Runtime Environment.

Overview

- Overview - What optional packages are and how to use them.

Extension Mechanism Architecture

- Extension Mechanism Architecture - Notes on the extension mechanism API and how optional packages use the Jar file format.

Enhancements Made in Previous Release J2SDK 1.3

- Enhancements to the extension mechanism in version 1.3 of the Java 2 SDK included an extended set of manifest attributes that can be used for checking vendor and versioning information of installed optional packages. If an applet requires an optional package that isn't installed, or that is installed but has the wrong version number or is not from the appropriate vendor, the Java Plug-in can download the needed extension from a specified URL. For more information, see Optional-Package Versioning.

API Specification

The following classes play a role in the extension mechanism:

- java.lang.ClassLoader
- java.lang.Package
- java.lang.Thread
- java.net.JarURLConnection
- java.net.URLClassLoader
- java.security.SecureClassLoader

See also the API Details section of the mechanism specification for notes on these APIs.

Tutorial

> Located on the Java Software website:

- The Java Extension Mechanism trail of the Java Tutorial.

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