Optional packages are packages of 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 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 JDK or JRE.
Overview - What optional packages are and how to use them.
The following classes play a role in the extension mechanism:
See also the Related APIs section of the mechanism specification for notes on these APIs.
Located on the Java Software website:
For details, see Enhancements.
Extension Mechanism Architecture - Notes on the extension mechanism API and how optional packages use the Jar file format.