The Java platform previously had an extension mechanism that gave developers the ability to make custom APIs available to all applications. As you can see in the following illustration, extensions are plugins of sorts, or add-ons to the Java platform. The APIs and classes in each extension are, by default, automatically available:
As the illustration suggests, Java applications have access both to the Java platform and extensions without requiring classpaths. This feature was depreciated in Java 8 and no longer exists in Java 9.
