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 from the following diagram, 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 diagram suggests, Java applications have access both to the Java platform and extensions without requiring classpaths. This feature was deprecated in Java 8 and no longer exists in the current version of Java.