Play 2.0 makes use of several popular Java libraries:

The reused parser code is not part of the standard Java libraries.

There is no implementation of getopt in the Java standard library.

All standard Java libraries can be used as well, and sophisticated functionality can be achieved.

As such, it provides algorithmic skeleton programming as a Java library.

Both of them provide nestable skeletons to the programmer as Java libraries.

Note that the Java libraries use constant interface pattern themselves, proving that it may be a reasonable choice in some situations.

Since Java 1.5, the Java standard library has included and functions.

The Java standard library provides utility classes to use with enumerations.

The Java library is released as an open source project.