Because the Java Platform is not dependent on a specific operating system, applications cannot rely on any of the platform-native libraries.
Instead, the Java Platform provides a comprehensive set of standard class libraries, containing the functions common to modern operating systems.
Almost all of JCL is stored in a single Java archive file called "rt.jar" which is provided with JRE and JDK distributions.
Following their promise to release a fully buildable JDK based on almost completely free and open-source code in the first half of 2007,[3] Sun released the complete source code of the Class Library under the GPL on May 8, 2007, except some limited parts that were licensed by Sun from third parties who did not want their code to be released under an open-source license.
[20] GNU Classpath is the other main free software class library for Java.