Java Class Library
The Java Class Library (JCL) is a set of dynamically loadable libraries that Java Virtual Machine (JVM) languages can call at run time. 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. JCL serves three purposes within the JVM: Implementation and configuration. JCL is almost entirely written in Java, except for the parts that need direct access to the hardware and operating system  (such as for I/O or bitmap graphics). The classes that give access to these functions commonly use Java Native Interface wrappers to access operating system APIs. Almost all of JCL is stored in a single Java archive file called "rt.jar" which is provided with JRE and JDK distributions. The Java Class Library (rt.jar) is located in the default bootstrap classpath and does not have to appear in the classpath declared for the application. The runtime uses the bootstrap class loader to find the JCL. The Java Module System (part of the Java 9 release) broke the monolithic "rt.jar" JAR  file and modularized the JCL itself in several modules with specified dependencies. Conformance. Any Java implementation must pass the Java Technology Compatibility Kit tests for compliance, which includes JCL tests. Main features. JCL Features are accessed through classes provided in packages. Licensing. Prior licenses. Before the release of OpenJDK, the JDK was based on a proprietary license. Following their promise to release a fully buildable JDK based on almost completely free and open-source code in the first half of 2007, 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. Sun's goal was to replace the parts that remain proprietary and closed source with alternative implementations and make the Class Library completely free and open source. Until December 2010, the remaining encumbered part of the JDK was made available by Sun then Oracle as "Binary Plugs" which were required to build the JDK but not necessary to run it. , the only part of the Class library that remained proprietary and closed-source (4% for OpenJDK  7, and less than 1% and OpenJDK 6) was: Since the first May 2007 release, Sun, with the help of the community, released as open-source or replaced with open-source alternatives almost all the encumbered code: Open source release. Beginning in December 2010, all the so-called "binary plugs" were replaced by open source replacements, making the entire JDK open. Alternative implementations. GNU Classpath is the other main free software class library for Java. Contrary to other implementations, it only implements the Class Library, and is used by many free Java runtimes (like Kaffe, SableVM, JamVM). Apache Harmony was  another free software class library. Its aim was to implement the other parts of the Java stack (Virtual Machine, Compiler, and other tools required for any Java implementation).