Java Class Library
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The Java Class Library (JCL) is a set of dynamically loadable libraries that Java applications 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 Java Platform:
- Like other standard code libraries, they provide the programmer a well-known set of useful facilities, such as container classes and regular expression processing.
- The library provides an abstract interface to tasks that would normally depend heavily on the hardware and operating system, such as network access and file access.
- Some underlying platforms may not support all of the features a Java application expects. In these cases, the library implementation can either emulate those features or provide a consistent way to check for the presence of a specific feature.
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.
Any Java implementation must pass the Java Technology Compatibility Kit tests for compliance, which includes JCL tests.
java.langcontains fundamental classes and interfaces closely tied to the language and runtime system.
- I/O and networking access the platform file system, and more generally networks through the
java.netpackages. For networking, SCTP is available through
- Mathematics package:
java.mathprovides mathematical expressions and evaluation, as well as arbitrary-precision decimal and integer number datatypes.
- Collections and Utilities : built-in Collection data structures, and utility classes, for Regular expressions, Concurrency, logging and Data compression.
- GUI and 2D Graphics: the AWT package (
java.awt) basic GUI operations and binds to the underlying native system. It also contains the 2D Graphics API. The Swing package (
javax.swing) is built on AWT and provides a platform-independent widget toolkit, as well as a Pluggable look and feel. It also deals with editable and non-editable text components.
- Sound: interfaces and classes for reading, writing, sequencing, and synthesizing of sound data.
java.textdeals with text, dates, numbers, and messages.
- Image package:
javax.imageioprovide APIs to write, read, and modify images.
- XML: SAX, DOM, StAX, XSLT transforms, XPath and various APIs for Web services, as SOAP protocol and JAX-WS.
- CORBA and RMI APIs, including a built-in ORB
- Security is provided by
java.securityand encryption services are provided by
- Databases: access to SQL databases via
- Access to Scripting engines: The
javax.scriptpackage gives access to any conforming Scripting language.
java.appletallows applications to be downloaded over a network and run within a guarded sandbox
- Java Beans:
java.beansprovides ways to manipulate reusable components.
- Introspection and reflection: java.lang.Class represents a class, but other classes such as Method and Constructor are available in
Following their promise to release a fully buildable JDK based almost completely on 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. as of May 2007[update], the only part of the Class library that remained proprietary and closed-source (4% as of May 2007[update] for OpenJDK 7, and less than 1% as of May 2008[update] 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:
- All the audio engine code, including the software synthesizer, became open source. The closed-source software synthesizer has been replaced by a new synthesizer developed specifically for OpenJDK called Gervill,
- All cryptography classes were released as open source,
- The code that scales and rasterizes fonts uses open source FreeType 
- The native color management uses open source LittleCMS. There is a pluggable layer in the JDK, so that the commercial release of Java can use the original, proprietary color management system and OpenJDK can use LittleCMS.
- The anti-aliasing graphics rasterizer code uses the open source Pisces renderer used in the phoneME project.
Open source release
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, CACAO).
- Java Platform, Standard Edition
- List of Java APIs
- Free Java implementations
- Standard library
- Java applet
- "How Classes are Found". Oracle Corporation. Retrieved 2015-12-05.
- "JDK Module Summary". Oracle Corporation. 2015-10-23. Retrieved 2015-11-29.
- Rich Green (2007-05-08). "Open JDK is here!". Sun Microsystems. Retrieved 2011-11-25.
- "OpenJDK Binary Plugs". Sun Microsystems. 2007-05-08. Retrieved 2011-11-25.
- Fitzsimmons, Thomas (2007-05-18). "Plans for OpenJDK". Retrieved 2007-05-22.
- Angel, Lillian (2008-03-13). "OpenJDK to replace IcedTea in Fedora 9". Retrieved 2008-04-05.
- Wade, Karsten (2008-03-13). "OpenJDK in Fedora 9!". redhatmagazine.com. Retrieved 2008-04-05.
Thomas Fitzsimmons updated the Fedora 9 release notes source pages to reflect that Fedora 9 would ship with OpenJDK 6 instead of the IcedTea implementation of OpenJDK 7. Fedora 9 (Sulphur) is due to release in May 2008.
- Herron, David (2007-10-04). "Plans for OpenJDK". Retrieved 2007-10-09.
- "OpenJDK 6 b10 source posted". 2008-05-30. Retrieved 2008-06-01.
- audio-engine project page
- "Gervill - Software Synthesizer". Retrieved 2008-06-01.
- "Crypto has been added to OpenJDK". 2007-09-27. Retrieved 2007-10-07.
- font-scaler projectpage
- Java2D project page
- "Freetype font rasteriser". 2007-08-07. Retrieved 2007-11-24.
- graphics-rasterizer project page
- Kelly O'Hair (December 2010). "OpenJDK7 and OpenJDK6 Binary Plugs Logic Removed". Oracle Corporation. Retrieved 2011-11-25.
- Java SE Main page
- Java SE 8 API Javadocs
- Java SE API documentation
- Java software development kits (status as of August 2006[update]):