The Java Class Library is a set of dynamically loadable libraries that Java applications can call at run time. Because the Java Platform is not dependent on any specific operating system, applications cannot rely on any of the existing libraries. Instead, the Java Platform provides a comprehensive set of standard class libraries, containing much of the same reusable functions commonly found in modern operating systems.

The Java class libraries serve 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 expressions.
• In addition, the class libraries provide an abstract interface to tasks that would normally depend heavily on the hardware and operating system. Tasks such as network access and file access are often heavily dependent on the native capabilities of the platform.
• Finally, some underlying platforms may not support all of the features a Java application expects. In these cases, the class libraries can either emulate those features using whatever is available, or provide a consistent way to check for the presence of a specific feature.

Architecture

The Java Class Library is almost entirely written in Java itself, 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 native interface wrappers to access the API of the operating system.

Almost all of the Java Class Library 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,^[1] and does not have to be found in the classpath declared for the application. The runtime uses the bootstrap class loader to find the Java class library.

Conformance

Any Java implementation must pass the Java Technology Compatibility Kit tests for compliance.

Main features

Features of the Class Library are accessed through classes grouped by packages.

• java.lang contains fundamental classes and interfaces closely tied to the language and runtime system.
• I/O and networking: access to the platform file system, and more generally to networks, is provided through the java.io, java.nio, and java.net packages.
• Mathematics package: java.math provides regular mathematical expressions, as well as arbitrary-precision decimals and integers numbers.
• Collections and Utilities : provide built-in Collection data structures, and various utility classes, for Regular expressions, Concurrency, logging and Data compression.
• GUI and 2D Graphics: the AWT package supports basic GUI operations and binds to the underlying native system. It also contains the 2D Graphics API. The Swing package 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: provides interfaces and classes for reading, writing, sequencing, and synthesizing of sound data.
• Text: the java.text package deals with text, dates, numbers, and messages.
• Image package: java.awt.image and javax.imageio provide APIs to write, read, and modify images.
• XML: built-in classes handle 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 and Cryptography
• Databases: access to SQL databases is provided through the java.sql package.
• Access to Scripting engines: the javax.script package gives access to any Scripting language that conforms to this API.
• Applets: java.applet allows applications to be downloaded over a network and run within a guarded sandbox
• Java Beans: java.beans provides ways to manipulate reusable components.

Licensing

Before May 2007

Before the release of OpenJDK, the JDK was based on a proprietary license.

OpenJDK release (May 2007)

Following their promise to release a fully buildable JDK based almost completely on free and open source code in the first half of 2007,^[2] 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 a free and open-source license.^[3]. 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.^[4]

Before December 2010

Until December 2010, the remaining encumbered part of the JDK was made available by Sun then Oracle as Binary Plugs^[5] which were required to build the JDK but not necessary to run it. as of May 2007, the only part of the Class library that remained proprietary and closed-source (4% as of May 2007 for OpenJDK 7,^[6] and less than 1% as of May 2008 and OpenJDK 6^[7][8]) was:^{[9] [10]}

• The SNMP implementation.^[10]

Since the first May 2007 release, Sun Microsystems, 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 synthetizer, has been released as Open-source.^[10][11] The closed-source software synthesizer has been replaced by a new synthesizer developed specifically for OpenJDK called Gervill,^[12]
• All cryptography classes used in the Class library have been released as Open-source,^[13]
• The code that scales and rasterizes fonts has been replaced by FreeType ^[14][15][16]
• The native color management system has been replaced by LittleCMS.^[15] There is a pluggable layer in the JDK, so that the commercial version can use the old color management system and OpenJDK can use LittleCMS.
• The anti-aliasing graphics rasterizer code has been replaced by the Open-sourced Pisces renderer used in the phoneME project.^[15][17][18] This code is fully functional, but still needs some performance enhancements,^[19]
• The JavaScript plugin has been open-sourced (the JavaScript engine itself was open-sourced from the beginning)^[20].

After December 2010

Beginning in December 2010, all the so called binary plugs were replaced by open source replacements, making the whole JDK open sourced and the binary plugs not necessary anymore^[21].

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, CACAO).

Apache Harmony was another free software class library. Its aim was to also implement the other parts of the Java stack (Virtual Machine, Compiler, and other tools required for any Java implementation).

See also

Java portal

• Java Platform, Standard Edition
• List of Java APIs
• OpenJDK
• Free Java implementations
• Standard library
• Java applet

References

External links

• Java SE Main page
• Java SE 7 API Javadocs
• Java SE API documentation
• Java software development kits (status as of August 2006):
  • 1.7 (early development)
  • 1.6 (stable, current)
  • 1.5 (stable)
  • 1.4 (stable)
  • 1.3 (obsolete)

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