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{{Short description|Core Java libraries}}
The '''Java Class Library''' ('''JCL''') is a set of dynamically loadable libraries that [[List of JVM languages|Java Virtual Machine (JVM) languages]] can call at [[Run time (program lifecycle phase)|run time]]. It is the [[Standard library|standard library]] of [[Java (programming language)|Java]] and other JVM languages, and is thus sometimes called the '''Java Standard Library'''.<ref>{{cite journal |author1=Huisman, M. |author2=Jacobs, B. P. F. |author3=van den Berg, J. |year=1999 |title=A case study in class library verification: Java's vector class |journal=FernUniversität |url=https://www.academia.edu/26405431/Formal_Techniques_for_Java_Programs|page=40 |access-date=6 October 2025 }}</ref> Because the Java Platform is not dependent on a specific [[Operating system|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. Since Java 9, the Java Class Library can be accessed through module path using the [[Java Platform Module System]].

JCL serves three purposes within the JVM:
* Like other [[Standard library|standard code libraries]], they provide the programmer a well-known set of useful facilities, such as container classes and [[Regular expression|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 [[Computer file|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.

It uses the namespaces <code>java.*</code>, <code>javax.*</code>, and <code>jdk.*</code>. Some bundled classes may reside in namespaces like <code>sun.*</code> or <code>com.sun.*</code> (for [[Company:Sun Microsystems|Sun Microsystems]]), but they are not considered part of the standard library.

==Implementation and configuration==
JCL is almost entirely written in Java, except for the parts that need direct access to the [[Computer hardware|hardware]] and [[Operating system|operating system]] (such as for I/O or [[Engineering:Rasterisation|bitmap graphics]]), which are instead written in [[C++]].<ref>{{cite web|url=https://github.com/openjdk/jdk|title=Based on the OpenJDK sources|access-date=23 October 2021|publisher=[[Software:OpenJDK|OpenJDK]]}}</ref> The classes that give access to these functions commonly use [[Java Native Interface]] wrappers to access operating system [[API]]s.

Almost all of JCL is stored in a single [[JAR (file format)|Java archive]] file called "{{mono|rt.jar}}" which is provided with JRE and [[Software:Java Development Kit|JDK]] distributions. The Java Class Library ({{mono|rt.jar}}) is located in the default bootstrap classpath<ref>{{cite web | url=http://java.sun.com/javase/6/docs/technotes/tools/findingclasses.html | title=How Classes are Found | publisher=[[Company:Oracle Corporation|Oracle Corporation]] | accessdate=2015-12-05 | archive-date=2009-04-25 | archive-url=https://web.archive.org/web/20090425152829/http://java.sun.com/javase/6/docs/technotes/tools/findingclasses.html | url-status=live }}</ref> and does not have to appear in the [[Classpath (Java)|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 "{{mono|rt.jar}}" JAR file and modularized the JCL itself in several modules with specified dependencies.<ref>{{cite web | url=http://cr.openjdk.java.net/~mr/jigsaw/ea/module-summary.html | title=JDK Module Summary | publisher=[[Company:Oracle Corporation|Oracle Corporation]] | date=2015-10-23 | accessdate=2015-11-29 | archive-date=2015-12-08 | archive-url=https://web.archive.org/web/20151208074800/http://cr.openjdk.java.net/~mr/jigsaw/ea/module-summary.html | url-status=dead}}</ref>

==Conformance==
Any Java implementation must pass the Java [[Engineering:Technology Compatibility Kit|Technology Compatibility Kit]] tests for compliance, which includes JCL tests.

==Main features==
{{Main|Java Platform, Standard Edition}}
JCL Features are accessed through classes provided in [[Java package|packages]].
* {{Javadoc:SE|package=java.lang|java/lang}} contains fundamental classes and [[Interface (Java)|interfaces]] closely tied to the language and runtime system.
* I/O and networking access the platform [[File system|file system]], and more generally [[Computer network|networks]] through the {{Javadoc:SE|package=java.io|java/io}}, {{Javadoc:SE|package=java.nio|java/nio}} and {{Javadoc:SE|package=java.net|java/net}} packages. For networking, SCTP is available through {{Javadoc:SE|package=com.sun.nio.sctp|com/sun/nio/sctp|module=jdk.sctp}}.
* Mathematics package: {{Javadoc:SE|package=java.math|java/math}} provides mathematical expressions and evaluation, as well as arbitrary-precision decimal and integer number datatypes.
* Collections and Utilities : built-in Collection [[Data structure|data structure]]s, and utility classes, for [[Regular expression|regular expression]]s, [[Concurrent computing|concurrency]], logging and [[Data compression|data compression]].
* GUI and [[2D computer graphics|2D Graphics]]: the [[Abstract Window Toolkit|AWT]] package ({{Javadoc:SE|package=java.awt|java/awt|module=java.desktop}}) basic GUI operations and binds to the underlying native system. It also contains the 2D Graphics API. The Swing package ({{Javadoc:SE|package=javax.swing|javax/swing|module=java.desktop}}) is built on AWT and provides a platform-independent [[Widget toolkit|widget toolkit]], as well as a [[Pluggable look and feel|pluggable look and feel]]. It also deals with editable and non-editable text components.
* Sound: interfaces and classes for reading, writing, [[Engineering:Music sequencer|sequencing]], and [[Engineering:Synthesizer|synthesizing]] of sound data.
* Text: {{Javadoc:SE|package=java.text|java/text}} deals with text, dates, numbers and messages.
* Image package: {{Javadoc:SE|package=java.awt.image|java/awt/image|module=java.desktop}} and {{Javadoc:SE|package=javax.imageio|javax/imageio|module=java.desktop}} provide APIs to write, read, and modify images.
* [[XML]]: [[Simple API for XML|SAX]], [[Document Object Model|DOM]], [[StAX]], XSLT transforms, [[XPath]] and various APIs for Web services, as [[SOAP|SOAP protocol]] and [[Software:Java API for XML Web Services|JAX-WS]].
* Security is provided by {{Javadoc:SE|package=java.security|java/security}} and encryption services are provided by {{Javadoc:SE|package=javax.crypto|javax/crypto}}.
* [[Database]]s: access to [[SQL]] databases via {{Javadoc:SE|package=java.sql|java/sql|module=java.sql}}
* Access to Scripting engines: The {{Javadoc:SE|package=javax.script|javax/script|module=java.scripting}} package gives access to any conforming [[Scripting language]].
* [[Java applet|Applets]]: {{Javadoc:SE|package=java.applet|java/applet|module=java.desktop}} allows applications to be downloaded over a network and run within a guarded sandbox
* Java Beans: {{Javadoc:SE|package=java.beans|java/beans|module=java.desktop}} provides ways to manipulate reusable components.
* Introspection and reflection: [https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/Class.html {{code|java.lang.Class}}] represents a class, but other classes such as Method and Constructor are available in {{Javadoc:SE|package=java.lang.reflect|java/lang/reflect}}.

===Packages outside of the main namespace===
Java bundles some packages outside of the main namespaces (<code>java.*</code>, <code>javax.*</code>, and <code>jdk.*</code>). Among these include:
* <code>org.ietf.jgss.*</code>: Generic Security Service API as specified by the [[Organization:Internet Engineering Task Force|Internet Engineering Task Force]].<ref>{{Cite web|title=org.ietf.jgss (Java SE 25 & JDK 25)|url=https://docs.oracle.com/en/java/javase/25/docs/api/java.security.jgss/org/ietf/jgss/package-summary.html|access-date=10 October 2025|website=docs.oracle.com|publisher=Oracle Corporation}}</ref>
* <code>org.omg.*</code>: [[Organization:Object Management Group|Object Management Group]] packages, such as [[Common Object Request Broker Architecture]] (CORBA) features. Deprecated since Java 9, removed since Java 11.<ref>{{Cite web|title=java.corba (Java SE 10 & JDK 10)|url=https://docs.oracle.com/javase/10/docs/api/java.corba-summary.html|website=docs.oracle.com|access-date=10 October 2025|publisher=Oracle Corporation}}</ref>
* <code>org.w3c.dom.*</code>: [[Document Object Model]] packages as specified by the [[Organization:World Wide Web Consortium|World Wide Web Consortium]].<ref>{{Cite web|title=org.w3c.dom (Java SE 25 & JDK 25)|url=https://docs.oracle.com/en/java/javase/25/docs/api/java.xml/org/w3c/dom/package-summary.html|access-date=10 October 2025|website=docs.oracle.com|publisher=Oracle Corporation}}</ref>
* <code>org.xml.sax.*</code>: [[Simple API for XML]] packages related to [[XML]].<ref>{{Cite web|title=org.xml.sax (Java SE 25 & JDK 25)|url=https://docs.oracle.com/en/java/javase/25/docs/api/java.xml/org/xml/sax/package-summary.html|access-date=10 October 2025|website=docs.oracle.com|publisher=Oracle Corporation}}</ref>

===Implementation packages===
The <code>sun.*</code> (which contains the [[Security of the Java software platform#The sun.misc.Unsafe class|<code>sun.misc.Unsafe</code>]] class, used to manipulate the CPU and hardware, directly manage memory, and other things)<ref>{{Cite web|title=The Unsafe Class: Unsafe at Any Speed|url=https://blogs.oracle.com/javamagazine/post/the-unsafe-class-unsafe-at-any-speed|website=blogs.oracle.com|author=Ben Evans|date=4 May 2020|publisher=Oracle Corporation}}</ref> and <code>com.sun.*</code> packages are all part of a module called <code>jdk.unsupported</code>.<ref>{{Cite web|title=jdk.unsupported|url=https://cr.openjdk.org/~jjg/java-javafx-jdk-docs/api/jdk.unsupported-summary.html|publisher=OpenJDK|website=cr.openjdk.org|access-date=10 October 2025}}</ref> This module contains packages not part of the Java standard library but bundled as part of the Java implementation.

==Licensing==
{{See also|Free Java implementations|OpenJDK}}

===Prior licenses===
Before the release of OpenJDK, the [[Software:Java Development Kit|JDK]] was based on a [[Proprietary software|proprietary license]].

Following their promise to release a fully buildable [[Software:Java Development Kit|JDK]] based on almost completely free and open-source code in the first half of 2007,<ref>{{Cite web |date=May 2007 |title=Free and Open Source Java - FAQ |url=http://www.sun.com/software/opensource/java/faq.jsp#b4 |url-status=dead |archive-url=https://web.archive.org/web/20070522170749/http://www.sun.com/software/opensource/java/faq.jsp#b4 |archive-date=May 22, 2007 |access-date=2024-06-27 |website=sun.com |language=en}}</ref> Sun released the complete [[Source code|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.<ref>{{cite web | url=http://mail.openjdk.java.net/pipermail/announce/2007-May.txt | title=Open JDK is here! | author=Rich Green | publisher=[[Company:Sun Microsystems|Sun Microsystems]] | date=2007-05-08 | accessdate=2011-11-25 | archive-date=2012-02-07 | archive-url=https://web.archive.org/web/20120207052347/http://mail.openjdk.java.net/pipermail/announce/2007-May.txt | url-status=live }}</ref> 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 [[Company:Sun Microsystems|Sun]] then [[Company:Oracle Corporation|Oracle]] as ''Binary Plugs''<ref>{{cite web | url=http://openjdk.java.net/legal/binary-plugs-2007-05-08.html | title=OpenJDK Binary Plugs | publisher=[[Company:Sun Microsystems|Sun Microsystems]] | date=2007-05-08 | accessdate=2011-11-25 | archive-url=https://web.archive.org/web/20120826021741/http://openjdk.java.net/legal/binary-plugs-2007-05-08.html | archive-date=2012-08-26 | url-status=dead}}</ref> which were required to build the JDK but not necessary to run it. {{As of|lc=y|2007|05}}, the only part of the Class library that remained proprietary and closed-source (4% {{as of|lc=y|2007|05}} for OpenJDK 7,<ref>{{cite web | url=http://fitzsim.org/blog/?p=17 | title=Plans for OpenJDK | last=Fitzsimmons|first=Thomas | date=2007-05-18 | accessdate=2007-05-22}}</ref> and less than 1% {{as of|lc=y|2008|05}} and OpenJDK 6<ref>{{cite web |url = http://langel.wordpress.com/2008/03/13/openjdk-in-fedora-9/ |title = OpenJDK to replace IcedTea in Fedora 9 |last = Angel |first = Lillian |date = 2008-03-13 |accessdate = 2008-04-05 |url-status = dead |archiveurl = https://archive.today/20121210052442/http://langel.wordpress.com/2008/03/13/openjdk-in-fedora-9/ |archivedate = 2012-12-10}}</ref><ref>{{cite web | url=http://developer.redhatmagazine.com/2008/03/12/openjdk-to-replace-icedtea-in-fedora-9/ | title=OpenJDK in Fedora 9! | last=Wade | first=Karsten | publisher=redhatmagazine.com | quote=''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.'' | date=2008-03-13 | accessdate=2008-04-05 | archive-url=https://web.archive.org/web/20080421100346/http://developer.redhatmagazine.com/2008/03/12/openjdk-to-replace-icedtea-in-fedora-9/ | archive-date=2008-04-21 | url-status=dead}}</ref>) was<ref>{{cite web |url = http://weblogs.java.net/blog/robogeek/archive/2007/10/openjdk_encumbr.html |title = Plans for OpenJDK |last = Herron |first = David |date = 2007-10-04 |accessdate = 2007-10-09 |url-status = dead |archiveurl = https://web.archive.org/web/20071011014156/http://weblogs.java.net/blog/robogeek/archive/2007/10/openjdk_encumbr.html |archivedate = 2007-10-11}}</ref> the [[Simple Network Management Protocol|SNMP]] implementation.<ref name="openjdkb10">{{cite web |url=http://mail.openjdk.java.net/pipermail/jdk6-dev/2008-May/000058.html |title=OpenJDK 6 b10 source posted |date=2008-05-30 |accessdate=2008-06-01 |archive-date=2009-06-18 |archive-url=https://web.archive.org/web/20090618054843/http://mail.openjdk.java.net/pipermail/jdk6-dev/2008-May/000058.html |url-status=live }}</ref>

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.<ref name="openjdkb10" /><ref>{{Cite web |url=http://openjdk.java.net/projects/audio-engine/ |title=audio-engine project page |access-date=2007-05-19 |archive-date=2007-05-16 |archive-url=https://web.archive.org/web/20070516132340/http://openjdk.java.net/projects/audio-engine/ |url-status=live }}</ref> The closed-source software synthesizer has been replaced by a new synthesizer developed specifically for OpenJDK called ''Gervill'',<ref>{{cite web |url = https://gervill.dev.java.net/ |archive-url = https://archive.today/20111004052704/https://gervill.dev.java.net/ |url-status = dead |archive-date = 2011-10-04 |title = Gervill – Software Synthesizer |accessdate = 2008-06-01}}</ref>
* All [[Cryptography|cryptography]] classes were released as open-source,<ref>{{cite web |url=http://mail.openjdk.java.net/pipermail/security-dev/2007-September/000015.html |title=Crypto has been added to OpenJDK |date=2007-09-27 |accessdate=2007-10-07 |archive-date=2007-12-25 |archive-url=https://web.archive.org/web/20071225035137/http://mail.openjdk.java.net/pipermail/security-dev/2007-September/000015.html |url-status=live }}</ref>
* The code that scales and [[Engineering:Rasterisation|rasterizes]] [[Computer font|fonts]] uses open source [[Software:FreeType|FreeType]]<ref>{{Cite web |url=http://openjdk.java.net/projects/font-scaler/ |title=font-scaler projectpage |access-date=2007-05-19 |archive-date=2015-06-10 |archive-url=https://web.archive.org/web/20150610021721/http://openjdk.java.net/projects/font-scaler/ |url-status=live }}</ref><ref name="2dproject">{{Cite web |url=http://openjdk.java.net/groups/2d/ |title=Java2D project page |access-date=2007-05-19 |archive-date=2007-05-16 |archive-url=https://web.archive.org/web/20070516132229/http://openjdk.java.net/groups/2d/ |url-status=live }}</ref><ref>{{cite web |url=http://mail.openjdk.java.net/pipermail/font-scaler-dev/2007-August/000011.html |title=Freetype font rasteriser |date=2007-08-07 |accessdate=2007-11-24 |archive-date=2009-01-07 |archive-url=https://web.archive.org/web/20090107195849/http://mail.openjdk.java.net/pipermail/font-scaler-dev/2007-August/000011.html |url-status=live }}</ref>
* The native [[Color management|color management]] uses open-source LittleCMS.<ref name="2dproject" /> 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 [[Spatial anti-aliasing|anti-aliasing]] graphics [[Engineering:Rasterisation|rasterizer]] code uses the open source Pisces renderer used in the [[PhoneME|phoneME]] project.<ref name="2dproject" /><ref>{{cite web|url=https://phoneme.dev.java.net/ phoneme.dev.java.net/ |title=Welcome to the PhoneME project |archive-url=https://web.archive.org/web/20070610234355/https://phoneme.dev.java.net/ |archive-date=2007-06-10}}</ref><ref>{{Cite web |url=http://openjdk.java.net/projects/graphics-rasterizer/ |title=graphics-rasterizer project page |access-date=2007-05-19 |archive-date=2007-05-16 |archive-url=https://web.archive.org/web/20070516132602/http://openjdk.java.net/projects/graphics-rasterizer/ |url-status=live }}</ref>
* The [[JavaScript]] plugin is open source (the [[Software:Rhino (JavaScript engine)|JavaScript engine]] itself was open source from the beginning).<ref>{{cite web | url=http://icedtea.classpath.org/bugzilla/show_bug.cgi?id=85 | title=Javascript is encumbered and there is no javascript plugin support. | publisher=[[Software:IcedTea|IcedTea]] | quote=''Changing Summary. JavaScript is no longer encumbered, but we still need liveconnect support.'' | date=2008-03-11 | accessdate=2008-06-01 | archive-date=2008-09-27 | archive-url=https://web.archive.org/web/20080927185957/http://icedtea.classpath.org/bugzilla/show_bug.cgi?id=85 | url-status=dead}}</ref>

===Open source release===
Beginning in December 2010, all the so-called ''binary plugs'' were replaced by [[Open-source model|open source]] replacements, making the entire JDK open.<ref>{{cite web | url=http://mail.openjdk.java.net/pipermail/build-dev/2010-December/003881.html | title=OpenJDK7 and OpenJDK6 Binary Plugs Logic Removed | author=Kelly O'Hair | publisher=[[Company:Oracle Corporation|Oracle Corporation]] | date=December 2010 | accessdate=2011-11-25 | archive-date=2022-04-07 | archive-url=https://web.archive.org/web/20220407115622/http://mail.openjdk.java.net/pipermail/build-dev/2010-December/003881.html | url-status=live }}</ref>

==Alternative implementations==
[[Software:GNU Classpath|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 implementations|free Java runtimes]] (like [[Software:Kaffe|Kaffe]], [[Software:SableVM|SableVM]], [[Software:JamVM|JamVM]]).

[[Software:Apache Harmony|Apache Harmony]] was another free software class library. Its aim was to implement the other parts of the Java stack ([[Java virtual machine|Virtual Machine]], [[Java compiler|Compiler]], and other tools required for any [[Free Java implementations|Java implementation]]).

==See also==
* [[Java Platform, Standard Edition]]
* [[Software:List of Java APIs|List of Java APIs]]
* [[Software:OpenJDK|OpenJDK]]
* [[Free Java implementations]]
* [[Standard library]]
* [[Java applet]]

==References==
{{reflist|1=2}}

==External links==
* [http://java.sun.com/j2se/ Java SE Main page]
* {{Javadoc:SE}}
* [http://java.sun.com/reference/api/index.html Java SE API documentation]
* Java software development kits (status {{As of|2006|alt=as of August 2006}}):
** [https://web.archive.org/web/20070610111552/https://jdk7.dev.java.net/ 1.7] (early development)
** [http://java.sun.com/javase/6/ 1.6] (stable, current)
** [http://java.sun.com/j2se/1.5.0/ 1.5] (stable)
** [http://java.sun.com/j2se/1.4.2/ 1.4] (stable)
** [http://java.sun.com/j2se/1.3/ 1.3] (obsolete)

{{Java (Sun)}}
{{Sun Microsystems}}

[[Category:Java (programming language)]]
[[Category:Java (programming language) libraries]]

{{Sourceattribution|Java Class Library}}