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Java Class Library

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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. 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 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.

Contents

• Implementation and configuration

• Conformance

• Main features

• Licensing

• Prior licenses

• Open source release

• Alternative implementations

• See also

• References

• External links

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), which are instead written in C++.cite-ref-1[1] 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 classpathcite-ref-2[2]

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.cite-ref-3[3]

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.

• java.lang contains 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.io, java.nio and java.net packages. For

networking, SCTP is available through com.sun.nio.sctp.

• Mathematics package: java.math provides 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.

• Text: java.text deals with text, dates, numbers and messages.

• Image package: java.awt.image and javax.imageio provide 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.

• Security is provided by java.security and encryption services are

provided by javax.crypto.

• Databases: access to SQL databases via java.sql

• Access to Scripting engines: The javax.script package gives access to

any conforming Scripting language.

• 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.

• Introspection and reflection: java.lang.Class represents a class, but

other classes such as Method and Constructor are available in

java.lang.reflect.

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,cite-ref-4[4] 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.cite-ref-5[5] 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 Plugscite-ref-6[6] 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,cite-ref-7[7] and less than 1% as of May 2008 and

OpenJDK 6cite-ref-8[8]cite-ref-9[9]) wascite-ref-10[10] the SNMP implementation.cite-ref-openjdkb10-11-0[11]

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.cite-ref-openjdkb10-11-1[11]cite-ref-12[12] The closed-source software synthesizer has been

replaced by a new synthesizer developed specifically for OpenJDK called

Gervill,cite-ref-13[13]

• All cryptography classes were released as open-source,cite-ref-14[14]

• The code that scales and rasterizes fonts uses open source

FreeTypecite-ref-15[15]cite-ref-2dproject-16-0[16]cite-ref-17[17]

• The native color management uses open-source LittleCMS.cite-ref-2dproject-16-1[16] 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.cite-ref-2dproject-16-2[16]cite-ref-18[18]cite-ref-19[19]

• The JavaScript plugin is open source (the JavaScript engine itself was

open source from the beginning).cite-ref-20[20]

Open source release

Beginning in December 2010, all the so-called binary plugs were replaced

by open source replacements, making the entire JDK open.cite-ref-21[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).

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).