Java refers to a number of computer software products and specifications from Sun Microsystems, a subsidiary of Oracle Corporation, that together provide a system for developing application software and deploying it in across-platform environment. Java is used in a wide variety of computing platforms fromembedded devices andmobile phones on the low end, to enterprise servers and supercomputers on the high end. Java is used in mobile phones, Web servers and enterprise applications, and while less common on desktop computers, Java appletsare sometimes used to provide improved and secure functionalities while browsing the World Wide Web.
Writing in the Java programming language is the primary way to produce code that will be deployed as Java bytecode, though there are bytecode compilersavailable for other languages such as Ada, JavaScript, Python, and Ruby. Several new languages have been designed to run natively on the Java Virtual Machine (JVM), such asScala, Clojure and Groovy. Java syntax borrows heavily from C and C++, but object-oriented features are modeled after Smalltalkand Objective-C.[1] Java eliminates certain low-level constructs such as pointers and has a very simple memory model where every object is allocated on the heap and all variables of object types arereferences. Memory management is handled through integrated automatic garbage collection performed by the JVM.
On November 13, 2006, Sun Microsystems made the bulk of its implementation of Java available under the GNU General Public License,[2] although there are still a few parts distributed as precompiled binaries due to copyright issues with code that is licensed (but not owned) by Sun.[3]
Platform
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An edition of the Java platform is the name for a bundle of related programs from Sun that allow for developing and running programs written in the Java programming language. The platform is not specific to any one processor or operating system, but rather an execution engine (called a virtual machine) and a compiler with a set of libraries that are implemented for various hardware and operating systems so that Java programs can run identically on all of them.
- Java Card: A technology that allows small Java-based applications (applets) to be run securely on smart cardsand similar small-memory-footprint devices.
- Java ME (Micro Edition): Specifies several different sets of libraries (known as profiles) for devices that are sufficiently limited that supplying the full set of Java libraries would take up unacceptably large amounts of storage.
- Java SE (Standard Edition): For general-purpose use on desktop PCs, servers and similar devices.
- Java EE (Enterprise Edition): Java SE plus various APIs useful for multi-tier client–server enterprise applications.
As of September 2009, the current version of the Java Platform is specified as either 1.6.0 or 6 (both refer to the same version). Version 6 is the product version, while 1.6.0 is the developer version.
The Java Platform consists of several programs, each of which provides a distinct portion of its overall capabilities. For example, the Java compiler, which converts Java source code into Java bytecode (an intermediate language for the Java Virtual Machine (JVM)), is provided as part of the Java Development Kit (JDK). The Java Runtime Environment (JRE), complementing the JVM with a just-in-time (JIT) compiler, converts intermediate bytecode into native machine code on the fly. Also supplied are extensive libraries, pre-compiled in which are several other components, some available only in certain editions.
The essential components in the platform are the Java language compiler, the libraries, and the runtime environment in which Java intermediate bytecode "executes" according to the rules laid out in the virtual machine specification.
Java Virtual Machine
The heart of the Java Platform is the concept of a "virtual machine" that executes Java bytecode programs. This bytecode is the same no matter what hardware or operating system the program is running under. There is a JIT compiler within the Java Virtual Machine, or JVM. The JIT compiler translates the Java bytecode into native processor instructions at run-time and caches the native code in memory during execution.
The use of bytecode as an intermediate language permits Java programs to run on any platform that has a virtual machine available. The use of a JIT compiler means that Java applications, after a short delay during loading and once they have "warmed up" by being all or mostly JIT-compiled, tend to run about as fast as native programs. Since JRE version 1.2, Sun's JVM implementation has included a just-in-time compiler instead of an interpreter.
Although Java programs are platform independent, the code of the Java Virtual Machine (JVM) that execute these programs is not; every supported operating platform has its own JVM.
Class libraries
In most modern operating systems, a large body of reusable code is provided to simplify the programmer's job. This code is typically provided as a set of dynamically loadable libraries that applications can call at runtime. Because the Java Platform is not dependent on any specific operating system, applications cannot rely on any of the pre-existing OS libraries. Instead, the Java Platform provides a comprehensive set of its own standard class libraries containing much of the same reusable functions commonly found in modern operating systems. The majority of the system library is also written in Java. For instance, Swing library paints the user interface and handles the events itself, eliminating many subtle differences between how different platforms handle even similar components.
The Java class libraries serve three purposes within the Java Platform. First, like other standard code libraries, the Java libraries provide the programmer a well-known set of functions to perform common tasks, such as maintaining lists of items or performing complex string parsing. Second, 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 intertwined with the distinctive implementations of each platform. The java.net and java.io libraries implement an abstraction layer in native OS code, then provide a standard interface for the Java applications to perform those tasks. Finally, when some underlying platform does not support all of the features a Java application expects, the class libraries work to gracefully handle the absent components, either by emulation to provide a substitute, or at least by providing a consistent way to check for the presence of a specific feature.
Languages
The word Java, by itself, usually refers to the Java programming language that was designed for use with the Java Platform. Programming languages are typically outside of the scope of the phrase "platform", although the Java programming language is listed as a core part of the Java platform. The language and runtime are therefore commonly considered a single unit.
Nevertheless, third parties have produced a number of compilers or interpreters that target the JVM. Some of these are for existing languages, while others are for extensions to the Java language itself. These include:
- BeanShell a lightweight scripting for Java
- Clojure
- Groovy
- JRuby, a Ruby interpreter
- Jython, a Python interpreter that includes jythonc, a Python-to-Java bytecode compiler
- Rhino, a JavaScript interpreter
- Scala
Similar platforms
The success of Java and its write once, run anywhere concept has led to other similar efforts, notably the Microsoft .NET platform, appearing since 2002, which incorporates many of the successful aspects of Java. .NET in its complete form (Microsoft's implementation) is currently only fully available on Windows platforms, whereas Java is fully available on many platforms. .NET was built from the ground-up to support multiple programming languages, while the Java platform was initially built to support only the Java language (although many other languages have been made for JVM since).
.NET includes a Java-like language called Visual J# (formerly known as J++) that is not compatible with the Java specification, and the associated class library mostly dates to the old JDK 1.1 version of the language; for these reasons, it is more a transitional language to switch from Java to the Microsoft .NET platform, than a first class Microsoft .NET language. Visual J# has been discontinued with the release of Microsoft Visual Studio 2008. The existing version shipping with Visual Studio 2005 will be supported until 2015 as per the product life-cycle strategy.
Java Development Kit
The Java Development Kit (JDK) is a Sun product aimed at Java developers. Since the introduction of Java, it has been by far the most widely used Java SDK. It contains a Java compiler and a number of other important development tools as well as a full copy of the Java Runtime Environment.
History
The Java platform and language began as an internal project at Sun Microsystems in December 1990, providing an alternative to the C++/C programming languages. Engineer Patrick Naughton had become increasingly frustrated with the state of Sun's C++ and C APIs (application programming interfaces) and tools. While considering moving to NeXT, Naughton was offered a chance to work on new technology and thus the Stealth Project was started.
The Stealth Project was soon renamed to the Green Project with James Gosling and Mike Sheridan joining Naughton. Together with other engineers, they began work in a small office on Sand Hill Roadin Menlo Park, California. They were attempting to develop a new technology for programming next generation smart appliances, which Sun expected to be a major new opportunity.[4]
The team originally considered using C++, but it was rejected for several reasons. Because they were developing an embedded system with limited resources, they decided that C++ demanded too large a footprint and that its complexity led to developer errors. The language's lack of garbage collection meant that programmers had to manually manage system memory, a challenging and error-prone task. The team was also troubled by the language's lack of portable facilities for security, distributed programming, and threading. Finally, they wanted a platform that could be easily ported to all types of devices.
Bill Joy had envisioned a new language combining Mesa and C. In a paper called Further, he proposed to Sun that its engineers should produce an object-oriented environment based on C++. Initially, Gosling attempted to modify and extend C++ (that he referred to as "C++ ++ --") but soon abandoned that in favor of creating an entirely new language, which he called Oak, after the tree that stood just outside his office.
By the summer of 1992, they were able to demonstrate portions of the new platform including the Green OS, the Oak language, the libraries, and the hardware. Their first attempt, demonstrated on September 3, 1992, focused on building a PDA device named Star7[2] that had a graphical interface and a smart agent called "Duke" to assist the user. In November of that year, the Green Project was spun off to become firstperson, a wholly owned subsidiary of Sun Microsystems, and the team relocated to Palo Alto, California.[5] The firstperson team was interested in building highly interactive devices, and when Time Warner issued an RFP for a set-top box, firstperson changed their target and responded with a proposal for a set-top box platform. However, the cable industry felt that their platform gave too much control to the user and firstperson lost their bid to SGI. An additional deal with The 3DO Company for a set-top box also failed to materialize. Unable to generate interest within the TV industry, the company was rolled back into Sun.
Java meets the Internet
In June and July 1994, after three days of brainstorming with John Gage, the Director of Science for Sun, Gosling, Joy, Naughton, Wayne Rosing, and Eric Schmidt, the team re-targeted the platform for the World Wide Web. They felt that with the advent of graphical web browsers like Mosaic, the Internet was on its way to evolving into the same highly interactive medium that they had envisioned for cable TV. As a prototype, Naughton wrote a small browser, WebRunner (named after the movie Blade Runner), later renamed HotJava.[4]
That year, the language was renamed Java after a trademark search revealed that Oak was used by Oak Technology.[6] Although Java 1.0a was available for download in 1994, the first public release of Java was 1.0a2 with the HotJava browser on May 23, 1995, announced by Gage at the SunWorld conference. His announcement was accompanied by a surprise announcement by Marc Andreessen, Executive Vice President of Netscape Communications Corporation, that Netscape browsers would be including Java support. On January 9, 1996, the JavaSoft group was formed by Sun Microsystems in order to develop the technology.[7]
Version history
The Java language has undergone several changes since JDK (Java Development Kit) 1.0 was released on (January 23, 1996), as well as numerous additions of classes and packages to the standardlibrary. Since J2SE 1.4, the evolution of the Java Language has been governed by the Java Community Process (JCP), which uses Java Specification Requests (JSRs) to propose and specify additions and changes to the Java platform. The language is specified by the Java Language Specification (JLS); changes to the JLS are managed under JSR 901.[8]
JDK 1.1 was released on February 19, 1997. Major additions included an extensive retooling of the AWT event model, inner classes added to the language, JavaBeans and JDBC.
J2SE 1.2 (December 8, 1998) — Codename Playground. This and subsequent releases through J2SE 5.0 were rebranded Java 2 and the version name "J2SE" (Java 2 Platform, Standard Edition) replaced JDK to distinguish the base platform from J2EE (Java 2 Platform, Enterprise Edition) and J2ME (Java 2 Platform, Micro Edition). Major additions included reflection, a Collections framework,Java IDL (an IDL implementation for CORBA interoperability), and the integration of the Swing graphical API into the core classes. A Java Plug-in was released, and Sun's JVM was equipped with a JIT compiler for the first time.
J2SE 1.3 (May 8, 2000) — Codename Kestrel. Notable changes included the bundling of the HotSpot JVM (the HotSpot JVM was first released in April, 1999 for the J2SE 1.2 JVM), JavaSound, Java Naming and Directory Interface (JNDI) and Java Platform Debugger Architecture (JPDA).
J2SE 1.4 (February 6, 2002) — Codename Merlin. This was the first release of the Java platform developed under the Java Community Process as JSR 59.[9] Major changes included regular expressions modeled after Perl, exception chaining, an integrated XML parser and XSLT processor (JAXP), and Java Web Start.
J2SE 5.0 (September 30, 2004) — Codename Tiger. Originally numbered 1.5, which is still used as the internal version number.[10] Developed under JSR 176, Tiger added a number of significant new language features including the for-each loop, generics, autoboxing and var-args.[11]
The current version, Java SE 6 (December 11, 2006) — Codename Mustang — is bundled with a database manager, facilitates the use of scripting languages (currently JavaScript using Mozilla's Rhinoengine) with the JVM and has Visual Basic language support. As of this version, Sun replaced the name "J2SE" with Java SE and dropped the ".0" from the version number.[12] Other major changes include support for pluggable annotations (JSR 269), lots of GUI improvements, including native UI enhancements to support the look and feel of Windows Vista, and improvements to the Java Platform Debugger Architecture (JPDA) & JVM Tool Interface for better monitoring and troubleshooting
Java SE 7 — Codename Dolphin. The Dolphin Project started in August 2006, with release estimated in mid 2011. New builds including enhancements and bug fixes are released approximately weekly. [13][14]
In addition to the language changes, much more dramatic changes have been made to the Java class library over the years, which has grown from a few hundred classes in JDK 1.0 to over three thousand in J2SE 5.0. Entire new APIs, such as Swing and Java 2D, have been introduced, and many of the original JDK 1.0 classes and methods have been deprecated.
Usage
Desktop use
According to Sun, the Java Runtime Environment is found on over 700 million PCs.[15] Microsoft has not bundled a Java Runtime Environment (JRE) with its operating systems since Sun Microsystems sued Microsoft for adding Windows-specific classes to the bundled Java runtime environment, and for making the new classes available through Visual J++. A Java runtime environment is bundled with Apple's Mac OS X (although as of the Java for Mac OS X 10.6 Update 3 release, the Apple-supplied runtime is deprecated and may be removed from future OS releases[16]), and many Linux distributions include the partially compatible free software package GNU Classpath[17] and increasingly mostly compatible IcedTea.
Some Java applications are in fairly widespread desktop use, including the NetBeans and Eclipse integrated development environments, and file sharing clients such as LimeWire and Vuze. Java is also used in the MATLAB mathematics programming environment, both for rendering the user interface and as part of the core system. Java provides cross platform user interface for some high end collaborative applications like Lotus Notes.
Mobile devices
Java ME has become popular in mobile devices, where it competes with Symbian, BREW, and the .NET Compact Framework.
The diversity of mobile phone manufacturers has led to a need for new unified standards so programs can run on phones from different suppliers - MIDP. The first standard was MIDP 1, which assumed a small screen size, no access to audio, and a 32kB program limit. The more recent MIDP 2 allows access to audio, and up to 64kB for the program size. With handset designs improving more rapidly than the standards, some manufacturers relax some limitations in the standards, for example, maximum program size.
Google's Android Operating System uses the Java language, but not its class libraries, therefore the Android platform cannot be called Java. Android executes the code on the Dalvik VM instead of the Java VM.
Web server and enterprise use
The Java platform has become a mainstay of enterprise IT development since the introduction of the Enterprise Edition in 1998, in two different ways:
- Through the coupling of Java to the web server, the Java platform has become a leading platform for integrating the Web with enterprise backend systems. This has allowed companies to move part or all of their business to the Internet environment by way of highly interactive online environments (such as highly dynamic websites) that allow the customer direct access to the business processes (e.g. online banking websites, airline booking systems and so on). This trend has continued from its initial Web-based start:
- The Java platform has matured into an Enterprise Integration role in which legacy systems are unlocked to the outside world through bridges built on the Java platform. This trend has been supported for Java platform support for EAI standards like messaging and Web services and has fueled the inclusion of the Java platform as a development basis in such standards as SCA,XAM and others.
- Java has become the standard development platform for many companies' IT departments, which do most or all of their corporate development in Java. This type of development is usually related to company-specific tooling (e.g. a booking tool for an airline) and the choice for the Java platform is often driven by a desire to leverage the existing Java infrastructure to build highly intelligent and interconnected tools.
- The Java platform has become the main development platform for many software tools and platforms that are produced by third-party software groups (commercial, open source and hybrid) and are used as configurable (rather than programmable) tools by companies. Examples in this category include Web servers, application servers, databases, enterprise service buses, BPM tools and content management systems.
Enterprise use of Java has also long been the main driver of open source interest in the platform. This interest has inspired open source communities to produce everything from simple function libraries to program development frameworks (e.g. the Spring Framework, Wicket, Dojo, Hibernate) to open source implementations of standards and tools (e.g. Apache Tomcat, the Glassfish application server, the Mule and ServiceMix ESBs).
Mascot
When Sun announced that Java SE and Java ME would be released under a free software license (the GNU General Public License), they released the Duke graphics under the free BSD license at the same time.[19]
[edit]Licensing
The source code for Sun's implementations of Java (that is the de-facto reference implementation) has been available for some time, but until recently the license terms severely restricted what could be done with it without signing (and generally paying for) a contract with Sun. As such these terms did not satisfy the requirements of either the Open Source Initiative or the Free Software Foundation to be considered open source or free software, Sun Java was therefore a proprietary platform.[20]
While several third-party projects (e.g. GNU Classpath and Apache Harmony) created free software partial Java implementations, the sheer size of the Sun libraries combined with the use of clean room techniques meant that their implementations of the Java libraries (the compiler and vm are comparatively small and well defined) were incomplete and not fully compatible. These implementations also tended to be a long way behind Sun's in terms of optimization[citation needed].
Free software
Sun announced in JavaOne 2006 that Java would become free and open source software,[21] and on October 25, 2006, at the Oracle OpenWorld conference, Jonathan I. Schwartz said that the company was set to announce the release of the core Java Platform as free and open source software within 30 to 60 days.[22]
Sun released the Java HotSpot virtual machine and compiler as free software under the GNU General Public License on November 13, 2006, with a promise that the rest of the JDK (that includes the JRE) would be placed under the GPL by March 2007 ("except for a few components that Sun does not have the right to publish in source form under the GPL").[23] According to Richard Stallman, this would mean an end to the "Java trap".[24] Mark Shuttleworth called the initial press announcement, "A real milestone for the free software community".[25]
Sun released the source code of the Class library under GPL on May 8, 2007, except some limited parts that were licensed by Sun from 3rd parties who did not want their code to be released under a free software and open-source license.[26] Some of the encumbered parts turned out to be fairly key parts of the platform such as font rendering and 2D rasterisation, but these were released as open-source later by Sun (see OpenJDK Class library).
Sun's goal is to replace the parts that remain proprietary and closed-source with alternative implementations and make the class library completely free and open source. A third party project calledIcedTea has created a completely free and highly usable JDK by replacing encumbered code with either stubs or code from GNU Classpath. IcedTea is currently available on Fedora 7 and Ubuntu.
In June 2008, it was announced that IcedTea6 (as the packaged version of OpenJDK on Fedora 9) has passed the Technology Compatibility Kit tests and can claim to be a fully compatible Java 6 implementation.[27]
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