Java is a high-level, class-based, object-oriented programming language that is designed to have as few implementation dependencies as possible. It is a general-purpose programming language intended to let programmers write once, run anywhere (WORA), meaning that compiled Java code can run on all platforms that support Java without the need to recompile. Java applications are typically compiled to bytecode that can run on any Java virtual machine (JVM) regardless of the underlying computer architecture. The syntax of Java is similar to C and C++, but has fewer low-level facilities than either of them. The Java runtime provides dynamic capabilities that are typically not available in traditional compiled languages.
A computing platform, digital platform, or software platform is an environment in which software is executed. It may be the hardware or the operating system (OS), a web browser and associated application programming interfaces, or other underlying software, as long as the program code is executed using the services provided by the platform. Computing platforms have different abstraction levels, including a computer architecture, an OS, or runtime libraries. A computing platform is the stage on which computer programs can run.
The GNU Compiler for Java (GCJ) is a discontinued free compiler for the Java programming language. It was part of the GNU Compiler Collection.
GNU Classpath is a free software implementation of the standard class library for the Java programming language. Most classes from J2SE 1.4 and 5.0 are implemented. Classpath can thus be used to run Java-based applications. GNU Classpath is a part of the GNU Project. It was originally developed in parallel with libgcj due to license incompatibilities, but later the two projects merged.
Kaffe is a discontinued "clean room design" version of a Java Virtual Machine. It comes with a subset of the Java Platform, Standard Edition, Java API, and tools needed to provide a Java runtime environment. Like most other Free Java virtual machines, Kaffe uses GNU Classpath as its class library.
Free Java implementations are software projects that implement Oracle's Java technologies and are distributed under free software licences, making them free software. Sun released most of its Java source code as free software in May 2007, so it can now almost be considered a free Java implementation. Java implementations include compilers, runtimes, class libraries, etc. Advocates of free and open source software refer to free or open source Java virtual machine software as free runtimes or free Java runtimes.
javac is the primary Java compiler included in the Java Development Kit (JDK) from Oracle Corporation. Martin Odersky implemented the GJ compiler, and his implementation became the basis for javac.
HotSpot, released as Java HotSpot Performance Engine, is a Java virtual machine for desktop and server computers, developed by Sun Microsystems and now maintained and distributed by Oracle Corporation. It features improved performance via methods such as just-in-time compilation and adaptive optimization. It is the de facto Java Virtual Machine, serving as the reference implementation of the Java programming language.
Apache Harmony is a retired open source, free Java implementation, developed by the Apache Software Foundation. It was announced in early May 2005 and on October 25, 2006, the board of directors voted to make Apache Harmony a top-level project. The Harmony project achieved 99% completeness for J2SE 5.0, and 97% for Java SE 6. The Android operating system has historically been a major user of Harmony, although since Android Nougat it increasingly relies on OpenJDK libraries.
Jikes Research Virtual Machine is a mature virtual machine that runs programs written for the Java platform. Unlike most other Java virtual machines (JVMs), it is written in the programming language Java, in a style of implementation termed meta-circular. It is free and open source software released under an Eclipse Public License.
Java is a set of computer software and specifications that provides a software platform for developing application software and deploying it in a cross-platform computing environment. Java is used in a wide variety of computing platforms from embedded devices and mobile phones to enterprise servers and supercomputers. Java applets, which are less common than standalone Java applications, were commonly run in secure, sandboxed environments to provide many features of native applications through being embedded in HTML pages.
OpenJDK is a free and open-source implementation of the Java Platform, Standard Edition. It is the result of an effort Sun Microsystems began in 2006. The implementation is licensed under the GPL-2.0-only with a linking exception. Were it not for the GPL linking exception, components that linked to the Java Class Library would be subject to the terms of the GPL license. OpenJDK is the official reference implementation of Java SE since version 7.
Comparison of the Java and .NET platforms.
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.
Azul Systems, Inc. develops runtimes for executing Java-based applications. Founded in March 2002, Azul Systems has headquarters in Sunnyvale, California.
Mika VM is an open-source implementation of the Java virtual machine specification, together with class libraries which implement the Connected Device Configuration of Java ME. Mika VM is based on Wonka VM, which was developed independently of any other implementation, including Sun Microsystem's RI. The same is true of most of the class libraries, but in this case some code is drawn from the GNU Classpath and Apache Harmony projects. MikaVM support MIDP 1.0 and 2.0, CDC Personal Profile.
The Java Development Kit (JDK) is a distribution of Java Technology by Oracle Corporation. It implements the Java Language Specification (JLS) and the Java Virtual Machine Specification (JVMS) and provides the Standard Edition (SE) of the Java Application Programming Interface (API). It is derivative of the community driven OpenJDK which Oracle stewards. It provides software for working with Java applications. Examples of included software are the virtual machine, a compiler, performance monitoring tools, a debugger, and other utilities that Oracle considers useful for a Java programmer.
Excelsior JET is a now-defunct proprietary Java SE technology implementation built around an ahead-of-time (AOT) Java to native code compiler. The compiler transforms the portable Java bytecode into optimized executables for the desired hardware and operating system (OS). Also included are a Java runtime featuring a just-in-time (JIT) compiler for handling classes that were not precompiled for whatever reason, the complete Java SE API implementation licensed from Oracle, and a toolkit to aid deployment of the optimized applications. Excelsior JET was developed by Excelsior LLC, headquartered in Novosibirsk, Russia.
GraalVM is a Java Development Kit (JDK), written in Java. The open-source distribution of GraalVM is based on OpenJDK, and the enterprise distribution is based on Oracle JDK. As well as just-in-time (JIT) compilation, GraalVM can compile a Java application ahead-of-time so that it starts instantly, provides peak performance with no warmup, and uses fewer resources. It provides additional programming languages and execution modes. The first production-ready release, GraalVM 19.0, was distributed in May 2019. The most recent release is GraalVM for JDK 21, made available in September 2023.
