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Virtual machine that runs Java programs

A Java virtual machine (JVM) is a virtual machine that enables a computer to run Java programs as well as programs written in other languages that are also compiled to Java bytecode. The JVM is detailed by a specification that formally describes what is required in a JVM implementation. Having a specification ensures interoperability of Java programs across different implementations so that program authors using the Java Development Kit (JDK) need not worry about idiosyncrasies of the underlying hardware platform. The JVM reference implementation is developed by the OpenJDK project as open source code and includes a JIT compiler called HotSpot. The commercially supported Java releases available from Oracle are based on the OpenJDK runtime. Eclipse OpenJ9 is another open source JVM for OpenJDK.

The Java virtual machine is an abstract (virtual) computer defined by a specification. It is a part of the Java runtime environment. The garbage collection algorithm used and any internal optimization of the Java virtual machine instructions (their translation into machine code) are not specified. The main reason for this omission is to not unnecessarily constrain implementers. Any Java application can be run only inside some concrete implementation of the abstract specification of the Java virtual machine. Starting with Java Platform, Standard Edition (J2SE) 5.0, changes to the JVM specification have been developed under the Java Community Process as JSR 924. as of 2006, changes to the specification to support changes proposed to the class file format (JSR 202) are being done as a maintenance release of JSR 924. The specification for the JVM was published as the blue book, whose preface states:
We intend that this specification should sufficiently document the Java Virtual Machine to make possible compatible clean room implementations. Oracle provides tests that verify the proper operation of implementations of the Java Virtual Machine. One of Oracle's JVMs is named HotSpot; the other, inherited from BEA Systems, is JRockit. Oracle owns the Java trademark and may allow its use to certify implementation suites as fully compatible with Oracle's specification.

The JVM operates on specific types of data as specified in Java Virtual Machine specifications. The data types can be divided into primitive types (integers, Floating point, long etc.) and Reference types. The earlier JVM were only 32 bit machines. long and double types, which are 64 bits, are supported natively, but consume two units of storage in a frame's local variables or operand stack, since each unit is 32 bits. boolean, byte, short, and char types are all sign extended (except char which is zero extended) and operated on as 32 bit integers, the same as int types. The smaller types only have a few type specific instructions for loading, storing, and type conversion. boolean is operated on as 8 bit byte values, with 0 representing false and 1 representing true. (Although boolean has been treated as a type since The Java Virtual Machine Specification, Second Edition clarified this issue, in compiled and executed code there is little difference between a boolean and a byte except for name mangling in method signatures and the type of boolean arrays. booleans in method signatures are mangled as Z while bytes are mangled as B. Boolean arrays carry the type boolean[] but use 8 bits per element, and the JVM has no built in capability to pack booleans into a bit array, so except for the type they perform and behave the same as byte arrays. In all other uses, the boolean type is effectively unknown to the JVM as all instructions to operate on booleans are also used to operate on bytes.) However the newer JVM releases (OpenJDK HotSpot JVM) support 64 bit, so you can either have 32 bit/64 bit JVM on a 64 bit OS. The primary advantage of running Java in a 64 bit environment is the larger address space. This allows for a much larger Java heap size and an increased maximum number of Java Threads, which is needed for certain kinds of large applications; however there is a performance hit in using 64 bit JVM compared to 32 bit JVM. The JVM has a garbage collected heap for storing objects and arrays. Code, constants, and other class data are stored in the "method area". The method area is logically part of the heap, but implementations may treat the method area separately from the heap, and for example might not garbage collect it. Each JVM thread also has its own call stack (called a "Java Virtual Machine stack" for clarity), which stores frames. A new frame is created each time a method is called, and the frame is destroyed when that method exits. Each frame provides an "operand stack" and an array of "local variables". The operand stack is used for operands to run computations and for receiving the return value of a called method, while local variables serve the same purpose as registers and are also used to pass method arguments. Thus, the JVM is both a stack machine and a register machine. In practice, HotSpot eliminates every stack besides the native thread/call stack even when running in Interpreted mode, as its Templating Interpreter technically functions as a compiler.

A JVM language is any language with functionality that can be expressed in terms of a valid class file which can be hosted by the Java Virtual Machine. A class file contains Java Virtual Machine instructions (Java byte code) and a symbol table, as well as other ancillary information. The class file format is the hardware and operating system independent binary format used to represent compiled classes and interfaces. There are several JVM languages, both old languages ported to JVM and completely new languages. JRuby and Jython are perhaps the most well known ports of existing languages, i. e. Ruby and Python respectively. Of the new languages that have been created from scratch to compile to Java bytecode, Clojure, Groovy, Scala and Kotlin may be the most popular ones. A notable feature with the JVM languages is that they are compatible with each other, so that, for example, Scala libraries can be used with Java programs and vice versa. Java 7 JVM implements JSR 292: Supporting Dynamically Typed Languages on the Java Platform, a new feature which supports dynamically typed languages in the JVM. This feature is developed within the Da Vinci Machine project whose mission is to extend the JVM so that it supports languages other than Java.

For each hardware architecture a different Java bytecode interpreter is needed. When a computer has a Java bytecode interpreter, it can run any Java bytecode program, and the same program can be run on any computer that has such an interpreter. When Java bytecode is executed by an interpreter, the execution will always be slower than the execution of the same program compiled into native machine language. This problem is mitigated by just in time (JIT) compilers for executing Java bytecode. A JIT compiler may translate Java bytecode into native machine language while executing the program. The translated parts of the program can then be executed much more quickly than they could be interpreted. This technique gets applied to those parts of a program frequently executed. This way a JIT compiler can significantly speed up the overall execution time. There is no necessary connection between the Java programming language and Java bytecode. A program written in Java can be compiled directly into the machine language of a real computer and programs written in other languages than Java can be compiled into Java bytecode. Java bytecode is intended to be platform independent and secure. Some JVM implementations do not include an interpreter, but consist only of a just in time compiler.

At the start of the Java platform's lifetime, the JVM was marketed as a web technology for creating Rich Web Applications. as of 2018, most web browsers and operating systems bundling web browsers do not ship with a Java plug in, nor do they permit side loading any non Flash plug in. The Java browser plugin was deprecated in JDK 9. The NPAPI Java browser plug in was designed to allow the JVM to execute so called Java applets embedded into HTML pages. For browsers with the plug in installed, the applet is allowed to draw into a rectangular region on the page assigned to it. Because the plug in includes a JVM, Java applets are not restricted to the Java programming language; any language targeting the JVM may run in the plug in. A restricted set of APIs allow applets access to the user's microphone or 3D acceleration, although applets are not able to modify the page outside its rectangular region. Adobe Flash Player, the main competing technology, works in the same way in this respect. as of June 2015 according to W3Techs, Java applet and Silverlight use had fallen to 0.1% each for all web sites, while Flash had fallen to 10.8%.

Since May 2016, JavaPoly allows users to import unmodified Java libraries, and invoke them directly from JavaScript. JavaPoly allows websites to use unmodified Java libraries, even if the user does not have Java installed on their computer.

With the continuing improvements in JavaScript execution speed, combined with the increased use of mobile devices whose web browsers do not implement support for plugins, there are efforts to target those users through transpilation to JavaScript. It is possible to either transpile the source code or JVM bytecode to JavaScript. Compiling the JVM bytecode, which is universal across JVM languages, allows building upon the language's existing compiler to bytecode. The main JVM bytecode to JavaScript transpilers are TeaVM, the compiler contained in Dragome Web SDK, Bck2Brwsr, and j2js compiler. Leading transpilers from JVM languages to JavaScript include the Java to JavaScript transpiler contained in Google Web Toolkit, Clojurescript (Clojure), GrooScript (Apache Groovy), Scala. js (Scala) and others.