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A Java virtual machine (JVM) is a
virtual machine In computing, a virtual machine (VM) is the virtualization/ emulation of a computer system. Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve specialized h ...
that enables a computer to run
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
programs as well as programs written in other languages that are also compiled to
Java bytecode In computing, Java bytecode is the bytecode-structured instruction set of the Java virtual machine (JVM), a virtual machine that enables a computer to run programs written in the Java programming language and several other programming langua ...
. 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 Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source model is a decentralized so ...
code and includes a JIT compiler called HotSpot. The commercially supported Java releases available from
Oracle An oracle is a person or agency considered to provide wise and insightful counsel or prophetic predictions, most notably including precognition of the future, inspired by deities. As such, it is a form of divination. Description The word ...
are based on the OpenJDK runtime. Eclipse
OpenJ9 Eclipse OpenJ9 (previously known as IBM J9) is a high performance, scalable, Java virtual machine (JVM) implementation that is fully compliant with the Java Virtual Machine Specification. OpenJ9 can be built from source, or can be used with pre ...
is another open source JVM for OpenJDK.


JVM specification

The Java virtual machine is an abstract (virtual) computer defined by a specification. It is a part of 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. , changes to 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'', The preface states: 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.


Class loader

One of the organizational units of JVM byte code is a
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently ...
. A class loader implementation must be able to recognize and load anything that conforms to the Java class
file format A file format is a standard way that information is encoded for storage in a computer file. It specifies how bits are used to encode information in a digital storage medium. File formats may be either proprietary or free. Some file format ...
. Any implementation is free to recognize other binary forms besides ''class'' files, but it must recognize ''class'' files. The class loader performs three basic activities in this strict order: #Loading: finds and imports the binary data for a type #Linking: performs verification, preparation, and (optionally) resolution #*Verification: ensures the correctness of the imported type #*Preparation: allocates memory for class variables and initializing the memory to default values #*Resolution: transforms symbolic references from the type into direct references. #Initialization: invokes Java code that initializes class variables to their proper starting values. In general, there are three types of class loader: bootstrap class loader, extension class loader and System / Application class loader. Every Java virtual machine implementation must have a bootstrap class loader that is capable of loading trusted classes, as well as an extension class loader or application class loader. The Java virtual machine specification doesn't specify how a class loader should locate classes.


Virtual machine architecture

The JVM operates on specific types of data as specified in Java Virtual Machine specifications. The data types can be divided into primitive types (
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the languag ...
s, Floating-point, long etc.) and Reference types. The earlier JVM were only
32-bit In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in 32- bit units. Compared to smaller bit widths, 32-bit computers can perform large calculati ...
machine. 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, 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 In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, program stack, control stack, run-time stack, or mac ...
(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 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 In computer science, computer engineering and programming language implementations, a stack machine is a computer processor or a virtual machine in which the primary interaction is moving short-lived temporary values to and from a push down ...
and a register machine. (In practice, HotSpot entirely eliminates every stack besides the native thread/call stack even when running in Interpreted mode, as its Templating Interpreter is technically a Compiler in disguise)


Bytecode instructions

The JVM has instructions for the following groups of tasks: The aim is binary compatibility. Each particular host
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
needs its own implementation of the JVM and runtime. These JVMs interpret the bytecode semantically the same way, but the actual implementation may be different. More complex than just emulating bytecode is compatibly and efficiently implementing the Java core API that must be mapped to each host operating system. These instructions operate on a set of common rather the
native data type In computer science and computer programming, a data type (or simply type) is a set of possible values and a set of allowed operations on it. A data type tells the compiler or interpreter how the programmer intends to use the data. Most prog ...
s of any specific
instruction set architecture In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ...
.


JVM languages

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 In computing, Java bytecode is the bytecode-structured instruction set of the Java virtual machine (JVM), a virtual machine that enables a computer to run programs written in the Java programming language and several other programming languag ...
) 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 Jython is an implementation of the Python programming language designed to run on the Java platform. The implementation was formerly known as JPython until 1999. Overview Jython programs can import and use any Java class. Except for some stand ...
are perhaps the most well-known ports of existing languages, i.e.
Ruby A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called ...
and Python respectively. Of the new languages that have been created from scratch to compile to Java bytecode, Clojure, Apache 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.


Bytecode verifier

A basic philosophy of Java is that it is inherently safe from the standpoint that no user program can crash the host machine or otherwise interfere inappropriately with other operations on the host machine, and that it is possible to protect certain methods and data structures belonging to trusted code from access or corruption by untrusted code executing within the same JVM. Furthermore, common programmer errors that often led to data corruption or unpredictable behavior such as accessing off the end of an array or using an uninitialized pointer are not allowed to occur. Several features of Java combine to provide this safety, including the class model, the garbage-collected heap, and the verifier. The JVM verifies all bytecode before it is executed. This verification consists primarily of three types of checks: * Branches are always to valid locations * Data is always initialized and references are always type-safe * Access to private or package private data and methods is rigidly controlled The first two of these checks take place primarily during the verification step that occurs when a class is loaded and made eligible for use. The third is primarily performed dynamically, when data items or methods of a class are first accessed by another class. The verifier permits only some bytecode sequences in valid programs, e.g. a jump (branch) instruction can only target an instruction within the same method. Furthermore, the verifier ensures that any given instruction operates on a fixed stack location, allowing the JIT compiler to transform stack accesses into fixed register accesses. Because of this, that the JVM is a stack architecture does not imply a speed penalty for emulation on register-based architectures when using a JIT compiler. In the face of the code-verified JVM architecture, it makes no difference to a JIT compiler whether it gets named imaginary registers or imaginary stack positions that must be allocated to the target architecture's registers. In fact, code verification makes the JVM different from a classic stack architecture, of which efficient emulation with a JIT compiler is more complicated and typically carried out by a slower interpreter. Additionally, the Interpreter used by the default JVM is a special type known as a Template Interpreter, which translates bytecode directly to native, register based machine language rather than emulate a stack like a typical interpreter (In many aspects the HotSpot Interpreter can be considered a JIT Compiler rather than a true Interpreter), meaning that the stack architecture that the bytecode targets is not actually used in the implementation, but merely a specification for the intermediate representation that can well be implemented in a register based architecture (Another instance of a stack architecture being merely a specification and implemented in a register based virtual machine is the Common Language Runtime). The original specification for the bytecode verifier used natural language that was incomplete or incorrect in some respects. A number of attempts have been made to specify the JVM as a formal system. By doing this, the security of current JVM implementations can more thoroughly be analyzed, and potential security exploits prevented. It will also be possible to optimize the JVM by skipping unnecessary safety checks, if the application being run is proven to be safe.


Secure execution of remote code

A virtual machine architecture allows very fine-grained control over the actions that code within the machine is permitted to take. It assumes the code is "semantically" correct, that is, it successfully passed the (formal) bytecode verifier process, materialized by a tool, possibly off-board the virtual machine. This is designed to allow safe execution of untrusted code from remote sources, a model used by Java applets, and other secure code downloads. Once bytecode-verified, the downloaded code runs in a restricted " sandbox", which is designed to protect the user from misbehaving or malicious code. As an addition to the bytecode verification process, publishers can purchase a certificate with which to digitally sign applets as safe, giving them permission to ask the user to break out of the sandbox and access the local file system,
clipboard A clipboard is a thin, rigid board with a clip at the top for holding paper in place. A clipboard is typically used to support paper with one hand while writing on it with the other, especially when other writing surfaces are not available. The ...
, execute external pieces of software, or network. Formal proof of bytecode verifiers have been done by the Javacard industry (Formal Development of an Embedded Verifier for Java Card Byte Code)


Bytecode interpreter and just-in-time compiler

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.


JVM in the web browser

At the start of the Java platform's lifetime, the JVM was marketed as a web technology for creating Rich Web Applications. , 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 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 ...
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 Adobe Flash Player (known in Internet Explorer, Firefox, and Google Chrome as Shockwave Flash) is computer software for viewing multimedia contents, executing rich Internet applications, and streaming audio and video content created on ...
, the main competing technology, works in the same way in this respect. according to W3Techs, Java applet and
Silverlight Microsoft Silverlight is a discontinued application framework designed for writing and running rich web applications, similar to Adobe's runtime, Adobe Flash. A plugin for Silverlight is still available for a very small number of browsers. Whi ...
use had fallen to 0.1% each for all web sites, while Flash had fallen to 10.8%.


JavaScript JVMs and interpreters

As of 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.


Compilation to JavaScript

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 compilation to JavaScript. It is possible to either compile 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 compilers are TeaVM, the compiler contained in Dragome Web SDK, Bck2Brwsr, and j2js-compiler. Leading compilers from JVM languages to JavaScript include the Java-to-JavaScript compiler contained in
Google Web Toolkit Google Web Toolkit (GWT ), or GWT Web Toolkit, is an open-source set of tools that allows web developers to create and maintain JavaScript front-end applications in Java. It is licensed under the Apache License 2.0. GWT emphasizes reusab ...
, Clojurescript (Clojure), GrooScript (Apache Groovy), Scala.js (Scala) and others.


See also

*
Common Language Runtime The Common Language Runtime (CLR), the virtual machine component of Microsoft .NET Framework, manages the execution of .NET programs. Just-in-time compilation converts the managed code (compiled intermediate language code) into machine instru ...
* List of Java virtual machines *
List of JVM languages This list of JVM Languages comprises notable computer programming languages that are used to produce computer software that runs on the Java virtual machine (JVM). Some of these languages are interpreted by a Java program, and some are compi ...
*
Comparison of Java virtual machines : Version information Technical information Supported CPU architectures Supported operating systems References {{Java Virtual Machine Java platform software Java virtual machine Software comparisons ...
* Comparison of application virtual machines * Automated exception handling * Java performance * Java processor


References

*
Clarifications and Amendments to the Java Virtual Machine Specification, Second Edition
' includes list of changes to be made to support J2SE 5.0 and JSR 45
JSR 45
specifies changes to the class file format to support source-level debugging of languages such as JavaServer Pages (JSP) and SQLJ that are translated to Java {{Sun Microsystems
Virtual machine In computing, a virtual machine (VM) is the virtualization/ emulation of a computer system. Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve specialized h ...
Java platform software
Virtual machine In computing, a virtual machine (VM) is the virtualization/ emulation of a computer system. Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve specialized h ...
Stack-based virtual machines