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Encyclopedia > Java virtual machine

A Java Virtual Machine (JVM) is a set of computer software programs and data structures which implements a specific virtual machine model. This model accepts a form of computer intermediate language, commonly referred to as Java bytecode, which conceptually represents the instruction set of a stack-oriented, capability architecture. This code is most often generated by Java language compilers, although the JVM can also be targeted by compilers of other languages. JVMs using the "Java" trademark may be developed by other companies as long as they adhere to the JVM standard published by Sun (and related contractual obligations). Image File history File links Java_Logo. ... In computer science, a virtual machine is software that creates a virtualized environment between the computer platform and its operating system, so that the end user can operate software on an abstract machine. ... In computer science, an intermediate language is the language of an abstract machine designed to aid in the analysis of computer programs. ... Java bytecode is the form of instructions that the Java virtual machine executes. ... A stack-oriented programming language is one that relies on a stack machine model for passing parameters. ... This article or section should be merged with Capability_based security and Capability. ... Java is a programming language originally developed by Sun Microsystems and released in 1995. ... A diagram of the operation of a typical multi-language, multi-target compiler. ...


The JVM is a crucial component of the Java Platform. Because JVMs are available for many hardware and software platforms, Java can be both middleware and a platform in its own right — hence the expression "write once, run anywhere." The use of the same bytecode for all platforms allows Java to be described as "compile once, run anywhere", as opposed to "write once, compile anywhere", which describes cross-platform compiled languages. The JVM also enables such unique features as Automated Exception Handling which provides 'root-cause' debugging information for every software error (exception) independent of the source code. The Java platform is the name for a computing environment, or platform, from Sun Microsystems which can run applications developed using the Java programming language and set of development tools. ... In computing, a platform describes some sort of framework, either in hardware or software, which allows software to run. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Write once, run anywhere (WORA), or sometimes also Write once, run everywhere (WORE), is a slogan created by Sun Microsystems to illustrate the cross-platform benefits of the Java language. ... A compiled language is a programming language whose implementations are typically compilers (translators which generate machine code from source code), and not interpreters (step-by-step executors of source code, where no translation takes place). ... Runtime engines such as those for the Java language or Microsoft . ...


Starting with J2SE 5.0, changes to the JVM specification have been developed under the Java Community Process as JSR 924[1]. As of 2006, changes to specification to support changes proposed to the class file format (JSR 202[2]) are being done as a maintenance release of JSR 924. The specification for the JVM is published in book form,[3] known as "blue book". The preface states: Java 2 Platform, Standard Edition or J2SE is a collection of java Application Programming Interfaces targeting Java platform applications running on a workstation. ... The Java Community Process or JCP, established in 1995, is a formalized process which allows interested parties to be involved in the definition of future versions and features of the Java platform. ... 2006 is a common year starting on Sunday of the Gregorian calendar. ... The Java source files (.java files) get compiled into . ...

We intend that this specification should sufficiently document the Java Virtual Machine to make possible compatible clean-room implementations. Sun provides tests which verify the proper operation of implementations of the Java Virtual Machine.

Sun's JVM is called HotSpot. Clean-room Java implementations include Kaffe and IBM J9 VM. Sun retains control over the Java trademark, which it uses to certify implementation suites as fully compatible with Sun's specification. HotSpot is the primary Java Virtual Machine for desktops and servers produced by Sun Microsystems. ... For the meaning of Cleanroom engineering in software development, see Cleanroom Software Engineering. ... Kaffe is a clean room design of a Java Virtual Machine. ... J9 is a Java toolchain from IBM that include a Java VM and compiler. ...

Contents

Execution environment

Programs intended to run on a JVM must be compiled into a standardized portable binary format, which typically comes in the form of .class files. A program may consist of many classes in different files. For easier distribution of large programs, multiple class files may be packaged together in a .jar file (short for Java archive). The Java source files (.java files) get compiled into . ... In computing, a JAR file (or Java ARchive) file used to distribute a set of Java classes. ...


The JVM runtime executes .class or .jar files, emulating the JVM instruction set by interpreting it, or using a just-in-time compiler (JIT) such as Sun's HotSpot. JIT compiling, not interpreting, is used in most JVMs today to achieve greater speed. In computer science, runtime or run time describes the operation of a computer program, the duration of its execution, from beginning to termination (compare compile time). ... This article is about emulation in computer science. ... It has been suggested that some sections of this article be split into a new article entitled instruction set architecture. ... An interpreter is a computer program that executes other programs. ... In computing, just-in-time compilation (JIT), also known as dynamic translation, is a technique for improving the performance of bytecode-compiled programming systems, by translating bytecode into native machine code at runtime. ... HotSpot is the primary Java Virtual Machine for desktops and servers produced by Sun Microsystems. ...


Like most virtual machines, the Java Virtual Machine has a stack-based architecture In computer science, a stack machine is a model of computation in which the computers memory takes the form of a stack. ...


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 functions 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 lead to data corruption or unpredictable behavior (accessing off the end of an array, using an uninitialized pointer, etc) 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" data and methods is rigidly controlled.

The first two of these checks take place primarily during the "verification" step which 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 function or method. Because of this, the fact that 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 need to be allocated to the target architecture's registers. In fact, code verification makes the JVM different from a classic stack architecture whose efficient emulation with a JIT compiler is more complicated and typically carried out by a slower interpreter. A branch (or jump on some computer architectures, such as the PDP-8 and Intel x86) is a point in a computer program where the flow of control is altered. ... In computer science, a subroutine (function, method, procedure, or subprogram) is a portion of code within a larger program, which performs a specific task and is relatively independent of the remaining code. ...


Code verification also ensures that arbitrary bit patterns cannot get used as an address. Memory protection is achieved without the need for a MMU. Thus, JVM is an efficient way of getting memory protection on simple architectures that lack a MMU. This is analogous to managed code in Microsoft's .NET CLR, and conceptually similar to capability architectures such as the Plessey 250, and IBM System/38. Memory protection is a system that prevents one process from corrupting the memory of another process running on the same computer at the same time. ... This 68451 MMU could be used with the Motorola 68010 MMU, short for memory management unit or sometimes called paged memory management unit as PMMU, is a class of computer hardware components responsible for handling memory accesses requested by the CPU. Among the functions of such devices are the translation... In Microsoft Windows terminology, managed code is computer instructions — that is, code — executed by a CLI-compliant virtual machine, such as Microsofts . ... The Common Language Runtime (CLR) is the virtual machine component of Microsofts . ... This article or section should be merged with Capability_based security and Capability. ... The Plessey 250 was a computer system manufactured by the Plessey company. ... The IBM System/38 was a computer. ...


Bytecode instructions

The JVM has instructions for the following groups of tasks: In computer science, an instruction typically refers to a single operation of a processor within a computer architecture. ...

The aim is binary compatibility. Each particular host operating system needs its own implementation of the JVM and runtime. These JVMs interpret the byte code semantically the same way, but the actual implementation may be different. More complicated than just the emulation of bytecode is compatible and efficient implementation of the Java core API which has to be mapped to each host operating system. Arithmetic tables for children, Lausanne, 1835 Arithmetic or arithmetics (from the Greek word αριθμός = number) is the oldest and most elementary branch of mathematics, used by almost everyone, for tasks ranging from simple daily counting to advanced science and business calculations. ... In computer science, type conversion or typecasting refers to changing an entity of one data type into another. ... In computer science, dynamic memory allocation is the allocation of memory storage for use in a computer program during the runtime of that program. ... Simple representation of a stack In computer science, a stack is a temporary abstract data type and data structure based on the principle of Last In First Out (LIFO). ... A branch (or jump on some computer architectures, such as the PDP-8 and Intel x86) is a point in a computer program where the flow of control is altered. ... In computer science, a subroutine (function, method, procedure, or subprogram) is a portion of code within a larger program, which performs a specific task and is relatively independent of the remaining code. ... Exception handling is a programming language construct or computer hardware mechanism designed to handle the occurrence of some condition that changes the normal flow of execution. ... A monitor is an approach to synchronizing two or more computer tasks that use a shared resource, usually a hardware device or a set of variables. ... It has been suggested that Maintenance OS be merged into this article or section. ... The Java Class Library is a set of dynamically loadable libraries that Java applications can call at runtime. ...


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. This is designed to allow safe execution of untrusted code from remote sources, a model used by Java applets. Applets run within a VM incorporated into a user's browser, executing code downloaded from a remote HTTP server. The remote code runs in a restricted "sandbox", which is designed to protect the user from misbehaving or malicious code. Publishers can purchase a certificate with which to digitally sign applets as "safe", giving them permission to break out of the sandbox and access the local file system and network. A Java applet is an applet delivered in the form of Java bytecode. ... HTTP (for HyperText Transfer Protocol) is the primary method used to convey information on the World Wide Web. ... For the Wikipedia sandbox for editing experiments see Wikipedia:Sandbox This article is about the computer security model. ... In cryptography, a digital signature or digital signature scheme is a type of asymmetric cryptography used to simulate the security properties of a signature in digital, rather than written, form. ...


References

  1. ^ JSR 924 – Specifies changes to the JVM specification starting with J2SE 5.0
  2. ^ JSR 202 – Specifies a number of changes to the class file format
  3. ^ The Java Virtual Machine Specification (the first and second editions are also available online)
  1. 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
  2. JSR 45 – Specifies changes to the class file format to support source-level debugging of languages such as JSP and SQLJ that are translated to Java

JavaServer Pages (JSP) is a Java technology that allows software developers to dynamically generate HTML, XML or other types of documents in response to a Web client request. ... SQLJ is an ISO standard (ISO/IEC 9075-10) for embedding SQL statements in Java programs. ...

See also

HotSpot is the primary Java Virtual Machine for desktops and servers produced by Sun Microsystems. ... This is a list of Java Virtual Machines. ... The Java Runtime Environment, or JRE, is a software bundle from Sun Microsystems that allows a computer system to run a Java application. ... Runtime engines such as those for the Java language or Microsoft . ... The Common Language Runtime (CLR) is the virtual machine component of Microsofts . ... Parrot is a register-based virtual machine being developed using the C programming language and intended to run dynamic languages efficiently. ... C to Java Virtual Machine compilers attempt to marry the highly popular C language with the platform independent Java Virtual Machine for Write Once Run Anywhere (WORA) using the C language. ... Java bytecode is the form of instructions that the Java virtual machine executes. ... The Java source files (.java files) get compiled into . ... Java is often perceived as significantly slower and more memory-consuming than natively compiled languages such as C or C++. However, Java programs execution speed have improved a lot, due to introduction of Just-In Time compilation[1] and mainly optimizations in the Java Virtual Machine itself introduced overtime. ...

External links


  Results from FactBites:
 
The lean, mean, virtual machine - Java World (568 words)
The JVM is "virtual" because it is generally implemented in software on top of a "real" hardware platform and operating system.
The JVM is central to Java's portability because compiled Java programs run on the JVM, independent of whatever may be underneath a particular JVM implementation.
The JVM is mean because it of its ambition.
virtual machine - a definition from Whatis.com (495 words)
Java was designed to allow application programs to be built that could be run on any platform without having to be rewritten or recompiled by the programmer for each separate platform.
The Java virtual machine specification defines an abstract rather than a real "machine" (or processor) and specifies an instruction set, a set of registers, a stack, a "garbage heap," and a method area.
A Java virtual machine can either interpret the bytecode one instruction at a time (mapping it to a real microprocessor instruction) or the bytecode can be compiled further for the real microprocessor using what is called a just-in-time compiler.
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