Java Virtual Machine

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A Java Virtual Machine (JVM) is a virtual machine that interprets and executes Java bytecode. This code is most often generated by Java language compilers, although the JVM can also be targeted by compilers of other languages. JVMs may be developed by other companies as long as they adhere to the JVM standard published by Sun.

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.

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:

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's. Sun retains control over the Java trademark, which it uses to certify implementation suites as fully compatible with Sun's specification.

Contents 1 Execution environment 1.1 Bytecode verifier 1.2 Bytecodes 1.3 Secure execution of remote code 2 References 3 See also 4 External links

[edit] 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.

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.

The Java Virtual Machine has a stack based architecture. In contrast, JIT compilers usually compile the byte code into register based machine code. Each thread has its own stack and program counter.

[edit] Bytecode verifier

The JVM verifies all bytecode before it is executed. 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. 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.

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.

[edit] Bytecodes

The JVM has instructions for these groups of tasks

• Load and store
• Arithmetic
• Type conversion
• Object creation and manipulation
• Operand stack management (push / pop)
• Control transfer (branching)
• Method invocation and return
• Throwing exceptions
• Monitor-based concurrency

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.

[edit] 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.

[edit] 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

[edit] See also

• List of Java virtual machines
• Parrot virtual machine
• Common Language Runtime
• C to Java Virtual Machine compilers
• Java Runtime Environment

[edit] External links

• List of languages which compile to the Java virtual machine.
• A decade after Java arrived, there have been improvements in the runtime performance of platform-independent virtual-machine based software.
• Kaffe.org - the Kaffe project
• JamVM - The Jam Virtual Machine
• The lean, mean, virtual machine - An introduction to the basic structure and functionality of the Java Virtual Machine
• Java Glossary - installing Java useful tips for installing Java for users and developers
• Test your Java Virtual Machine
• A list of Java VM-s used in mobile devices

Java (Sun)
Major Technologies:	Java (programming language) | Java Platform | Java Development Kit | Java Virtual Machine | Java Runtime Environment
History:	Java version history | Criticism of Java | Java Community Process | Sun Microsystems
Language Features:	Bytecode | Syntax | Applets | Servlets | JavaServer Pages