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. JVM's 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. The availability of JVMs on many types of hardware and  software platforms enables Java to function both as middleware and  a platform in its own right. Hence the expression "Write once, run anywhere."

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.

Kaffe is  an example of a clean-room Java implementation. 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 which case every class will be in a different file. For easier distribution of large programs, multiple class files may be packaged together in a .jar file.

Execution of .class or  .jar files is  then handled by the JVM runtime which carries out emulation of the JVM instruction set by interpreting it  or by applying a just-in-time compiler (JIT) such as Sun's HotSpot. JIT compilation is  used (rather than interpreters) in most JVM's 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. This means that only a limited amount of bytecode sequences form 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 the following 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

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.
• 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

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