Java virtual machine

Java Sun Logo

Java Sun Logo

A Java Virtual Machine (JVM) is a set of computer software programs and data structures which implement 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).

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.

Starting with J2SE 5.0, changes to the JVM specification have been developed under the Java Community Process as JSR 924. As of 2006, 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 is published in book form, 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 J9 VM. Sun retains control over the Java trademark, which it uses to certify implementation suites as fully compatible with Sun's specification.

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 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.

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 ot "trusted" code to from access or corruption by "untrusted" code executing within the same JVM. Further, 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.

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.

Bytecode instructions

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
• 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.

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.

References

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

See also

• HotSpot, the primary Java Virtual Machine produced by Sun
• List of Java virtual machines
• Java Runtime Environment
• Automated Exception Handling
• Common Language Runtime, the equivalent C# Virtual Machine
• Parrot virtual machine, the upcoming Perl Virtual Machine
• C to Java Virtual Machine compilers
• Java bytecode
• the Java .class file format

External links

• The Java Virtual Machine Specification
• 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