Java virtual machine
A Java virtual machine (JVM) is a virtual machine that can execute Java bytecode. It is the code execution component of the Java platform. Sun Microsystems has stated that there are over 5.5 billion JVM-enabled devices.
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Oracle's Java execution environment is termed the Java Runtime Environment, or JRE.
Programs intended to run on a JVM must be compiled into Java bytecode, a standardized portable binary format which typically comes in the form of .class files (Java 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 application launchers,
javaw offer a standard way of executing Java code, with or without a console window respectively.
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 Oracle's HotSpot. JIT compiling, not interpreting, is used in most JVMs today to achieve greater speed. There are also ahead-of-time compilers that enable developers to precompile class files into native code for particular platforms.
Like most virtual machines, the Java virtual machine has a stack-based architecture akin to a microcontroller/microprocessor. However, the JVM also has low-level support for Java-like classes and methods, which amounts to a highly idiosyncratic[clarification needed] memory model and capability-based architecture.
Although the JVM was primarily aimed at running compiled Java programs, many other languages can now run on top of it. The JVM has currently no built-in support for dynamically typed languages: the existing JVM instruction set is statically typed, although the JVM can be used to implement interpreters for dynamic languages. Java 7 supports dynamically typed languages  which started as Sun's "Da Vinci Machine" project 
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 lead 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.
Code verification also ensures that arbitrary bit patterns cannot get used as an address. Memory protection is achieved without the need for a memory management unit (MMU). Thus, JVM is an efficient way to get memory protection on simple architectures that lack an MMU. This is analogous to managed code in Microsoft's .NET Common Language Runtime, and conceptually similar to capability architectures such as the Plessey 250, and IBM System/38.
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 proved to be safe.
The JVM has instructions for the following groups of tasks:
- Load and store
- 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 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.
- The young generation stores short-lived objects that are created and immediately garbage collected.
- Objects that persist longer are moved to the old generation (also called the tenured generation).
Originally there was no permanent generation, and objects and classes were stored together in the same area. But as class unloading occurs much more rarely than objects are collected, moving class structures to a specific area allows significant performance improvements.
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 ask the user to break out of the sandbox and access the local file system, clipboard, execute external pieces of software, or network.
C to bytecode compilers
|This section relies on references to primary sources. (November 2011)|
From the viewpoint of a compiler, the Java virtual machine is just another processor with an instruction set, Java bytecode, for which code can be generated. The JVM was originally designed to execute programs written in the Java language. However, the JVM provides an execution environment in the form of a bytecode instruction set and a runtime system that is general enough that it can be used as the target for compilers of other languages.
Because of its close association with the Java language, the JVM performs the strict runtime checks mandated by the Java specification. That requires C to bytecode compilers to provide their own lax machine abstraction, for instance producing compiled code that uses a Java array to represent main memory (so pointers can be compiled to integers), and linking the C library to a centralized Java class that emulates system calls. Most or all of the compilers listed below use a similar approach.
Several C to bytecode compilers exist:
- NestedVM translates C to MIPS machine language first before converting to Java bytecode.
- Cibyl works similarly to NestedVM but targets J2ME devices.
- LLJVM compiles C to LLVM IR, which is then translated to JVM bytecode.
- C2J is also GCC-based, but it produces intermediary Java source code before generating bytecode. Supports the full ANSI C runtime. Available as a Win32 binary or as a Java executable.
- Java Backend for GCC, possibly the oldest project of its kind, was developed at The University of Queensland in 1999.
- Javum is an attempt to port the full GNU environment to the JVM, and includes one of the above compilers packaged with additional utilities.
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. As of 2006[update], 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. Oracle provides tests that verify the proper operation of implementations of the Java Virtual Machine.
One of Oracle's JVMs is named HotSpot. Clean-room Java implementations include Kaffe and IBM J9. Oracle retains control over the Java trademark, which it uses to certify implementation suites as fully compatible with Oracle's specification.
- List of Java virtual machines
- Comparison of Java virtual machines
- Comparison of application virtual machines
- Automated exception handling
- Java performance
- List of JVM languages
- Java processor
- Common Language Runtime
- Learn about Java Technology
- "The java and javaw commands". IBM. 2010. Retrieved 2011-01-16. "The javaw command is identical to java, except that javaw has no associated console window."
- 1996, possibly the first new language specifically designed to run on the JVM)
- Tolksdorf, Robert (2005). "Languages for the Java VM". Retrieved 2008-06-08.
- Nutter, Charles (2007-01-03). "InvokeDynamic: Actually Useful?". Retrieved 2008-01-25.
- "Oracle Technology Network for Java Developers". Java.sun.com. Retrieved 2013-01-30.
- Krill, Paul (2008-01-31). "Sun's Da Vinci Machine broadens JVM coverage". Retrieved 2008-02-06.
- "The Verification process". The Java Virtual Machine Specification. Sun Microsystems. 1999. Retrieved 2009-05-31.
- Stephen N. Freund and John C. Mitchell. 1999. A formal framework for the Java bytecode language and verifier. In Proceedings of the 14th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications (OOPSLA '99), A. Michael Berman (Ed.). ACM, New York, NY, USA, 147-166. DOI=10.1145/320384.320397 http://doi.acm.org/10.1145/320384.320397
- "Frequently Asked Questions about Garbage Collection in the Hotspot Java Virtual Machine". Sun Microsystems. 6 February 2003. Retrieved 7 February 2009.
- Masamitsu, Jon (28 November 2006). "Presenting the Permanent Generation". Retrieved 7 February 2009.
- Nutter, Charles (11 September 2008). "A First Taste of InvokeDynamic". Retrieved 7 February 2009.
- "C2J Converter FAQ". Tech.novosoft-us.com. Retrieved 2013-01-30.
- JSR 924, specifies changes to the JVM specification starting with J2SE 5.0
- JSR 202, specifies a number of changes to the class file format
- The Java Virtual Machine Specification (the first and second editions are also available online)
- 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
- The Java Virtual Machine Specification
- Java implementations at the Open Directory Project
- Java Virtual Machine Download Link
- JVM implementation in pure Java