Java Virtual Machine
Encyclopedia
A Java virtual machine is a virtual machine
capable of executing Java bytecode
. It is the code execution component of the Java software platform. Sun Microsystems
stated that there are over 4.5 billion JVM-enabled devices.
s. A JVM provides an environment in which Java bytecode can be executed, enabling such features as automated exception handling
, which provides "root-cause" debugging information for every software error (exception
), independent of the source code. A JVM is distributed along with a set of standard class libraries
that implement the Java application programming interface
(API). Appropriate APIs bundled together with JVM form the Java Runtime Environment (JRE).
JVMs are available for many hardware and software platforms
. The use of the same bytecode for all JVMs on all platforms allows Java to be described as a "write once, run anywhere
" programming language, as opposed to "write once, compile anywhere
", which describes cross-platform compiled language
s. Thus, the JVM is a crucial component of the Java platform.
Java bytecode is an intermediate language
which is typically compiled from Java, but it can also be compiled from other programming languages. For example, Ada
source code can be compiled to Java bytecode and executed on a JVM.
Oracle
, the owner of Java, produces a JVM, but JVMs using the "Java" trademark may be developed by other companies as long as they adhere to the JVM specification published by Oracle and to related contractual obligations.
The Oracle JVM (HotSpot) is written in the C++ programming language.
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 application launcher,
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 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. Prior to Java 7 the JVM had a limited support for dynamically modifying existing classes and methods; this only worked in a debugging environment
, where new classes and methods can be added dynamically.
Better support for dynamic languages is included in JVM since Java 7.
The JVM verifies all bytecode before it is executed. This verification consists primarily of three types of checks:
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 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 memory management unit
(MMU). Thus, JVM is an efficient way of getting 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 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.
) for dynamic memory allocation. The heap
is split up into "generations":
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.
s. 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
or network.
, 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 compiler
s 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:
Compilers targeting Java bytecode have been written for other programming languages, including Ada
and COBOL
.
5.0, changes to the JVM specification have been developed under the Java Community Process
as JSR 924. , 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:
Oracle's JVM is called 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.
Virtual machine
A virtual machine is a "completely isolated guest operating system installation within a normal host operating system". Modern virtual machines are implemented with either software emulation or hardware virtualization or both together.-VM Definitions:A virtual machine is a software...
capable of executing Java bytecode
Java bytecode
Java bytecode is the form of instructions that the Java virtual machine executes. Each bytecode opcode is one byte in length, although some require parameters, resulting in some multi-byte instructions. Not all of the possible 256 opcodes are used. 51 are reserved for future use...
. It is the code execution component of the Java software platform. Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
stated that there are over 4.5 billion JVM-enabled devices.
Overview
A Java virtual machine is software that is implemented on non-virtual hardware and on standard operating systemOperating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s. A JVM provides an environment in which Java bytecode can be executed, enabling such features as automated exception handling
Automated Exception Handling
Automated exception handling is a computing term referring to the computerized handling of errors. Runtime engines such as those for the Java language or Microsoft .Net lend themselves to an automated mode of exception or error handling. In these environments software errors do not 'crash' the...
, which provides "root-cause" debugging information for every software error (exception
Exception handling
Exception handling is a programming language construct or computer hardware mechanism designed to handle the occurrence of exceptions, special conditions that change the normal flow of program execution....
), independent of the source code. A JVM is distributed along with a set of standard class libraries
Java Class Library
The Java Class Library is a set of dynamically loadable libraries that Java applications can call at run time. Because the Java Platform is not dependent on any specific operating system, applications cannot rely on any of the existing libraries...
that implement the Java application programming interface
Application programming interface
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...
(API). Appropriate APIs bundled together with JVM form the Java Runtime Environment (JRE).
JVMs are available for many hardware and software platforms
Platform (computing)
A computing platform includes some sort of hardware architecture and a software framework , where the combination allows software, particularly application software, to run...
. The use of the same bytecode for all JVMs on all platforms allows Java to be described as a "write once, run anywhere
Write once, run anywhere
"Write once, run anywhere" , or sometimes write once, run everywhere , is a slogan created by Sun Microsystems to illustrate the cross-platform benefits of the Java language. Ideally, this means Java can be developed on any device, compiled into a standard bytecode and be expected to run on any...
" programming language, as opposed to "write once, compile anywhere
Write once, compile anywhere
Write once, compile anywhere refers to writing software that compiles, and is thus usable, on all platforms. As opposed to Sun's write once, run anywhere slogan, cross-platform compatibility is implemented at the source code level, rather than at the compiled binary code level.There are many...
", which describes cross-platform compiled language
Compiled language
A compiled language is a programming language whose implementations are typically compilers , and not interpreters ....
s. Thus, the JVM is a crucial component of the Java platform.
Java bytecode is an intermediate language
Intermediate language
In computer science, an intermediate language is the language of an abstract machine designed to aid in the analysis of computer programs. The term comes from their use in compilers, where a compiler first translates the source code of a program into a form more suitable for code-improving...
which is typically compiled from Java, but it can also be compiled from other programming languages. For example, Ada
Ada (programming language)
Ada is a structured, statically typed, imperative, wide-spectrum, and object-oriented high-level computer programming language, extended from Pascal and other languages...
source code can be compiled to Java bytecode and executed on a JVM.
Oracle
Oracle Corporation
Oracle Corporation is an American multinational computer technology corporation that specializes in developing and marketing hardware systems and enterprise software products – particularly database management systems...
, the owner of Java, produces a JVM, but JVMs using the "Java" trademark may be developed by other companies as long as they adhere to the JVM specification published by Oracle and to related contractual obligations.
The Oracle JVM (HotSpot) is written in the C++ programming language.
Execution environment
Sun Microsystem's Java execution environment is termed the Java Runtime Environment, or JRE.Programs intended to run on a JVM must be compiled into a standardized portable binary format, which typically comes in the form of .class
Class (file format)
In the Java programming language, source files are compiled into machine-readable class files which have a .class extension. Since Java is a platform-independent language, source code is compiled into an output file known as bytecode, which it stores in a .class file. If a source file has more...
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
JAR (file format)
In software, JAR is an archive file format typically used to aggregate many Java class files and associated metadata and resources into one file to distribute application software or libraries on the Java platform.JAR files are built on the ZIP file format and have the .jar file extension...
file (short for Java archive).
The Java application launcher,
java, offers a standard way of executing Java code. Compare javaw.The JVM runtime
Run-time system
A run-time system is a software component designed to support the execution of computer programs written in some computer language...
executes .class or .jar files, emulating
Emulator
In computing, an emulator is hardware or software or both that duplicates the functions of a first computer system in a different second computer system, so that the behavior of the second system closely resembles the behavior of the first system...
the JVM instruction set
Instruction set
An instruction set, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...
by interpreting
Interpreter (computing)
In computer science, an interpreter normally means a computer program that executes, i.e. performs, instructions written in a programming language...
it, or using a just-in-time compiler
Just-in-time compilation
In computing, just-in-time compilation , also known as dynamic translation, is a method to improve the runtime performance of computer programs. Historically, computer programs had two modes of runtime operation, either interpreted or static compilation...
(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
AOT compiler
An ahead-of-time compiler is a compiler that implements ahead-of-time compilation. This refers to the act of compiling an intermediate language, such as Java bytecode, .NET Common Intermediate Language , or IBM System/38 or IBM System i "Technology Independent Machine Interface" code, into a...
that enable developers to precompile class files into native code for particular platforms.
Like most virtual machines, the Java virtual machine has a stack
Stack machine
A stack machine may be* A real or emulated computer that evaluates each sub-expression of a program statement via a pushdown data stack and uses a reverse Polish notation instruction set....
-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 memory model
Java Memory Model
The Java memory model describes how threads in the Java programming language interact through memory. Together with the description of single-threaded execution of code, the memory model provides the semantics of the Java programming language....
and capability-based architecture.
JVM languages
| Language | On JVM |
|---|---|
| Erlang | Erjang |
| JavaScript JavaScript JavaScript is a prototype-based scripting language that is dynamic, weakly typed and has first-class functions. It is a multi-paradigm language, supporting object-oriented, imperative, and functional programming styles.... |
Rhino Rhino (JavaScript engine) Rhino is an open source JavaScript engine. It is developed entirely in Java and managed by the Mozilla Foundation. The Foundation also provides another implementation of JavaScript engine written in C known as SpiderMonkey.... |
| Pascal Pascal (programming language) Pascal is an influential imperative and procedural programming language, designed in 1968/9 and published in 1970 by Niklaus Wirth as a small and efficient language intended to encourage good programming practices using structured programming and data structuring.A derivative known as Object Pascal... |
Free Pascal Free Pascal Free Pascal Compiler is a free Pascal and Object Pascal compiler.In addition to its own Object Pascal dialect, Free Pascal supports, to varying degrees, the dialects of several other compilers, including those of Turbo Pascal, Delphi, and some historical Macintosh compilers... |
| PHP PHP PHP is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document... |
Resin Resin Server Resin is a software product, a web server and Java application server from Caucho Technology. Resin is provided in two versions, Resin Professional and Resin Open Source .... |
| Python Python (programming language) Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive... |
Jython Jython Jython, successor of JPython, is an implementation of the Python programming language written in Java.-Overview:Jython programs can seamlessly import and use any Java class. Except for some standard modules, Jython programs use Java classes instead of Python modules... |
| REXX REXX REXX is an interpreted programming language that was developed at IBM. It is a structured high-level programming language that was designed to be both easy to learn and easy to read... |
NetRexx NetRexx NetRexx is an open source, originally IBM's, variant of the REXX programming language to run on the Java virtual machine. It supports a classic REXX syntax, with no reserved keywords, along with considerable additions to support object-oriented programming in a manner compatible with Java's object... |
| Ruby Ruby (programming language) Ruby is a dynamic, reflective, general-purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was first developed and designed by Yukihiro "Matz" Matsumoto... |
JRuby JRuby JRuby is a Java implementation of the Ruby programming language, being developed by the JRuby team. It is free software released under a three-way CPL/GPL/LGPL license... |
| Tcl Tcl Tcl is a scripting language created by John Ousterhout. Originally "born out of frustration", according to the author, with programmers devising their own languages intended to be embedded into applications, Tcl gained acceptance on its own... |
Jacl |
| MIDletPascal MIDletPascal MIDletPascal is an Pascal compiler and IDE specifically designed to create software for mobiles. It generates Java bytecode that runs on any Java ME device. In September 2009, Niksa Orlic, who wrote MIDlet pascal, transmitted the source code to the Russian Boolean.name development community for... |
| Clojure Clojure Clojure |closure]]") is a recent dialect of the Lisp programming language created by Rich Hickey. It is a general-purpose language supporting interactive development that encourages a functional programming style, and simplifies multithreaded programming.... |
| Groovy |
| Scala |
| Fantom |
| BBj BBj BBj is a software development framework which consists of a dynamic programming language by the same name, an integrated development environment, a GUI forms designer, a Rapid Application Development tool called for generating database-driven graphical applications.BBj is Java-integrated and... |
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
Java bytecode
Java bytecode is the form of instructions that the Java virtual machine executes. Each bytecode opcode is one byte in length, although some require parameters, resulting in some multi-byte instructions. Not all of the possible 256 opcodes are used. 51 are reserved for future use...
is statically typed, although the JVM can be used to implement interpreters for dynamic languages. Prior to Java 7 the JVM had a limited support for dynamically modifying existing classes and methods; this only worked in a debugging environment
Java Platform Debugger Architecture
The Java Platform Debugger Architecture is a collection of APIs to debug Java code.* Java Debugger Interface - defines a high-level Java language interface which developers can easily use to write remote debugger application tools....
, where new classes and methods can be added dynamically.
Better support for dynamic languages is included in JVM since Java 7.
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 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
Branch (computer science)
A branch is sequence of code in a computer program which is conditionally executed depending on whether the flow of control is altered or not . The term can be used when referring to programs in high level languages as well as program written in machine code or assembly language...
can only target an instruction within the same method
Method (computer programming)
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
. 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
Register machine
In mathematical logic and theoretical computer science a register machine is a generic class of abstract machines used in a manner similar to a Turing machine...
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 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
Memory protection
Memory protection is a way to control memory access rights on a computer, and is a part of most modern operating systems. The main purpose of memory protection is to prevent a process from accessing memory that has not been allocated to it. This prevents a bug within a process from affecting...
is achieved without the need for a memory management unit
Memory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...
(MMU). Thus, JVM is an efficient way of getting memory protection on simple architectures that lack an MMU. This is analogous to managed code
Managed code
Managed code is a term coined by Microsoft to identify computer program code that requires and will only execute under the "management" of a Common Language Runtime virtual machine ....
in Microsoft's .NET Common Language Runtime
Common Language Runtime
The Common Language Runtime is the virtual machine component of Microsoft's .NET framework and is responsible for managing the execution of .NET programs. In a process known as just-in-time compilation, the CLR compiles the intermediate language code known as CIL into the machine instructions...
, and conceptually similar to capability architectures such as the Plessey 250, and IBM System/38
System/38
The System/38 was a midrange computer server platform manufactured and sold by the IBM Corporation. The system offered a number of innovative features, and was the brainchild of IBM engineer Dr. Frank Soltis...
.
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.
Bytecode instructions
The JVM has instructions for the following groups of tasks:- Load and store
- ArithmeticArithmeticArithmetic or arithmetics is the oldest and most elementary branch of mathematics, used by almost everyone, for tasks ranging from simple day-to-day counting to advanced science and business calculations. It involves the study of quantity, especially as the result of combining numbers...
- Type conversionType conversionIn computer science, type conversion, typecasting, and coercion are different ways of, implicitly or explicitly, changing an entity of one data type into another. This is done to take advantage of certain features of type hierarchies or type representations...
- Object creation and manipulation
- Operand stack management (push / pop)Stack (data structure)In computer science, a stack is a last in, first out abstract data type and linear data structure. A stack can have any abstract data type as an element, but is characterized by only three fundamental operations: push, pop and stack top. The push operation adds a new item to the top of the stack,...
- Control transfer (branching)Branch (computer science)A branch is sequence of code in a computer program which is conditionally executed depending on whether the flow of control is altered or not . The term can be used when referring to programs in high level languages as well as program written in machine code or assembly language...
- Method invocation and returnSubroutineIn computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
- Throwing exceptionsException handlingException handling is a programming language construct or computer hardware mechanism designed to handle the occurrence of exceptions, special conditions that change the normal flow of program execution....
- Monitor-based concurrencyMonitor (synchronization)In concurrent programming, a monitor is an object or module intended to be used safely by more than one thread. The defining characteristic of a monitor is that its methods are executed with mutual exclusion. That is, at each point in time, at most one thread may be executing any of its methods...
The aim is binary compatibility. Each particular host operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer 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
Java Class Library
The Java Class Library is a set of dynamically loadable libraries that Java applications can call at run time. Because the Java Platform is not dependent on any specific operating system, applications cannot rely on any of the existing libraries...
that must be mapped to each host operating system.
Heap
The Java virtual machine heap is the area of memory used by the JVM (and specifically HotSpotHotSpot
HotSpot is a Java virtual machine for desktops and servers, maintained and distributed by Oracle Corporation. It features techniques such as just-in-time compilation and adaptive optimization designed to improve performance.-History:...
) for dynamic memory allocation. The heap
Heap (data structure)
In computer science, a heap is a specialized tree-based data structure that satisfies the heap property: if B is a child node of A, then key ≥ key. This implies that an element with the greatest key is always in the root node, and so such a heap is sometimes called a max-heap...
is split up into "generations":
- The young generation stores short-lived objectsObject (computer science)In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...
that are created and immediately garbage collectedGarbage collection (computer science)In computer science, garbage collection is a form of automatic memory management. The garbage collector, or just collector, attempts to reclaim garbage, or memory occupied by objects that are no longer in use by the program...
. - Objects that persist longer are moved to the old generation (also called the tenured generation).
- The permanent generation (or permgen) is used for classClass (file format)In the Java programming language, source files are compiled into machine-readable class files which have a .class extension. Since Java is a platform-independent language, source code is compiled into an output file known as bytecode, which it stores in a .class file. If a source file has more...
definitions and associated metadata.
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 appletJava applet
A Java applet is an applet delivered to users in the form of Java bytecode. Java applets can run in a Web browser using a Java Virtual Machine , or in Sun's AppletViewer, a stand-alone tool for testing applets...
s. 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
Digital signature
A digital signature or digital signature scheme is a mathematical scheme for demonstrating the authenticity of a digital message or document. A valid digital signature gives a recipient reason to believe that the message was created by a known sender, and that it was not altered in transit...
applets as "safe", giving them permission to ask the user to break out of the sandbox and access the local file system, clipboard
Clipboard (software)
The clipboard is a software facility that can be used for short-term data storage and/or data transfer between documents or applications, via copy and paste operations...
or network.
C to bytecode compilers
From the point of view of a compilerCompiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
, the Java virtual machine is just another processor with an instruction set, Java bytecode
Java bytecode
Java bytecode is the form of instructions that the Java virtual machine executes. Each bytecode opcode is one byte in length, although some require parameters, resulting in some multi-byte instructions. Not all of the possible 256 opcodes are used. 51 are reserved for future use...
, 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 compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
s of other languages
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
.
Because of its close association with the Java language, the JVM performs the strict runtime checks mandated by the Java specification. That requires C
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
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.
- Axiomatic Multi-Platform C supports full ANSI C 1989, SWT, and J2ME CDC 1.1 for mobile devices.
- 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.
Compilers targeting Java bytecode have been written for other programming languages, including Ada
Ada (programming language)
Ada is a structured, statically typed, imperative, wide-spectrum, and object-oriented high-level computer programming language, extended from Pascal and other languages...
and COBOL
COBOL
COBOL is one of the oldest programming languages. Its name is an acronym for COmmon Business-Oriented Language, defining its primary domain in business, finance, and administrative systems for companies and governments....
.
Licensing
Starting with J2SEJava Platform, Standard Edition
Java Platform, Standard Edition or Java SE is a widely used platform for programming in the Java language. It is the Java Platform used to deploy portable applications for general use...
5.0, changes to the JVM specification have been developed under the Java Community Process
Java Community Process
The Java Community Process or JCP, established in 1998, is a formalized process that allows interested parties to get involved in the definition of future versions and features of the Java platform....
as JSR 924. , changes to specification to support changes proposed to the class file format
Class (file format)
In the Java programming language, source files are compiled into machine-readable class files which have a .class extension. Since Java is a platform-independent language, source code is compiled into an output file known as bytecode, which it stores in a .class file. If a source file has more...
(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:
Oracle's JVM is called HotSpot
HotSpot
HotSpot is a Java virtual machine for desktops and servers, maintained and distributed by Oracle Corporation. It features techniques such as just-in-time compilation and adaptive optimization designed to improve performance.-History:...
. Clean-room
Clean room design
Clean room design is the method of copying a design by reverse engineering and then recreating it without infringing any of the copyrights and trade secrets associated with the original design. Clean room design is useful as a defense against copyright and trade secret infringement because it...
Java implementations include Kaffe
Kaffe
Kaffe is a clean room design of a Java Virtual Machine. It comes with a subset of the Java Platform, Standard Edition , Java API, and tools needed to provide a Java runtime environment. Like most other Free Java virtual machines, Kaffe uses GNU Classpath as its class library.Kaffe, first released...
and IBM J9
IBM J9
J9 is a Java Virtual Machine developed by IBM. The J9 VM is the basis of multiple IBM Java offerings, including WebSphere Micro Edition, as well as the basis of all IBM Java Development kits since version 5...
. Oracle retains control over the Java trademark, which it uses to certify implementation suites as fully compatible with Oracle's specification.
See also
- List of Java virtual machines
- Comparison of application virtual machinesComparison of Application Virtual MachinesThis article lists some software virtual machines that are typically used for allowing application bytecode to be portably run on many different computer architectures and operating systems. The application is usually run on the computer using an interpreter or just-in-time compilation...
- Automated Exception HandlingAutomated Exception HandlingAutomated exception handling is a computing term referring to the computerized handling of errors. Runtime engines such as those for the Java language or Microsoft .Net lend themselves to an automated mode of exception or error handling. In these environments software errors do not 'crash' the...
- Java performanceJava performanceThe performance of a compiled Java program will depend on how smartly its particular tasks are going to be managed by the host JVM, and how well the JVM takes advantage of the features of the hardware and OS in doing so. Thus, any Java performance test or comparison has to always report the...
- List of JVM languages
- Java processorJava processorA Java processor is the implementation of the Java Virtual Machine in hardware.In other words the bytecodes that make up the instruction set of the abstract machine become the instruction set of a concrete machine.- Implementations :...