• A Java Virtual Machine (JVM) enables a set of computer software programs and data structures to use a virtual machine model for the execution of other computer programs and scripts. The model used by a JVM accepts a form of computer intermediate language commonly referred to as Java bytecode.

JVM, JRE, SDK,BYTECODE

Smitha N Pai

• A JVM can also implement programming languages other than Java. For example, Ada source code can be compiled to Java bytecode, which may then be executed by a JVM. JVMs can also be released by other companies besides Sun (the developer of Java) — JVMs using the "Java" trademark may be developed by other companies as long as they adhere to the JVM specification published by Sun and to related contractual obligations

• Java was conceived with the concept of WORA: "write once, run anywhere". This is done using the Java Virtual Machine. The JVM is the environment in which java programs execute. It is software that is implemented on non-virtual hardware and on standard operating systems

• 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.

• The JVM, which is the instance of the 'JRE' (Java Runtime Environment), comes into action when a Java program is executed. When execution is complete, this instance is garbage collected. JIT is the part of the JVM that is used to speed up the execution time. JIT compiles parts of the bytecode that have similar functionality at the same time, and hence reduces the amount of time needed for compiling.

• 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.

• 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).

• A virtual machine (VM) is a software implementation of a programmable machine, where the software implementation is constrained within another computer at a higher or lower level of symbolic abstraction

• Bytecode is a term which has been used to denote various forms of instruction sets designed for efficient execution by a software interpreter as well as being suitable for further compilation into machine code

• The name bytecode stems from instruction sets which have one-byte opcodes followed by optional parameters. Intermediate representations such as bytecode may be output by programming language implementations to ease interpretation, or it may be used to reduce hardware and operating system dependence by allowing the same code to run on different platforms. Bytecode may often be either directly executed on a virtual machine (i.e. interpreter), or it may be further compiled into machine code for better performance

• A bytecode program may be executed by parsing and directly executing the instructions, one at a time. This kind of bytecode interpreter is very portable. Some systems, called dynamic translators, or "just-in-time" (JIT) compilers, translate bytecode into machine language as necessary at runtime: this makes the virtual machine unportable, but doesn't lose the portability of the bytecode itself.

• In computing, just-in-time compilation (JIT), also known as dynamic translation, is a method to improve the runtime performance of computer programs

• Interpreted code is translated from a high-level language to a machine code continuously during every execution, whereas statically compiled code is translated into machine code before execution, and only requires this translation once.

• JIT compilers represent a hybrid approach, with translation occurring continuously, as with interpreters, but with caching of translated code to minimize performance degradation. It also offers other advantages over statically compiled code at development time, such as handling of late-bound data types and the ability to enforce security guarantees.

• In a bytecode-compiled system, source code is translated to an intermediate representation known as bytecode. Bytecode is not the machine code for any particular computer, and may be portable among computer architectures. The bytecode may then be interpreted by, or run on, a virtual machine. A just-in-time compiler can be used as a way to speed up execution of bytecode. At the time the bytecode is run, the just-in-time compiler will compile some or all of it to native machine code for better performance.

• A common goal of using JIT techniques is to reach or surpass the performance of static compilation, while maintaining the advantages of bytecode interpretation: Much of the "heavy lifting" of parsing the original source code and performing basic optimization is often handled at compile time, prior to deployment: compilation from bytecode to machine code is much faster than compiling from source. The deployed bytecode is portable, unlike native code. Since the runtime has control over the compilation, like interpreted bytecode, it can run in a secure sandbox. Compilers from bytecode to machine code are easier to write, because the portable bytecode compiler has already done much of the work.

• JDK = Java Development Kit - if you want to develop or just compile Javaprograms (*.java -> *.class -> *.jar)JRE = Java Runtime Environment - if you want to execute a Java program

Just my simple descriptions:J2SE = Java 2 Standard Edition - (JDK+JRE) - for common purposes for exampleon PCsJ2EE = Java 2 Enterprise Edition - AFAIK framework and methodology toproduce applications in 3-layer technology (robust applications)J2ME = Java 2 Micro Edition - (JDK+JRE) - for small devices (cellularphones, palmtops etc.)

SDK = Software Development Kit.It follows some fundamental infos from Sun's website:--------------------------J2SE-------------------------- The premier solution for rapidly developing and deploying mission-critical,enterprise applications, J2SETM provides the essential compiler, tools,runtimes, and APIs for writing, deploying, and running applets andapplications in the Java programming language.

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--------------------------J2EE-------------------------- J2EETM technology and its component based model simplifies enterprisedevelopment and deployment. The J2EE platform manages the infrastructure andsupports the Web services to enable development of secure, robust andinteroperable business applications. The J2EE platform is the foundationtechnology of the Sun ONE platform and Sun's Web services strategy.

--------------------------J2ME--------------------------A highly optimized Java runtime environment, J2METM technology specificallyaddresses the vast consumer space, which covers the range of extremely tinycommodities such as smart cards or a pager all the way up to the set-topbox, an appliance almost as powerful as a computer.

• 1. A JRE software package is needed to use Java applications while a JDK software package is needed to create Java applications2. A JDK software package contains everything in a JRE package along with the development tools3. JREs are available for a much wider platform range compared to JDK4. The JDK is meant for programmers and software developers while the JRE is for the common everyday user

• 1. An SDK is a set of development tools that allow applications to be created for certain software packages or platforms; the JDK is the most widely used SDK and is an extension of the SDK responsible for writing and running Java programs.

• 2. An SDK includes sample code and technical notes or other supporting documentation; the JDK includes components that are a selection of programming tools.

• Java is a software platform that allows a unified code base for creating and deploying applications across a wide range of operating systems. Prior to Java, you would need to recompile, and even edit your code, in order to make it run on another operating system. With Java, you can write a program once, and be sure that it will run on a wide range of operating systems, provided there is a JVM, or Java Virtual Machine, that executes the Java application, and acts as a middle man between the application and the operating system. The JVM is packaged along with a collection of software, and referred to as a JRE, or Java Runtime Environment.

• The JRE not only contains the JVM, which is the most important software in the package, but extra software as well, that extends the functionalities of Java. This includes AWT, Swing, and a lot of other libraries that can be used by Java applications.

• You can find a JRE for any operating system, including those for smartphones, like Windows Mobile and Google Android, and even standard mobile phones. There are different specifications for computers and mobile phones though, as they do not share the same hardware, and computer applications often do not run on mobile phones

• 1. Java is a software platform, while JRE is a software package.

• 2. JRE contains the necessary software for running Java applications.

• 3. Java is the same, regardless of the operating system, while the JRE differs.