Java virtual machine

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Overview of a Java virtual machine (JVM) architecture based on The Java Virtual Machine Specification Java SE 7 Edition

A Java virtual machine (JVM) is an abstract computing machine. There are three notions of the JVM: specification, implementation, and instance. An instance of the JVM can execute any executable computer program compiled into Java bytecode. It is the code execution component of the Java platform. The Java virtual machine is called "virtual" because it is an abstract computer defined by a specification. JVM specification omits implementation details that are not essential to insure interoperability. For example, the memory layout of run-time data areas, the garbage-collection algorithm used, and any internal optimization of the Java virtual machine instructions (their translation into machine code). The main reason for this omission is to not unnecessarily constrain the creativity of implementors. Any Java application can be run only inside a run-time instance of some concrete implementation of the abstract specification of the Java virtual machine.[1]

JVM languages[edit]

Versions of non-JVM languages
Language On JVM
Erlang Erjang
JavaScript Rhino
Pascal Free Pascal
PHP Quercus
Python Jython
REXX NetRexx[2]
Ruby JRuby
Tcl Jacl
Languages designed expressly for JVM

A JVM language is any language with functionality that can be expressed in terms of a valid class file which can be hosted by the Java Virtual Machine. A class file contains Java Virtual Machine instructions (or bytecode) and a symbol table, as well as other ancillary information. The class file format, the hardware- and operating system- independent binary format used to represent compiled classes and interfaces.[3] Java 7 JVM implements JSR 292: Supporting Dynamically Typed Languages on the Java Platform, a new feature which supports dynamically typed languages in the JVM. This feature is developed within the Da Vinci Machine Project whose mission is to extend the JVM so that it supports languages other than Java.[4]

Class loader[edit]

A class loader implementation must be able to recognize and load classes and interfaces stored in binary files that conform to the Java class file format. Any implementation is free to recognize other binary forms besides class files, but it must recognize class files.

The class loader performs three basic activities in this strict order:

  1. Loading: finds and imports the binary data for a type
  2. Linking: performs verification, preparation, and (optionally) resolution
    • Verification: ensures the correctness of the imported type
    • Preparation: allocates memory for class variables and initializing the memory to default values
    • Resolution: transforms symbolic references from the type into direct references.
  3. Initialization: invokes Java code that initializes class variables to their proper starting values.

In general, there are two types of class loader: bootstrap class loader and user defined class loader.

Every Java virtual machine implementation must have a bootstrap class loader, capable of loading trusted classes. The Java virtual machine specification doesn't specify how a class loader should locate classes.

Bytecode instructions[edit]

Main article: Java bytecode

The JVM has instructions for the following groups of tasks:

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 Java virtual machine heap is the area of memory used by the JVM, specifically HotSpot, for dynamic memory allocation.[5] The heap is divided into generations:

  • 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).

The permanent generation (or permgen) was used for class definitions and associated metadata prior to Java 8. Permanent generation was not part of the heap.[6][7] The permanent generation was removed from Java 8.[8]

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 allowed significant performance improvements.[6]

Secure execution of remote code[edit]

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.

Bytecode interpreter and just-in-time compiler[edit]

For each hardware architecture a different Java bytecode interpreter is needed. When a computer has a Java bytecode interpreter, it can run any Java bytecode program, and the same program can be run on any computer that has such an interpreter.

When Java bytecode is executed by an interpreter, the execution will be always slower than the execution of the same program compiled into native machine language. This problem is mitigated by just-in-time (JIT) compilers for executing Java bytecode. A just-in-time compiler may translate Java bytecode into native machine language while executing the program. The translated parts of the program can then be executed much more quickly than they could be interpreted. This technique gets applied to those parts of a program frequently executed. This way a just-in-time compiler can significantly speed up the overall execution time.

There is no necessary connection between Java and Java bytecode. A program written in Java can be compiled directly into the machine language of a real computer and programs written in other languages than Java can be compiled into Java bytecode.

Java bytecode is intended to be platform-independent and secure.[9] Some JVM implementations do not include an interpreter, but consist only of a just-in-time compiler.[10]


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.[11] As of 2006, changes to specification to support changes proposed to the class file format (JSR 202)[12] are being done as a maintenance release of JSR 924. The specification for the JVM is published in book form,[13] 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, the other, inherited from BEA Systems is JRockit. Clean-room Java implementations include Kaffe and IBM J9. Oracle owns the Java trademark, and may allow its use to certify implementation suites as fully compatible with Oracle's specification.

See also[edit]


  1. ^ Inside the Java Virtual Machine by Bill Venners, Chapter 5
  2. ^ 1996, possibly the first new language specifically designed to run on the JVM)
  3. ^ The Java® Virtual Machine Specification Java SE 7 Edition
  4. ^ New JDK 7 Feature: Support for Dynamically Typed Languages in the Java Virtual Machine
  5. ^ "Frequently Asked Questions about Garbage Collection in the Hotspot Java Virtual Machine". Sun Microsystems. 6 February 2003. Retrieved 7 February 2009. 
  6. ^ a b Masamitsu, Jon (28 November 2006). "Presenting the Permanent Generation". Retrieved 7 February 2009. 
  7. ^ Nutter, Charles (11 September 2008). "A First Taste of InvokeDynamic". Retrieved 7 February 2009. 
  8. ^ "JEP 122: Remove the Permanent Generation". Oracle Corporation. 2012-12-04. Retrieved 2014-03-23. 
  9. ^ Introduction to Programming Using Java, Seventh Edition, Version 7.0, August 2014 by David J. Eck Section 1.3 The Java Virtual Machine
  10. ^ Oracle® JRockit Introduction Release R28 2. Understanding Just-In-Time Compilation and Optimization
  11. ^ JSR 924, specifies changes to the JVM specification starting with J2SE 5.0
  12. ^ JSR 202, specifies a number of changes to the class file format
  13. ^ The Java Virtual Machine Specification (the first and second editions are also available online)


External links[edit]