Java virtual machine
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 part of the Java virtual machine's specification. 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 
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. 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.
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:
- Loading: finds and imports the binary data for a type
- 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.
- 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.
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).
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. The permanent generation was removed from Java 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.
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.
Bytecode interpreter and just-in-time compiler
If the Java Virtual Machine is an abstract computer then its machine language is the Java bytecode. For each type of real computer 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.
There are two basic reasons for having the Java bytecode interpreter:
Simplicity: The Java compiler is itself a complex program. A Java bytecode interpreter, is a relatively small, simple program. This makes it easy to write a bytecode interpreter for a new type of computer; once that is done, that computer can run any compiled Java program. It would be much harder to write a Java compiler for the same computer.
Security: The bytecode interpreter is a buffer between the user and the program the user downloads. The user is really running the interpreter, which runs the downloaded program indirectly. The interpreter can protect the user from potentially dangerous actions on the part of that program.
Since Java bytecode was executed by an interpreter, the execution will be always slower than the execution of the same program compiled into native machine language. This problem was resolved by introducing just-in-time (JIT) compilers for executing Java bytecode. A just-in-time compiler translates Java bytecode into native machine language. It does this while it is executing the program. The input to a just-in-time compiler is a Java bytecode program, and its task is to execute that program.
While it is executing the program, it also translates parts of the program into machine language of the underlying computer. 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.
Some JVM implementations do not include an interpreter which way the just-in-time compilation of the byte code into native machine code has to occur before a method executes. The JIT compilation is performed the first time a Java method is called.
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, the other, inherited from BEA Systems is JRockit. 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
- Inside the Java Virtual Machine by Bill Venners, Chapter 5 http://www.artima.com/insidejvm/ed2/index.html
- 1996, possibly the first new language specifically designed to run on the JVM)
- The Java® Virtual Machine Specification Java SE 7 Edition http://docs.oracle.com/javase/specs/jvms/se7/jvms7.pdf
- New JDK 7 Feature: Support for Dynamically Typed Languages in the Java Virtual Machine http://www.oracle.com/technetwork/articles/javase/dyntypelang-142348.html
- "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.
- "JEP 122: Remove the Permanent Generation". Oracle Corporation. 2012-12-04. Retrieved 2014-03-23.
- http://math.hws.edu/javanotes/c1/s3.html Introduction to Programming Using Java, Seventh Edition, Version 7.0, August 2014 by David J. Eck Section 1.3 The Java Virtual Machine
- http://docs.oracle.com/cd/E15289_01/doc.40/e15058/underst_jit.htm Oracle® JRockit Introduction Release R28 2. Understanding Just-In-Time Compilation and Optimization
- 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 DMOZ
- Java Virtual Machine Download Link
- JVM implementation in pure Java