Java Virtual Machine (JVM)

The Java Virtual Machine is the cornerstone of the Sun Microsystem Java programming language. It is the component of the Java technology responsible for Java's cross-platform delivery, the small size of its compiled code, and its ability to protect users from malicious programs.

The Java Virtual Machine is an abstract computing machine. Like a real computing machine, it has an instruction set and uses various memory areas. It is reasonably common to implement a programming language using a virtual machine; the best-known virtual machine may be the P-Code machine of UCSD Pascal. The Java Virtual Machine does not assume any particular implementation technology or host platform. It is not inherently interpreted, and it may just as well be implemented by compiling its instruction set to that of a real CPU, as for a conventional programming language. It may also be implemented in microcode, or directly in silicon.

The Java Virtual Machine knows nothing of the Java programming language, only of a particular file format, the .class file format. A .class file contains Java Virtual Machine instructions (or bytecodes) and a symbol table, as well as other ancillary information.