Java virtual machine
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The Java virtual machine is a Virtual machine that runs Java byte codes, the codes produced by a Java compiler or compilers from other languages which produce this code.
The specification for this virtual machine are published in book form and html and anybody is allowed to write an implementation of it. The preface states We intend that this specification should sufficiently document the Java Virtual Machine to make possible compatible clean-room implementations. Sun provides tests which verify the proper operation of implementations of the Java Virtual Machine. . www.kaffe.org shows an example of a group of developers having done this.
List of languages which compile to the Java virtual machine.