Learning Java

A Virtual Machine

Java is both a compiled and an interpreted language. Java source code is turned into simple binary instructions, much like ordinary microprocessor machine code. However, whereas C or C++ source is refined to native instructions for a particular model of processor, Java source is compiled into a universal format—instructions for a virtual machine.

Compiled Java byte-code , also called J-code, is executed by a Java runtime interpreter. The runtime system performs all the normal activities of a real processor, but it does so in a safe, virtual environment. It executes the stack-based instruction set and manages a storage heap. It creates and manipulates primitive datatypes, and loads and invokes newly referenced blocks of code. Most importantly, it does all this in accordance with a strictly defined open specification that can be implemented by anyone who wants to produce a Java-compliant virtual machine. Together, the virtual machine and language definition provide a complete specification. There are no features of Java left undefined or implementation-dependent. For example, Java specifies the sizes of all its primitive data types, rather than leave it up to each implementation.

The Java interpreter is relatively lightweight and small; it can be implemented in whatever form is desirable for a particular platform. On most systems, the interpreter is written in a fast, natively compiled language like C or C++. The interpreter can be run as a separate application, ...