Learning Java, 4th Edition

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 reduced to native instructions for a particular model of processor, Java source is compiled into a universal format—instructions for a virtual machine.

Compiled Java bytecode is executed by a Java runtime interpreter. The runtime system performs all the normal activities of a hardware processor, but it does so in a safe, virtual environment. It executes a stack-based instruction set and manages memory like an operating system. It creates and manipulates primitive data types 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 the base Java language left undefined or implementation-dependent. For example, Java specifies the sizes and mathematical properties of all its primitive data types rather than leaving it up to the platform implementation.

The Java interpreter is relatively lightweight and small; it can be implemented in whatever form is desirable for a particular platform. The interpreter may be run as a separate application or it can be embedded in another piece of software, ...