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A key component of Java is the Java Virtual Machine -- a virtual computer, typically implemented in software on top of a "real" hardware platform and operating system, that runs compiled Java programs. This article is a hands-on introduction to the JVM.
(This article, which first appeared in JavaWorld in May 1996, was the very first technical article I ever published and contains the first Java applet I ever wrote: "EternalMath.")
Welcome to the first installment of "Under The Hood." In this column I'd like to explore topics concerning the inner workings of Java. Each month I'll focus on one area and attempt to demystify it. My aim is to help programmers understand what is actually going on when they compile and run their Java programs. In this installment, I provide an introduction to the basic structure and functionality of the Java Virtual Machine.
What is the Java Virtual Machine? Why is it here?
The Java Virtual Machine, or JVM, is an abstract computer that runs compiled Java programs. The JVM is "virtual" because it is generally implemented in software on top of a "real" hardware platform and operating system. All Java programs are compiled for the JVM. Therefore, the JVM must be implemented on a particular platform before compiled Java programs will run on that platform.
The JVM plays a central role in making Java portable. It provides a layer of abstraction between the compiled Java program and the underlying hardware platform and operating system. The JVM is central to Java's portability because compiled Java programs run on the JVM, independent of whatever may be underneath a particular JVM implementation.
What makes the JVM lean and mean? The JVM is lean because it is small when implemented in software. It was designed to be small so that it can fit in as many places as possible -- places like TV sets, cell phones, and personal computers. The JVM is mean because it of its ambition. "Ubiquity!" is its battle cry. It wants to be everywhere, and its success is indicated by the extent to which programs written in Java will run everywhere.
Java programs are compiled into a form called Java bytecodes. The JVM executes Java bytecodes, so Java bytecodes can be thought of as the machine language of the JVM. The Java compiler reads Java language source (.java) files, translates the source into Java bytecodes, and places the bytecodes into class (.class) files. The compiler generates one class file per class in the source.
To the JVM, a stream of bytecodes is a sequence of instructions. Each instruction consists of a one-byte opcode and zero or more operands. The opcode tells the JVM what action to take. If the JVM requires more information to perform the action than just the opcode, the required information immediately follows the opcode as operands.
A mnemonic is defined for each bytecode instruction. The mnemonics can be thought of as an assembly language for the JVM. For example, there is an instruction that will cause the JVM to push a zero onto the stack. The mnemonic for this instruction is iconst_0, and its bytecode value is 60 hex. This instruction takes no operands. Another instruction causes program execution to unconditionally jump forward or backward in memory. This instruction requires one operand, a 16-bit signed offset from the current memory location. By adding the offset to the current memory location, the JVM can determine the memory location to jump to. The mnemonic for this instruction is goto, and its bytecode value is a7 hex.