David Flanagan - Java in a Nutshell, 6th Edition

The JVM is a program that provides the runtime environment necessary for Java programs to execute. Java programs cannot run unless there is a JVM available for the appropriate hardware and OS platform we wish to execute on.

Fortunately, the JVM has been ported to run on a large number of environmentsanything from a set-top box or Blu-ray player to a huge mainframe will probably have a JVM available for it.

Java programs are typically started by a command line, such as:

This brings up the JVM as an operating system process that provides the Java runtime environment, and then executes our program in the context of the freshly started (and empty) virtual machine.

It is important to understand that when the JVM takes in a Java program for execution, the program is not provided as Java language source code. Instead, the Java language source must have been converted (or compiled) into a form known as Java bytecode. Java bytecode must be supplied to the JVM in a format called class fileswhich always have a .class extension.

The JVM is an interpreter for the bytecode form of the programit steps through one bytecode instruction at a time. However, you should also be aware that both the JVM and the user program are capable of spawning additional threads of execution, so that a user program may have many different functions running simultenously.

The design of the JVM built on many years of experience with earlier programming environments, notably C and C++ , so we can think of it as having several different goalswhich are all intended to make life easier for the programmer:

• Comprise a container for application code to run inside
• Provide a secure execution environment as compared to C/C++
• Take memory management out of the hands of developers
• Provide a cross-platform execution environment

These objectives are often mentioned together when discussing the platform.

Weve already mentioned the first of these goals, when we discussed the JVM and its bytecode interpreterit functions as the container for application code.

Well discuss the second and third goals in , when we talk about how the Java environment deals with memory management.

The fourth goal, sometimes called write once, run anywhere (WORA), is the property that Java class files can be moved from one execution platform to another, and they will run unaltered provided a JVM is available.

This means that a Java program can be developed (and converted to class files) on an Apple Mac machine running OS X, and then the class files can be moved to Linux or Microsoft Windows (or other platforms) and the Java program will run without any further work needed.