Learning Java, 4th Edition

The Java VM

A Java virtual machine (VM) is software that implements the Java runtime system and executes Java applications. It can be a standalone application like the java command that comes with the JDK or built into a larger application like a web browser. Usually the interpreter itself is a native application, supplied for each platform, which then bootstraps other tools written in the Java language. Tools such as Java compilers and IDEs are often implemented directly in Java to maximize their portability and extensibility. NetBeans, for example, is a pure-Java application.

The Java VM performs all the runtime activities of Java. It loads Java class files, verifies classes from untrusted sources, and executes the compiled bytecode. It manages memory and system resources. Good implementations also perform dynamic optimization, compiling Java bytecode into native machine instructions.