JManager and the Java Runtime EnvironmentIM:JM

Inside Macintosh: JManager Reference /: Chapter 1 - JManager Reference

JManager and the Java Runtime Environment
Java is an object-oriented programming language that you can use to construct programs that can run without modification on multiple platforms. To accomplish this, Java requires that each platform provide a virtual machine (VM) that can interpret and execute compiled Java code. A virtual machine is software that simulates an abstract microprocessor, complete with its own registers and instruction set. This virtual machine executes "system software" that

loads and executes Java programs
creates windows and graphics to interact with the user
provides a secure environment for executing untrusted code
provides networking capabilities and security measures
enables garbage collection (automatic memory deallocation and cleanup)

Each platform that supports Java must contain software that emulates this virtual machine. The combination of the virtual machine and the associated system software is called the Java runtime environment.
You can think of the Java runtime environment as a black box platform running within the Mac OS. This platform can handle multiple programs, and each program can contain virtual windows, buttons, and text. In order for the virtual machine to interact with the outer world, the actions that occur within the Java VM must be mapped to a similar action on the Mac OS. For example, if a Java applet decides to create a window, a Mac OS application must map that window to an actual one that the user can see. Similarly, a mouse click by the user must be passed by the application to the Java VM so the applet can take proper action. JManager is the interface that handles these translations.
You can use JManager to accomplish the following on the Mac OS platform:

Create an instantiation of the Java runtime environment (the Java virtual machine and its associated system software).
Create execution environments within the Java runtime environment that can be mapped to the Mac OS user interface. The environment and its associated thread is called the Abstract Window Toolkit context (AWT context). Each applet or application runs in its own AWT context.
Find and instantiate Java applets.
Set security options and specify proxy servers when accessing remote Java code.
Pass user events and window manipulations between the Mac OS user interface and the abstract interface provided by the Java runtime environment.
Call Java methods from Mac OS code.
Gain access to the Java Runtime Interface.