PATHJava Developer Documentation > Mac OS Runtime for Java > JManager > Programming With JManager

JManager and the Java Runtime Environment

Java is an object-oriented programming language you can use to construct programs that will run without modification on multiple platforms. To support this capability, 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 its 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 similar actions on the Mac OS. To do so requires an embedding application that accesses the Java runtime environment. This application can be a full-featured program (such as a Web browser) or a simple "wrapper" whose sole purpose is to run Java programs (such as JBindery).

For example, if a Java applet creates 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 transactions. Figure 1-1 shows the relationship between the Java runtime environment, JManager, and a Mac OS application.

Figure 1-1 The Java runtime environment, JManager, and a Mac OS application

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.
• find and instantiate Java applets or applications.
• 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 Native Interface and the Java Runtime Interface.