1.2 A Virtual Machine
Java is both a compiled and an interpreted language. Java source code is turned into simple binary instructions, much like ordinary microprocessor machine code. However, whereas C or C++ source is refined to native instructions for a particular model of processor, Java source is compiled into a universal format--instructions for a virtual machine.
Compiled Java byte-code, also called J-code, is executed by a Java run-time interpreter. The run-time system performs all the normal activities of a real processor, but it does so in a safe, virtual environment. It executes the stack-based instruction set and manages a storage heap. It creates and manipulates primitive data types, and loads and invokes newly referenced blocks of code. Most importantly, it does all this in accordance with a strictly defined open specification that can be implemented by anyone who wants to produce a Java-compliant virtual machine. Together, the virtual machine and language definition provide a complete specification. There are no features of Java left undefined or implementation dependent. For example, Java specifies the sizes of all its primitive data types, rather than leave it up to each implementation.
The Java interpreter is relatively lightweight and small; it can be implemented in whatever form is desirable for a particular platform. On most systems, the interpreter is written in a fast, natively compiled language like C or C++. The interpreter can be run as a separate application, or it can be embedded in another piece of software, such as a Web browser.
All of this means that Java code is implicitly portable. The same Java application can run on any platform that provides a Java run-time environment, as shown in Figure 1.1. You don't have to produce alternate versions of your application for different platforms, and you never have to distribute source code to end users.
The fundamental unit of Java code is the class. As in other object-oriented languages, classes are application components that hold executable code and data. Compiled Java classes are distributed in a universal binary format that contains Java byte-code and other class information. Classes can be maintained discretely and stored in files or archives on a local system or on a network server. Classes are located and loaded dynamically at run-time, as they are needed by an application.
In addition to the platform-specific run-time system, Java has a number of fundamental classes that contain architecture-dependent methods. These native methods serve as Java's gateway to the real world. These methods are implemented in a native language on the host platform. They provide access to resources such as the network, the windowing system, and the host filesystem. The rest of Java is written entirely in Java, and is therefore portable. This includes fundamental Java utilities like the Java compiler, which is also a Java application and is therefore immediately available on all Java platforms.
In general, interpreters are slow, but because the Java interpreter runs compiled byte-code, Java is a fast interpreted language. Java has also been designed so that software implementations of the run-time system can optimize their performance by compiling byte-code to native machine code on the fly. This is called "just in time" compilation. Sun claims that with just in time compilation, Java code can execute nearly as fast as native compiled code and maintain its transportability and security. The one performance hit that natively compiled Java code will always suffer is array bounds checking. But on the other hand, some of the basic design features of Java place more information in the hands of the compiler, which allows for certain kinds of optimizations not possible in C or C++.