Java Architecture

The Architecture

Java's architecture arises out of four distinct but interrelated technologies:

• the Java programming language
• the Java class file format
• the Java Application Programming Interface
• the Java virtual machine

When you write and run a Java program, you are tapping the power of these four technologies. You express the program in source files written in the Java programming language, compile the source to Java class files, and run the class files on a Java virtual machine. When you write your program, you access system resources (such as I/O, for example) by calling methods in the classes that implement the Java Application Programming Interface, or Java API. As your program runs, it fulfills your program's Java API calls by invoking methods in class files that implement the Java API. You can see the relationship between these four parts in Figure 1-1.



Figure 1-1. The Java programming environment. Together, the Java virtual machine and Java API form a "platform" for which all Java programs are compiled. In addition to being called the Java runtime system, the combination of the Java virtual machine and Java API is called the Java Platform (or, starting with version 1.2, the Java 2 Platform). Java programs can run on many different kinds of computers because the Java Platform can itself be implemented in software. As you can see in Figure 1- 2, a Java program can run anywhere the Java Platform is present.