Learning Java

The Class Path

The concept of a path should be familiar to anyone who has worked on a DOS or Unix platform. It’s an environment variable that provides an application with a list of places to look for some resource. The most common example is a path for executable programs. In a Unix shell, the PATH environment variable is a colon-separated list of directories that are searched, in order, when the user types the name of a command. The Java CLASSPATH environment variable, similarly, is a list of locations that can be searched for packages containing Java class files. Both the Java interpreter and the Java compiler use CLASSPATH when searching for packages and classes on the local host.

A location on the class path can be a directory name or the name of a class archive file. Java supports archives of class files in its own Java archive ( JAR) format, and in the conventional ZIP format. JAR and ZIP are really the same format, but JAR archives include extra files that describe each archive’s contents. JAR files are created with the SDK’s jar utility; many tools for creating ZIP archives are publicly available. The archive format enables large groups of classes to be distributed in a single file; the Java interpreter automatically extracts individual class files from an archive, as needed.

The precise means and format for setting the class path vary from system to system. On a Unix system, you set the CLASSPATH environment variable with a colon-separated list of directories and class ...