Java Cookbook

Using CLASSPATH Effectively

Problem

You need to keep your class files in a common directory or you’re wrestling with CLASSPATH.

Solution

Set CLASSPATH to the list of directories and/or JAR files that contain the classes you want.

Discussion

CLASSPATH is one of the more interesting aspects of using Java. You can store your class files in any of a number of directories, JAR files, or zip files. Just like the PATH your system uses for finding programs, the CLASSPATH is used by the Java runtime to find classes. Even when you type something as simple as java HelloWorld, the Java interpreter looks in each of the places named in your CLASSPATH until it finds a match. Let’s work through an example.

The CLASSPATH can be set as an environment variable on systems that support this (at least Unix and MS-Windows). You set it in the same syntax as your PATH environment variable. PATH is a list of directories to look in for programs; CLASSPATH is a list of directories or JAR files to look in for classes.

Alternatively, you can set your CLASSPATH right on the command line:

java -classpath \c:\ian\classes MyProg

Suppose your CLASSPATH were set to C:\classes;. on MS-Windows, or ~/classes:. on Unix (on the Mac, you can set the CLASSPATH with JBindery). Suppose you had just compiled a file named HelloWorld.java into HelloWorld.class, and went to run it. On Unix, if you run one of the kernel tracing tools (trace, strace, truss, ktrace) you would probably see the Java program open (or stat, or access ...