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 archive files:

CLASSPATH=/home/vicky/Java/classes:/home/josh/oldstuff/foo.zip:.

On a Windows system, the CLASSPATH environment variable is set with a semicolon-separated list of directories and class archive files:

set CLASSPATH=D:\users\vicky\Java\classes;.

The first example above, for Unix, specifies a class path with three locations: a directory in the user’s home, a ZIP file in another user’s directory, and the current directory, which is always specified with a dot (.). The last component of the class path, the current directory, is useful when tinkering with classes, but as a general rule, it’s bad practice to put the current directory in any kind of path.

The Java interpreter and the other command-line tools also know how to find core classes, which are the classes included in every Java installation. The classes in the java.lang, java.io, java.net, and javax.swing packages, for example, are all core classes. You don’t need to include these classes in your class path; the Java interpreter and the other tools can find them by themselves.

To find other classes, the Java interpreter searches the locations on the class path in order. The search combines the path location and the fully qualified class name. For example, consider a search for the class animals.birds.BigBird. Searching the class path directory /usr/lib/java means the interpreter looks for an individual class file at /usr/lib/java/animals/birds/BigBird.class. Searching a ZIP or JAR archive on the class path, say /home/vicky/Java/utils/classutils.jar, means that the interpreter looks for component file animals/birds/BigBird.class in the archive.

For the Java interpreter, java, and the Java compiler, javac, the class path can also be specified with the -classpath option:

% javac -classpath /pkg/sdk/lib/classes.zip:/home/pat/java:. Foo.java

If you don’t specify the CLASSPATH environment variable, it defaults to the current directory (.); this means that the files in your current directory are always available. If you change the class path and don’t include the current directory, these files will no longer be accessible.