CLASSPATH Explained

A Bit of Introduction

An environment variable is a bit of information maintained by the operating system that is available to every program. They are usually used to specify the location of important files that an application needs when it starts, or commonly used command line options. The CLASSPATH is such an environment variable. It is used by Java to locate the "class" files it needs to use. Depending on the operating system the environment variables may be set differently, and this is where most problems arise. Below is an attempt to render assistance to users with any Java supported platform.

Mac Users: DECAF will only run under MacOS X and higher. Since Mac OS X is at heart a flavor of UNIX, use the information in the that section.

UNIX

In UNIX the environment variables are set through the shell. Some shells sets these variables differently and when practical those differences are listed.

First, you might want to see if you already have a CLASSPATH variable defined and what is in it. Do this type echo $CLASSPATH at the prompt. There are three possible outcomes. "CLASSPATH: Undefined variable." means that no CLASSPATH variable is currently set up in the environment. A list of paths separated by colons means a CLASSPATH has already been defined. If nothing is printed except another command prompt, then that may mean that a CLASSPATH variable has been created, but nothing is in it.

Now that you know whether or not you have a class path you need to know how to set it. This is where differences in the shell come in.

• C Shells (includes csh and tcsh):
  You will need to open the shell resource file (usually .cshrc, but could be .tcshrc) and add the following line:
  setenv CLASSPATH .:~/decaf_dir/decaf.jar:~/decaf_dir/planeditor.jar:$CLASSPATH
  Now whenever you open a new shell it will add the location of the DECAF files to the CLASSPATH.
• Bourne, Bourne Again(BASH), Korn(a.k.a. Kourne), and Z Shells (sh, bsh, ksh, zsh)
  Since all of these shells are routed in the original Bourne syntax, they behave similarly in many situations. To set a variable you alter the resource file to say
  CLASSPATH=.:~/decaf_dir/decaf.jar:~/decaf_dir/planeditor.jar:$CLASSPATH
  Each shell uses a different resource file, but it is usually a period, the shell name, and then rc (i.e. .zshrc). Some shells can be configured to use different files, so if you don't see one that looks obvious ask a system administrator.

Some simple things to keep in mind:

• Make sure '.' is the first item in the list.
• Each path is separated by a colon ':' and no spaces are needed, especially before or after the = sign if you are using that syntax.
• You should use fully-qualified path names, so no '~', but you can use shell variables like $HOME if they are defined earlier in the file.
• Only include the trailing :$CLASSPATH if you already had a CLASSPATH. This appends the previous CLASSPATH to your new one. This is important as other programs might rely on the CLASSPATH as well.

Windows

How you format and set the CLASSPATH on Windows machines is a little different then on UNIX. Go to Start→Control Panels→System and then click on the "Advanced" tab. Under Windows 2000 and XP there is a button at the bottom that says "Environment Variables". Select the button, a new window will open. There should be two lists, the top is User Variables and the bottom is System wide. Look in the variables at the top and see if CLASSPATH exists.

If you already have a CLASSPATH variable, select it and click on the "Edit" button. If there is no CLASSPATH, then select the "New" button, and type CLASSPATH as the name. For the value you want to enter a period '.' as the first path. Then enter a semi-colon ';' to separate between paths. Then enter either the directory where all the decaf files are located, or the path with the jar file. If you are using decaf in the jar file, then the CLASSPATH must include the name of the jar file, not just the enclosing directory. Next, enter another semi-colon and the path to, and the name of, the planeditor jar file.

If there was already a CLASSPATH here then you will need another semi-colon, if not then you are done.