CLASSPATH

CLASSPATH is at the heart of the JRE. Whenever the JRE needs a class, it looks for it in a certain set of directories. This set of directories is called the CLASSPATH.

By default, only the current working directory is included in the CLASSPATH.
But it can be configured to contain any directory on your computer.

In this sense it is no different from the PATH environment variable. Just like your OS looks at the directories in the PATH environmental variable to find a program it needs to run, the JRE looks in the CLASSPATH to find a class it needs.

CLASSPATH can be configured as a command line argument or as a CLASSPATH environment variable.

THE STRANGE THING is that CLASSPATH can be dynamically changed by the Java program that is running!!!

This leads to a phenomenon called CLASSLOADER HELL! Because when you are running Java programs inside a Web Server, it gets very hard to know what exactly the CLASSPATH is! Because different WebServers (Tomcat, Weblogic, JBoss, ..) have different ways to dynamically configure the CLASSPATH! There seems to be NO STANDARD in this regard!

If a class is not found in the CLASSPATH, you get a NoClassDefFoundError exception.

So, if we try to run the HelloWorld program form a different directory (i.e. a directory other than the one in which HelloWorld.class is located), we will get the NoClassDefFoundError exception. Because remember that by default, only the current directory is included in the CLASSPATH.

See example below:

 C:\users\LavanniM\confucius>java HelloWorld  
 Hello World!  
   
 C:\users\LavanniM\confucius>cd ..  
   
 C:\users\LavanniM>java HelloWorld  
 Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld  
 Caused by: java.lang.ClassNotFoundException: HelloWorld  
     at java.net.URLClassLoader$1.run(Unknown Source)  
     at java.security.AccessController.doPrivileged(Native Method)  
     at java.net.URLClassLoader.findClass(Unknown Source)  
     at java.lang.ClassLoader.loadClass(Unknown Source)  
     at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)  
     at java.lang.ClassLoader.loadClass(Unknown Source)  
 Could not find the main class: HelloWorld. Program will exit.  
   
 C:\users\LavanniM>