When do we typically need to add the path of the directory of .class files into CLASSPATH variable.
Is that useful for compiling the servlets files? I am not getting the exact use of it.
I read about this in a book named Core Servlets Volume-I 2nd Edition. Page 57 , Second chapter, Topic - Updating your CLASSPATH.
I am using Tomcat 5.5.28.
If you are working with servlets then you have to add servlet-api.jar (found in the tomcat's lib directory) to the CLASSPATH in order to compile your servlets. CLASSPATH is for pointing the compiler which places to look for classes/jar files etc.. which are not part of the core Java library but from the third parties. And you need that same additional classes/jar files when you run the program in order to let the JVM use them too.
However - it is important to remember that Tomcat does NOT use the CLASSPATH environment variable to run servlets. Tomcat has its own - rather complicated - rules which control where it looks for classes and library jar files. Your Tomcat installation should have a "class-loader-howto.html" file which explains this.