I wrote a little article on how J2EE classloaders work, especially the WebSphere classloaders. So, if you want to know how classloaders work, then you could read this. Not sure if that's what you're after though:
What is a classloader?
Java applications run on a Java Virtual Machine (JVM).
Our J2EE applications run on a Java Virtual Machine as well, after all, they are written in Java.
In the WebSphere world, the JVM that our J2EE applications run on is often referred to as an �application server,� since it is responsible for serving our J2EE applications up to clients. Under the covers though, an application server is nothing more than a souped-up JVM running on the server.
If you are so inclined, you can even monitor the JVM responsible for serving up your J2EE applications by bringing up the Windows Task Manager and looking for the java.exe process that�s hogging your memory and consuming all of your processor�s clock cycles. That�s the Java Virtual Machine that enables your WebSphere applications.
J2EE applications are written in Java, and JVMs have the job of running and executing the Java code you have written.
When a client calls a Servlet, or an EJB calls the constructor of a JavaBean, compiled Java code, also known as bytecode, must be loaded and subsequently run on a JVM. Finding the appropriate Java bytecode file, and throwing that bytecode onto a piping hot JVM is the job of a very special, and extremely important component known as a classloader.
Classloaders locate Java bytecode files and load that Java bytecode on the JVM. Without a classloaders, our J2EE applications simply would not work.