A few weeks ago I started a thread on a similar subject (http://forum.java.sun.com/thread.jspa?threadID=709140&tstart=15).
Reading 'The basics of Java class loaders' (http://www.javaworld.com/javaworld/jw-10-1996/jw-10-indepth.html) and 'Understanding the Java ClassLoader' (http://www-128.ibm.com/developerworks/edu/j-dw-javaclass-i.html) should help you.