Three types of classes
Bootstrap classes
* Core Java classes
* Specified by system property sun.boot.class.path
* -Xbootclasspath switches bootstrap classpath
Extension classes
* Specified by system property -Djava.ext.dirs
* Defaults to java.home/lib/ext
* Can use soft link in Unix or junction in Windows to switch to a different extension directory
User classes
* User classes are searched last
References
The Java Extension Mechanism