why do we need user defined classloader in java

Question

why tomcat has its own classloader. what is the advantage of having user defined classloader

Answer 1

Tomcat (and other application containers) need to be able to handle loading classes from WAR files etc. How would you be able to do that without using user-defined classloaders?

EDIT: Basically you need user-defined classloaders if you need to load classes or resources in "unusual" ways... such as from an EAR or WAR file. As another example, you might load classes from a database, or from some secure storage.