when a user-defined class loader is required??

[jacob deiter]

In java based application .JVM will load all possible class when it required, then please explain in which scenario a user-defined class loader should be used in an application??

[David Newton]

If you want to modify the byte code at load time. If you want to compile a JVM script language script from the source on load. All sorts of other reasons.

[Campbell Ritchie]

Class-loaders are hardly a beginner's topic. Moving.

[Paul Clapham]

Depends on what you mean by "user-defined".

If you count a URLClassLoader as a "user-defined" classloader, then you might do that to implement a plug-in architecture.

[Ulf Dittmer]

Plugins: http://www.javaranch.com/journal/200607/Journal200607.jsp#a1