Why are more class loaders needed?

Question

I know that hierarchy of class loaders in java is :

1.Bootstrap class loader (in native code)
2. Extension class loader (sun.misc.Launcher$ExtClassLoader)
3. System class loader (sun.misc.Launcher$AppClassLoader class)
4. Custom class loaders (i.e. app server, ear classloader, war classloader)

It is not clear for me why additional child class loaders are needed. I could understand the need for 'pure java' class loader after the one in native code.
I have some thoughts of possible reasons but can someone provide me with clear explanation for this behavior / need for hierarchies of class loaders?
In general but applicable for j2ee as well.

Answer 1

Classloaders are useful for providing a variety of behavior and for protecting different Java programs running in the same VM from each other. There are three major categories that come to mind:

• Classloaders can implement the process of loading classes differently. For example, you might have a classloader that retrieves the bytecode for a class from some server and then installs it into the JVM, or you might have a classloader that actually generates the bytecode on the fly instead of reading it out of a file.
• Classloaders can form partitions inside a single JVM so that different code "trees" can have different copies of classes with the same names. A common example of this is a servlet container like Tomcat, where several wars might be installed that all have different versions of a library (say, Apache Commons-Lang). OSGi also uses this approach to ensure that each bundle can only access the classes it's specifically requested and can't "leak" API.
• Classloaders can be used to implement different policies about when to garbage-collect actual classes themselves. Using just the default classloader keeps classes around indefinitely, leading to the dreaded PermGen errors; an application that used lots of temporary classes (maybe ones generated on the fly) might want to make sure that they were freed when no longer necessary by using disposable classloaders.