Re: About classloader

I have a couple question regarding classloader.

<quote>
Each class loader also keeps a reference to a parent class loader,
defining a tree of class loaders with the bootstrap loader at the root.
When an instance of a particular class (identified by name) is needed,
whichever class loader initially handles the request normally checks
with its parent class loader first before trying to load the class
directly. This applies recursively if there are multiple layers of
class loaders, so it means that a class will normally be visible not
only within the class loader that loaded it, but also to all descendant
class loaders. It also means that if a class can be loaded by more than
one class loader in a chain, the one furthest up the tree will be the
one that actually loads it.

There are many circumstances where multiple application classloaders
are used by Java programs. One example is within the J2EE framework.
Each J2EE application loaded by the framework needs to have a separate
class loader to prevent classes in one application from interfering
with other applications. The framework code itself will also use one or
more other class loaders, again to prevent interference to or from
applications. The complete set of class loaders make up a
tree-structured hierarchy with different types of classes loaded at
each level.
</quote>

1) Why multiple application classloaders is ever needed?
I thought the only thing needs to be done is just load the class, it
shouldn't matter which classloader loads it as the result matters not
the way how it is loaded.

2). When should I explicitly need to know about classloader? usually
the class is loaded for me by system. Could anybody give me some
example.

3). I saw some program that uses
ClassLoader cl = Thread.currentThread().getContextClassLoader();
and
ClassLoader.getSystemClassLoader().getResource(..)

what's the difference?