In trying to learn about Java class loaders from Wikipedia, I think I can see why they have the three major class loaders:
1) Bootstrap class loader
2) Extensions class loader
3) System class loader
They go on to say you can define your own classloader. I'm not sure I see the value in defining your own, but the following quote from Wikipedia really makes me wonder:
The most complex JAR hell problems arise in circumstances that take advantage of the full complexity of the classloading system. A Java program is not required to use only a single "flat" classloader, but instead may be composed of several (potentially very many) nested, cooperating classloaders. Classes loaded by different classloaders may interact in complex ways not fully comprehended by a developer, leading to errors or bugs that are difficult to analyze, explain, and resolve.
If it's so complex, why bother with it? Shouldn't the three already-defined classloaders be enough?
(And yes, for those curious, I did run into a ClassCastException that I didn't think should have happened, much like the graphic labelled Figure 2. Class identity crisis. I'm trying to understand the background is all.)
Certain use cases require custom classloaders. A few examples: