If you are interested in code generation, model driven architecture or in metaprogramming generally, have a look at
Eclipse EMF.

To put it simply, you can create an entire Java GUI application (okay – an Eclipse Plugin) by defining your model in
Java classes
(or it can import a Rational Rose model
or use the
Omondo UML plugin for Eclipse).

I found the existing functionality amazing: Following the tutorial
it took me 30 minutes to have a fully functioning Eclipse plugin that allowed data editing and persistance to XML. The only
code that I needed to write was some interfaces that defined how my data interacted using the JavaBeans model, and everything else
was generated.

The crazy thing is that this application generation I'm raving about is really just a sample for the tutorial – developers
are supposed to use EMF as a tool to generate code however they like. Anyway, if I was considering writing a XDoclet plugin
or something similar right now I would look very hard at using EMF instead.