If you plan on doing anything in the open social world you really need to start learning Guice. All of the modules are written as Guice modules. This is a dependency injection framework that uses Aspect like programming techniques to create more modular code.
The presentation below is an excellent primer to better understand what dependency injection is and how Guice fits into Java. It’s now funny that the presenters tout Google Wave (may it rest in peace), but the technique, compiler and overall presentation is very good.