java.lang.RuntimeException: Uncompilable source code - what can cause this?

I guess you are using an IDE (like Netbeans) which allows you to run the code even if certain classes are not compilable. During the application's runtime, if you access this class it would lead to this exception.

If it is Netbeans, try to uncheck "Compile on save" setting in the project properties (Build -> Compiling). This is the only thing which helped me in a similar situation.