Internal Java Language? (Language ran within java)

I have recently started a large project that requires a fair share of dynamic code saving, as it needs to develop with new methods on it's own.

My only problem is that I haven't a clue on how I can achieve this. I was considering writing a simple language to fit the basic needs of my project, but for the sake of development time, I wish to have this done at the utmost speed, in which case I need a library for a different language within Java (I am currently storing the code within a master, transferable XML file.

Any help would be appreciated.