It is GWT, so it is transpiled to JS. I remember when I first saw this project years ago, I was impressed. Back then GWT was the best choice for statically-typed, large JS project development IMO.
We just chopped GWT out of our project a month or so ago; I'll fully admit that I didn't know much about it, but every step of the way I felt like I was battling it - it ended up being easier, faster and more testable to write our own JS for the front-end.
It's a Google App Engine-based Java monster:
https://github.com/lewish/asciiflow