I don't know why this was downvoted. Spark was actually one of the alternatives I considered before I concluded that I couldn't find any framework that suits me, and started writing grumpyrest.

The main issue I had with spark was, from what I could see, the lack of good JSON support. A major part of grumpyrest is its JSON serialization/deserialization framework (it's roughly half of the whole codebase!) which applies the same principles as grumpyrest does for REST, to JSON. (I even called it grumpyjson in anticipation that I might one day break this out as a standalone project).

Now don't get me wrong, I consider Jackson a high-quality framework, and I like very much how its author takes care of even the smallest details. It's not at all like Spring. However, in making it work as I wanted there were too many things that I could not solve to my satisfaction -- I could not abstract them in a way that was truly re-usable, and every API method would have to deal with these things again. This was even more true for Gson. So in the end, I used them (Gson, to be precise) as a low-level JSON library that basically translates between a JSON AST and serialized JSON, and did the high-level mapping to application classes myself.