This API is not as smart as JAX-RS or Scalatra. Spark code contains much clutter and redundancy (repeated paths). Frameworks like this would profit from Java 8 lamdas.
JAX-RS is just not fun! That's at least my opinion. I just took a brief look to spark-java but it looks more fun than JAX-RS, because it looks very simple and straight forward. That's at least my first impression.
The code looked well commented and readable to me. Scalatra seems to be for Scala and my eyes don't get along with Scala syntax.