Sylk is a 2D game engine written in Java and is currently in development. With goals of being able to write a good looking and optimized game in Sylk, it is constantly being worked on a iterated upon.
You can use Sylk to write your own games and deploy them as a bundled executable jar file for others to play and can be integrated within your maven or gradle project. A more indepth wiki about Sylk and how to use it can be found here.