EDIT: removed reference to C# as the only accepted answer is about Java. If someone needs information about websocket server implementation in C#, ask a new
Take a look at the Bristleback Framework. It is a high level overlay for commonly used Java Websocket Servers, like Jetty, Netty or Tomcat. If you like Spring Framework, you must definitely try Bristleback!
Disclaimer: I'm a contributor in Bristleback Framework project.