The position of things like Netty and Undertow at the top of the techempower benchmarks has me a little intrigued. How much hassle is it to write websites with servlets like that?
A bit of a hassle, but Comsat will make it dead-easy (right now Comsat runs on top of any servlet container, but it will also run directly on top of Netty/Undertow). Take a look at our Comsat web actors: http://blog.paralleluniverse.co/2014/01/28/web-actors-1/
Undertow provides a Servlet container, so if you are already using a Servlet-based framework, the level of effort is fairly minimal. Though, Resin remains a very high-performance Servlet container as well in our benchmarks and if you need a broad set of Servlet features, Resin may be a better fit.
Another option is WildFly, which is the new JBoss application server that uses Undertow below the hood.
We are presently adapting our in-house framework to use Undertow natively (without the Servlet layer) and the effort to do so has been low, all things considered. If you're comfortably working with the internals of your framework, you may be able to do the same without a great deal of pain.
I've had a great experience building web APIs (i.e. returning JSON rather than HTML) in Scala with Spray; it requires you to think a bit more about when things happen (because everything is async), but it's lovely and smooth to use (there's this routing configuration DSL that looks almost like a config file, but then you realize it's all Scala so you can use things like for/yield with it).
In terms of actual websites I understand Play is migrating to run on top of Spray, but I'm not aware of any mature frameworks for doing it right now.
