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.