Besides serving pages, controller's methods are able to return serialized objects as JSON, XML, base64 or serving any type of files using REST urls. However developers are free to develop their own serializers (XML, JSON, CSV, or whatever they need).
This project is opensource under Apache 2.0 license, hosted at Github & at Sourceforge. Also its artifacts can be found in Maven Central repository.
You can find more information about how to use it at http://serfj.sourceforge.net.