I should add that my "90% of the way there" gets you a service that easily speaks JSON via REST calls. Its all you need if your frontend is backbone based or its just a service getting called by another layer. If you need to actually render web pages in response you'll need something for views/templating, freemarker is a reasonable option and JSF isn't terrible if you avoid half of it (looking at you ViewScoped). But, honestly, I would do the page rendering elsewhere, I don't think there is a good Java answer for this, maybe the Play Framework, haven't used it much yet.

In my case the Java backend exposes RESTful models via JSON which are consumed by a rails app using Her[0] a great gem that is what ActiveResource should have been. It wires up to Jersey quite easily.

[0] https://github.com/remiprev/her