Naively, it seems like a bit of overkill. You don't need a full stack web app framework to create REST endpoints, right?
If you're truly building a one-page style app, Rails is, as you say, probably not your first choice. Sinatra maybe?
Rails scaffolding creates RESTful resources by default (and Ember.js plays nicely with it too). Starting from a "blank slate" Ubuntu server install, you could have a dev server providing this groundwork in about 7 commands in under 5 minutes. If you were to do this in J2EE (nothing is stopping you), setting up the backend would take significantly longer than the frontend ideas being explained.