Here’s a little sort of template toy project regarding contemporary JVM-based REST APIs powering HTML5/JS SPAs:
https://github.com/robi42/boot-rest

Built with JDK 8, Spring Boot, Elasticsearch, Jersey 2, Gradle, Yeoman, Bootstrap, AngularJS, fun, and all that jazz…

PS: look ma’, no XML! :)

Update 2015: now with ES6 a.k.a. ES 2015 via JSPM/SystemJS…

Update 2016: these days, “isomorphic” Redux/React is all the rage on the FE side of things…
One may want to check out Este.js & co.
On the BE, Spring Boot’s playing nicely with Kotlin, and Spring MVC’s favourable to JAX-RS, IMHO.