Checkout this project in my playground that shows how to achieve interplay between a SPA application and a REST backend using the RESTEasy JAX-RS implementation and deployed in the JBoss EAP 6.4.0 server.

Also, this particular application (further into the above project) shows how to achieve JAX-RS deployment with the RESTEasy implementation using either the "scanner" or the "application" approach.

In the context of this travail, I posted this SO question and drew heavily (but not without changes) from this blog post.

I've tested the below with Spring Web MVC 4.3.5:

    import org.springframework.web.bind.annotation.RequestMapping;
    import org.springframework.web.bind.annotation.RequestParam;
    import org.springframework.web.bind.annotation.RequestMethod;
    import org.springframework.web.bind.annotation.ResponseBody;

    import org.springframework.http.ResponseEntity;
    import org.springframework.http.MediaType;
    import org.springframework.http.HttpHeaders;
    import org.springframework.http.HttpStatus;

    @RequestMapping(path="/example-of-method-1-of-generating-a-JSON-response", method=RequestMethod.GET)
    public ResponseEntity fooBar(final HttpServletRequest request
                         , @RequestParam(value="json", required=true) String json) {
        final HttpHeaders httpHeaders= new HttpHeaders();
        httpHeaders.setContentType(MediaType.APPLICATION_JSON);
        return new ResponseEntity(GsonHelper.toJson(new FooBar(json)), httpHeaders, HttpStatus.OK);
    }

    @ResponseBody
    @RequestMapping(path="/example-of-method-2-of-generating-a-JSON-response", method=RequestMethod.GET)
    public String fooBar2(final HttpServletRequest request
                         , @RequestParam(value="json", required=true) String json) {
        return GsonHelper.toJson(new FooBar(json));
    }

    ...
    class FooBar {
        public String echo;
        public FooBar(String echo) {
            this.echo = echo;
        }
    }