adam bien's blog

Wrapping a list of instances with a Response:

    @GET
    public Response workshops() {
        List<Workshop> workshops = ...//a list of entities
        return Response.ok(workshops).build();
    }

Leads to a type loss carried by the Collection and the following (or similar) exception:

MessageBodyWriter not found for media type=application/json, type=class java.util.Arrays$ArrayList, 
genericType=class java.util.Arrays$ArrayList

JAX-RS comes with GenericEntity which carries the generic type. You only have to wrap the Collection with the GenericEntity to solve the problem:

import javax.ws.rs.core.GenericEntity;
//...
  @GET
    public Response workshops() {
        List<Workshop> workshops = ...//a list of entities
        GenericEntity<List<Workshop>> l
        ist = new GenericEntity<List<Workshop>>(workshops) {
        };
        return Response.ok(list).build();
    }

See you at Java EE Workshops at Munich Airport, particularly at: Effective Java EE 7!