code: public final Result json(Object obj) {
          final Result response = ok().contentType(MediaType.APPLICATION_JSON).renderable(obj);
          holder.setControllerResult(response);
          return response;
      }
docs: This method will send a JSON response to the client.
  It will not use any layouts.
  Use it to build app.services and to support AJAX.
  
  @param obj
  @return {@link Result}, to accept additional information. The response is automatically
  has its content type set to "application/json"