code: private <R> void notifyHandlers() {
          // the response may or may not exist at this point
          response.forEach(t -> eventHandlers.stream().forEach(h -> h.notify(t)));
      }
docs: Invoke all handlers with the value of this Future. <br>
  The response may or may not exist at this point. <br>
  A filter is applied to make sure we only notify handlers that have not been notified before.