code: public void handler(final Request req, final Handler handler) throws Exception {
      this.handler = requireNonNull(handler, "Handler is required.");
      if (initializer != null) {
        initializer.run(req, this);
      }
    }
docs: Setup a handler for this deferred. Application code should never call this method: INTERNAL USE
  ONLY.
  
  @param req Current request.
  @param handler A response handler.
  @throws Exception If initializer fails to start.