code: protected void flushListeners (long now)
      {
          if (_listeners.size() > 0) {
              long then = now - _listenerMaxAge;
              Iterator<ListenerMarshaller> iter = _listeners.values().iterator();
              while (iter.hasNext()) {
                  ListenerMarshaller lm = iter.next();
                  if (then > lm.mapStamp) {
  //                     Log.info("Flushing marshaller " + lm + ".");
                      iter.remove();
                  }
              }
          }
      }
docs: Flushes listener mappings that are older than {@link #_listenerMaxAge} milliseconds. An
  alternative to flushing listeners that did not explicitly receive a response within our
  expiry time period is to have the server's proxy listener send a message to the client when
  it is finalized. We then know that no server entity will subsequently use that proxy
  listener to send a response to the client. This involves more network traffic and complexity
  than seems necessary and if a user of the system does respond after their listener has been
  flushed, an informative warning will be logged. (Famous last words.)