LISTEN/NOTIFY pgconnection goes down java?

Question

I am using PostgreSQL DB and applying it\'s LISTEN/NOTIFY functionality. So my listener is at my AS (Application Server) and I have triggers configured on my DB suc

Answer 1

The notification listeners are internally maintained by that library as weak references meaning that you have to hold a hard reference externally so they won't be garbage collected. Check out the BasicContext class lines 642 - 655:

public void addNotificationListener(String name, String channelNameFilter, NotificationListener listener) {

    name = nullToEmpty(name);
    channelNameFilter = channelNameFilter != null ? channelNameFilter : ".*";

    Pattern channelNameFilterPattern = Pattern.compile(channelNameFilter);

    NotificationKey key = new NotificationKey(name, channelNameFilterPattern);

    synchronized (notificationListeners) {
      notificationListeners.put(key, new WeakReference<NotificationListener>(listener));
    }

}

If the GC picks up your listener, calls to "get" on the weak reference will return null and will not fire as seen from lines 690 - 710

  @Override
  public synchronized void reportNotification(int processId, String channelName, String payload) {

    Iterator<Map.Entry<NotificationKey, WeakReference<NotificationListener>>> iter = notificationListeners.entrySet().iterator();
    while (iter.hasNext()) {

      Map.Entry<NotificationKey, WeakReference<NotificationListener>> entry = iter.next();

      NotificationListener listener = entry.getValue().get();
      if (listener == null) {

        iter.remove();
      }
      else if (entry.getKey().channelNameFilter.matcher(channelName).matches()) {

        listener.notification(processId, channelName, payload);
      }

    }

}

To fix this, add your notification listeners as such:

/// Do not let this reference go out of scope!
    PGNotificationListener listener = new PGNotificationListener() {

    @Override
    public void notification(int processId, String channelName, String payload) {
        // interesting code
    };
};
    pgConnection.addNotificationListener(listener);

Quite an odd use-case for weak references in my opinion...