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 such that when CRUD operations are performed on a table a NOTIFY request is sent on AS.
LISTENER class in java:
@Singleton
@Startup
NotificationListenerInterface.class)
public class NotificationListener extends Thread implements NotificationListenerInterface {
@Resource(mappedName="java:/RESOURCES")
private DataSource ds;
@PersistenceContext(unitName = "one")
EntityManager em;
Logger logger = Logger.getLogger(NotificationListener.class);
private Connection Conn;
private PGConnection pgConnection = null;
private NotifyRequest notifyRequest = null;
@PostConstruct
public void notificationListener() throws Throwable {
System.out.println("Notification****************");
try
{
Class.forName("com.impossibl.postgres.jdbc.PGDriver");
String url = "jdbc:pgsql://192.xx.xx.126:5432/postgres";
Conn = DriverManager.getConnection(url,"postgres","password");
this.pgConnection = (PGConnection) Conn;
System.out.println("PG CONNECTON: "+ pgConnection);
Statement listenStatement = Conn.createStatement();
listenStatement.execute("LISTEN notify_channel");
listenStatement.close();
pgConnection.addNotificationListener(new PGNotificationListener() {
@Override
public void notification(int processId, String channelName, String payload){
System.out.println("*********INSIDE NOTIFICATION*************");
System.out.println("Payload: " + jsonPayload);
}
So as my AS is up, I have configured that at startup the listener class is called (@Startup annotation) and it's start listening on the channel.
Now this works fine if like say for testing I edit my table in DB manually, the notification is generated and the LISTENER receives it.
However, when I programmatically send a UPDATE request on the table, the UPADTE is performed successfully but LISTENER is not receiving anything.
I feel my connection of the LISTENER goes down when I send a request (it also makes a connection to edit entities), but I am not sure. I read about permanent connections and pooled connections, but not able to decide how to pursue that.
I am using pgjdbc (http://impossibl.github.io/pgjdbc-ng/) jar for async notifications as jdbc connection requires polling.
EDIT:
When I try the above listener with polling by using the standard jdbc jar (not pgjdbc), I get the notifications.
I do
PGNotification notif[] = con.getNotifications()
and I get notifications, however doing it asynchronously like below I don't get notifications.
pgConnection.addNotificationListener(new PGNotificationListener() {
@Override
public void notification(int processId, String channelName, String payload){
System.out.println("*********INSIDE NOTIFICATION*************");
}
SOLVED:
My listener was going out of scope after the function execution was completed as my listener had the function scope. So kept it into a member variable of my startup bean class and then it worked.
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...
来源：https://stackoverflow.com/questions/37916489/listen-notify-pgconnection-goes-down-java