LISTEN/NOTIFY pgconnection关闭java？

Question

我正在使用PostgreSQL DB并应用它的LISTEN/NOTIFY功能.所以我的监听器在我的AS(应用服务器)上,并且我在我的数据库上配置了触发器,这样当在表上执行CRUD操作时,NOTIFY在AS上发送请求.

java中的LISTENER类:

        @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);

}

因此,当我的AS启动时,我已经配置了在启动时调用侦听器类(@Startup annotation)并且它开始在通道上侦听.

现在这样可以正常工作,如果说测试我手动编辑DB中的表,生成通知并且LISTENER接收它.

但是,当我以编程方式在表上发送UPDATE请求时,UPADTE成功执行但LISTENER没有收到任何内容.

当我发送请求时,我觉得我的LISTENER连接断开了(它也与编辑实体建立了连接),但我不确定.我读到了永久连接和汇集连接,但无法决定如何实现这一点.

我正在使用pgjdbc(http://impossibl.github.io/pgjdbc-ng/)jar作为异步通知,因为jdbc连接需要轮询.

编辑:

当我使用标准的jdbc jar(而不是pgjdbc)通过轮询尝试上面的监听器时,我收到通知.

我这样做 PGNotification notif[] = con.getNotifications() ,我收到通知,但是如下所示异步,我没有收到通知.

    pgConnection.addNotificationListener(new PGNotificationListener() {

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

            System.out.println("*********INSIDE NOTIFICATION*************");
         }

解决了:

我的听众是走出去的范围在函数执行完毕后,我的听众有功能范围.所以将它保存到我的启动bean类的成员变量中然后它工作.

Answer 1

通知侦听器由该库在内部维护为弱引用,这意味着您必须在外部持有硬引用,以便它们不会被垃圾回收.查看BasicContext类的第642 - 655行:

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));
    }

}

如果GC选择了你的监听器,那么对弱引用的"get"调用将返回null并且不会从第690 - 710行看到

  @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);
      }

    }

}

要解决此问题,请添加通知侦听器:

/// 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);

在我看来,弱引用的用例相当奇怪......