while (true) {
        Statement stmt = conn.createStatement();
        ResultSet rs = stmt.executeQuery("SELECT 1");
        rs.close();
        stmt.close();
        org.postgresql.PGNotification notifications[] = pgconn.getNotifications();
        handleNotifications(notifications);
        Thread.sleep(100);
    }

The problem with this approach[1] is there's a 100ms of latency between polling attempts (the sleep call).

LISTEN/NOTIFY is very useful, particularly the transactional nature and not requiring any additional tech stack (e.g. no need for a separate MQ server), but for frequent/high-value/lower latency signalling you're better off using something that doesn't require sleeping/polling.

[1]: That and the lack of exception handling or resource cleanup.