Listen / Notify
Listen and Notify provide a simple signal or interprocess communication mechanism for a collection of processes accessing the same PostgreSQL® database. For more information on notifications, consult the main server documentation. This section only deals with the JDBC-specific aspects of notifications.
Standard LISTEN, NOTIFY, and UNLISTEN commands are issued via the standard Statement interface. To retrieve and process notifications, the Connection must be unwrapped as the PostgreSQL-specific extension interface PGConnection. From there the getNotifications() method can be used to retrieve any outstanding notifications.
NOTE
pgJDBC has no background reader thread, so client code must call
getNotifications()itself to drain the notifications the server has pushed onto the socket. The no-argument form returns immediately with whatever is buffered;getNotifications(timeoutMillis)blocks up totimeoutMillis(or forever whentimeoutMillis == 0) until at least one notification arrives. While that call is blocking, other threads cannot issue statements on the same connection; other connections are unaffected.
Example: receiving notifications
import java.sql.*;
public class NotificationTest {
public static void main(String args[]) throws Exception {
String url = "jdbc:postgresql://localhost:5432/test";
// Create two distinct connections, one for the notifier
// and another for the listener to show the communication
// works across connections although this example would
// work fine with just one connection.
Connection lConn = DriverManager.getConnection(url, "test", "");
Connection nConn = DriverManager.getConnection(url, "test", "");
// Create two threads, one to issue notifications and
// the other to receive them.
Listener listener = new Listener(lConn);
Notifier notifier = new Notifier(nConn);
listener.start();
notifier.start();
}
}
class Listener extends Thread {
private Connection conn;
private org.postgresql.PGConnection pgconn;
Listener(Connection conn) throws SQLException {
this.conn = conn;
this.pgconn = conn.unwrap(org.postgresql.PGConnection.class);
Statement stmt = conn.createStatement();
stmt.execute("LISTEN mymessage");
stmt.close();
}
public void run() {
try {
while (true) {
org.postgresql.PGNotification notifications[] = pgconn.getNotifications();
// If this thread is the only one that uses the connection, pass a timeout
// to wake up as soon as a notification arrives (up to the timeout):
// org.postgresql.PGNotification notifications[] = pgconn.getNotifications(10000);
for (int i = 0; i < notifications.length; i++) {
// PGNotification also exposes getParameter() (the payload from NOTIFY channel,
// 'payload', supported since PG 9.0) and getPID() of the sending backend.
System.out.println("Got notification: " + notifications[i].getName());
}
// wait a while before checking again for new
// notifications
Thread.sleep(500);
}
} catch (SQLException sqle) {
sqle.printStackTrace();
} catch (InterruptedException ie) {
ie.printStackTrace();
}
}
}
class Notifier extends Thread {
private Connection conn;
public Notifier(Connection conn) {
this.conn = conn;
}
public void run() {
while (true) {
try {
Statement stmt = conn.createStatement();
stmt.execute("NOTIFY mymessage");
stmt.close();
Thread.sleep(2000);
} catch (SQLException sqle) {
sqle.printStackTrace();
} catch (InterruptedException ie) {
ie.printStackTrace();
}
}
}
}