MessageListener

When an incoming message arrives, the notifyIncomingMessage() method is called. The application MUST retrieve the message using the receive() method of the MessageConnection. MessageListener should not call receive() directly. Instead, it can start a new thread which will receive the message or call another method of the application (which is outside of the listener) that will call receive(). For an example of how to use MessageListener, see A Sample MessageListener Implementation.

The listener mechanism allows applications to receive incoming messages without needing to have a thread blocked in the receive() method call.

If multiple messages arrive very closely together in time, the implementation has the option of calling this listener from multiple threads in parallel. Applications MUST be prepared to handle this and implement any necessary synchronization as part of the application code, while obeying the requirements set for the listener method.

A Sample MessageListener Implementation

The following sample code illustrates how lightweight and resource-friendly a MessageListener can be. In the sample, a separate thread is spawned to handle message reading. The MIDlet life cycle is respected by releasing connections and signalling threads to terminate when the MIDlet is paused or destroyed.

// Sample message listener program.
import java.io.IOException;
import javax.microedition.midlet.*;
import javax.microedition.io.*;
import javax.wireless.messaging.*;
public class Example extends MIDlet implements MessageListener {
MessageConnection messconn;
boolean done;
Reader reader;
// Initial tests setup and execution.
public void startApp() {
try {
// Get our receiving port connection.
messconn = (MessageConnection)
Connector.open("sms://:6222");
// Register a listener for inbound messages.
messconn.setMessageListener(this);
// Start a message-reading thread.
done = false;
reader = new Reader();
new Thread(reader).start();
} catch (IOException e) {
// Handle startup errors
}
}
// Asynchronous callback for inbound message.
public void notifyIncomingMessage(MessageConnection conn) {
if (conn == messconn) {
reader.handleMessage();
}
}
// Required MIDlet method - release the connection and
// signal the reader thread to terminate.
public void pauseApp() {
done = true;
try {
messconn.close();
} catch (IOException e) {
// Handle errors
}
}
// Required MIDlet method - shutdown.
// @param unconditional forced shutdown flag
public void destroyApp(boolean unconditional) {
done = true;
try {
messconn.setMessageListener(null);
messconn.close();
} catch (IOException e) {
// Handle shutdown errors.
}
}
// Isolate blocking I/O on a separate thread, so callback
// can return immediately.
class Reader implements Runnable {
private int pendingMessages = 0;

// The run method performs the actual message reading.
public void run() {
while (!done) {
synchronized(this) {
if (pendingMessages == 0) {
try {
wait();
} catch (Exception e) {
// Handle interruption
}
}
pendingMessages--;
}

// The benefit of the MessageListener is here.
// This thread could via similar triggers be
// handling other kind of events as well in
// addition to just receiving the messages.

try {
Message mess = messconn.receive();
} catch (IOException ioe) {
// Handle reading errors
}
}
}

public synchronized void handleMessage() {
pendingMessages++;
notify();
}

}
}