Problem with MulticastSocket on Java-Android

Question

I'm starting to code with MulticastSocket, trying to make a simple app with a client and a server to send messages.

The code I have for the server:

    import java.io.IOException;
    import java.net.DatagramPacket;
    import java.net.InetAddress;
    import java.net.MulticastSocket;
    import java.net.SocketException;


    public class Servidor {
 private static MulticastSocket ms;
 public static void main(String[] args) throws IOException{

  InetAddress sessAddr = InetAddress.getByName("224.2.76.24");
     try{
    sessAddr = InetAddress.getByName("224.2.76.24");
       ms = new MulticastSocket(5500);
       ms.joinGroup(sessAddr);

       while (true)
       {
       byte[] mensaje = new byte[1024];
       mensaje = "aa".getBytes();
       DatagramPacket dp = new DatagramPacket(mensaje, mensaje.length,sessAddr,5500);
       ms.send(dp);
       }
      }
      catch (SocketException se) {
        System.err.println(se);
      }

      ms.leaveGroup(sessAddr);

    }

}

And this on the client:

    package com.example;
    import java.io.IOException;
    import java.net.DatagramPacket;
    import java.net.InetAddress;
    import java.net.MulticastSocket;
    import java.net.UnknownHostException;

    import android.app.Activity;
    import android.os.Bundle;
    import android.widget.EditText;
    import android.widget.TextView;

    public class ClienteMultiCast extends Activity {
    /** Called when the activity is first created. */


    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        TextView Mensaje;
        Mensaje =(TextView)findViewById(R.id.Mensaje);


        InetAddress ia = null;
        byte[] buffer = new byte[65535];
        MulticastSocket ms = null;
        int port = 5500;
        try {
        ia = InetAddress.getByName("224.2.76.24");
        DatagramPacket dp = new DatagramPacket(buffer, buffer.length,ia,port);
        ms = new MulticastSocket(port);
            ms.joinGroup(ia);
            while (true) {
                ms.receive(dp);
                String s = new String(dp.getData(),0,dp.getLength());
                Mensaje.setText(s);
            }

            } catch (UnknownHostException e) {Mensaje.setText(e.getMessage());} catch (IOException e) {Mensaje.setText(e.getMessage()); }

            try {
            ms.leaveGroup(ia);
             } catch (IOException e) {
            Mensaje.setText(e.getMessage());
  }
    }
}

The problem is that when I start both, nothing happens. The client doesn't get any message.

Any idea what's wrong?

Answer 1

Diego,

By default, the Android WiFi stack filters out multicast packets. Take a look at http://developer.android.com/reference/android/net/wifi/WifiManager.MulticastLock.html.

You need something along the lines of:

  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    /* Turn off multicast filter */
    MulticastLock mcastLock = new MulticastLock();
    mcastLock.acquire();

    /* Process Multicast Packets */

  }

Answer 2

It appears that Multicast support in Android is not as solid as some of use might hope. See http://codeisland.org/2012/udp-multicast-on-android/

Ie whether it actually works out or may be device dependent. It is not working on my Nexus5.

https://code.google.com/p/android/issues/detail?id=51195