Datagram socket asks for destination address

Question

When send a data using Datagram socket connection it fires exception:

Destination address is null

DatagramSocket ds = new DatagramSocket(1050);
                ds.setBroadcast(true);
                InetAddress broadcastAdress = getBroadcastAdd();
                DatagramPacket pack = new DatagramPacket(data, data.length, broadcastAdress, 1050);
                ds.send(pack);
                ds.close();

Why does it say that if UDP means broadcasting, so the receiver is everyone, no particular address?

What address must be used and where should it be placed?

Well I was using that code to get that address:

private InetAddress getBroadcastAdd() throws UnknownHostException {
    WifiManager wifi = (WifiManager) context.getSystemService(Context.WIFI_SERVICE);
    DhcpInfo dhcp = wifi.getDhcpInfo();
    int broadcast = (dhcp.ipAddress & dhcp.netmask) | ~dhcp.netmask;
    byte[] quads = new byte[4];
    for (int k = 0; k < 4; k++)
        quads[k] = (byte) (broadcast >> (k * 8));
    return InetAddress.getByAddress(quads);
}

But I still have the null Exception:

UDP does *not* mean broadcasting, and even if you are broadcasting you still have to (a) set broadcast mode on the socket and (b) provide a destination broadcast address in the datagram. Unclear what you're asking. — user207421, Aug 03 '17 at 08:23
Possible duplicate of [Android Broadcast Address](https://stackoverflow.com/questions/2993874/android-broadcast-address) — Cédric Julien, Aug 03 '17 at 08:57
The image clearly shows that you are not getting a null exception, but rather a [`NetworkOnMainThreadException`](https://developer.android.com/reference/android/os/NetworkOnMainThreadException.html) exception: "*The exception that is thrown when an application attempts to perform a networking operation on its main thread.*" You can't perform network operations in the main thread, you need to use a worker thread or asynchronous task instead. — Remy Lebeau, Aug 03 '17 at 18:58

Cédric Julien · Accepted Answer · 2017-08-03T10:14:17.793

2

If you want to use UDP to broadcast a message, then you should use the setBroadcast on your socket, and use the broadcast address (an address is always needed, even in case of broadcasting).

DatagramSocket udpSocket = new DatagramSocket(1050);
udpSocket.setBroadcast(true);
InetAddress broadcastAdress = getAdressBroadcast();
DatagramPacket pack = new DatagramPacket(data, data.length, broadcastAdress, 1050);
udpSocket.send(pack);

With the getAddressBroadcast like this :

public static String getAdressBroadcast() throws SocketException {
    System.setProperty("java.net.preferIPv4Stack", "true");
    for (Enumeration<NetworkInterface> networkInterfaceEnum = NetworkInterface.getNetworkInterfaces(); networkInterfaceEnum.hasMoreElements();) {
        NetworkInterface networkInterface = networkInterfaceEnum.nextElement();
        if (!networkInterface.isLoopback()) {
            for (InterfaceAddress interfaceAddress : networkInterface.getInterfaceAddresses()) {
                return interfaceAddress.getBroadcast().toString().substring(1);
            }
        }
    }
    return null;
}

As a side note, do not forgot to do those action (networking operation) should not be done in the main activity thread, but in an Async task

edited Aug 03 '17 at 10:14

answered Aug 03 '17 at 08:03

Cédric Julien

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There is no 'otherwise' about it. An address is always needed, whether unicast, broadcast, or multicast. Your own code exhibits that. – user207421 Aug 03 '17 at 08:26
Hello.. what is getAddressBroadcast? – Aug 03 '17 at 08:46
would it be just InetAddress.getByName("0.0.0.0"); ? – Aug 03 '17 at 08:55
@AlisherBasmati : I've updated my answer, but check this question : https://stackoverflow.com/questions/2993874/android-broadcast-address my code was originally inspired by it. – Cédric Julien Aug 03 '17 at 08:58
@AlisherBasmati No. Can you possibly look up what a broadcast address actually is? Instead of all this guessing? – user207421 Aug 03 '17 at 08:59
ok, I also updated Q. why I used 0.0;0;0 because in .NET in would be just any address – Aug 03 '17 at 09:35
1

Cedric, if you add this reference into your answer I will mark it as Answer. Also, pls, write that the Networking Operation should be performed in Async way, not is the main Activity form. The reference: https://stackoverflow.com/questions/6343166/how-do-i-fix-android-os-networkonmainthreadexception – Aug 03 '17 at 09:54
1

@AlisherBasmati I've added a reference to Async task. – Cédric Julien Aug 03 '17 at 10:17

Datagram socket asks for destination address

What address must be used and where should it be placed?

1 Answers1