Java MulticastSocket causing null pointer exception

Question

I'm trying to get a multicast socket working on a 'server' app, which will spit info at a bunch of android phones. The code snippet responsible for setting up the socket and sending a some data is as follows:

    private MulticastSocket multisocket;
    private DatagramPacket packet;
    private InetAddress addr;
    private Question question;

    byte[] buffer = "Some text to be sent".getBytes();
    packet = new DatagramPacket(buffer, buffer.length);

    try {
        addr = InetAddress.getByName("228.5.6.7");
        multisocket = new MulticastSocket(4446);
        multisocket.joinGroup(addr);

    } catch (IOException e) {
        e.printStackTrace();
    }
    try {
        System.out.println("Sending...");
        multisocket.send(packet); // This is the line it dies on...     
        System.out.println("Text sent, closing socket");
        multisocket.close();

    } catch (IOException e) {
        e.printStackTrace();
    }

What happens is that it gets to the multisocket.send(packet); line and dies with the following stack trace info:

Exception in thread "SendThread" java.lang.NullPointerException: null address || null buffer
    at java.net.PlainDatagramSocketImpl.send(Native Method)
    at java.net.DatagramSocket.send(Unknown Source)
    at Model.QuestionSendThread.run(CommServer.java:158)

...and I am at a loss as to why.

One question I do have (and excuses the n00bishness of it) - Is the IP address you set in the multicast socket just a placeholder, or do you really have to have your IP address set to that? Half of me thinks you probably don't - the other half is screaming the opposite - but I can't find anything to confirm that when googling for an answer - just that it has to be an address in a fairly narrow range. If I've got this wrong (my IP is 192.168.1.3), then is that the problem? Or is it something else.

Thanks in advance

Steve

Answer 1

MulticastSocket and its joinGroup() are about receiving multicast packets. Sending multicast packets is the same as sending regular UDP packets to the address of group, therefore you need to set destination address and port for each packet:

packet = new DatagramPacket(buffer, buffer.length, group, 4446); 
...
multisocket.send(packet);

See also:

• MulticastSocket