用 Java 中的 UDP 广播

Question

I have been struggling to understand broadcasting with UDP and haven't been able to implement it in my program.

Currently, I have a Client & Server which both run 2 Threads (one for receiving, one for sending).

The Client can send SET/GET queries to a Server it is connected to, to store/get Key-Value pairs. The Idea is that, with multiple Servers, the Key we are looking for may be on another Server. Therefore the GET query should be broadcast to all other servers.

private void send(String value, int port) throws IOException {
    System.out.println("Send: " + value);
    byte[] buffer = (value).getBytes();
    DatagramPacket packet = new DatagramPacket(buffer, buffer.length, address, connectingPort);
    socket.send(packet);
}

This is currently the send function of the Sending Thread for the Client.

As InetAddress I use localhost.

The connectingPort = the Port of the Server that this client is connected to

public ClientSendingThread() throws UnknownHostException, SocketException {
    address = InetAddress.getByName("localhost");
    stdin = new BufferedReader(new InputStreamReader(System.in));
    //Registering yourself upon creation
    try {
        socket = new DatagramSocket();
        socket.setBroadcast(true);
        receiver = new ClientReceivingThread(socket);
        System.out.println("Enter port of the Server you would like to connect to: ");
        System.out.println("localhost is used automatically as address");
        connectingPort = Integer.parseInt(stdin.readLine());
        Thread receivingThread = new Thread(receiver);
        receivingThread.start();
        send("Client", connectingPort);
    } catch (IOException ex) {
        System.out.println(ex);
    }
}

That's the constructor of the ClientSendingThread, were I simply ask for User Input for the Port Number to which they want to connect to. Then I start the receiving Thread and send "Client" to the Server, such that it will "register" the Client.

My questions about broadcasting:

What Address do I even use? Which port?

Answer 1

What you want is technically called 'multicast', not 'broadcast'. It allows a server or servers to register a multicast address, to which clients can send messages.

Both client and server create a MulticastSocket . You'll need to pick a port number and a multicast address in the 224.xxx space; 224.0.0.1 is likely suitable, though it's been a while since I did this so I may be a bit rusty. You may also pick an 'unused' 224 address for your specific use.

The difference between multicast and broadcast is that broadcast is 'everyone', including random routers, webcams, etc., that really do not care about your application. Broadcast access tends to be a privileged operation. The broadcast address is all-ones in your network. So, if your network is 192.168.1.0/24, then the broadcast address is 192.168.1.255.