address already in use error in java on udp server

Question

I have an application that does many things. I'd like to add a method that enables this application to behave like a udp server on a socket. In particular it listens on the localhost address on port 8888. I tried implementing this behaviour with the following code but i get an Address already in use error. Plus the whole application is stuck on this udp server method. I guess its probably due to the fact that its all running on one thread.

1) Can you please show me how to correct my method. In particular how to make this udp server listener start on a new thread.

2) This server will listen forever for pks from the client. Depending on wether the server receveid a specific packet or not it needs to do certain things. Is the logic correct: if the packetReceived is != null and the packet has not been processed then process it. repeat forever (as show in the code)?

public void startSocketListening(){
        byte[] receiveData = new byte[1024];
        DatagramPacket receivePacket;
        System.out.println("Waiting to receive...");
        while (true) {
            receivePacket = new DatagramPacket(receiveData, receiveData.length);
            try {
                DatagramSocket serverSocket = new DatagramSocket(8888);
                serverSocket.receive(receivePacket);
                //if i receive a packet and it doesn't already have a flow rule process it
                if ((receivePacket != null) && (newOFRuleAdded == false)){
                    this.rlocAddress = new String(receivePacket.getData());
                    System.out.println("RECEIVED: " + rlocAddress);
                    System.out.println("RLOC: " + rlocAddress);
                    //process the message
                    newOFRuleAdded = true;
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

It is impossible for receivePacket to have become null at the point you are testing for that, by the rules of Java. — user207421

Sorcen Sorcen · Accepted Answer · 2016-01-29T15:57:49

For 1.) First move the socket creation (serverSocket = new DatagramSocket(8888);) out of the loop. That is where your bind failed errors come from.

Best put your whole startSocketListening() code in the run() method of a class extending Thread or implementing Runnable (see code below for threadexample)

For 2.) there might be a misunderstanding. "serverSocket.receive(receivePacket);" will wait till a packet is received. So whatever comes later on is for the newly received packet that has never been processed. The while loop just gets you back to waiting for a new packet.

Something like this might do. If you are unsure debug it step by step how it behaves when a packet is received.

public class UdpListener extends Thread
{
    public void run()
    {
        byte[] receiveData = new byte[1024];
        DatagramPacket receivePacket;
        System.out.println("Waiting to receive...");
        DatagramSocket serverSocket;
        try {
            serverSocket = new DatagramSocket(8888);    
            while (true) {
                receivePacket = new DatagramPacket(receiveData, receiveData.length);
                serverSocket.receive(receivePacket);
                //if i receive a packet and it doesn't already have a flow rule process it
                if ((receivePacket != null) ){
                    System.out.println("First byte of received package is: " + receivePacket.getData()[0]);
                }
            }
        } catch (SocketException e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

Called like this:

Thread t = new UdpListener();
t.start();

address already in use error in java on udp server

1 Answers