Question

I am trying to create a UDP listener that will listen on a separate thread. It works fine the first time but when I stop the connection and then start it again it gives me errors.

listenerRunnable = new Runnable() {
        public void run() {
            //This thread will listen keep listening to the UDP traffic and put it to the log source string
            try {
                sock = new DatagramSocket(portNumber);
            } catch (SocketException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
            }
            while(keepListening) {
                try {
                    pack = new DatagramPacket(recievedData, BUFFERSIZE);
                    sock.receive(pack);

                    String data = new String(pack.getData(), 0, pack.getLength());
                    addToLog(data);
                    System.out.println(data);
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                sock.close();
            }
        }
    };

/**
 * Function to start the listening thread.
 */
public void startListening(int portNum) {
    keepListening = true;
    portNumber = portNum;

    listenerThread = new Thread(listenerRunnable);

    logSource_buffer = "";
    logSourcehtml_buffer = "";
    logSourcehtml_temp = "";
    ipListIndex_beg = 0;
    ipListIndex_end = -1;

    if(!listenerThread.isAlive()) {
        listenerThread.start();
    }
}

/**
 * stops the listening thead.  When the listening thread sees that keepListening is set to false
 * it will reach the end of its loop, close the socket, and the thread will die.
 */
public void stopListening() {
    keepListening = false;
}

It gives me the following error:

logUpdatingThread has entered synchronized block!!!
java.net.SocketException: Unrecognized Windows Sockets error: 0: Cannot bind
at java.net.PlainDatagramSocketImpl.bind0(Native Method)
at java.net.PlainDatagramSocketImpl.bind(Unknown Source)

which points to the line with sock.recieve(pack);
It seems like for some reason the socket isn’t closing because, I think, its waiting at sock.recieve(pack) and never gets out of the while loop to close the socket. How would I get around this though?

Thanks

Answer 1

You’ll have to add a setSoTimeout(timeout) before calling receive. This will regularly throw SocketTimeoutExceptions, but keeping the Datagram socket open. This will allow you to regularly check the loop variable.

Additionally you should move the loop inside the first try-catch block and add a finally block to close the socket.

like :

        try {
            sock = new DatagramSocket(portNumber);
            sock.setSoTimeout(250);
            while(keepListening) {
                try {
                    pack = new DatagramPacket(recievedData, BUFFERSIZE);
                    sock.receive(pack);

                    String data = new String(pack.getData(), 0, pack.getLength());
                    addToLog(data);
                    System.out.println(data);
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        } catch (SocketException e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
        } finally {
            sock.close();
        }