Server-Client in UDP

Question

My Datagram simple client server program is giving error.Client takes information from server and server also gets information from client.

Here is my Server:

package udp;

import java.net.*;

public class DatagramServer implements Runnable {

DatagramPacket pack;
DatagramSocket sock;

DatagramServer() {
    new Thread(this).start();
}

public void run() {
    try {
        while (true) {
            recieve();
            send();
        }
    } catch (Exception e) {
        System.out.println(e);
    }
}

public void send() throws Exception {
    byte data[] = "This is a \n datagram packet".getBytes();
    InetAddress add = InetAddress.getByName("localhost");
    pack = new DatagramPacket(data, data.length, add, 8000);
    sock = new DatagramSocket();
    sock.send(pack);
    sock.close();
}

public void recieve() throws Exception {

    byte[] buffer = new byte[65536];
    DatagramPacket incoming = new DatagramPacket(buffer, buffer.length);
    sock = new DatagramSocket(8000);
    sock.receive(incoming);
    byte[] data = incoming.getData();
    String s = new String(data, 0, incoming.getLength());
}

public static void main(String[] args) {
    new DatagramServer();
}
}

And here is my Client :

package udp;

import java.net.*;

public class DatagramClient {

DatagramPacket pack;
DatagramSocket sock;

DatagramClient() {
    Thread t1 = new Thread() {
        public void run() {
            while (true) {
                try {
                    receive();
                    Thread.sleep(5000);
                } catch (Exception e) {
                    System.out.println(e);
                }
            }
        }
    };
    t1.start();
    Thread t2 = new Thread() {
        public void run() {
            while (true) {
                try {
                    send();
                    Thread.sleep(5000);
                } catch (Exception e) {
                    System.out.println(e);
                }
            }
        }
    };
    t2.start();
}

public void receive() throws Exception {
    byte data[] = new byte[1000];
    pack = new DatagramPacket(data, data.length);
    sock = new DatagramSocket(8000);
    sock.receive(pack);
    System.out.println("Data::" + new String(pack.getData(), "UTF-8"));
    System.out.println("Port::" + pack.getPort());
    sock.close();
}

public void send() throws Exception {

    String s = "KOLI SDF DFEF XFFFSS";
    byte[] b = new byte[1000];
    b = s.getBytes();
    DatagramPacket dp = new DatagramPacket(b, b.length, InetAddress.getByName("localhost"), 8000);
    sock= new DatagramSocket(8000);
    sock.send(dp);
}

public static void main(String[] args) {
    new DatagramClient();
}
}

I'm getting errors like:

java.net.BindException: Address already in use: Cannot bind
java.net.BindException: Address already in use: Cannot bind
java.net.BindException: Address already in use: Cannot bind
java.net.BindException: Address already in use: Cannot bindBUILD STOPPED  (total time: 4 seconds)

What should i do?

Answer 1

Your client and server has some issues such as open same port 8000 in different threads so I have fixed those within the following code.

Client

package udp;

import java.net.*;

public class DatagramClient {

    DatagramPacket pack;
    DatagramSocket sock;

    DatagramClient() {
        Thread t1 = new Thread() {
            public void run() {
                while (true) {
                    try {
                        receive();
                        Thread.sleep(5000);
                    } catch (Exception e) {
                        System.out.println(e);
                    }
                }
            }
        };
        t1.start();
    }

    public void receive() throws Exception {
        byte data[] = new byte[1000];
        pack = new DatagramPacket(data, data.length);
        sock = new DatagramSocket(8000);
        sock.receive(pack);
        System.out.println("Data::" + new String(pack.getData(), "UTF-8"));
        System.out.println("Port::" + pack.getPort());
        sock.close();
    }

    public void send() throws Exception {

        String s = "KOLI SDF DFEF XFFFSS";
        byte[] b = new byte[1000];
        b = s.getBytes();
        DatagramPacket dp = new DatagramPacket(b, b.length, InetAddress.getByName("localhost"), 8000);
        sock= new DatagramSocket(8000);
        sock.send(dp);
    }

    public static void main(String[] args) {
        new DatagramClient();
    }
}

Server

package udp;

import java.net.*;

public class DatagramServer implements Runnable {

    DatagramPacket pack;
    DatagramSocket sock;

    DatagramServer() {
        new Thread(this).start();
    }

    public void run() {
        try {
            while (true) {
                //recieve();
                send();
            }
        } catch (Exception e) {
            System.out.println(e);
        }
    }

    public void send() throws Exception {
        byte data[] = "This is a \n datagram packet".getBytes();
        InetAddress add = InetAddress.getByName("localhost");
        pack = new DatagramPacket(data, data.length, add, 8000);
        sock = new DatagramSocket();
        sock.send(pack);
        sock.close();
    }

    public void recieve() throws Exception {

        byte[] buffer = new byte[65536];
        DatagramPacket incoming = new DatagramPacket(buffer, buffer.length);
        sock = new DatagramSocket(8000);
        sock.receive(incoming);
        byte[] data = incoming.getData();
        String s = new String(data, 0, incoming.getLength());
    }

    public static void main(String[] args) {
        new DatagramServer();
    }
}

Answer 2

The problem is your server code is trying to bind to the 8000 port each time receive () called. And receive is being called repeatedly from within the while(true) loop. So it tries to bind to the same port again and again.

To fix it remove the UDP socket instantiation i.e

sock = new DatagramSocket(8000);

From the receive method and put it before your while loop begins.