Question

**Here is a simple code of chat between two peers. According to me, the code does what it should have but I am facing difficulty solving this SocketException error.
This code worked fine till i made the last modifications. And now I am finding it difficult to trace the error.

import java.awt.BorderLayout;
import java.awt.FlowLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.*;
import java.util.*;
import java.net.*;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextArea;
import javax.swing.JTextField;

public class UDPchat extends Thread implements ActionListener
{
    JFrame f;
    JPanel p;
    JTextField text;
    JButton b;
    JTextArea ta;
    private final static String newline = "\n";
    private final static BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

    int porttor=7777; // port to send/receive datagrams on
    //int porttos=5555;
    String remoteIPaddress= ("127.0.0.1");      // IP to send datagrams

    public UDPchat() throws Exception
    {
        start();      // start thread to receive and display datagrams
        f=new JFrame("CHAT WINDOW");
        p= new JPanel();
        text = new JTextField();
        text.setColumns(25);

        b = new JButton("SEND");

        b.addActionListener(this);

        ta = new JTextArea("Chat messages",20,50);

        ta.setEditable(false);

        f.setLayout(new FlowLayout());

        p.add(text,BorderLayout.NORTH);

        p.add(b,BorderLayout.SOUTH);

        f.setLayout(new BorderLayout());

        f.add(ta,BorderLayout.NORTH);

        f.add(p,BorderLayout.SOUTH);

        f.setSize(400, 400);

        f.setVisible(true);
    }

    @Override

    public void actionPerformed(ActionEvent e)
    {

        try
        {

            String s = text.getText();  // read a String

            text.setText(" ");

            ta.append(newline);

            ta.append(s);

            //System.out.println("Sending to " + remoteIPaddress + " socket " + port + " data: " + s);

            byte[] data = s.getBytes();  // convert to byte array

            DatagramSocket theSocket = new DatagramSocket();  // create datagram socket and the datagram

            DatagramPacket   theOutput = new DatagramPacket(data, data.length, InetAddress.getLocalHost(), 5555);

            theSocket.send(theOutput);     

        }

        catch(Exception et)
        {

            System.out.println("Error sending datagram" + et);

        }

    }

    // thread run method, receives datagram and display contents as a string
    public void run()                
    {

        try
        {

            // open DatagramSocket to receive 

            DatagramSocket ds = new DatagramSocket(7777);

            // loop forever reading datagrams from the DatagramSocket

            while (true)
            {

                byte[] buffer = new byte[65507];  // array to put datagrams in

                DatagramPacket dp = new DatagramPacket(buffer, buffer.length);  // DatagramPacket to hold the datagram

                try {

                    ds.receive(dp);

                } catch (IOException e) {

                    // TODO Auto-generated catch block

                    System.err.println("chat error2 " + e);

                }   // wait for next datagram

                String s = new String(dp.getData(),0,dp.getLength()); // get contents as a String

                System.out.println("UDP datagram length " + s.length()+ "  from IP " + dp.getAddress() + " received: " + s );

                ta.append(newline);

                ta.append(s);

            }

        }

        catch (SocketException se)
        {

             System.err.println("chat error1 " + se);

        }

        //catch (IOException se) {System.err.println("chat error2 " + se);}

        //System.exit(1);   // exit on error

    }

    public static void main(String args[]) throws Exception
    {

        UDPchat c=new UDPchat();

    }

}

Answer 1

Check this you making two types of DatagramSockets in your code, try to provide both the same 7777 port. One in your actionPerformed() Method as :

DatagramSocket theSocket = new DatagramSocket();  // create datagram socket and the datagram

and the one in your run Method as :

// open DatagramSocket to receive 

DatagramSocket ds = new DatagramSocket(7777);

The Upper constructor is creating a datagram socket and binding it to any available port on the local host machine, and the second one is trying to reach port 7777. Try to initialize both by same port number, that will sort things out for you.

Hope that might help in some way.

Regards