JavaNIO multithreaded server not working

Question

Hello Experts
can somebody please indentify the problem with this server why this is unable to connect more then one client

import java.io.*;
import java.net.*;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.*;

public class MultithreadedServer extends Thread {

    private ServerSocketChannel ssChannel;

    private Thread tRunSer = new Thread(this, "ServerSelectThread");

    public static void main(String argv[]) throws Exception {
        new MultithreadedServer();
    }

    public MultithreadedServer() throws Exception {
        this.start();
    }

    public void run() {
        while (true) {
            try {

                ssChannel = ServerSocketChannel.open();
                ssChannel.configureBlocking(false);
                int port = 2345;
                ssChannel.socket().bind(new InetSocketAddress(port));
            } catch (Exception e) {
            }
        }
    }
}

class Connect extends Thread {
    private ServerSocketChannel ssChannel;
    private SimManager SM;
    private BallState BS = new BallState(10, 5);

    public Connect(ServerSocketChannel ssChannel) {
        this.ssChannel = ssChannel;

        SM = new SimManager(BS);
        SM.start();
    }

    public void run() {
        try {
            SocketChannel sChannel = ssChannel.accept();
            while (true) {
                ObjectOutputStream oos = new ObjectOutputStream(sChannel
                        .socket().getOutputStream());
                oos.writeObject(BS);
                System.out.println("Sending String is: '" + BS.X + "'" + BS.Y);
                oos.flush();

            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

}

my intention is to send the objects on network.

please help

new code:

import java.io.*;
import java.net.*;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.*;

public class MultithreadedServer extends Thread {

    private ServerSocketChannel ssChannel;
    private SimManager SM;
    private BallState BS = new BallState(10, 5);
    private Thread tRunSer = new Thread(this, "ServerSelectThread");

    public static void main(String argv[]) throws Exception {
        new MultithreadedServer();
    }

    public MultithreadedServer() throws Exception {
        this.start();
    }

    public void run() {
        // create the server socket once
        try {
            ssChannel = ServerSocketChannel.open();
            ssChannel.configureBlocking(false);
            ssChannel.socket().bind(new InetSocketAddress(2345));
        } catch (IOException e1) {
            e1.printStackTrace();
        }

        while (true) {
            // accept new connections on the socket

            SocketChannel accept;
            try {
                accept = ssChannel.accept();
                ObjectOutputStream oos;
                oos = new ObjectOutputStream(accept.socket().getOutputStream());

                oos.writeObject(BS);
                System.out.println("Sending String is: '" + BS.X + "'" + BS.Y);
                oos.flush();

            }

            catch (IOException e) {
                e.printStackTrace();
            }

        }
    }
}

Answer 1

You are creating a new server socket for each loop iteration (using the same port over and over). You must create the server socket only once, and then accept new incoming connections.

Something like:

 public void run() {
    // create the server socket once
    ssChannel = ServerSocketChannel.open();
    ssChannel.configureBlocking(false);
    ssChannel.socket().bind(new InetSocketAddress(2345));

    while (true) {
        // accept new connections on the socket
        try {
            SocketChannel accept = ssChannel.accept();
            System.out.println("new client: " + accept.getRemoteAddress());
        } catch (Exception e) {
            System.out.println("exception: " + e.getMessage());
        }
    }
}

Answer 2

If you put something in your catch block you will probably find it yourself. (e.printStackTracer() might help for the time being).

Answer 3

Here is the reason for your NPE:

If this channel is in non-blocking mode then this method will immediately return null if there are no pending connections.

This is from ServerSocketChannel.accept() .

Your accept call returns null , and you then try to call a method on this null object.