Send data from server to all clients in socket programming in Java

Question

I have a client and server class, the client has GUI. If I open the server and then the client, everything is ok. But If I open the server and two clients, the clients don't messaging each other, only messaging with server like echo client. I guess I'm jumping some details in my code, but I can't figure out where I wrong.

CLIENT

package program;

import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.Socket;
import java.net.UnknownHostException;

    public class gui
    {
        private Socket socket        = null;
        private DataInputStream input = null;
        private DataOutputStream out     = null;
        public String data = "0";

        server _server;
        JFrame pencere;
        JButton button;
        JTextArea area;
        JTextField type;
        public gui(){


            try
            {
                socket = new Socket("127.0.0.1",4000);
                System.out.println("Connected");

                // takes input from terminal
                input = new DataInputStream(socket.getInputStream());

                // sends output to the socket
                out = new DataOutputStream(socket.getOutputStream());
            }
            catch(UnknownHostException u)
            {
                System.out.println(u);
            }
            catch(IOException i)
            {
                System.out.println(i);
            }



            pencere = new JFrame("oxChat");
            pencere.setSize(640,480);
            pencere.setLayout(null);
            button = new JButton("gönder");
            button.addActionListener( new ActionListener()
            {
                @Override
                public void actionPerformed(ActionEvent e)
                {
                    try {
                        out.writeUTF(type.getText());
                    } catch (IOException ex) {
                        ex.printStackTrace();
                    }
                }
            });
            area = new JTextArea();
            type = new JTextField();
            pencere.add(type);
            pencere.add(area);
            pencere.add(button);
            area.setBounds(0,0,640,350);
            type.setBounds(0,370,640,25);
            button.setBounds(640/2-80/2,400,80,30);
            pencere.setVisible(true);
            pencere.setResizable(false);
            pencere.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            new Thread (new dagitici(socket,this)).start();
        }

        public static void main(String[] args) throws IOException {
            gui app = new gui();
        }

        String getData(){
            return data;
        }
        void setData(String dataa){
            this.data = dataa;
            area.append(this.data+"\n");
        }

    }

    class dagitici extends Thread{
        private Socket socket        = null;
        private DataInputStream input = null;
        private DataOutputStream out     = null;
        gui g;
        public String okunan="";
        public dagitici(Socket socket,gui g){
            this.socket = socket;
            this.g = g;
        }
        @Override
        public void run() {

            try {
                input = new DataInputStream(socket.getInputStream());
                out = new DataOutputStream(socket.getOutputStream());
            } catch (IOException e) {
                e.printStackTrace();
            }

            while(true){
                try {
                    System.out.println("a");
                    okunan=input.readUTF();
                    g.setData(okunan);
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

SERVER

package program;

// A Java program for a Server
import com.sun.jdi.PathSearchingVirtualMachine;

import javax.swing.*;
import java.net.*;
import java.io.*;

public class server
{
    private Socket socket;
    private ServerSocket server;
    public static   String data;
    // constructor with port
    public void start(int port){
        try {
            server = new ServerSocket(port);
            while(true){
                socket = server.accept();
                new Thread (new ConnectionHandler(socket)).start();
            }
        }catch(IOException i){

        }
    }

    public static void main(String[] args) {
        server _server = new server();
        _server.start(4000);
    }
}

class ConnectionHandler extends Thread{
    gui app;
    private String data;
    private Socket       socket = null;
    private DataInputStream in   = null;
    private DataOutputStream out     = null;
    public ConnectionHandler(Socket socket){
        this.socket=socket;
    }
    @Override
    public void run() {
        try
        {
            System.out.println("Waiting for a client ...");
            System.out.println("Client accepted");
            in = new DataInputStream(new BufferedInputStream(socket.getInputStream()));
            out = new DataOutputStream(socket.getOutputStream());

            String line = "";

            // reads message from client until "Over" is sent
            while (!line.equals("Over"))
            {
                try
                {
                    line = in.readUTF();
                    out.writeUTF(line);
                }
                catch(IOException i)
                {
                    System.out.println(i);
                }
            }
            System.out.println("Closing connection");

            // close connection
            socket.close();
            in.close();
        }
        catch(IOException i)
        {
            System.out.println(i);
        }
    }
    public String getServerData(){
        return  data;
    }
}

Did you read my answer to your last question? I already explained how to do that. In this code you only send the data back on the same socket it comes from. — Johannes Kuhn
– Johannes Kuhn, Commented Oct 29, 2019 at 17:14
I'm thinking to make for loop to send data through sockets. line = in.readUTF(); for(int a=0;a<threads.length-1;a++){ PrintStream os = new PrintStream(threads[a].getSocket().getOutputStream()); out.writeUTF(line); } But how can I find socket of any thread ? getSocket() won't work. — ShooterLens Aim
– ShooterLens Aim, Commented Oct 29, 2019 at 17:30

Johannes Kuhn · Accepted Answer · 2019-10-29 17:33:28Z

Add a List<ConnectionHandler> to your server class:

List<ConnectionHandler> clients = new ArrayList<>();

And for good measure, a lock:
```
Object lock = new Object();
```

Then we need to add any new connected client to that list:

socket = server.accept();
ConnectionHandler client = new ConnectionHandler(this, socket)
synchronized (lock) {
    clients.add(client);
}
new Thread(client).start();

Now we just need a method to distribute all the incoming messages in the server class:

void distributeMessage(String message) {
    List<ConnectionHandler> clientsCopy;
    synchronized (lock) {
        clientsCopy = new ArrayList<>(clients);
    }
    for (ConnectionHandler client : clientsCopy) {
        client.sendMessage(message);
    }
}

Now we need to change the ConnectionHandler, and we start by cleaning up the fields:
```
private Socket socket;
private DataInputStream in;
private DataOutputStream out;
private server server;
```
Those are all the fields that we need.

Next we need to change the constructor of this class:

public ConnectionHandler(server server, Socket socket) {
    this.server = server;
    this.socket = socket;
    this.in = new DataInputStream(new BufferedInputStream(socket.getInputStream()));
    this.out = new DataOutputStream(new BufferedOutputStream(socket.getOutputStream()));
}

All fields should be initialized in the constructor if possible.

Then we have to add our new sendMessage(String message) method:

public void sendMessage(String message) {
    try {
        out.writeUTF(message);
        out.flush();
    } catch (IOException e) {
        // TODO: Here you HAVE to check if the connection was closed
        // And if it was closed, call a method in the server class to
        // remove this client.
        e.printStackTrace();
    }
}

List<ConnectionHandler> clients = new ArrayList<>(); solved my problem thanks !

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