Java how to make server send a message to every client connected

Question

I'm trying to create a chat program where a client sends a message to the server and then the server sends that message to all connected clients to display it. The program works but when a client sends a message only it receives it back and the rest of the connected clients don't get anything.

client code:

public class Client {

    protected static JTextArea textArea = new JTextArea(20, 30);
    protected static String sendMSG, getMSG;

    public static void main(String[] args) throws IOException {
        String hostName = args[0];
        String Username = args[1];
        boolean sending = true;

        try (
            Socket socket = new Socket(hostName, 1010);
            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
            BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        ) {
            BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));

            //frame setup
            JFrame frame = new JFrame("chat client");
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

            //text area
            JScrollPane scrollPane = new JScrollPane(textArea);

            //text field
            JTextField MSGText = new JTextField(5);

            //"send" button
            JButton sMSGB = new JButton("send");
            sMSGB.setPreferredSize(new Dimension(60, 30));
            sMSGB.addActionListener(new ActionListener() {

                public void actionPerformed(ActionEvent event) {
                    sendMSG = MSGText.getText();
                    MSGText.setText("");
                    out.println("<" + Username + ">: " + sendMSG);
                }

            });

            //panel
            JPanel p = new JPanel();
            p.setLayout((new BoxLayout(p, BoxLayout.PAGE_AXIS)));
            p.add(Box.createVerticalStrut(5));
            p.add(scrollPane);
            p.add(Box.createVerticalStrut(5));
            p.add(MSGText);
            p.add(Box.createVerticalStrut(5));
            p.add(sMSGB);
            p.add(Box.createVerticalStrut(5));
            frame.getContentPane().add(p);

            //set frame visible
            frame.pack();
            frame.setVisible(true);

            System.out.println("<Client>: opened stream");

            while(sending) {
                while((getMSG = in.readLine()) != null) {
                    System.out.println(getMSG);
                    textArea.append(getMSG + "\n");
                }
            }
        } 
    }       
}   

server code:

public class Server {

    public static void main(String[] args) {
        boolean listening = true;
        try (ServerSocket serverSocket = new ServerSocket(1010)) {
            while(listening) {
                new ServerThread(serverSocket.accept()).start();
                System.out.println("opened thread");
            }

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

    }
}

Server thread code:

public class ServerThread extends Thread {

    private Socket socket = null;

    public ServerThread(Socket socket) {
        super("ServerThread");
        this.socket  = socket;
    }

    public void run() {
        try (
            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
            BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        ) {
            System.out.println("stream opened");
            String getMSGs;

            while((getMSGs = in.readLine()) != null) {
                out.println(getMSGs);
                System.out.println("msg received and sent");
            }

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


}

Thanks in advance.

Answer 1

Very simple decision create a PrintWriter holder like a List. Don't forget create close mechanism for this collection! And think about multithreading.

public class ServerThread extends Thread {
    private final Socket socket;
    private final List<PrintWriter> outs;

    public ServerThread(Socket socket, List<PrintWriter> outs) {
        super("ServerThread");
        this.socket  = socket;
        this.outs = outs;
        System.out.println("Opened outs: " + outs.size());
    }

    private void sendToAll(String msg) throws IOException {
        for(PrintWriter out: outs) {
            out.println(msg);
        }
    }

    public void run() {
        try (
                PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
                BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        ) {
            System.out.println("stream opened");
            outs.add(out);
            String getMSGs;
            while((getMSGs = in.readLine()) != null) {
                System.out.println("msg received and sent " + getMSGs);
                sendToAll(getMSGs);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

If it is a big project better create queue for messaging