How to send data from Server to multiple Clients using Sockets?

Question

I have simple server-clients programm:

public class Server extends Thread {
    private ServerSocket server;

    public Server(int port) {
        try {
            this.server = new ServerSocket(port);
            System.out.println("New server initialized!");
            this.start();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public void run() {
        while (true) {
            try {
                Socket client = server.accept();
                System.out.println(client.getInetAddress().getHostName()
                        + " connected");
                new ServerHandler(client);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

Server handler must send message in stream:

public class ServerHandler extends Thread {
    protected Socket client;
    protected String userInput;
    protected PrintWriter out;
    protected BufferedReader console;

    public ServerHandler(Socket client) {
        this.client = client;
        this.userInput = null;
        this.start();
    }

    public void run() {
        System.out.println("New Communication Thread Started");
        System.out.println("Enter message:");
        try {
            this.out = new PrintWriter(client.getOutputStream(), true);
            this.console = new BufferedReader(new InputStreamReader(System.in));
            while ((this.userInput = console.readLine()) != null) {
                this.out.println(userInput);
            }               
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Client side receives that message :

public class Client {
    protected Socket client;
    protected BufferedReader in;

    public Client(String hostName, int ip) {
        try {
            this.client = new Socket(hostName, ip);
            this.in = new BufferedReader(new InputStreamReader(
                    this.client.getInputStream()));
            String buffer = null;
            while ((buffer = in.readLine()) != null) {
                System.out.println(buffer);
            }
        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

It works well with one client,but when I start new client problem with receiving message from server arises. What am I doing wrong?

Not sure if this is related to your problem, but in your Server class you create a new instance of ServerHandler but you don't assign it to anything. With nothing referencing it it can be garbage collected at any time. — John Sheridan
– John Sheridan, Commented Aug 3, 2015 at 12:09
you need in your server an arraylist of socket connections. put each client on a thread and then create a thread in your server that sends data to your clients. make sure you synchronize on your arraylist tho — rush2sk8
– rush2sk8, Commented Aug 3, 2015 at 14:30
@JohnSheridan That's not correct. It extends Thread, and its constructor calls start(), so it isn't elegible for GC until it exits. — user207421
– user207421, Commented Aug 4, 2015 at 9:30
@EJP You're absolutely right. Must have been sleepwalking. Forgot that a running Thread is treated as a GC root and immune from GC. Thanks for picking this up. — John Sheridan
– John Sheridan, Commented Aug 4, 2015 at 11:45
Changed my code but still have problem.Read my answer below please. — Ruslan Lomov
– Ruslan Lomov, Commented Aug 4, 2015 at 14:55

Ruslan Lomov · Accepted Answer · 2015-08-04 18:28:15Z

I found the answer:

public class Server extends Thread {
    private ServerSocket server;
    protected List<ClientHandler> clients;

    public Server(int port) {
        try {
            this.server = new ServerSocket(port);
            System.out.println("New server initialized!");
            clients = Collections
                    .synchronizedList(new ArrayList<ClientHandler>());
            this.start();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    public void run() {
        while (true) {
            try {
                Socket client = server.accept();
                System.out.println(client.getInetAddress().getHostName()
                        + " connected");
                ClientHandler newClient = new ClientHandler(client);
                clients.add(newClient);
                new SendMessage(clients);

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

ClientHandler instead of Serverhandler class:

public class ClientHandler {
    protected Socket client;
    protected PrintWriter out;

    public ClientHandler(Socket client) {
        this.client = client;
        try {
            this.out = new PrintWriter(client.getOutputStream());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Thread can be added to Client class but works fine without it.I assume because when main method called new thread automatically created:

public class Client {
    protected Socket client;
    protected BufferedReader in;

    public Client(String hostName, int ip) {
        try {
            this.client = new Socket(hostName, ip);
            this.in = new BufferedReader(new InputStreamReader(
                    this.client.getInputStream()));
            String buffer = null;
            while ((buffer = in.readLine()) != null) {
                System.out.println(buffer);
            }
        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }       
}

Class that sends data to clients:

 public class SendMessage extends Thread {
        protected List<ClientHandler> clients;
        protected String userInput;
        protected BufferedReader console;

        public SendMessage(List<ClientHandler> clients) {
            this.clients = clients;
            this.userInput = null;
            this.start();
        }

        public void run() {
            System.out.println("New Communication Thread Started");
            if (clients.size() == 1) {
                System.out.println("Enter message:");
            }
            try {
                if (clients.size() > 0) {
                    this.console = new BufferedReader(new InputStreamReader(
                            System.in));
                    while ((this.userInput = console.readLine()) != null) {
                        if (userInput != null & userInput.length() > 0) {
                            for (ClientHandler client : clients) {
                                client.out.println(userInput);
                                client.out.flush();
                            Thread.currentThread();
                            Thread.sleep(1 * 1000);
                            }
                        }
                    }
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

Now it works! Need to make SendMessage thread sleep after flushing data. Main for server starter:

public static void main(String[] args) {
new Server(1200);
}

Main for client starter:

public static void main(String[] args) {
new Client("127.233.0.1", 1200);
}

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