Multithreaded Server receives data from just one client JAVA

Question

I ve created a simple MultiThreaded server. Everytime it accepts client a DataInputStream and DataOutputStream is created and the comunication start

Server:

public class Connection implements Runnable{
boolean isAlreadyOpened = false;

@Override
public void run() {
    // TODO Auto-generated method stub  
    try {           
        ServerSocket ss = new ServerSocket(7000);
        while(true){
            System.out.println("Il Server sta cercando Connessioni");
            Socket s = ss.accept();
            System.out.println("Il Server ha accettato un Client");

            Thread t2 = new Thread(new Runnable(){
                public void run(){             
                   try {

                        DataInputStream dis = new DataInputStream(s.getInputStream());
                        isAlreadyOpened = true;
                        DataOutputStream dos = new DataOutputStream(s.getOutputStream());
                        while(true){
                            String test = dis.readUTF();
                            dos.writeUTF(test);
                            System.out.println(test);
                            dos.flush();
                        }                       
                    } catch (IOException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                        isAlreadyOpened = false;
                    }  
                }           
            });
            t2.start();
        }
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
 }  
}

I ve tried to connect an Android application and a JavaApp; every 40 seconds both of these programs send a string via writeUTF. The connection is established correctly from both Clients but the server receives Data just from the last one that connects to server. How can I allow the server to receive/send the data from/to ALL the clients?

EDIT:

I ve tried to set this:

public class Connection implements Runnable {

 @Override
  public void run() {

    try {
        ServerSocket ss = new ServerSocket(7000);
        while (true) {
            System.out.println("Server is listening");
            Socket s = ss.accept();
            System.out.println("Client Accepted");

            Thread t2 = new Thread(new Runnable() {
                public void run() {
                    try {
                        DataInputStream dis = new DataInputStream(s.getInputStream());
                        DataOutputStream dos = new DataOutputStream(s.getOutputStream());
                        while (true) {
                            String test = dis.readUTF();
                            dos.writeUTF(test);
                            System.out.println(test);
                            dos.flush();
                        }
                    } catch (IOException 
                        e.printStackTrace();
                    } finally {
                        try {
                            s.close();
                        } catch (IOException e) {
                            e.printStackTrace();
                        }
                    }
                }
            });                 
    t2.start();
        }
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

But the result is the same

EDIT:

Andorid Client Code

public class Connection implements Runnable {
@Override
public void run() {
    try {
        Socket s = new Socket("127.0.0.1", 7000);
        DataInputStream dis = new DataInputStream(s.getInputStream());
        DataOutputStream dos = new DataOutputStream(s.getOutputStream());

        while(true){
           dos.writeUTF("FromAndroid");
            Log.d("InputStreammmm", dis.readUTF());
         Thread.sleep(10000)
        }

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

}

JAVA APP CLIENT CODE

@Override
public void run() {
     try {

        Socket socket = new Socket("127.0.0.1", 7000);
        System.out.println("Connessooo");
         DataInputStream dis = new DataInputStream(socket.getInputStream());
         DataOutputStream dos = new DataOutputStream(socket.getOutputStream());

         while(true){
         dos.writeUTF("Invioooooooooooooooooooooooooo");
         result = dis.readUTF();
         System.out.println(result);
         Thread.sleep(10000);
         }

    } catch (IOException e) {

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

        e.printStackTrace();
    }
}

EDIT WITH LIST OF SOCKET:

public class Connection implements Runnable {
List<Socket> sList = new ArrayList<>();
Socket s;
int i = 0;

@Override
public void run() {
    // TODO Auto-generated method stub
    try {
        ServerSocket ss = new ServerSocket(7000);
        while (true) {
            System.out.println("Server Listening");
            s = ss.accept();
            sList.add(s);

            System.out.println("Accepted Client --- " +s.toString());
            Thread t2 = new Thread(new Runnable() {
                public void run() {
                    try {
                        DataInputStream dis = new DataInputStream(s.getInputStream());  

                        while (true) 
                        {
                            String test = dis.readUTF();
                            System.out.println("Message sent from -- " + sList.get(i).toString());
                            System.out.println(test);

                            while(i < sList.size()){
                                 DataOutputStream dos = new DataOutputStream(sList.get(i).getOutputStream());
                                 dos.writeUTF(test);    
                                 System.out.println("Message Sent to -- " + sList.get(i).toString());
                                 dos.flush();
                                 ++i;
                             }
                            i=0;
                        }
                    } catch (IOException e) 
                    {
                        e.printStackTrace();
                    } finally
                    {
                        try
                        {                      
                            System.out.println("Closing Socket --- " + sList.get(i).toString());
                            sList.get(i).close();
                            sList.remove(i);

                        }
                        catch (IOException e)
                        {
                            e.printStackTrace();
                        }
                    }
                }
            });
            t2.start();
        }
    } catch (IOException e) {
        e.printStackTrace();
    }

}

Doing in this way my problem is solved as suggested by EJP...

You're never closing the accepted socket. But otherwise this code should work. Are you sure the problem isn't at the client end? But how does this code compile when `s` isn't final? — user207421, Feb 11 '16 at 22:14
What if the 3 clients connect quickly but don't immediately send data to the server. These variables all go out of scope. Of course this code is not OK. — peter.petrov, Feb 11 '16 at 22:15
@peter.petrov The code will block in `readUTF()` until data is received or an exception occurs. Nothing wrong here. — user207421, Feb 11 '16 at 22:17
@EJP OK, but who keeps reference to `s` and `t2`? Aren't they subject to garbage collection almost immediately? Maybe I am wrong, I just read the code, didn't try anything. — peter.petrov, Feb 11 '16 at 22:17
@EJP I m not closing the socket becasue the server should listen for data from clients.. ClientSide I never close the socket connection.. I simply have a writeutf called everyt tot seconds — Alessio Trecani, Feb 11 '16 at 22:17
You must close the accepted socket `s` after the loop. The `isAlreadyOpened` variable is pointless: remove it. — user207421, Feb 11 '16 at 22:19
@peter.petrov There's a mystery about why `s` isn't final, but a socket won't be garbage-collected while there are references to it or its streams, and a thread won't be garbage-collected while it's running. — user207421, Feb 11 '16 at 22:20
@EJP I ve tried to put s.close() after dos.flush but it throws immediatly an exception "Socket is Closed".. I can post the Client code if it can help? — Alessio Trecani, Feb 11 '16 at 22:25
Of course. That's not where I said to put it, but it should be closed in a `finally` block after the `catch`, actually. — user207421, Feb 11 '16 at 22:26
I ve tried also after "dos.flush}" ( I mean outside the while loop) but eclipse says Unreacheble code. I ve tried to declare the Socket outside the thread but it is the same result — Alessio Trecani, Feb 11 '16 at 22:30
It should be closed in a `finally` block after the `catch`, actually. Not that I'm claiming it will solve your problem. There's no reason to declare the socket outside the thread. — user207421, Feb 11 '16 at 22:31
Yes, but can you please format your code correctly? Very difficult to read like this. — user207421, Feb 11 '16 at 22:38
I ve tried to format it a little bit.. probably now is more readable — Alessio Trecani, Feb 11 '16 at 22:47
@peter.petrov Can we please delete our irrelevant discussion? — user207421, Feb 11 '16 at 23:15

score 1 · Accepted Answer · answered Feb 11 '16 at 23:02

Cannot reproduce.

Test program, copy-pasted from your edited code and added final to Socket s as I am using Java 7, and added client code:

public class Connection implements Runnable
{

    @Override
    public void run()
    {

        try
        {
            ServerSocket ss = new ServerSocket(7000);
            while (true)
            {
                System.out.println("Server is listening");
                final Socket s = ss.accept();
                System.out.println("Client Accepted");

                Thread t2 = new Thread(new Runnable()
                {
                    public void run()
                    {
                        try
                        {
                            DataInputStream dis = new DataInputStream(s.getInputStream());
                            DataOutputStream dos = new DataOutputStream(s.getOutputStream());
                            while (true)
                            {
                                String test = dis.readUTF();
                                dos.writeUTF(test);
                                System.out.println(test);
                                dos.flush();
                            }
                        }
                        catch (IOException e)
                        {
                            e.printStackTrace();
                        }
                        finally
                        {
                            try
                            {
                                s.close();
                            }
                            catch (IOException e)
                            {
                                e.printStackTrace();
                            }
                        }
                    }
                });
                t2.start();
            }
        }
        catch (IOException e)
        {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }


    public static void main(String[] args) throws InterruptedException
    {
        Connection c = new Connection();
        Thread  t1 = new Thread(c);
        t1.setDaemon(true);
        t1.start();
        Runnable    r = new Runnable()
        {
            public void run()
            {
                try (Socket s = new Socket("localhost", 7000))
                {
                    DataOutputStream    dos = new DataOutputStream(s.getOutputStream());
                    DataInputStream dis = new DataInputStream(s.getInputStream());
                    for (int i = 0; i < 10; i++)
                    {
                        dos.writeUTF("Hello from "+Thread.currentThread().getName());
                        String  reply = dis.readUTF();
                        Thread.sleep(10*1000);
                    }
                }
                catch (IOException|InterruptedException exc)
                {
                    exc.printStackTrace();
                }
            }
        };
        Thread  t2 = new Thread(r);
        Thread  t3 = new Thread(r);
        t2.start();
        t3.start();
        t2.join();
        t3.join();
    }
}

Output:

Server is listening
Client Accepted
Server is listening
Client Accepted
Server is listening
Hello from Thread-1
Hello from Thread-2
Hello from Thread-2
Hello from Thread-1
Hello from Thread-2
Hello from Thread-1
Hello from Thread-2
Hello from Thread-1
Hello from Thread-1
Hello from Thread-2
Hello from Thread-1
Hello from Thread-2
Hello from Thread-2
Hello from Thread-1
Hello from Thread-1
Hello from Thread-2
Hello from Thread-2
Hello from Thread-1
Hello from Thread-2
Hello from Thread-1

My code is the same.. but it doesn't work.. Can it be a problem of java8? Or can it be something related to the difference btween my Clients ( one JavaApp and one Android)? — Alessio Trecani, Feb 11 '16 at 23:09
Ok I ve figured it out.. My serve now can listen from both Clients.. So the server displays all the messages.. but the Clients Don't receive the message sent by the other one. I m posting the clients code. — Alessio Trecani, Feb 11 '16 at 23:18
I don't see any reason why those clients won't work. The clients aren't *supposed* to 'receive the messages sent by the other one'. Unclear what you're asking. — user207421, Feb 11 '16 at 23:27
Yes this is the point.. How can I set the client to be able to receive data also from other Client — Alessio Trecani, Feb 11 '16 at 23:29
The server will have to *send* it to the other client. So it will need a list of client sockets and traverse them all when a message is received. — user207421, Feb 11 '16 at 23:31
Ok I though it would be this the solution.. I will try to create A Socket[] that storres all clients.. And Inside the loop I should create a for loop that write data(dos.wrtieUTF) to all the Clients inside Socket[].. I will post something when this is done Thanks so much for the help — Alessio Trecani, Feb 11 '16 at 23:33
I strongly suggest a `List` instead of an array. Make sure you clean it up when the accepted socket gets closed. — user207421, Feb 11 '16 at 23:36
Really appreciated.. I will try it.. Thanks really much for your time — Alessio Trecani, Feb 11 '16 at 23:37
I have updated the code and everything is solved.. But in this way when an exception is thrown My code close every time the first Socket in the list.. How can i understand which socket thrown the exception and close it? — Alessio Trecani, Feb 12 '16 at 18:32

Multithreaded Server receives data from just one client JAVA

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