Client-Server-Client communication using Sockets

Question

I am building a small chat application in which client A wants to send something to client C with server B in between. First of all is this a correct approach for the proble

Answer 1

When your clients connect to the server, your server creates a Socket for it, here it is Socket socket = ss.accept();, your socket variable will be holding that client.

now if you just keep adding your client socket to a arraylist in your while loop, you will have a list of clients actively connected with your server like:

after the accept:

clients = new ArrayList<DataOutputStream>();
Socket socket = ss.accept();
os = new DataOutputStream(socket.getOutputStream());
clients.add(os);

Now as you have all the clients in that clients arraylist, you can loop through it, or with some protocol define which client should i send the data after reading.

Iterator<DataOutputStream> it = clients.iterator();
while ((message = reader.readLine()) != null) { //reading    
    while (it.hasNext()) {
        try {
            DataOutputStream oss = it.next();
            oss.write(message);//writing
            oss.flush();
        }
        catch (Exception e) { }
     }
 }

This will loop through all the available clients in the arraylist and will send to all. you can define ways to send to only some.

For example: maintain a ActiveClients arraylist and with some GUI interaction may be or maybe, define what all clients you want to send the message. Then add just those clients outputStreams to ActiveClients

ActiveClients.add(clients.get(2));

or remove them, if you don't want them.

ActiveClients.remove(clients.get(2));

and now just loop through this arraylist to send the data as above.

Answer 2

You can create message queue for each client:

1. Client A sends message 'Hi' with address Client C to server B.
2. Server B receives message and adds it to message queue of client C.
3. Thread in server B which communicates with client C check message queue, retrieve message and sends it to client C.
4. Client C receives message.

Answer 3

If I am not mistaken, you must be having a problem with receiving a message from the Server or SSocket class. What happens with your code is that when you send a message from the client to the server the Server class receives your messages also gives an echo of the message in the client. However, when you send a message from the Server class, you don't get any messages in the Client Class.

To get this to work, you would have to modify your code in the following fashion:

SSocket Class

String line = null;
while (true) {
    line = dIn.readUTF();
    System.out.println("Recievd the line----" + line);
    dOut.writeUTF(line + " Comming back from the server");
    dOut.flush();
    System.out.println("waiting for the next line....");
}

You should add these lines:

String Line2 = take.nextLine(); //The user types a message for the client
dOut.writeUTF(Line2 + " Comming back from the server"); //The message is sent to the client

Replace the while loop with this one and it will work fine.

while (true) {       
    line = dIn.readUTF(); //Takes the msg from the client
    System.out.println("Recievd the line----" + line); //Prints the taken message

    String Line2 = take.nextLine(); //The user types a message for the client
    dOut.writeUTF(Line2 + " Comming back from the server"); //The message is sent to the client
    dOut.flush();
    System.out.println("waiting for the next line....");
}