套接字服务器-通过传递客户端消息

Question

I am creating a chat client. I have multiple clients connecting but I am having a hard time getting them to talk to each other. How do I get the server to send the message incoming from client1 to the rest of the clients without client1 having an echo? I haven't discovered any way to identify each client.

public class connect1 extends Thread {
    public void run() {
        ServerSocket serverSocket = null;
        try {
            serverSocket = new ServerSocket(4444); 
            while (acceptMore) {
                Socket send1socket = serverSocket.accept();
                new Thread(new sendRunnable(send1socket)).start();
            }
        } catch (IOException exp) {
            exp.printStackTrace();
        }
    }
} //this thread starts my Runnable where I have my PrintWriter

Answer 1

The simplest way to do it is have an Array or list of Sockets, and add to it every time a new client connects/subtract when client leaves.

Then when a message is received, loop through that array/list and send message to other clients. (make sure to check the client you are on against the current client in list so you dont send message to the client that sent it to server)

While this is fine on a small scale, it will cause problem on a large scale as you have to loop through a large list each time a new message is received.

to have chatRooms or something, you can have a multidimensional array ( Array[room][socket] ) then you can do loop through certain rooms and only send to those clients, or loop through all, etc.

To send to specific users only, you can have a User.java class that has a username and a socket in it. then, instead of doing an array of sockets, use an array of users and in your loop, check username and only send to the required users.

Flow: UserA connects to server, sends username to be bob server recieves connections, makes new User object, sets username to bob and sets socket to users socket, then adds that User object to User[]

UserB connects to server, sends username john same deal, server makes new user object etc etc.

john sends a message to server and tells it to only send the message to bob Server loops through User[] and checks username of them, if username matches bob server calls getSocket on that user object, this returns the socket connection for that user. server then gets the outputStream of that socket, and creates a printwriter with it. the it sends the message received from john through the newly create printwriter.

Answer 2

If your question is how to send/receive messages over a socket, the following is one way to do it.

To send:

String message = "..." // message to send
output = new DataOutputStream(socket.getOutputStream());
output.writeInt(message.getBytes().length);  // first you send how long the message is
output.write(message.getBytes(), 0, message.getBytes().length);  // then you send the message

And to read:

DataInputStream input = new DataInputStream(socket.getInputStream());
if(input.available() > 0) {  // important - makes sure the code doesn't "block" and wait for input
    byte[] bytes = new byte[input.readInt()]; // makes a byte[] of the size of the message it's going to read
    input.read(bytes,0,bytes.length);  // reads the actual message to the byte array
    String message = new String(bytes,0,bytes.length,"UTF-8");  // convert the message back into a String
}

You should see Socket#getOutputStream() and Socket#getInputStream() , and then use methods in the DataInputStream and DataOutputStream class. input.ava

As for your other question, if you want a client to be able to send messages to specific other client(s), the client must know exactly which client(s) it wants to send the message to. When the client sends a message to the server, that information has to be part of the message. The server would use that to selectively send the message to specific Sockets .

There would have to be a system in place of course. For example, for a chat app each client might have a username. You could have a global class HashSet<User> variable - where a User has a Socket and a String (username). The client's message to the server could then be a (series of) username(s) followed by the actual message. The server would then iterate through the HashSet<User> and send the actual message only to Users with matching usernames.

Source: I'm making a chat app right now as well, and this is how I'm doing it. That's not to say there aren't other ways, or even better ways.