Java - 如何检查 ServerSocket.accept() 是否已连接？

Question

I'm trying to write simple server-client chat solution. For test purposes, I'm creating an array of 2 serverThreads, which are responsible for sending and receiving messages from the clients connected.

I'd like a server to reject a connections after the number of connected clients reach a maximum value. However, even though the server do not accept the connection, the socket on client side is created. Methods socket.isBound and isConnected both return true value.

So back to the main question. Do you have any ideas how could I reject the client from connecting when the ServerSocket will not be able to .accept() additional connection?

Here's the code of the Server class.

public class Server {

private ServerSocket serverSocket = null;
private ServerThread serverThread[] = new ServerThread[2];
protected volatile int clientCount = 0;

public Server (int port){
   try {
       System.out.println("Binding to port " + port + " ...");
       serverSocket = new ServerSocket (port);
       System.out.println("Binded to port " + port + ".");
   } catch (IOException e) {
       System.out.println("Failed binding to the port: " + e.getMessage());
        }
}

public void addThread (Socket socket){

   System.out.println ("Client connected at socket: " + socket);
   serverThread[clientCount] = new ServerThread (this, socket);
   try {
       serverThread[clientCount].open();
       serverThread[clientCount].start();
   } catch (IOException e) {e.getMessage();}

}

public void waitForClient () {

   boolean isLogPrinted = false;

   while (true){
       try {
           if (clientCount < serverThread.length){
               System.out.println ("Waiting for connection...");
               isLogPrinted = false;
               addThread (serverSocket.accept());
               clientCount++;
               System.out.println("Client count: " + clientCount);
           }
           else {
               if (!isLogPrinted){
                    System.out.println("MAXIMUM NUMBER OF CLIENTS REACHED! (" + clientCount + ").");
                    isLogPrinted = true;
               }
           }
       } catch (IOException e) {
           System.out.println("Error while waiting for new clients to connect: " + e.getMessage());
            }
   }
}

public synchronized void broadcastMessages (String msg){
   for (int i = 0; i < clientCount; i++)
       serverThread[i].sendMessage(msg);
}

public static void main (String args[]){
   Server server = new Server (4200);
   server.waitForClient();
 }

}

Answer 1

I'd like a server to reject a connections after the number of connected clients reach a maximum value.

Close the server socket.

However, even though the server do not accept the connection, the socket on client side is created. Methods socket.isBound and isConnected both return true value.

Correct. That's because TCP maintains a 'backlog queue' of incoming connections which have been completed but not yet accepted by the server application.

So back to the main question. Do you have any ideas how could I reject the client from connecting when the ServerSocket will not be able to .accept() additional connection?

Close the server socket while the number of connections is at its maximum.

However due to the backlog this technique can never be perfect. There is no perfect solution. You could have the server immediately close excess connections, but the excess clients won't detect that until they try to send something. If you need perfection you will probably have to introduce an application protocol whereby the server sends something like 'ACCEPTED' or 'REJECTED' accordingly.

Answer 2

Instead of while true in you waitForClient method try this

private final int allowedClients = 10;
private int connectedClients = 0;
public void waitForClient () {

boolean isLogPrinted = false;

while (connectedClients <= allowedClients){
    try {
       if (clientCount < serverThread.length){
           System.out.println ("Waiting for connection...");
           isLogPrinted = false;
           addThread (serverSocket.accept());
           connectedClients++;
           System.out.println("Client count: " + clientCount);
       }
       else {
           if (!isLogPrinted){
                System.out.println("MAXIMUM NUMBER OF CLIENTS REACHED! (" + clientCount + ").");
                isLogPrinted = true;
           }
       }
   } catch (IOException e) {
       System.out.println("Error while waiting for new clients to connect: " + e.getMessage());
        }

} }

Answer 3

I know this is very late to answer, but I think it will help many. You can check for the existing socket if any by below code.

SocketAddress socketAddress = new InetSocketAddress("localhost", 8091);
    Socket socket = new Socket();
    ServerSocket serverSocket = null;
    try {
        socket.connect(socketAddress);
    } catch (IOException e) {
        e.printStackTrace();
    }
    if(socket == null) {
        try {
            serverSocket = new ServerSocket(8091);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

if not found a active socket on the same port and IP then it will start a new server socket or you can change it start socket only else you can connect to the existing socket.