Java TCP Client listening and writing to TCP Server

Question

I have one TCP-Client and one TCP-Server written in Java. The Server is waiting for instructions from the Client but should also be able to send instructions to the client. I can make the Client send something to the Server and wait for a reply. But I can not make it like waiting for a message without sending something before.

TCP-Client

    public class TCPClient {

    static DataOutputStream toServer;
    static BufferedReader fromServer;
    static Socket socket;

    public static void main(String[] args) throws Exception {
        BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
        System.out.println("*************************");
        System.out.println("*       Client          *");
        System.out.println("*************************");
        System.out.println("INSTRUCTION       | EFFECT");
        System.out.println("aktiv             | ready to do something");
        System.out.println("exit              | disconnect");
        System.out.println();
        System.out.print("Please enter the IP-Address of the Server: ");
        String ip = input.readLine();
        System.out.println();

        try {
            socket = new Socket(ip, 9999);
        } catch (Exception e) {
            System.out.println("Can not connect to Server!");
        }

        toServer = new DataOutputStream(socket.getOutputStream());  // Datastream FROM Server 
        fromServer = new BufferedReader(new InputStreamReader(socket.getInputStream()));     
        while (sendRequest()) {              
            receiveResponse();                 
        }
        socket.close();
        toServer.close();
        fromServer.close();
    }

    private static boolean sendRequest() throws IOException {
        BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
        String line;
        boolean holdTheLine = true;          // Connection exists

        System.out.print("> ");
        line = input.readLine();

        switch (line) {
            case "aktiv":
                toServer.writeBytes("active" + '\n');
                break;
            case "exit":
                holdTheLine = false;
                break;
            default:
                break;
        }

        return holdTheLine;
    }

    private static void receiveResponse() throws IOException {
        System.out.println("Server: " + fromServer.readLine() + '\n');
    }
}

TCP-Server

public class TCPServer {
    static boolean connected = true;
    public static void main(String[] args) throws Exception {
        System.out.println("********************************");
        System.out.println("*         Server               *");
        System.out.println("********************************");
        System.out.println("INSTRUCTION | EFFECT");
        System.out.println("ok          | send an ok to client");

        ServerSocket listenSocket = new ServerSocket(9999);

        while (true) {
            final Socket client = listenSocket.accept();

            Thread newClientThread = new Thread(new Runnable() {
                @Override
                public void run() {
                    multithreadedServer(client);
                }
            });
            newClientThread.start();
        }
    }

    public static void multithreadedServer(Socket client) {
        String line;
        final BufferedReader fromClient;
        final DataOutputStream toClient;
        Thread cmdForClient;

        try {
            fromClient = new BufferedReader(new InputStreamReader(client.getInputStream()));
            toClient = new DataOutputStream(client.getOutputStream());

            while (connected) {
                cmdForClient = new Thread(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            String line = fromClient.readLine();
                            System.out.println("Client: " + line);
                            if (line.equals("exit")) {
                                connected = false;
                            }
                        } catch (Exception e) {
                            System.out.println(e);
                        }
                    }
                });
                cmdForClient.start();

                final BufferedReader input = new BufferedReader(new InputStreamReader(System.in));

                try {
                    String reply = input.readLine();
                    if (reply.equals("ok")) {
                            toClient.writeBytes("OK." + '\n');
                    }
                } catch (Exception e) {
                    System.out.println(e);
                }
            }
            fromClient.close();
            toClient.close();
            client.close();
        } catch (Exception e) {
            System.out.println(e);
        }
    }

}

Answer 1

The typical operation for a client-server scenario is for the client sending requests to the server. However, in peer to peer applications, both endpoints might act as both clients and servers. The only difference would be which endpoint that opened the connection. In your case, the problem is that only the "server" is using a receiver thread. Start a receiver thread at the client side and your problem should be solved. You should pretty much be able to just reuse your threaded code from the server in the client. Just pass the socket to the receive thread after opening the connection to the server.

EDIT:

In your client:

            Thread newServerThread = new Thread(new Runnable() {
                @Override
                public void run() {
                    multithreadedServer(socket);
                }
            });
            newServerThread.start();

where socket, is the socket to the server. You may need to update multithreadedServer for any specifics or differences for the clients operations, but the principle should be the same.