Java TCP Server不会将数据发送到客户端

Question

I need to make a TCP Server which accepts sockets from clients and receives data from them and send received data to all clients. to do that my Server class saves accepted connections to an ArrayList<Socket> and while wants to send data to clients:

Arraylist<Socket> sockets=Server.getSockets();
for(Socket current: sockets)
{
   ObjectOutputStream out=new ObjectOutputStream (current.getOutputStream());
   out.flush();
   out.writeObject(object);
   out.flush();
   out.close();
 }

but it doesn't work. what's wrong?

Answer 1

There is nothing wrong with your code snippet, the only problem which I can feel is that how your client code is implemented. Let us say your client code is something like this.

Socket clientSocket = new Socket(ip, port);
BufferedReader inFromUser = new BufferedReader( new InputStreamReader(System.in));
DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
while ( true ){
String sentence = inFromUser.readLine();
outToServer.writeBytes(sentence + '\n');
outToServer.flush();

DataInputStream is = new DataInputStream(clientSocket.getInputStream());
System.out.println(is.readLine());
}

Now this client is reading input from console and sending it to server, post that it blocks to wait for response. Lets assume server will send data to all[Which is what you want] if he receive a message 'fire' . Now there are two clients connected to server, ClientA and ClientB .

1. ClientA blocked at user-input (console)
2. User enter 'abc'
3. ClientA moves on sends 'abc' to server
4. ClientA blocks to read data from server
5. Server sends back 'abc' to ClientA [Assuming its an echo server]
6. ClientA reads the data, print it
7. ClientA moves back(while loop), blocked again for user input.

Same thing happen for ClientB, so now both are blocked for user input from console.
Lets say now ClientA user send our magic word ['fire' in this case], server identifies that its a magic word and start sending data to ClientA and ClientB,

Now here the important thing to notice, which is the state of ClientA and ClientB , at this point ClientA has just send the data and is at point 4, hence is blocked to read data from server, so when server sends the message it can read and display, whereas clientB is at point 1, blocked for getting data from console.. so even though server has send data ClientB has data to read, but it can't since its stuck at point 1, also you are closing the connection from server after write, so if ClientB somehow after sometime moves from point 1 to point 4, socket is already closed, hence again it will not be able to read. Though server has send data to both ClientA and ClientB, only ClientA is able to get it.

Phew!! Quite a long explanation, I guess it gives you some direction to solve your problem

Server code for reference

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.ObjectOutputStream;
import java.io.OutputStreamWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;


public class BroadCastTCPServer {

    private static ArrayList<Socket> sockets = new ArrayList<Socket>();
    //Usage: java TCPServer port
    public static void main(String args[]){

        BroadCastTCPServer server = new BroadCastTCPServer();
        try {
            server.startServer("9999");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private void startServer(String portStr) throws IOException {

        int port = Integer.parseInt(portStr);
        ServerSocket  serverSocket = new ServerSocket(port);

         System.out.println("Listening on IP:" +  serverSocket.getInetAddress().getHostAddress() + " Port:"+port);

         while(true){
            Socket connectionSocket = serverSocket.accept();
            sockets.add(connectionSocket);
            System.out.println("New client connection:" + connectionSocket.getRemoteSocketAddress());
            ClientHandler cl = new ClientHandler(connectionSocket);
            Thread clientThread = new Thread(cl);
            clientThread.start();
         }

    }

    public void sendAll() throws Exception{
        System.out.println("No of connections:" + sockets.size());
        for(Socket current: sockets){
            System.out.println(current.getRemoteSocketAddress());
           ObjectOutputStream out=new ObjectOutputStream (current.getOutputStream());
           out.flush();
           out.writeObject("Tata\n");
           out.flush();
           //out.close();
         }
    }

    class ClientHandler implements Runnable{

        Socket socket;
        public ClientHandler(Socket socket){
            this.socket = socket;
        }
        @Override
        public void run() {
            BufferedReader inFromClient = null;
            ObjectOutputStream out = null;
            BufferedWriter br = null;
            try {
                inFromClient = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                out=new ObjectOutputStream (socket.getOutputStream());
                OutputStreamWriter or= new OutputStreamWriter(out);
                br = new BufferedWriter(or);
            } catch (IOException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
            }
            while(true){
            String clientSentence= null;
            try {
                clientSentence = inFromClient.readLine() ;
            } catch (IOException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
            }
            System.out.println("Received: " + clientSentence);
            if (clientSentence.equalsIgnoreCase("fire")) {
                try {
                    sendAll();
                } catch (Exception e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }else{
                try {
                    br.write(clientSentence + "\n" );
                    br.flush();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            }
        }

    }

}