服务器未将确认发送回客户端

Question

I have my server code below over here:

public void startServer() {
        ServerSocket listener = selectUnusedPortFromRange(1024, 65535);
        Thread t = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    String command = null;
                    while (true) {
                        Socket socket = listener.accept();
                        System.out.println("Got a connection from: " + socket.getLocalPort());
                        BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                        command = in.readLine();
                        System.out.println("GOT HERE"); //Not being printed out
                        if (command != null && !"".equals(command)) {
                            if ("connection".equals(command)) {
                                Writer writer = new BufferedWriter(new OutputStreamWriter(socket.getOutputStream()));
                                writer.write("success\n");
                                writer.flush();
                            }
                        }
                    }
                }
            }
        }
        t.start();
}

This is my client side:

public void makeConnection() {
    try {
        Socket socket = new Socket(IP, PORT);
        Writer writer = new PrintWriter(socket.getOutputStream(), true);
        writer.write("connection\n");
        BufferedReader socketRead = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        String str;
        while ((str = socketRead.readLine()) != null) {
            if ("success".equals(str)) {
                System.out.println("Successfully saved all hosts to: " + listOfHosts.get(i));
                socketRead.close();
                socket.close();
                iStream.close();
                writer.close();
            }
        }
    }catch (Exception e) {
         System.out.println(e.getMessage());
    }
}

On the client side after I create my socket to the connect to the server I write "connection" into the outputStream of the socket and wait for an acknowledgement back from the server saying success. For some reason the connection is not being made to the server. In the server System.out.println("Got a connection from: " + socket.getLocalPort()); this line is not being printed out.

Is there something wrong that I am doing. I can't spot it. And I am not getting an exception thrown when I try to connect to my server.

Answer 1

1) Make sure you use the same port for both the Client and Server. They must communicate over the same port. It seems you may be using different ports currently.

2) Make sure you actually start your server thread. As-is in your code above, you make a new Thread, but never start it. t.start() must be called somewhere.

3) If this is on your local machine, you may be better off using localhost instead of the actual IP address. Firewalls might treat your external IP differently.

4) Terminate your messages with a newline character, such as \\n , so that your BufferedReader can use it's readLine() method. For good measure, also follow-up by flushing the writer's buffer, just in case the newline character didn't trigger that. writer.flush();

And lastly, make sure you terminate the JVM before trying to run your code again. Your code has not shutdown mechanism to un-bind the server from the port... so you may get an exception thrown telling you the port and/or address are already in use. If that happens, either change ports, or kill the java process running in the background.

Here is your code, slightly modified to run on my system. It's working as you might expect it to. I tried to change as little as possible just to get it working on my system. One note is, I hard-coded the port number into the server and client - that's not required, it was just convenient for me to test with:

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.Writer;
import java.net.ServerSocket;
import java.net.Socket;

public class Test {
    public static void main(String[] args) throws IOException {
        Test test = new Test();
        test.startServer();
        test.makeConnection();
    }

    public void startServer() throws IOException {
        final ServerSocket listener = new ServerSocket(60001);
        Thread t = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    String command = null;
                    while (true) {
                        Socket socket = listener.accept();
                        System.out.println("Got a connection from: " + socket.getLocalPort());
                        BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                        command = in.readLine();
                        System.out.println("GOT HERE");
                        if (command != null && !"".equals(command)) {
                            if ("connection".equals(command)) {
                                Writer writer = new BufferedWriter(new OutputStreamWriter(socket.getOutputStream()));
                                writer.write("success\n");
                                writer.flush();
                            }
                        }
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
        t.start();
    }

    public void makeConnection() {
        System.out.println("Making Connection");;
        try {
            Socket socket = new Socket("localhost", 60001);
            Writer writer = new PrintWriter(socket.getOutputStream(), true);
            writer.write("connection\n");
            writer.flush();
            BufferedReader socketRead = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            String str;
            while ((str = socketRead.readLine()) != null) {
                if ("success".equals(str)) {
                    System.out.println("Successfully saved all hosts to: "); //+ listOfHosts.get(i));
                    socketRead.close();
                    socket.close();
                    //iStream.close();
                    writer.close();
                }
            }
        }catch (Exception e) {
             System.out.println(e.getMessage());
        }
    }

}

Answer 2

I was facing the exact same issue. I overcame it by using an ACK mechanism (Wasn't my idea, it was suggested to me). The idea is that client would make a request to server and keep the socket connection alive (and the ouput stream open) till server responds back an agreed ACK message over the same channel . Once the client receives the ACK message, only then it would close the connection.

Below is the code for Server :-

final ServerSocket listener = new ServerSocket(11111);
    Thread t = new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                String command = null;
                 while (true) {
                    System.out.println("About to accept");
                    Socket socket = listener.accept();
                    System.out.println("Got a connection from: " + socket.getLocalPort());
                    DataInputStream inputStream = new DataInputStream(socket.getInputStream());
                    StringBuilder str = new StringBuilder(inputStream.readUTF());
                    //command = in.readLine();
                    System.out.println("GOT HERE. Msg received : "+str); 
                    if (str != null && !"".equals(str.toString())) {
                        command = str.toString();
                        if ("connection".equals(command)) {
                            System.out.println("Got connection message");
                             DataOutputStream outputStream = new DataOutputStream(socket.getOutputStream());
                            outputStream.writeUTF("connection");
                            outputStream.close();

                        }
                    }
                    inputStream.close();
                    System.out.println("Done");
                }
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
            }
        }
});
    t.start();

}

Client :-

public void makeConnection() {
    try {
        System.out.println("In makeConnection");
        Socket socket = new Socket("127.0.0.1", 11111);
        DataOutputStream outputStream = new DataOutputStream(socket.getOutputStream());
        outputStream.writeUTF("connection");
        InputStream inputStream = socket.getInputStream();
        DataInputStream dataInputStream = new DataInputStream(inputStream);
        StringBuilder str;
        do {
            str = new StringBuilder(dataInputStream.readUTF()); 
            } while (!str.toString().equals("connection"));
        System.out.println("Successfully saved all hosts to: ");
        outputStream.close();
        dataInputStream.close();
        socket.close();
        outputStream.close();
    }catch (Exception e) {
         System.out.println(e.getMessage());
    }
}

A call to start the proceedings :-

public void start() throws IOException, InterruptedException {
    System.out.println("Starting server");
    startServer();
    Thread.sleep(1000);
    System.out.println("Starting connection");
    makeConnection();
}