Question

I’m working on a project which expects a TCP client and Server, where server echoes the message back to client. Following is from the assignment:

The server application shall:

1. Listen for TCP connections on a well known IP address and port
2. Accept connections initiated on that port
3. Receive messages from the client and echo them back
4. Continue to do this until the client drops the connection.

The client application shall:

1. Establish a connection with the server at its well known IP address and port
2. Send messages in an asynchronous manner to the server. The format of the message is
   of your choice; however, it must contain enough information in order for it to be
   recognized on its return from the server.

I have completed the coding for Server, and this is what i’ve come up with for the client.

My questions:

• What does it mean that Server listens for TCP connections on a well known IP and Port In my implementation, i’ve used ServerSocket which accepts the port server listens on. Did i interpret it correctly?

• In my current implementation of TCPClient, client sends messages to Server, but the println() seems to be a blocking call, which makes it Synchronous. What can i do to make my client asynchronous?

For brevity, I havent added the code of TCPServer, let me know if it is needed

UPDATE**
Based on the feedback, i have modified by TCPClient class. After receiving client request, i spawn two threads ReceiveMessage and SendMessage. Doing that gives me following exception:

[Client] Message sent: Message from Client 97
[Client] Message sent: Message from Client 98
[Client] Message sent: Message from Client 99
[Client] Done Sending all the messages
java.net.SocketException: Socket closed
    at java.net.SocketInputStream.socketRead0(Native Method)
    at java.net.SocketInputStream.read(SocketInputStream.java:129)
    at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:264)
    at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:306)
    at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:158)
    at java.io.InputStreamReader.read(InputStreamReader.java:167)
    at java.io.BufferedReader.fill(BufferedReader.java:136)
    at java.io.BufferedReader.readLine(BufferedReader.java:299)
    at java.io.BufferedReader.readLine(BufferedReader.java:362)
    at org.chanders.client.ReceiveMessage.run(ReceiveMessage.java:18)
    at java.lang.Thread.run(Thread.java:680)

Following is the new Client Code:

public class TCPClient {
    Socket clientSocket = null;
    OutputStream out = null;
    BufferedReader in = null;
    String message = "Hello from Client";
    int messagecount = 100;

    // server credentials
    private static final String SERVER_ADDRESS = "localhost";
    private static final int SERVER_PORT = 50001;

    protected void execute() {
        try {
            clientSocket = new Socket(SERVER_ADDRESS, SERVER_PORT);
            Thread send = new Thread(new SendMessage(clientSocket.getOutputStream()));
            Thread receive = new Thread(new ReceiveMessage(clientSocket.getInputStream()));

            send.start();
            receive.start();

            //For server to wait until send and receive threads finish
            send.join();
            receive.join();

        } catch (UnknownHostException uhe) {
            System.err.println("Couldnt find host:  "  + SERVER_ADDRESS);
            uhe.printStackTrace();
            System.exit(-1);

        }catch(IOException ioe) {
            System.err.println("Couldnt get I/O:  "  + SERVER_ADDRESS);
            ioe.printStackTrace();
            System.exit(-1);

        }catch(InterruptedException ie) {
            System.err.println("Thread.join failed:  ");
            ie.printStackTrace();
            System.exit(-1);
        }
        finally {
            //cleanup();
        }
    }

    private void cleanup() {
        try {
            clientSocket.close();
        }catch(Exception e) {
            e.printStackTrace();
            System.exit(-1);
        }
    }

    public static void main(String[] args) {
        TCPClient client = new TCPClient();
        client.execute();
    }

public class SendMessage implements Runnable {
    OutputStream out = null;
    String message = "Message from Client";
    int messageCount = 100;

    public SendMessage(OutputStream out) {
        this.out = out;
    }

    public void run() {
        PrintWriter writer = new PrintWriter(out);
        try {

            for (int i = 0; i < messageCount; i++) {
                String m = message + " " + i;
                writer.println(m);
                System.out.println("[Client] Message sent: " + m);
            }
            System.out.println("[Client] Done Sending all the messages");

        } catch (Exception e) {
            e.printStackTrace();
            System.exit(-1);
        } finally {
            cleanup();
        }

    }
    private void cleanup() {
        try {
            out.close();
        }catch(Exception e) {
            e.printStackTrace();
            System.exit(-1);
        }
    }
}

public class ReceiveMessage implements Runnable {
    InputStream in = null;
    String message;

    public ReceiveMessage(InputStream in) {
        this.in = in;
    }

    public void run() {
        BufferedReader reader = new BufferedReader(new InputStreamReader(in));
        try {

            while ((message = reader.readLine()) != null) {

                System.out.println("[Client] Received message from Server: "
                        + message);
            }

            System.out.println("[Client] Done Receiving messages from Server");

        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            cleanup();
        }

    }
    private void cleanup() {
        try {
            in.close();
        }catch(Exception e) {
            e.printStackTrace();
            System.exit(-1);
        }
    }
}

Answer 1

Use a separate thread for each client. When you write something, in the server end , there must be a method which accepts the string. Otherwise it will be blocking. Paste your server code.