Question

been trying to figure this problem out for about 5 hours but cant seem to see it, although all the steps are done to send data, I can only receive messages to the server, but not from server to client. I’m in the early stages of building/learning how to do a chat client program in command line. The following is the server code:

The CServer class:

public class CServer {

private static int port=2008, maxConnections=0;
private static String shutDownServer = "no";

public static void main(String[] args) throws IOException{


    ServerSocket listen = new ServerSocket(port);
    Socket server;

    while(shutDownServer.equalsIgnoreCase("no")){

        doComm connection;

        System.out.println("\nWaiting for clients to connect...");

        server = listen.accept(); // accept incomming connections from client
        System.out.println("Client connected. Location: " + server.getInetAddress().getHostName());
        connection = new doComm(server);

        Thread thread = new Thread(connection);
        thread.start();
    }



}

public void shutDownServer(String command){

    this.shutDownServer = command;
}
}

Now the doComm class that handles each client in thread:

public class doComm implements Runnable{

Socket server;
private String clientData;

  public doComm(Socket server){

      this.server = server;
  } 

  public void run(){

     try {

      BufferedReader fromClient = new BufferedReader(new InputStreamReader(server.getInputStream()));
      DataOutputStream toClient = new DataOutputStream(server.getOutputStream());

      clientData = fromClient.readLine();
         System.out.println("Client sent: "+clientData);

(( The problem -imo- may be either this statement: ))

      toClient.writeBytes("Recieved your sentence '"+clientData+"' and more to come :)!");

    //server.close();

  } catch (IOException e) {
    System.out.println("IOException on socket listen: " + e);
    e.printStackTrace();
  }

  }
}

Now the client class CClient:

    public class CClient {

    static String address = "localhost";

static int port = 4444;
static Socket echoSocket;

     public CClient(int port, String addr){

         changePort(port);
         changeAddr(addr);
     }

     public static void main(String[] args) throws IOException, UnknownHostException{

         Scanner scan = new Scanner(System.in);

         System.out.println("Please enter the port to connect to: ");
         int temp_port = Integer.parseInt(scan.nextLine());
         System.out.println("Please enter the address of server: ");
         System.out.flush();
         String temp_addr = scan.nextLine();

         CClient client = new CClient(temp_port,temp_addr);

            PrintWriter out = null;
            BufferedReader in = null;

         try{
             System.out.flush();
             echoSocket =  new Socket(address,port);
             out = new PrintWriter(echoSocket.getOutputStream(), true);
             in = new BufferedReader(new InputStreamReader(echoSocket.getInputStream()));
         }
         catch(IOException e){

             System.err.println("IOException error: " + e.getStackTrace());
         }

         BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));
    String userInput;

    while ((userInput = stdIn.readLine()) != null) {

        out.println(userInput);
            System.out.println("thingy prints right after this...");

(( or here: ))

System.out.println("echo: " + in.readLine());
}
 }

 public void changePort(int port){

     this.port=port;
 }

 public void changeAddr(String addr){

     this.address=addr;
 }
}

Answer 1

  clientData = fromClient.readLine();

  toClient.writeBytes("Recieved your sentence '"+clientData+"' and more to come :)!");

This is a very common problem whose root cause is the failure to document and specify the protocol being used for communication. Here you are receiving lines but sending bytes. If you had a protocol document, it would either specify that lines were exchanged or that arbitrary units of bytes were exchanged. That would show that one of these lines of code is wrong, and you could fix it. But without a protocol specification, you can’t even tell which side is wrong.

Please, take my advice from years of painful lessons — document a protocol before you implement. Then, if it doesn’t work, you can follow this three step process:

1. Does the client follow the documented protocol? If not, it is broken.

2. Does the server follow the documented protocol? If not, it is broken.

3. The protocol specification is broken.

In this case, the protocol specification would document what constitutes a “message” for your protocol. It would then be each side’s responsibility to send complete messages and find these message boundaries on receive. However, in your case, one piece of code expects a line terminator to mark a message boundary and the other side doesn’t send one.

Is the sender wrong to omit a message boundary? Is the receiver wrong to insist on receiving one? Nobody knows because there’s no specification to say what’s the right thing to do.