Java < - > C＃Socket：无法接收消息

Question

I need a socket communication between my own written java server and C# client, the problem is that the C# client can't receive any messages from my java server, but sending messages to my java server works.

my workflow:

1. Java: Create Server
2. Java: Waiting for Client connection
3. c#: Create Client
4. c#: Build connection to the server
5. c#: send a msg to the server
6. Java: msg received
7. java: send msg to c# client
8. c#: receiving msg from server <- this is the point where the client waits for a message but never get.

Java Server code:

public class Communicator {

   private int m_port;
   private Socket m_socket;
   private ServerSocket m_serverSocket;

   public Communicator(int port) {
        this.m_port = port;
        initConnection();
    }

    private void initConnection() {

        try {
            System.out.println("Creating Server");
            m_serverSocket = new ServerSocket(m_port);
            System.out.println("Waiting for client connection");
            m_socket = m_serverSocket.accept();
            System.out.println("Connection made");

        } catch (IOException e) {
            e.printStackTrace();
        }
    }

   public String sendMsg(JSONMessage msg) {
        try {

     //get msg
     BufferedReader bufferedReader = new BufferedReader(
           new InputStreamReader(m_socket.getInputStream()));
     System.out.println("Waiting for msg...");
       String answer = bufferedReader.readLine();
       System.out.println("Received: " + answer);

       //send msg
       PrintWriter writer = new PrintWriter(m_socket.getOutputStream(),true);
       writer.print(msg.getMsg());
       System.out.println("Sending: " + msg.getMsg());

       writer.flush();

    } catch (IOException e) {
        e.printStackTrace();
    }

        return "";
    }

}

My C# client code:

class Communicator
    {
        private int m_port;
        private Thread mainThread;

        public Communicator(int port)
        {
            m_port = port;
            mainThread = new Thread(new ThreadStart(this.initConnection));
            mainThread.Start();
        }

        public void initConnection()
        {
            IPEndPoint ip = new IPEndPoint(IPAddress.Parse("127.0.0.1"), m_port);
            Socket server = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);

            try
            {
                Console.WriteLine("Trying to build connection");
                server.Connect(ip);
                Console.WriteLine("Connection successful");

                NetworkStream ns = new NetworkStream(server);
                StreamReader sr = new StreamReader(ns);
                StreamWriter sw = new StreamWriter(ns);
                string data;

                string welcome = "Hello";
                Console.WriteLine("Sending: " + welcome);
                sw.WriteLine(welcome);
                sw.Flush();

                Console.WriteLine("Receiving...");
                data = sr.ReadLine();
                // --> NEVER REACHING THIS POINT <---
                Console.WriteLine("Received: " + data);                                         

            }
            catch (SocketException e)
            {
                Console.WriteLine("Connection failed.");
                return;
            }

        }
    }

Does somebody has any idea why it never reaches my client code Console.WriteLine("Received: " + data); ?

I already tried with waits on both sides. I'm not getting any exceptions or error so I don't have really an idea where my problem is.

Thanks

Answer 1

If your receiver is expecting lines , your sender has to send lines . Your sender does not send lines, so the receiver waits forever until it gets one.

To avoid these kinds of problems in the future, you should always make a specification document that explains how your protocol works, ideally at the byte level. It should specify whether the protocol contains messages and if so, how the sender marks message boundaries and how the receiver identifies them.

Here, your receiver identifies message boundaries by looking for line endings. But your sender doesn't mark message boundaries with line endings. So the receiver waits forever.

If you had a protocol specification, this would have been obvious. In the future, I strongly urge you to invest the time to specify every protocol you implement.

Answer 2

You need to use println() which adds a new line instead of print(). In Java, readLine waits for a new line and I would expect it to do the same in C#. Also println will auto-flush, so you don't need to flush as well.

If you intend to use this connection mreo than once, you need to keep the BufferedReader and PrintWriter for the connection. (So I suggest you create these after the socket is created/accepted) Creating these multiple times for the same socket can be error prone and confusing.