Question

I have written a simple Java dispatcher with a daemon thread to handle the incoming traffic, and using another thread to send out commands.

The problem comes when the server receives the first message, then the client/server system gets stuck on where the server trying to send out a response to the client. The sockets on both end just simply freeze when the server sends out data.

I have simplified my original problem into a echo server and client; I guess I must have a very very stupid mistake on the code. The code and result on my machine are reproduced below. Can anyone explain what is going wrong?

Thanks!

Here is the results, we can see the server and client stuck once the first message received.

Echo Server listening port...
Echo Server: Waiting from client connection.
Connecting to the server.
Connected to the server.
Dispatcher send: 10
Dispatcher send: 11
Dispatcher send: 12
Dispatcher send: 13
Dispatcher read...
Dispatcher read...
Dispatcher readed 10

Code:

EchoTest:

import java.io.*;
import java.net.*;

public class EchoTest {
  public static void main(String[] args) {
    EchoServer.listen();
    EchoClient client = new EchoClient();
    EchoServer server = EchoServer.accept();
  }
}

Client and Server:

class EchoClient implements Runnable {
  private static final int PORT = 13244;

  Socket socket;
  Disp disp;
  Thread client;

  public EchoClient() {
    client = new Thread(this);
    client.start();
  }

  public void run() {
    try {
      System.out.println("Connecting to the server.");
      Socket socket = new Socket("localhost", PORT);
      System.out.println("Connected to the server.");
      disp = new Disp(socket);

      disp.send(10);
      disp.send(11);
      disp.send(12);
      disp.send(13);


    } catch(IOException e) {
      System.out.println("Would not connect to local host: " + PORT);
      System.exit(-1);
    }
  }

  public void send(int m) {
    disp.send(m);
    System.out.println("Sent message " + m);

    int echo = disp.getMsg();

    if(m == echo) {
      System.out.println("Message " + m + "sent and received.");
    } else {
      System.out.println("Message " + m + "cannot be echoed correctly.");
    }
  }
}

class EchoServer implements Runnable{
  private static final int PORT = 13244;
  private static ServerSocket serverSocket;

  Disp disp;

  public EchoServer(Socket s) {
    disp = new Disp(s);
  }

  public static void listen() {
    System.out.println("Echo Server listening port...");

    try {
      serverSocket = new ServerSocket(PORT);
    } catch (IOException e) {
      System.out.println("Could not listen on port: " + PORT);
      System.exit(-1);
    }
  }

  public static EchoServer accept(){
    try {
      System.out.println("Echo Server: Waiting from client connection.");
      return new EchoServer(serverSocket.accept());
    } catch(IOException e) {
      System.out.println("Couldn't accept connection from client.");
      System.exit(-1);
    }

    return null;
  }

  public void run() {
    while(true) {
      int m = disp.getMsg();
      disp.send(m);
    }
  }
}

Disp:

class Disp implements Runnable{
  int msg = -1;
  Socket socket;
  BufferedInputStream input;
  DataInputStream dis;
  BufferedOutputStream output;
  DataOutputStream dos;
  Thread daemon;

  public Disp(Socket s) {
    this.socket = s;

    try{
      input = new BufferedInputStream(socket.getInputStream());
      dis = new DataInputStream(input);

      output = new BufferedOutputStream(socket.getOutputStream());
      dos = new DataOutputStream(output);
    }catch(IOException e) {
    }
    daemon = new Thread(this);
    daemon.start();
  }

  public void run() {
    while(true) {
      int m = get();
      setMsg(m);
    }
  }

  public void send(int m) {
    synchronized(dos) {
      try{
        System.out.println("Dispatcher send: " + m);
        dos.writeInt(m);
        dos.flush();
      } catch(IOException e) {
      }
    }
  }

  public int get() {
    System.out.println("Dispatcher read...");
    synchronized(dis) {
      try{
        int m = dis.readInt();
        System.out.println("Dispatcher readed " + m);
        return m;
      } catch(IOException e) {
      }
    }

    return -1;
  }

  synchronized public void setMsg(int m) {
    while(true) {
      if(msg == -1) {
        try {
          wait();
        } catch(InterruptedException e) {
        } 
      } else {
        msg = m;
        notifyAll();
      }
    }
  }

  synchronized public int getMsg() {
    while(true) {
      if(msg != -1) {
        try {
          wait();
        } catch(InterruptedException e) {
        } 
      } else {
        notifyAll();
        return msg;
      }
    }
  }
}

Answer 1

That’s an awful lot of code. First of all I suggest cutting it down.

Secondly, you never seem to call EchoServer.run, but it’s difficult to see.