[SOLVED] Help needed debugging a simple server supporting multiple clients (Java)

gregorian · 09-24-2009, 10:49 PM

I don't have any problem when I'm communicating with one client. The problem starts when I connect another client.The server eventually ignores the older clients and responds only to the latest one.

The working mechanism is simple: Accept connections and print messages received from multiple clients.

Client 1 (telnet localhost 5000)

Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
1
1
1
1
1
1
1
1
1
111
Connection closed by foreign host

Client 2 (telnet localhost 5000) Latest client. Works properly.
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
2
2
2
2
2
2
2
2
2
2
2
222
Connection closed by foreign host.

Server output (java DebugServer 5000)

Server started
Incoming connection accepted
RECEIVED: 1
RECEIVED: 1
RECEIVED: 1
RECEIVED: 1
RECEIVED: 1
RECEIVED: 1
Incoming connection accepted
RECEIVED: 2
RECEIVED: 2
RECEIVED: 2
RECEIVED: 2
RECEIVED: 2
RECEIVED: 2
RECEIVED: 2
RECEIVED: 2
RECEIVED: 1
RECEIVED: 2
RECEIVED: 2
RECEIVED: 2
RECEIVED: 222

You can see that it isn't receiving the messages from the first client eventually. I tried debugging it using Netbeans and it shows nothing happens at bfr.readLine() when Client 1 sends a message after some messages have been sent.

Code:

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

public class DebugServer {
	ServerSocket serverSocket;
	BufferedReader bfr;

	public static void main(String args[])
	{
		DebugServer server = new DebugServer();
		server.go(Integer.parseInt(args[0]));

	}
	
	public void go(int portNo)	
	{
		System.out.println("Server started");
		try {
		serverSocket = new ServerSocket(portNo);
		while(true) {
		Socket socket = serverSocket.accept();
		System.out.println("Incoming connection accepted");
		ClientSession c = new ClientSession(socket);
		Thread t = new Thread(c);
		t.start();
		}
		} catch(IOException e) 
		{
			System.out.println("Server IOException");
			e.printStackTrace();
		}
	}


class ClientSession implements Runnable{
	Socket socket;
	
	ClientSession(Socket socket)
	{
		this.socket = socket;
	}

	public void run() {
	try{
			InputStreamReader isr = new InputStreamReader(socket.getInputStream());
			bfr = new BufferedReader(isr);
			String s;

		for(;;) {
			while((s = bfr.readLine()) !=  null) {
				System.out.println("RECEIVED: "+s);
			}
		}
      } catch(IOException e) {
				System.out.println("Server IO Exception");
	} 
	}
	
		}
	
}

Thank you for your help.

nadroj · 09-25-2009, 07:17 PM

you declare "bfr" as a class-wide variable to "DebugServer". theres only one instance of this variable in the server, and for each client you overwrite the previous client's reference to this variable. "bfr" should be declared as local to the "run" method in class "ClientSession".

i suggest one ("named") class per file. if you must have more than one, then declare each one in its own block. your declaring the "ClientSession" class within the scope of "DebugServer". if you would have declared them as two separate blocks, you would have seen the compiler error, thus finding the problem immediately! of course there are other reasons besides this for structuring the classes in separate files too.

you have something like:

Code:

public class Debug Server
{
  bfr;
  //...

  class ClientSession
  {
    //...
    bfr...
  }
}

you would have seen the compiler error if you structured it this way:

Code:

public class Debug Server
{
  bfr;
  //...
}

class ClientSession
{
  //...
  bfr... // variable not defined in this scope!  better make a local one to the run method that uses it
}

gregorian · 09-25-2009, 08:41 PM

Thank you very much! Could you tell me how you found it? Did you just "spot" the error or did you arrive at it through analysis. It seems obvious on hindsight but I'd like to know how you found it. Thank you once again.

nadroj · 09-25-2009, 08:45 PM

this was one of those trickier ones to find. sure, its a simple solution, but when you run into something you cant solve we often look past simple things like where a variable is declared, and think theres something deeper that we are misunderstanding. so, at first no, i did not spot the error and was curious why you were seeing the behaviour (my initial guess was that somehow telnet was screwing it up, and i was going to suggest you write your own java client to talk to your server, rather than telnet). anyway, your welcome.

gregorian · 09-26-2009, 12:14 AM

Well I did write my java client originally and had the same problem. I confirmed that the problem was with the server because telnet wasn't working either. Anyway thank you! This bug had been escaping me for days and I was on the verge of giving up.