Chat Server Issue

[pasztorpisti]

I detailed a solution/workaround with blocking sockets. Nonblocking sockets is a bit more complicated so search for a good tutorial if you want to learn it. In my workaround solution there is one thread per user, and 1 thread to accept new users. As I described in the solution the user thread sends the incoming messages to the user and then receives the message of the user. It does send some kind of data periodically (every half/one sec) even if there are no incoming messages or message sent by the user. The thread should have an array for incoming messages into which other user threads can put incoming messages. lets say a user thread receives a message from the user - in this case the user thread iterates over the other users and puts this message to their incoming message queue. The other user threads will send this message out to other clients when their thread gets into the state of "sending out incoming messages". You have to use locks on data structures that are used by multiple threads, for example on the list of users. This list is used by both the accept threads and the user threads. You also need to use a lock on the incoming message queue of user threads because it is accessed by both the user thread and other user threads who put messages to it.

[pasztorpisti]

Thinking it over again, a single threaded non-blocking solution might be easier to implement than a multithreaded blocking if someone has no experience with threading. I had a multithreaded async reactor pattern in my mind that put the async solution to a more difficult level than the blocking one... EDIT: I think the OP should try to write both solutions to get the feeling of it if he is interested!

[User 9293174]

What is a non blocking solution, how would it work?

[pasztorpisti]

I dont know how to do it in java so use google for java solutions. To get the grasp you can read these: http://www.scottklement.com/rpg/socktut/nonblocking.html[^] Blocking vs Non blocking socket calls in C++[^]

[Joshua Waring]

OKAY so here comes the update, I've added it into threads but now a unknown error occurs. Thanks to a debugger I know where and when it occurs but I don't know why. I get a exceptionNullPointer at line 11 of Users.java br = new BufferedReader(new InputStreamReader(socket.getInputStream()));

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

public class Room implements Runnable{
public void run(){
ServerSocket serversocket;
Socket socket;
Clients client = new Clients();
try{
// Created a socket in which clients can connect to.
serversocket = new ServerSocket(7776);
// Waits for a client to connect to the server //
while(true){
socket = serversocket.accept();
System.out.println("Client connected" + socket);
// Starts their thread
client.newThread(socket);
}
}catch(IOException e){
System.out.println(e);
}
}
public static void main(String[] args){
(new Thread(new Room())).start();
}
}

import java.io.BufferedReader;
import java.io.IOException;
import java.net.Socket;

public class Clients implements Runnable{
Socket socket1;
public void run(){
Socket socket2 = socket1;
Users userfunction = new Users();
BufferedReader bf;
bf = userfunction.addStream(socket2);
while(true){
// Receives a Message.
try{
String out = bf.readLine();
System.out.println(out);
if(out != null)
userfunction.sendMessageALL(out);

		}catch(IOException e){
			System.out.println(e);
		}
		try{
			Thread.sleep(10);
		} catch (InterruptedException e) {}
	}
}
public void newThread(Socket socket){
	// Creates one thread for ever client, to parse the socket I sent it to socket1 which is then
	// received in the thread which is taken to socket2..
	socket1 = socket;
	(new Thread(new Clients())).start();
}

}

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

public class Users {
ArrayList recieveAL = new ArrayList();
ArrayList sendAL = new ArrayList();
// Creates the BufferedReader and PrintWriter then returns the Reader since the Writer it maintained in the class for the sendMessageAll method.
public BufferedReader addStream(Socket socket){
BufferedReader br;
PrintWriter pw;
try{
br = new BufferedReader(new InputStreamReader(socket.getInputStream()));
pw = new PrintWriter(socket.getOutputStream(), true);
recieveAL.add(br);
sendAL.

[pasztorpisti]

probably because there is a null pointer there. why don't you debug your code?

[pasztorpisti]

OK, here are a few suggestions: 1. I see you are coming from C because you declare all your function local variables at the beginning of the functions (sorry, methods). Declare your variables where you use it, or at least most most-inner scope where you need it. 2. Room does not have to implement the Runnable interface and you dont have to create a new thread for the room in your main(), call the run() method directly and execute it with the main thread. 3. The biggest mistake that causes the nullpointer is in Client.newThread: you should pass in this instead of new Clients(). I would do the work of Client.newThread in the constructor of Client and would create Client only after accepting the socket.

[Joshua Waring]

A null pointer is when it returns a Null, right? I don't see the problem because that part of the code is unchanged. so why return an error now

[pasztorpisti]

You have a serious bug in your newThread() method that causes the nullpointer exception.

[Joshua Waring]

I really don't understand, this is currently beyond me at the moment.

[pasztorpisti]

Well, object oriented programming is not easy at first. Socket/network programming is also hard. Threading is another beast. And you are mixing all of these, are you sure this is what you wanna do?

[Joshua Waring]

I'm committed to learn it and since I'm self taught, I've decided to throw myself in and learn how it all works then learn in more depth.

[pasztorpisti]

I don't recommend doing so, but good luck!

[Joshua Waring]

Parsing the Socket into the thread was my screw up, so It was replaced with parameters of the thread.