Reply to all clients Multithreading

[ManInTheMiddle]

I have a server/client written. At the moment it's replying to individual clients (User one receives user one's post only) and I need multiple users to receive all posts.


Server

import java.net.*;
import java.io.*;
import java.util.*;
 
class ServerThread extends Thread{
	private Socket socket;
 
	ServerThread(Socket socket){
		this.socket = socket;
	}	
 
	public void run(){
		try
		{
			String message = null;
			PrintWriter pw = new PrintWriter(socket.getOutputStream(), true);
			BufferedReader br = new BufferedReader(new InputStreamReader(socket.getInputStream()));
			br.readLine();
			while((message = br.readLine()) != null){ // while messages read them
				pw.println(message);
			}
 
			socket.close();
 
		}	
			catch(IOException e)
			{
				System.out.println("Client disconnected");
			}
	}
}
 
// The server
public class Server{
	public static final int PORT = 7777;
	public ArrayList<Socket> connections = new ArrayList<>();
 
	public static void main(String[] args) throws IOException {
			new Server().run();
	}
 
	public void run() throws IOException{
		ServerSocket serverSocket = new ServerSocket(PORT); 			// wait for request from PORT
		System.out.println("Server is listening for connections");			
 
		while(true){
			Socket socket = serverSocket.accept();						// listen and accept connection
			connections.add(socket);
			new ServerThread(socket).start();
			System.out.println("Client " + connections.size());
		}
	}
}

Client

import java.net.*;
import java.io.*;
import java.util.*;
 
 
 
public class Client{
 
    public static void main(String[] args) throws UnknownHostException, IOException 
	{
		String name = args[0];
		Socket socket = new Socket("localhost", 7777);
		BufferedReader brFromClient = new BufferedReader(new InputStreamReader(socket.getInputStream()));
		PrintWriter pw = new PrintWriter(socket.getOutputStream(), true);
		pw.println(name);
		BufferedReader brFromCmd = new java.io.BufferedReader(new InputStreamReader(System.in));
 
		while(true){
			String readInput = brFromCmd.readLine();
			pw.println(name + ": " + readInput);
			System.out.println(brFromClient.readLine());
		}
 
	}
}

Any help is greatly appreciated..

[Norm]

Save a list of clients to send messages to. When sending a message, loop through the list and send the message to each client in the list.

[ManInTheMiddle]

what would I use to hold the clients?

a socket Array?

[Norm]

Use an ArrayList to hold the objects needed to do the posts.

[ManInTheMiddle]

Sorry, I meant an ArrayList<Socket> ?

[Norm]

You might want to keep more info about the connection than just the Socket. Define a class that contains the Socket and allows for other info about the client to be saved.

[ManInTheMiddle]

Cheers, I'll give it a go. Much appreciated..

--- Update ---

I have, class ServerThread and class Server in my Server, can it be done in the ServerThread? As the ServerThread class holds the socket already .

[Norm]

can it be done

What is the "it" that is to be done?

[ManInTheMiddle]

You might want to keep more info about the connection than just the Socket. Define a class that contains the Socket and allows for other info about the client to be saved.

Can this be done in the ServerThread class?

[Norm]

It should be done where you want to save the data about a connection to a client.
When a connection to the server is made, create an instance of the class with a reference to the Socket and save that in the list to be used for sending messages to connected clients.

[ManInTheMiddle]

I came up with this Data Structure

class CountConnections extends Thread{
	private ArrayList<Socket> connections = new ArrayList<>();
	private Socket socket;
 
	CountConnections(){
		socket = null;
	}	
 
	void add(Socket s){
		connections.add(s);
	}
 
	void remove(Socket s){
		connections.remove(s);
	}
 
	void print(){
		System.out.println(connections.size());
	}
}

If I add the information to the ArrayList then should be stored correctly for sending?

[Norm]

Why does the class extend Thread?
How would that class be useful? The only thing it saves is a Socket. Is the all the info about a connection that needs to be saved? What about userid, status, last message received, message being sent,...? I don't know what the app might want to save about a client that is connected.

[ManInTheMiddle]

Actually, this class is just to save the number of connections. So, it displays on the server eg Client 1... Client 2...Client 3 then if someone leaves the chat it decrease eg. Client 2... Client 1 etc.. This is also in my assignment.

Sorry, I moved to a different problem late last night.

So, to send information to all clients userid, status, last message received and sent needs to be stored then sent?
I'll have a go even though it's really sunny here in Ireland today haha and we don't get much of that

[ManInTheMiddle]

Right, so, I'v come up with this

class ServerStatus {
	private final ArrayList<Socket> clients;
    private final ArrayList<String> names;
	private final ArrayList<String> messages;
 
    public ServerState() {
    	this.clients = new ArrayList<Socket>();
    	this.names = new ArrayList<String>();
		this.message = new ArrayList<String>();
    }
 
    public synchronized void addUser(String name, Socket client) {
    	this.clients.add(client);
    	this.names.add(name);
 
    }
 
    public void removeUser(String name, Socket client) throws IOException {
   		this.clients.remove(client);
   		this.names.remove(name);	
   		client.close();
    }
 
	public synchronized void addMessage(String message) {
    	this.message.add(message);
    }
 
    public List<String> getUsers() {
    	return this.names;
    }
 
    public List<Socket> getClients() {
    	return this.clients;
    }
 
	public List<String> getMessage() {
    	return this.messages;
    }
 
}

Would this work??

[Norm]

Parallel arraylists are going to be a problem. Put all the data into one class and save an instance of that class in the arraylist.

[ManInTheMiddle]

I don't quite follow. Can I add different data types ? As i'm adding Sockets and Strings.

would I use?

ArrayList<ClassName> ListName = new ArrayList<>();

[Norm]

Something like this:

class ClassToHoldData {
  SomeCls item1;
  SomeOtherCls item2;
  AnotherCls item3;
 
  ClassToHoldData(SomeCls i1, SomeOtherCls i2, AnotherCls ac) {
      item1 = i1;     //  save data in class variables
      item2 = i2;
      item3 = ac;
   } //  end constructor
 
   //  There would be getters and other methods here
 
} // end class
 
//  somewhere else in the code
   ArrayList<ClassToHoldData> listOfStuff = new ArrayList<>();
   ...
   listOfStuff.add(new ClassToHoldData(the data to be saved goes here));

[ManInTheMiddle]

Ah cool, much appreciated man.. I've never coded like that before so it's a bit tricky to get my head around..

[ManInTheMiddle]

so, I'v come up with this but it's showing a NullPointerException. Any suggestions?

class ServerThread extends Thread
{
	private Socket socket; 
	private String name, message;
    private ArrayList <ServerThread> clientList;
	private ArrayList <String> nameList;
	private PrintWriter out; 
    private BufferedReader in;
 
	public synchronized void sendToAll(String message)
	{
    	for(ServerThread c: clientList)
		{
			c.out.println(message);				
		}
    }
 
	ServerThread(Socket socket, ArrayList <ServerThread> clientList)
	{
    	super("ServerThread");
    	this.socket = socket;
    	this.clientList = clientList;
    }	
 
	public void run(){
		try
		{
			out = new PrintWriter(socket.getOutputStream(), true);	// be able to echo back message to client
			in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
			name = in.readLine();
 
			if(!nameList.contains(name))
			{
				nameList.add(name);
			}	
 
			sendToAll( name + " has joined the chatroom ....\n");
 
			while((message = in.readLine()) != null)
			{ 			
				sendToAll(name+": "+ message);	
			}
 
			clientList.remove(currentThread());
 
			System.out.println("Clients :" +clientList.size() + "\n");
			out.close();
			in.close();
			socket.close();
		}	
			catch(IOException e)
			{
				 e.printStackTrace();
			}
	}
}
 
// The server
public class Server
{
	public static final int PORT = 7777;
	private ArrayList <ServerThread> clientList = new ArrayList<>();
 
	public static void main(String[] args) throws IOException {
			new Server().run();
	}
 
	public void run() throws IOException{
		ServerSocket serverSocket = new ServerSocket(PORT); 			// wait for request from PORT
		System.out.println("Server is listening for connections...");			
 
		while(true){
			try{
				Socket socket = serverSocket.accept();						
				ServerThread client = new ServerThread(socket,clientList);
				//add client to clientList and start the thread
				clientList.add(client);
				System.out.println("Clients :" + clientList.size() + "\n");
				client.start();
			} 
			catch (IOException e)
			{
				e.getMessage();
			}
 
			serverSocket.close();
		}
	}
}

[Norm]

it's showing a NullPointerException

Copy the full text of the error message and paste it here.
Find the variable with the null value and backtrack in the code to see why it does not have a valid value.

[ManInTheMiddle]

Exception in thread "ServerThread" java.lang.NullPointerException
at ServerThread.run(Server.java:36)

it's the

if(!nameList.contains(name)) // line 36
			{
				nameList.add(name);
			}

[Norm]

What variable has the null value? Why doesn't it have a valid value?

[ManInTheMiddle]

It should be adding a Clients name if the name doesn't already exist, see previous post.

I'v an ArrayList<String> nameList that should be getting the client from clientList.add(client);

So, for some reason there is no data been added when I run a client

[Norm]

What variable has the null value?
An example of how to get a NPE:

  String aStr = null;
  int len = aAStr.length();  //   NPE here because aStr is null

[ManInTheMiddle]

The private String name is null. But it should be receiving data from

                        in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
			name = in.readLine();