Java - Chat async

Question

I want to implement a chat in java, which works asynchronously, and I wanted to know the best architecture to do this.

I managed to make a public chat using sockets(Netty), but I got to the following problem:

1. The server sends messages to everyone, rather than a specific group. (How to select the recipient?)

If you have an example project, thank you.

Answer 1

I don’t know Netty, but from the link you passed, I believe he’s sending messages to everyone in this method:

@Override
protected void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {
// Send the received message to all channels but the current one.

    for (Channel c: channels) {
        if (c != ctx.channel()) {
            c.writeAndFlush("[" + ctx.channel().remoteAddress() + "] " + msg + '\n');
            } else {
                c.writeAndFlush("[you] " + msg + '\n');
            }
        }
}

And apparently every Channel represents a user connected to the server. If you choose only one of them, only this client will receive the message.

Anyway, I believe that Netty offers a number of features that you shouldn’t need at the moment. I would start by taking a look at java Docs tutorials. There is a ready example of server and client.

Although they are very simple, with some modifications, it is possible to turn them into a complete chat program, with separate messages for each user.

Here’s an experience of mine, based on the javadocs tutorial.

I included a ArrayList<> to save the users who connect:

public static ArrayList<SCThread> lista = new ArrayList<SCThread>();

And when the server receives a message, I loop like Netty’s, but this time looking for the recipient of the message.

Example of the server:

Example of customers (in my version, x <msg> sends message to all):

Already <número> <msg> sends the message to the user with this number:

Servidorchat.java:

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

public class ServidorChat {

    public static void main(String[] args) throws IOException {

        if (args.length != 1) {
            System.err.println("Utilizacao: java ServidorChat <porta>");
            System.exit(1);
        }

        System.out.println("ServidorChat");
        int porta = Integer.parseInt(args[0]);
        boolean portaAberta = true;

        try (ServerSocket servidorSocket = new ServerSocket(porta)) {
            while (portaAberta) {
                SCThread s = new SCThread(servidorSocket.accept());
                s.start();
                Usuarios.adicionar(s);
            }
        } catch (IOException e) {
            System.err.println("Erro ao abrir a porta " + porta);
            System.exit(-1);
        }
    }

}

Scthread.java:

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

public class SCThread extends Thread {
    private Socket socket = null;

    public SCThread(Socket socket) {
        super("ServidorChatThread");
        this.socket = socket;
    }

    public void run() {

        try (BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));) {
            String inputLine;

            while ((inputLine = in.readLine()) != null) {

                for (int i = 0; i < Usuarios.lista.size(); i++) {

                    if (inputLine.startsWith(i + " ")) {
                        Usuarios.lista.get(i).envia(this.getName() + " te diz: " + inputLine.substring(2));
                    }
                }

                if (inputLine.startsWith("x ")) {

                    for (SCThread usuario : Usuarios.lista) {

                        if (usuario != this) {
                            usuario.envia(this.getName() + ": " + inputLine.substring(2));
                        }
                    }
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void envia(String msg) throws IOException {
        socket.getOutputStream().write((msg + "\n").getBytes());

    }

}

Chatcliente.java.

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

public class ChatCliente {

    static Socket s;

    public static void main(String[] args) throws IOException {

        if (args.length != 2) {
            System.err.println("Utilizacao: java ChatCliente <endereco> <porta>");
            System.exit(1);
        }

        String hostName = args[0];
        int portNumber = Integer.parseInt(args[1]);

        s = new Socket(hostName, portNumber);

        ClienteThread cT = new ClienteThread(s);
        cT.start();

        Scanner scan = new Scanner(System.in);

        while (scan.hasNextLine()) {

            enviar(scan.nextLine() + "\n");

        }
        scan.close();

    }

    public static void enviar(String str) throws IOException {

        s.getOutputStream().write(str.getBytes());

    }
}

Clientethread.java

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

public class ClienteThread extends Thread {

    BufferedReader in;

    public ClienteThread(Socket s) throws IOException {

        in = new BufferedReader(new InputStreamReader(s.getInputStream()));
    }

    String mensagem;

    public void run() {

        try {
            while ((mensagem = in.readLine()) != null)

                System.out.println(mensagem);
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

}

Users.

import java.io.IOException;
import java.util.ArrayList;

public class Usuarios {

    public static ArrayList<SCThread> lista = new ArrayList<SCThread>();

    public static void adicionar(SCThread u) throws IOException {

        lista.add(u);
        u.setName("Usuario " + (lista.size() - 1));
        u.envia("Servidor: Voce eh o " + u.getName());
        System.out.println("Quantidade de usuarios: " + lista.size());
    }

}

Note that this is just a general idea. Many steps have been set aside, and many things need to be implemented. For example:

1. This code is not thread safe (if many users connect at the same time, it will crash). Most notable is the ArrayList<> which should be replaced, as said here.

2. There is no deal for connection break - if a user disconnects, it will start giving error too (I removed to reduce the code size).