My own fixed version of http://www.instructables.com/id/Creating-a-Chat-Server-Using-Java/

README.md

what is this

original source is http://www.instructables.com/id/Creating-a-Chat-Server-Using-Java/

I tested that code, but that doesnot work. I rewrite many part of that.

other

You can use your own lisk.

I feel weird about class named ServerThread is in client package (also ClientThread is in server package).

I'm not native English speaker. Correct my English welcome.

ChatServer.java

// place this file the path such ends with: ChatServer/server/ChatServer.java

package ChatServer.server;

import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;
import java.util.List;

public class ChatServer {

    private static final int portNumber = 4444;

    private int serverPort;
    private List<ClientThread> clients; // or "protected static List<ClientThread> clients;"

    public static void main(String[] args){
        ChatServer server = new ChatServer(portNumber);
        server.startServer();
    }

    public ChatServer(int portNumber){
        this.serverPort = portNumber;
    }

    public List<ClientThread> getClients(){
        return clients;
    }

    private void startServer(){
        clients = new ArrayList<ClientThread>();
        ServerSocket serverSocket = null;
        try {
            serverSocket = new ServerSocket(serverPort);
            acceptClients(serverSocket);
        } catch (IOException e){
            System.err.println("Could not listen on port: "+serverPort);
            System.exit(1);
        }
    }

    private void acceptClients(ServerSocket serverSocket){

        System.out.println("server starts port = " + serverSocket.getLocalSocketAddress());
        while(true){
            try{
                Socket socket = serverSocket.accept();
                System.out.println("accepts : " + socket.getRemoteSocketAddress());
                ClientThread client = new ClientThread(this, socket);
                Thread thread = new Thread(client);
                thread.start();
                clients.add(client);
            } catch (IOException ex){
                System.out.println("Accept failed on : "+serverPort);
            }
        }
    }
}

Client.java

// place this file the path such ends with: ChatServer/client/Client.java

package ChatServer.client;

import java.io.IOException;
import java.net.Socket;
import java.util.Scanner;

public class Client {

    private static final String host = "localhost";
    private static final int portNumber = 4444;

    private String userName;
    private String serverHost;
    private int serverPort;
    private Scanner userInputScanner;

    public static void main(String[] args){
        String readName = null;
        Scanner scan = new Scanner(System.in);
        System.out.println("Please input username:");
        while(readName == null || readName.trim().equals("")){
            // null, empty, whitespace(s) not allowed.
            readName = scan.nextLine();
            if(readName.trim().equals("")){
                System.out.println("Invalid. Please enter again:");
            }
        }

        Client client = new Client(readName, host, portNumber);
        client.startClient(scan);
    }

    private Client(String userName, String host, int portNumber){
        this.userName = userName;
        this.serverHost = host;
        this.serverPort = portNumber;
    }

    private void startClient(Scanner scan){
        try{
            Socket socket = new Socket(serverHost, serverPort);
            Thread.sleep(1000); // waiting for network communicating.

            ServerThread serverThread = new ServerThread(socket, userName);
            Thread serverAccessThread = new Thread(serverThread);
            serverAccessThread.start();
            while(serverAccessThread.isAlive()){
                if(scan.hasNextLine()){
                    serverThread.addNextMessage(scan.nextLine());
                }
                // NOTE: scan.hasNextLine waits input (in the other words block this thread's process).
                // NOTE: If you use buffered reader or something else not waiting way,
                // NOTE: I recommends write waiting short time like following.
                // else {
                //    Thread.sleep(200);
                // }
            }
        }catch(IOException ex){
            System.err.println("Fatal Connection error!");
            ex.printStackTrace();
        }catch(InterruptedException ex){
            System.out.println("Interrupted");
        }
    }
}

ClientThread.java

// place this file the path such ends with: ChatServer/server/ClientThread.java

package ChatServer.server;

import java.io.IOException;
import java.io.PrintWriter;
import java.net.Socket;
import java.util.Scanner;

public class ClientThread implements Runnable {
    private Socket socket;
    private PrintWriter clientOut;
    private ChatServer server;

    public ClientThread(ChatServer server, Socket socket){
        this.server = server;
        this.socket = socket;
    }

    private PrintWriter getWriter(){
        return clientOut;
    }

    @Override
    public void run() {
        try{
            // setup
            this.clientOut = new PrintWriter(socket.getOutputStream(), false);
            Scanner in = new Scanner(socket.getInputStream());

            // start communicating
            while(!socket.isClosed()){
                if(in.hasNextLine()){
                    String input = in.nextLine();
                    // NOTE: if you want to check server can read input, uncomment next line and check server file console.
                    // System.out.println(input);
                    for(ClientThread thatClient : server.getClients()){
                        PrintWriter thatClientOut = thatClient.getWriter();
                        if(thatClientOut != null){
                            thatClientOut.write(input + "\r\n");
                            thatClientOut.flush();
                        }
                    }
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

ServerThread.java

// place this file the path such ends with: ChatServer/client/ServerThread.java

package ChatServer.client;

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

public class ServerThread implements Runnable {
    private Socket socket;
    private String userName;
    private boolean isAlived;
    private final LinkedList<String> messagesToSend;
    private boolean hasMessages = false;

    public ServerThread(Socket socket, String userName){
        this.socket = socket;
        this.userName = userName;
        messagesToSend = new LinkedList<String>();
    }

    public void addNextMessage(String message){
        synchronized (messagesToSend){
            hasMessages = true;
            messagesToSend.push(message);
        }
    }

    @Override
    public void run(){
        System.out.println("Welcome :" + userName);

        System.out.println("Local Port :" + socket.getLocalPort());
        System.out.println("Server = " + socket.getRemoteSocketAddress() + ":" + socket.getPort());

        try{
            PrintWriter serverOut = new PrintWriter(socket.getOutputStream(), false);
            InputStream serverInStream = socket.getInputStream();
            Scanner serverIn = new Scanner(serverInStream);
            // BufferedReader userBr = new BufferedReader(new InputStreamReader(userInStream));
            // Scanner userIn = new Scanner(userInStream);

            while(!socket.isClosed()){
                if(serverInStream.available() > 0){
                    if(serverIn.hasNextLine()){
                        System.out.println(serverIn.nextLine());
                    }
                }
                if(hasMessages){
                    String nextSend = "";
                    synchronized(messagesToSend){
                        nextSend = messagesToSend.pop();
                        hasMessages = !messagesToSend.isEmpty();
                    }
                    serverOut.println(userName + " > " + nextSend);
                    serverOut.flush();
                }
            }
        }
        catch(IOException ex){
            ex.printStackTrace();
        }

    }
}

Sorry for procrastinate.

Some of the questions are about how to execute it, so let me explain.
I was also interested in the question of running it in Eclipse, so I tried it, even though I don't usually use Eclipse.
I only explain with Eclipse 2020.
Of course, you don't need to use Eclipse to run these programs.

note: This answer is a machine translation, so the sentence may be unnatural. I lightly checked the English text, though.

To run these programs, do like following:

1. Run a server first.
   right click inside ChatServer.java source code
   
   Choose Run > Java Application
   

2. Then run a client.

Perform the same operation in Client.js.

You can see the runner pain like below:
note that: This is not current running list. This is a list of execution setting.

3. Switch the consoles to input and check output.

You can switch the console by clicking icon.

dude can u tell me how i chage localhost to ip adress for use in lan chat pls

I'm getting a null pointer access on the readName conditional in the Client class while(readName != null || readName.trim().isEmpty())

Edit: nevermind, I see that I wrote != instead of == for the lhs of the condition haha

There's issues with your scanners. The are not being closed and are causing leaks.

Hello. I have compiled it with IntelliJ and I have successfully accepted from clients.

Here is in Server:
server starts port = 0.0.0.0/0.0.0.0:4444
accepts : /127.0.0.1:52503

Here is in Client:
Input username:
client1

But when im trying to send message, i tried to type in both client and server terminal, nothing happened. What should i do?

Here is in Server:
server starts port = 0.0.0.0/0.0.0.0:4444
accepts : /127.0.0.1:52503
hello

hi
wait?

client1

Here is in Client:
Input username:
client1
hello
send request
wahy

Just one question, How would you change this code to make it work with SSL to encrypt it?

Thank you so much for sharing! This really helps me a lot on understanding concurrent programming and networking.