Multiplayer game in Java. Connect client (player) to game that was created by other client

Question

I am working on multiplayer game and I cant to find out how can I connect other clients to the created game. I mean that client A create a socket connection to the server an

Answer 1

Another client cannot be connected to the Client A because of his firewall.

You can create two majors kinds of network:

• Server-Client

• Peer-to-Peer

But a client can save some data to the server and the server can send them to all the clients (you don't need a Peer-to-Peer network for allow the Client B to send some data to the Client A).

Example: The Client B send his map position to the server, the server send the data to all the clients, so the Client A is able to draw a character tileset at the position of the Client B.

For connect two PCs together, you need to forward a port from the modem of your server to the PC used as server, and open the port from the firewall of the PC used as server.

You can also take a look here Creating a Multiplayer game in python, I give an example where the clients was able to connect them together with IRC and play at a Tic-Tac-Toe game (so you didn't have to manage a server). I have add an example in Java at the end of this post.

A simple server example:

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Date;


public class Server
{
    public static void main(String[] args) throws Exception
    {
        ServerSocket listener = new ServerSocket(4000);
        String line;
        try
        {
            while (true)
            {
                Socket socket = listener.accept();
                BufferedReader readerChannel = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                BufferedWriter writerChannel = new BufferedWriter(new OutputStreamWriter(socket.getOutputStream()));
                try
                {
                    writerChannel.write(new Date().toString() + "\n\r");
                    writerChannel.flush();

                    while ((line = readerChannel.readLine()) != null)
                    {
                        System.out.println(line);
                    }
                }
                finally
                {
                    socket.close();
                }
            }
        }
        finally
        {
            listener.close();
        }
    }
}

A simple client example:

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.net.Socket;
import java.util.Date;


public class Client
{
    public static void main(String[] args) throws Exception
    {
        Socket socket = new Socket("127.0.0.1", 4000);
        BufferedWriter writerChannel = new BufferedWriter(new OutputStreamWriter(socket.getOutputStream()));
        BufferedReader readerChannel = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        String line;

        writerChannel.write(new Date().toString() + "\n\r");
        writerChannel.flush();

        while ((line = readerChannel.readLine()) != null)
        {
            System.out.println(line);
        }
    }
}

Also take a look at:

import javax.net.ssl.SSLSocket;
import javax.net.ssl.SSLSocketFactory;


public class Client
{
    public static void main(String[] args) throws Exception
    {
        SSLSocketFactory socketBuilder = (SSLSocketFactory) SSLSocketFactory.getDefault();
        SSLSocket socket = (SSLSocket) socketBuilder.createSocket("127.0.0.1", 4000);
    }
}

A simple IRC example:

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import javax.net.ssl.SSLSocket;
import javax.net.ssl.SSLSocketFactory;


public class Client
{
    public static void main(String[] args) throws Exception
    {
        SSLSocketFactory socketBuilder = (SSLSocketFactory) SSLSocketFactory.getDefault();
        SSLSocket socket = (SSLSocket) socketBuilder.createSocket("irc.freenode.net", 6697);
        BufferedWriter writerChannel = new BufferedWriter(new OutputStreamWriter(socket.getOutputStream()));
        BufferedReader readerChannel = new BufferedReader(new InputStreamReader(socket.getInputStream()));

        String line, computerName, nick, login, channel = "#bot", channelPassword = "";
        long id = 1;

        computerName = java.net.InetAddress.getLocalHost().getHostName();
        nick = computerName + "_" + id;
        login = computerName + "_" + id;
        writerChannel.write("NICK " + nick + "\r\n"); // Join IRC with a specific Nick
        writerChannel.write("USER " + login + " 8 * :" + login + " \r\n"); // Join IRC with a specific User
        writerChannel.flush();

        while ((line = readerChannel.readLine()) != null)
        {
            if (line.indexOf("004") != -1) // If connected
            {
                break;
            }
            else if (line.indexOf("433") != -1) // If Nick already in use
            {
                id++;
                nick = computerName + "_" + id;
                login = computerName + "_" + id;
                writerChannel.write("NICK " + nick + "\r\n");
                writerChannel.write("USER " + login + " 8 * :" + login + " \r\n");
                writerChannel.flush();
            }
        }

        writerChannel.write("JOIN " + channel + " " + channelPassword + "\r\n"); // Join a channel
        writerChannel.flush();

        while ((line = readerChannel.readLine()) != null)
        {
            try
            {
                line = line.substring(line.indexOf("#"));
                channel = line.substring(0, line.indexOf(" "));

                if (line.toLowerCase().startsWith("ping")) // avoid ping time-out
                {
                    writerChannel.write("PONG :" + line.substring(5) + "\r\n");
                    writerChannel.flush();
                }
                else if (line.toLowerCase().contains("!ping"))
                {
                    writerChannel.write("PRIVMSG " + channel + " :pong\r\n");
                    writerChannel.flush();
                }
                else if (line.toLowerCase().contains("!join"))
                {
                    String newChannel = line.substring(line.indexOf("!join") + 6);
                    int stringPosition;
                    if ((stringPosition = newChannel.indexOf(" ")) != -1)
                    {
                        String newPassword = newChannel.substring(stringPosition + 1);
                        newChannel = newChannel.substring(0, stringPosition);
                        writerChannel.write("JOIN " + newChannel + " " + newPassword + "\r\n");
                        writerChannel.flush();
                    }
                    else
                    {
                        writerChannel.write("JOIN " + newChannel + "\r\n");
                        writerChannel.flush();
                    }
                }
                else if (line.toLowerCase().contains("!leave"))
                {
                    writerChannel.write("PART " + channel + "\r\n");
                    writerChannel.flush();
                }
                else if (line.toLowerCase().contains("!quit"))
                {
                    writerChannel.write("QUIT\r\n");
                    writerChannel.flush();
                    System.exit(0);
                }
            }
            catch (Exception e)
            {

            }
        }
    }
}

I cannot give you an example for a Peer-to-Peer network because I have never do it. This is really difficult and you have to do a lot of research on internet.

More informations:

• https://docs.oracle.com/javase/7/docs/api/java/net/Socket.html

• https://docs.oracle.com/javase/tutorial/networking/sockets/

• http://www.oracle.com/technetwork/java/socket-140484.html

• You need a multithreaded server for handle many different connections.

Hint - I have already answer at some similar questions. Even if the programming language are sometime different, I give you the link, the logic is always the same so it can maybe help you:

• Creating a Multiplayer game in python

• Xcode Mass Multiplayer (Not What You're Probably Thinking)

• How would an MMO deal with calculating and sending packets for thousands of players every tick for a live action game?

Answer 2

Here is one way to handle it. When a player wants to create a game, his copy of the application should open a ServerSocket on a known port - a port number that the application knows - and maybe display to the player the IP address that the socket was opened on.

Then when another player wants to join a game, she should enter that same IP address and her copy of the application should connect using a regular client Socket, the IP address entered, and the known port that the application knows.

Check the Socket and ServerSocket javadoc for details.