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Java RMI cannot connect to server via WAN while it works in LAN

I'm working on a project using Java RMI and with a client-server implementation.
On the server side, this is how I initialize my server:
final String url = "localhost";
final int port = 8090;
LocateRegistry.createRegistry(port);
IServerGame serverGame = new ServerGame();
Naming.rebind("//"+url+":"+port+"/undercover", serverGame);
System.out.println("Server running at //" + url + ":" + port + "/undercover")
Undercover is the name of my application. We chose to use the port 8090
arbitrarily.
Now, here is how I initialize the connection client-side:
try {
server = (IServerGame) Naming.lookup("//"+address+":8090/undercover");
System.out.println("Connected to server " + address + ".");
} catch (Exception e) {
System.out.println("Connection failed.");
e.printStackTrace();
}
address is a string in the parameter of the method which initializes the connection and is the IP of the server. We ask the client to enter this IP to allow us to connect to different servers if we want.
When I run my application in local, whether I use localhost or my private IP 192.168.x.x as address, everything works fine. The client connects to the server and I can use the application. The problem is that when I want to use the application via WAN, sending the client to a friend and starting the server on my local machine, it doesn't work anymore. I get this error:
java.rmi.ConnectException: Connection refused to host: (private IP of host machine); nested exception is:
java.net.ConnectException: Connection timed out: connect
I've already checked a lot of posts in StackOverflow with the exact same problem and the usual answer is to either set the SystemProperty(java.rmi.server.hostname,"192.168.x.x") or do it via the -Djava.rmi.server.hostname in command line. I'm using Gradle to compile. I run the application via the run task. The client is a basic fx application too. None of this works unfortunately. I've also tried to open the port 8090 in my firewall but it doesn't solve the problem either.
Now maybe I'm doing this wrong. I've also tried to replace my private IP 192.168.x.x, which is IPv4 that I found via ipconfig in the command shell, with my public IP 79.95.x.x. But, again, it doesn't work.
I tried adding the SystemProperty(java.rmi.server.hostname,"192.168.x.x") like the first line of code that appears in the server code I showed you above.
I'm connected to internet via 4G. I don't have a box connection, so I can't really go to the box settings to allow certain ports, if that's ever a thing that could fix the issue.
EDIT :
So i've tried to switch from naming implementation in server-side to Registry implementation as it was suggested bellow but it didn't make any difference. As i thought it could be just a connection problem, i asked a friend to ping the server with telnet on the port 8090 and in fact it didn't work. He ran telnet [domain name or ip] [port] and the error was :
Unable to connect to remote host: Connection timed out
So the problem is that the external clients cannot reach my server or connect to the port. As i'm using 4G as internet connection as i mentioned above, any idea on how i could make the port reachable ?

Try using class java.rmi.registry.Registry rather than class java.rmi.Naming.
On the server side:
final int port = 8090;
Registry registry = LocateRegistry.createRegistry(port);
ServerGame serverGame = new ServerGame();
IServerGame stub = (IServerGame) UnicastRemoteObject.exportObject(serverGame, 0);
registry.bind("undercover", stub);
On the client side:
Registry registry = LocateRegistry.getRegistry("server host name or IP", 8090);
IServerGame serverGame = (IServerGame) registry.lookup("undercover");

Can't connect to Android server using hostname

i'm trying to make a Java application that connects to an Android application.
Both my pc and my phone are connected to the same network.
This is the Java client wich runs on my pc:
client = new Socket("muffin", port);
System.out.println("Connected");
output = new ObjectOutputStream(client.getOutputStream());
output.flush();
input = new ObjectInputStream(client.getInputStream());
System.out.println("Streams ready");
And this is the Android application wich works as the server:
server = new ServerSocket(port);
socket = server.accept();
Log.i("Server", "Connected");
output = new ObjectOutputStream(socket.getOutputStream());
output.flush();
input = new ObjectInputStream(socket.getInputStream());
In the manifest i added the permissions:
<uses-permission android:name="android.permission.INTERNET"></uses-permission>
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"></uses-permission>
As you can see i'm trying to connect from my pc to the android server by using its hostname (i changed my android hostname to "muffin"), but it doesn't connect and it throws an exception:
java.net.UnknownHostException: muffin
If instead of the hostname i use the ip address, it works without problems.
It looks like it cannot find a device on LAN called "muffin", but you can see from this screenshot of my modem page that the name is right:
That said, i tryed to use the android application as the client and the java program as the server, but it looks like android has some problems because it didn't connect to my pc even by using the ip address instead of the hostname.
Do you have any idea on how to fix this problem?
Thanks in advance and sorry for my english, it's not my mother tongue.

I solved the problem, i've had to allow the traffic on the specific port through the windows firewall.
Now i can connect to the android application using its hostname.

You can't do:
new Socket("muffin", port);
There is a difference between Hostname, and Host, so:
java.net.UnknownHostException: muffin
means that the host "muffin" doesn't exist, and that's true: the only existing host is 192.168.1.105, who has a hostname who is "muffin".
So you should have done:
new Socket("192.168.1.105", port);
It is not possible to get a host from it's hostname : so if it is the only way for you to do it, you will have to do a huge scan of all the local network, and then see which host that is connected has the good hostname. As this method is much much harder, i realy recommand you to find another way to do it :)

Java Socket Bind Error

I have been having trouble about socket usage in java. First of all, let me explain what I want to do with sockets in java. I want to connect my laptop over the Internet via it. My laptop has a server and a client must connect over the internet. Because I have a router to handle my local network and I do not want to lead a port on the router to my laptop, I need to follow the path "internet->router->localNetwork->mylaptop". The problem is I have found a way to use both the internet ip address and the local ip address; However, it throws an exception : "Exception in thread "main" java.net.BindException: Address already in use"
The code I try is :
InetAddress addr = InetAddress.getByName("XXX.XXX.XXX.XXX");
InetAddress local = InetAddress.getByName("YYY.YYY.YYY.YYY");
Socket socket = new Socket(addr, 1111, local, 1111); // The line I have got exception
With leading router port to my laptop, I can run this code for the similar purposes:
Socket socket = new Socket("XXX.XXX.XXX.XXX", 1111);
*Xs stand for internet ip address
*Ys stand for local ip address
*Codes are belongs to client side of code

As far as I understand, you have a router with Internet (WAN) IP XXX.XXX.XXX.XXX with NAT, and you have a laptop with Local (LAN) IP YYY.YYY.YYY.YYY connected to the router, and you assume that
Socket socket = new Socket(InetAddress.getByName("XXX.XXX.XXX.XXX"), 1111, InetAddress.getByName("YYY.YYY.YYY.YYY"), 1111);
Will connect to the laptop. That is not correct.
Documentation of the constructor of Socket class you are using tells:
Creates a socket and connects it to the specified remote host on the specified remote port. The Socket will also bind() to the local address and port supplied.
That is not what you want.
You can't connect to a device behind NAT like this. You have to "lead a port on the router".

You don't need to specify the local address:port of the Socket, and you are doing so incorrectly. Remove the last two parameters.

Java sockets over internet

I have written a client and server using java sockets. The two machines are on the same network. When I connect using the local IP addresses there are no issues. However if I use the public IP address the connection times out and the client throws "SocketException: connection reset", and the server throws "SocketException: connection reset by peer: socket write error". This happens most of the time, but once in a blue moon the connection actually succeeds. I can successfully ping the servers local address and the router from the client machine.
I have gone into my router, assigned the server machine to a permanent IP address, and forwarded all traffic on the relevant port to that IP address. Unfortunately that was the only thing that I thought could have solved the issue and it did not. I have also turned off windows firewall on the server, still no luck.
The client: (ip taken from whatismyip.com)
Socket s = new Socket("xx.xxx.xxx.xx", 27499);
to connect.
The server:
InetAddress ip = InetAddress.getLocalHost();
ServerSocket ss = new ServerSocket(27499, 0, ip);
Socket cs = ss.accept();
Any help would be appreciated. I can provide more code if necessary.

"SocketException: connection reset by peer: socket write error"
This means the server is actually rejecting the connection, could be a firewall issue, the server rejecting the connection, the server actually being down or overloaded.

Unable to connect on socket across different networks

I am having trouble connecting my online application to others across another network. I am able to give them the hostAddress to connect when we are on the same network but when we are doing it across the internet the generated host address doesn't allow a connection, nor does using the ip address gotten from online sites such as whatismyip.com
My biggest issue isn't debugging this code, because it works over intra-network but The server doesn't see attempts when we try to move to different networks. Also, the test port I am using is 2222.
InetAddress addr = InetAddress.getLocalHost();
String hostname = addr.getHostName();
System.out.println("Hostname: " + hostname);
System.out.println("IP: " + addr.getHostAddress());
I display the host to the server when it is starting
if (isClient) {
System.out.println("Client Starting..");
clientSocket = new Socket(host, port_number);
} else {
System.out.println("Server Starting..");
echoServer = new ServerSocket(port_number);
clientSocket = echoServer.accept();
System.out.println("Warning, Incoming Game..");
}

If it works on your local lan but not across the internet then one or both peers are probably on a NAT'ed connection, meaning that the public IP address you see on the internet is not the same as the IP address of the machine you are trying to talk to. You would probably need to set up some kind of port forwarding to allow your app to connect.

The issue is probably firewall configuration.
Assuming you're testing this at home (it would usually be more complex from a university or company).
Usually you'll need to configure your router to let port 2222 open (you can also open port 5555 and tell your router to redirect to the host you want on your lan (there might be many), and port 2222).
To sum up:
other user ----> internet ----> [your modem] internet_IP -> [your router] lan_IP -> your computer lan_IP2
internet_IP is given by your ISP; find it here: http://www.whatismyip.com/
lan_IP: you defined in your router configuration. Typically: 192.168.0.1
lan_IP2: usually given to your PC by the router (DHCP). Find it by typing ipconfig (Windows) or ifconfig (Linux).
You need to tell your router to open port 2222, and route it to lan_IP2 on port 2222.
Configuring the router is usually done by connecting on its http interface: http://192.168.0.1

Some additional information might be helpful. Can you ping the machine from where you are? Are you attempting to go through a firewall? You say they work over localhost or local network - those are a bit different. Do you mean a network using local space (i.e. 10...* or 192.168.. or the like)? You say you are using a test ip of 2222 - that is not an ip address. Is that the domain of the address? Or is that the port number?