java rmi 服务器主机名

Question

I have two RMI java applications:

server

System.setProperty("java.rmi.server.hostname", "0.tcp.ngrok.io");
System.setProperty("java.security.policy", "security.policy");
if (System.getSecurityManager() == null) {
    System.setSecurityManager(new SecurityManager());
}

Registry r = LocateRegistry.createRegistry(1099);
r.rebind("Calc", new CalculatorImpl());

client

System.setProperty("java.rmi.server.hostname", "0.tcp.ngrok.io");
System.setProperty("java.security.policy", "security.policy");
Calculator c = (Calculator) Naming.lookup("//0.tcp.ngrok.io:18872"+"/Calc"); 
int sum = c.add(1, 5);

Communication between the two is done with the help of Ngrok:

ngrok tcp 1099

result

   Session Status                online
   Account                       nival (Plan: Free)
   Version                       2.2.8
   Region                        United States (us)
   Web Interface                 http://127.0.0.1:4040
   Forwarding                   tcp://0.tcp.ngrok.io:18872 -> localhost:1099

   Connections                   ttl     opn     rt1     rt5     p50     p90
                                  5       0       0.00    0.00    5.88    17.03

My problem is :

 RemoteException
 java.rmi.ConnectException: Connection refused to host: 0.tcp.ngrok.io; nested exception is: 
 java.net.ConnectException: Connection timed out: connect

I think the problem is at

System.setProperty("java.rmi.server.hostname","0.tcp.ngrok.io");

what should I put on the server and client at "java.rmi.server.hostname" ?

(Sorry for my bad english)

Answer 1

In general it's only required on server side to show on what hostname exported stubs will be available to the world.

It is useful for example, when your server is in the intranet and listens on some interface lets say 192.168.2.1 . NAT proxy can have ports forwarded to that host from internet. Normally RMI registry would report stubs being exported on address 192.168.2.1:someport . But obviously you cannot hit that address directly java.rmi.server.hostname comes in action. By setting that property, RMI registry will announce stubs being exported on given hostname insteed on actual intranet address, allowing clients to get to the stubs from outside world trough forwarded ports on NAT gateway.

So if your server is behind NAT, it should specify the domain that points to your NAT gateway from the Internet. If your server has external IP and its listening on that IP, then this property is not needed.