尝试连接到RMI注册表，但得到java.net.ConnectException：连接被拒绝：connect

Question

I have two computers. On one of them, is running an RMI Registry - which was created from this code alone:

package main;
import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
import java.util.Scanner;

public class TheRegistry{

    public static void main(String[] args) {
        try {
            Registry reg = LocateRegistry.createRegistry(2020);
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        finally{
            System.out.println("Registry Created");
            Scanner input = new Scanner(System.in);
            input.nextInt();
            System.exit(0);
        }
    }
}

The other computer has a server that is trying to register an Object on this registry, however, it gets an exception. Here is the code for the server:

import java.rmi.registry.LocateRegistry;
import java.rmi.registry.Registry;
import java.rmi.server.UnicastRemoteObject;
import java.rmi.*;

public class TextScramblerServer implements TextScramblerInterface 
{
    private static Remote obj; 
    // main method to export
    @Override //Return input text as-is.
    public String testInputText(String inputText) {

        return "Your input text is: " + inputText;
    }

    @Override //Return the string reversed.
    public String reverse(String inputText) {
        String reversedInput = "";
        for(int i=0; i<inputText.length();i++)
        {
            reversedInput=reversedInput+inputText.charAt((inputText.length()-1)-i);
        }
        return "Result: "+reversedInput;
    }

    @Override //Return the string scrambled.
    public String scramble(String inputText) {
        String scrambledInput="";

        for(int i=0; i<inputText.length();i++)
        {
            if(i%2==0)
            {
                scrambledInput=scrambledInput+inputText.charAt(i);
            }
            else
            {
                scrambledInput=inputText.charAt(i)+scrambledInput;
            }
        }
        return "Result: "+scrambledInput;
    }

    public void exportServer() throws Exception {
        System.setSecurityManager(new RMISecurityManager());
        obj = UnicastRemoteObject.exportObject(this, 2022);
        Registry registry = LocateRegistry.getRegistry("132.205.94.50", 2020);
        registry.bind("test", obj);
    }

    public static void main(String[] args) {
        try {
            (new TextScramblerServer()).exportServer();
            System.out.println("Server is up and running");
        }
        catch(Exception e){
            e.printStackTrace();
            try {
                UnicastRemoteObject.unexportObject(obj, true); //close port
            } catch (NoSuchObjectException e1) {
                // TODO Auto-generated catch block
                e1.printStackTrace();
            }
        }
    }
}

I keep getting the error:

java.rmi.ConnectException: Connection refused to host: 132.205.94.50; nested exception is: 
    java.net.ConnectException: Connection refused: connect
    at sun.rmi.transport.tcp.TCPEndpoint.newSocket(Unknown Source)
    at sun.rmi.transport.tcp.TCPChannel.createConnection(Unknown Source)
    at sun.rmi.transport.tcp.TCPChannel.newConnection(Unknown Source)
    at sun.rmi.server.UnicastRef.newCall(Unknown Source)
    at sun.rmi.registry.RegistryImpl_Stub.bind(Unknown Source)
    at TextScramblerServer.exportServer(TextScramblerServer.java:57)
    at TextScramblerServer.main(TextScramblerServer.java:62)
Caused by: java.net.ConnectException: Connection refused: connect
    at java.net.DualStackPlainSocketImpl.connect0(Native Method)
    at java.net.DualStackPlainSocketImpl.socketConnect(Unknown Source)
    at java.net.AbstractPlainSocketImpl.doConnect(Unknown Source)
    at java.net.AbstractPlainSocketImpl.connectToAddress(Unknown Source)
    at java.net.AbstractPlainSocketImpl.connect(Unknown Source)
    at java.net.PlainSocketImpl.connect(Unknown Source)
    at java.net.SocksSocketImpl.connect(Unknown Source)
    at java.net.Socket.connect(Unknown Source)
    at java.net.Socket.connect(Unknown Source)
    at java.net.Socket.<init>(Unknown Source)
    at java.net.Socket.<init>(Unknown Source)
    at sun.rmi.transport.proxy.RMIDirectSocketFactory.createSocket(Unknown Source)
    at sun.rmi.transport.proxy.RMIMasterSocketFactory.createSocket(Unknown Source)
    ... 7 more
java.rmi.NoSuchObjectException: object not exported
    at sun.rmi.transport.ObjectTable.unexportObject(Unknown Source)
    at java.rmi.server.UnicastRemoteObject.unexportObject(Unknown Source)
    at TextScramblerServer.main(TextScramblerServer.java:68)

I can't figure out why this is happening. I think I've tried everything

Answer 1

I ran your code and it worked for me after configuring the security policy.

Your ConnectionRefused exception means that the underlying TCP connection cannot be established. It's network issue, not an RMI issue.

Try running both the server and registry on the same host, and use localhost as the hostname. If it works, the problem is likely a firewall issue between the two hosts.

You can do a simple test of a TCP connection to the specific port using telnet. If the port isn't listening, telnet will give you a similar connection refused message. If the port is listening, you'll get something like this on the terminal:

Connected to localhost.
Escape character is '^]'.

Control-C to get out of the session.

The specific telnet output may vary based on your OS, but they are all about the same.

If it is a firewall issue, you'll have to open up the ports. How to do that depends on OS, but it's easy to find.

Answer 2

Either your Registry has been garbage-collected, you got the IP address wrong, or it is a public IP address and you haven't configured port forwarding.

You need to store the Registry reference in a static object to overcome garbage collection, although what the point of that program is when rmiregistry.exe already exists escapes me completely.

You're barking up the wrong tree anyway. You can only bind to an RMI Registry that is running in the local host. There is therefore never any need to use a Registry hostname other than "localhost" when binding or unbinding.

The reason you got the NoSuchObjectException is that you are trying to unexport the stub, which is referred to by obj , which is the result of UnicastRemoteObject.exportObject() , which returns the stub. See the Javadoc. You need to save the result of new TextScramblerServer() and unexport that.