Simple RMI application with client-side exception

Question

I use Java 8 and I created a simple RMI application but I have a client-side exception that I don't understand. Using Eclipse the structure of my application is:

---RMI_project     
-----bin
-------client
----------ImplementazioneMyClassServer_Stub.class
----------InterfacciaMyClassServer.class
----------MainClient.class
-------server
----------ImplementazioneMyClassServer.class
----------InterfacciaMyClassServer.class
----------ImplementazioneMyClassServer_Stub.class
----------ImplementazioneMyClassServer_Skel.class
----------MainServer.class
-----src 
-------client
----------InterfacciaMyClassServer.java
----------MainClient.java
-------server
----------ImplementazioneMyClassServer.java
----------InterfacciaMyClassServer.java
----------MainServer.java

here's the code:

InterfacciaMyClassServer.java

package client;

import java.rmi.*; // necessaria per estendere interfaccia Remote

public interface InterfacciaMyClassServer extends Remote {
 int somma(int a, int b) throws RemoteException;        //unico metodo
}

ImplementazioneMyClassServer.java

package server;

import java.rmi.*;
import java.rmi.server.*;  // per utilizzare UnicastRemoteObject

public class ImplementazioneMyClassServer extends UnicastRemoteObject implements InterfacciaMyClassServer{

    private static final long serialVersionUID = 1L;

    public ImplementazioneMyClassServer() throws RemoteException {      // costruttore
        System.out.println("ok1");
    }

    public int somma(int a, int b) throws RemoteException{      // implementazione metodo somma definito dall'interfaccia
        return a+b;
    }
}

MainServer.java

package server;

import java.rmi.*;

public class MainServer {

    public static void main(String[] args) {

        try{
            ImplementazioneMyClassServer s1 = new   ImplementazioneMyClassServer();

            System.out.println("ok2");

            Naming.rebind("oggetto1", s1);

            System.out.println("ok3");
        }
        catch(Exception e){
            System.out.println(e);
        }
    }

}

MainClient.java

package client;

import java.net.MalformedURLException;
import java.rmi.*;

public class MainClient {

    public static void main(String[] args) throws MalformedURLException, RemoteException, NotBoundException {

        String nomeServer="localhost";        // nome del server o suo    indirizzo IP (per l'esempio localhost=127.0.0.1)
        String nomeOggettoRemoto="oggetto1";   // nome dell'oggetto remoto da richiedere al server 
        String protocollo="rmi";              //protocollo usato, può essere: rmi,ftp,http
        String URLoggettoRemoto=protocollo+"://"+nomeServer+"/"+nomeOggettoRemoto;  // URL completo usato dal client per ottenere riferimento oggetto remoto sul server 

        // cerca all'URL specificato all'interno del registro rmi l'oggetto remoto con nome "oggetto1"
        // e restituisce nella var oggetto il suo riferimento per cui nelle istruzioni successive
        // è possibile riferirsi all'oggetto remoto come se fosse sul client.
        InterfacciaMyClassServer oggetto = (InterfacciaMyClassServer) Naming.lookup(URLoggettoRemoto);

        // uso oggetto remoto
        System.out.println(oggetto.somma(2, 3));
    }

}

I do not want to download dynamically stub class. That is why I got the class stub and skel and then I copied the stub class in bin\client. To get the stub and skel classes I followed the following procedure:

1) set classpath=C:\JavaWorkspace\RMI_project\src
2) cd C:\JavaWorkspace\RMI_project\src
3) C:\JavaWorkspace\RMI_project\src>javac server/InterfacciaMyClassServer.java
4) C:\JavaWorkspace\RMI_project\src>javac  server/ImplementazioneMyClassServer.java
5) C:\JavaWorkspace\RMI_project\src>rmic -v1.1 server.ImplementazioneMyClassServer

To run the entire application I followed the procedure:

by prompt1:

1) set classpath=C:/JavaWorkspace/RMI_project/bin
2) start rmiregistry
3) java server.MainServer

and I got properly:

ok1
ok2
ok3

by prompt2:

1) set classpath=C:/JavaWorkspace/RMI_project/bin
2) java client.MainClient

Where I got the exception:

Exception in thread "main" java.lang.ClassCastException: server.ImplementazioneMyClassServer_Stub cannot be cast to client.InterfacciaMyClassServer 
at client.MainClient.main(MainClient.java:18)

The exception is the line 18 in MainClient but I do not understand what is wrong.

https://docs.oracle.com/javase/8/docs/technotes/guides/rmi/hello/hello-world.html — MarkOfHall, Jun 13 '15 at 14:59

score 0 · Accepted Answer · answered Jun 13 '15 at 23:14

0

You have the remote interface declared in two packages. The server side uses the server package's remote interface; the client side uses the client package's remote interfaces. These two remote interfaces are different. You can't cast an implementation of one to the other.

You can't do this. You need to use the same remote interface, in the same package, at both ends.

answered Jun 13 '15 at 23:14

user207421

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Then I can create another package "common" in which to place the remote interface and then import it in the server package and in the client package? @EJP – Fobi Jun 14 '15 at 15:33
Certainly, that's what's usually done. That package also needs to include any other classes that are common to both server and client, that is to say classes that are named in the remote interface, classes they depend on, and so on. If you use generated stubs they will also be in that package. – user207421 Jun 14 '15 at 22:37

Simple RMI application with client-side exception

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