Java RMI proxy-casting issue

Question

I'm trying to get a RMI program to work. So far, the server starts up correctly but the client fails casting the remote object to the interface.

Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: com.sun.proxy.$Proxy0 cannot be cast to MonitorClient.InterfaceMonitor

All other answers I've found are for cases where the end user has cast the equivalent of InterfaceMonitorImpl (unknown to the client) instead of the Interface instead. This is not my case and I'm really at a loss here — RMI is nightmare-ish.

Server side

Main:

    InterfaceMonitor obj;
    try {
        LocateRegistry.createRegistry(1099);

        InterfaceMonitor stub = (InterfaceMonitor) UnicastRemoteObject.exportObject(new InterfaceMonitorImpl(), 0);

        Registry registry = LocateRegistry.getRegistry();
        registry.bind("imon", stub);

        System.out.println("Server ready");
    } catch (RemoteException | AlreadyBoundException ex) {
        System.out.println("Server error: " + ex.toString());
    }

InterfaceMonitor.java:

public interface InterfaceMonitor extends Remote {
    int checkAge() throws RemoteException; 
}

InterfaceMonitorImpl.java:

public class InterfaceMonitorImpl implements InterfaceMonitor {

    public InterfaceMonitorImpl() throws RemoteException {

    }

    @Override
    public int counter() throws RemoteException {
        return 10;
    }

}

Client side

    try {
        Registry reg = LocateRegistry.getRegistry(null);
        InterfaceMonitor im = (InterfaceMonitor) reg.lookup("imon");

        int counter = im.counter();
        System.out.println("Counter: " + counter);
    } catch (NotBoundException | RemoteException ex) {
        Logger.getLogger(MonitorGUI.class.getName()).log(Level.SEVERE, null, ex);
    }

The InterfaceMonitor.java is also on the client side.

Thanks for your time!

It has to be the same interface not just an interface with the same name and methods. Also is this the code as used? The interface has a checkAge method whereas the implementattion has a counter method. — M. Deinum
– M. Deinum, Commented May 22, 2014 at 10:48

user207421 · Accepted Answer · 2014-05-22 10:49:32Z

2

Obviously you must have two copies of InterfaceMonitor: one in MonitorClient and one in what may be something like MonitorServer. That makes two different classes. Not two copies of the same class. The class name, package, method declarations, inheritance, ... all have to be the same.

answered May 22, 2014 at 10:49

user207421

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2 Comments

Adrián Navarro Over a year ago

Thanks! Now I'm getting a "java.rmi.NoSuchObjectException: no such object in table" error, after cross-importing the InterfaceMonitor from the server package (not happy with it, but it can be run locally as a demo)

user207421 Over a year ago

That means that the remote object has been unexported. The solution is to store the result of LocateRegistry.createRegistry(1099) into a static variable.

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