Java RMI

[Gaurav Raje]

Please excuse me if i am asking the obvious or something which has been previously discussed. I searched a little for this.... but i didnot know what exactly to search. I will keep searching on google to find out if I get an answer.

I have been exposed to RMI for the first time after learning Languages. One thing felt a little striking.

 
public interface RMIInterface extends remote{
 public String sayHello() throws Exception;


}

Consider the implementation

 
public class RMIImplementation extends UnicastRemoteObject implements RMIInterface{


//Security manager code goes here

//

public String sayHello() throws Exception{

return "Hello World";

}

public static void main(String args[]){

RMIImplementation rmii = new RMIImplementation();
Naming.bind("sayHello",rmii);

}


}

Now the client

   
public static void main(String args[]) throws Exception{


//Security manager and other boilerplate code goes here


//Consider the two options

//1) 
// RMIImplementation rmii = (RMIImplementation)Naming.lookup(RMI_SERVER+"sayHello");

//2)
// RMIInterface rmii = (RMIInterface)Naming.lookup(RMI_SERVER+"sayHello");

}
 

I realized that the 2nd optioin is the right option. However, if I use the first, it gives me an error at runtime(Invalid cast). Now as far as I know, since the abstract interface method sayHello is defined only in the RMIImplementation class. So effectively any class which is returned by Naming.lookup will be a subclass of RMIImplementation and thus of RMIInterface.
If this is the case, why is there a problem in the casting?

What exacatly is the type of the returned object if it is not RMIImplementation but still is a type of RMIInterface

[Gaurav Raje]

Please assume minor mistakes are taken care of before compiling. and the boilerplate code is correct

[Ernest Friedman-Hill]

It will be a class created automatically by the JVM for this purpose; it won't be related to "RMIImplementation" in any way. In fact, the client program doesn't need the RMIImplementation class file to be on its class path.

[Rob Spoor]

First of all, I'll move this to our Distributed Java forum - the place for RMI on JavaRanch.

Gaurav Raje wrote:I realized that the 2nd optioin is the right option. However, if I use the first, it gives me an error at runtime(Invalid cast). Now as far as I know, since the abstract interface method sayHello is defined only in the RMIImplementation class. So effectively any class which is returned by Naming.lookup will be a subclass of RMIImplementation and thus of RMIInterface.
If this is the case, why is there a problem in the casting?

What exacatly is the type of the returned object if it is not RMIImplementation but still is a type of RMIInterface

It's RMIImplementation_Stub. The rmic compiler creates two helper classes for your RMIImplementation class: RMIImplementation_Skel and RMIImplementation_Stub. These two wrap around your RMIImplementation class to handle the communication. The Stub class also implements the interface, and that's why casting to the interface works and casting to the original class doesn't.
You can verify as such:

RMIInterface rmii = (RMIInterface)Naming.lookup(RMI_SERVER+"sayHello");
System.out.println(rmii.getClass());

[Gaurav Raje]

Thanks for the reply....
I did check the class of the returned object. But i always thought it was still a subclass of the implementation. Hence was confused...

Thanks for clearing my doubt.