Is "pass by value" enforced for calls within the same VM to objects that implement java.rmi.Remote? In other words, if I'm writing an object that implements java.rmi.Remote, can I assume that calls to it that originate within the local VM will enjoy the same "copy by value" rules for serializable parameters?

Created Feb 1, 2000

Tim Rohaly

Say I have a remote interface:

public interface Hello extends Remote {
    public String sayHello() throws RemoteException;
}

and an implementation like:

public class HelloImpl extends UnicastRemoteObject implements Hello {
    public String sayHello() {
        return "Hello!";
    }
}

Here, the return parameter of the sayHello() method is a serializable object (String).

If you obtain a reference by simply instantiating HelloImpl, then its semantics are that of a normal Java object. i.e., if you do:

Hello hello = new HelloImpl();
String result = hello.sayHello();

then the "result" reference points to the same object as the local String object created within HelloImpl.sayHello() - a copy of the String object is not made.

But, if we obtain a remote reference to the object, for example:

Hello hello = (Hello) Naming.lookup("rmi://localhost/helloserver");
hello.sayHello();

then the semantics are different, because we don't have a normal reference created by "new", we have a remote reference obtained by contacting the RMI registry. In this second case, the result string is serialized and returned, effectively creating a copy. Note that this is true whether or not the actual implementation object lives in the same VM. The deciding factor is how the reference was obtained.