Does narrowing the return type break RMI backward compatibility?

Question

Assume I have the following RMI function:

Object foo();

The only implementation is

Object foo() { return "some string"; }

Can I narrow the return type to String without breaking backward compatibility if client and server are not released in one step?

Answer 1

If you change the remote interface on the server side to be String foo(); but attempt to run the client using the older remote interface class file that has Object foo(); you will get an exception like the following:

java.rmi.UnmarshalException: unrecognized method hash: method not supported by remote object

RMI hashes the name, argument types, and return types and looks for an exact match. The fact that the newer version might be compatible is not considered.

As EJP said this is also a Java language question. If you attempt to run the client binary (without recompiling) against the new interface, you will get a NoSuchMethodError because this change is not binary compatible.

This is, however, a source-compatible change, as existing client sources probably do something like

String result = (String)stub.foo();

which works perfectly fine at the source code level if foo() is changed from returning Object to returning String.

Answer 2

This is a Java language question, not just an RMI question. The answer is 'no': you will get a NoSuchMethodError.