3.3 The Stub/Skeleton Layer

The stub/skeleton layer is the interface between the application layer and the rest of the RMI system. This layer does not deal with specifics of any transport, but transmits data to the remote reference layer via the abstraction of marshal streams. Marshal streams employ a mechanism called object serialization which enables Java objects to be transmitted between address spaces. Objects transmitted using the object serialization system are passed by copy to the remote address space, unless they are remote objects, in which case they are passed by reference.

A stub for a remote object is the client-side proxy for the remote object. Such a stub implements all the interfaces that are supported by the remote object implementation. A client-side stub is responsible for:

• Initiating a call to the remote object (by calling the remote reference layer).
• Marshaling arguments to a marshal stream (obtained from the remote reference layer).
• Informing the remote reference layer that the call should be invoked.
• Unmarshaling the return value or exception from a marshal stream.
• Informing the remote reference layer that the call is complete.

A skeleton for a remote object is a server-side entity that contains a method which dispatches calls to the actual remote object implementation. The skeleton is responsible for:

• Unmarshaling arguments from the marshal stream.
• Making the up-call to the actual remote object implementation.
• Marshaling the return value of the call or an exception (if one occurred) onto the marshal stream.

The appropriate stub and skeleton classes are determined at run time and are dynamically loaded as needed, as described in Section 3.8, "Dynamic Class Loading". Stubs and skeletons are generated using the rmic compiler.