It is important to understand the distinction between these two ways of integrating Java with CORBA.

RMI-IIOP is for Java programmers who want to program to the RMI interfaces but use IIOP as the underlying transport. RMI-IIOP provides interoperability with other CORBA objects implemented in various languages, but only if all the remote interfaces are originally defined as Java RMI interfaces. It is of particular interest to programmers using Enterprise JavaBeans (EJBs), because the remote object model for EJB is RMI-based.

Java IDL is for CORBA programmers who want to program in Java based on interfaces defined in CORBA IDL. This is "business as usual" CORBA programming, supporting Java in exactly the same way as other languages like C++ or COBOL.