JavaRemoteMethodInvocation and CommonObjectRequestBrokerArchitecture share a great deal in common.  The main difference is that RMI stubs must interface to a Java interface, while CORBA stubs interface to a special interface language called InterfaceDefinitionLanguage.  There's a lot more to it than just this, of course.

See http://java.sun.com/pr/1997/june/statement970626-01.faq.html for an overview of RMI vs CORBA. 

See http://www.javasoft.com/docs/books/tutorial/idl/summary/index.html for another overview of RMI vs CORBA. Including some future directions.

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CategoryCorba