The following syntax:

archive.addClass(MyClass.class);

...requires that the running CL has "MyClass" visible to it. This may not always be the case. Take for instance a test of EJB3 beans; the test class may not be able to load the EJB3 Bean Implementation class, as this may leak other internals out. The test only needs the interface in able to test it, though it'd still like to package up the impl class.

Support:

archive.addClass(String fqn, ClassLoader cl)

...where the archive implementation may then use the specified CL to load the class of the specified name