The document() function and Base URIs

I have a stylesheet that is loaded into Xalan using an http:// URL,
which happens to point to a Java servlet. The stylesheet contains an
import instruction:

Xalan is attempting to retrieve the imported stylesheet from the
filesystem, and failing - it seems to be using the docBase of the
servlet context, which I suppose is equivalent to using an empty Base URI.

So how does a stylesheet come to know what its URI is? How do you 'tell'
a stylesheet where it came from?

Thanks,
Jack.