Although you can supply a SAXSource as the input to your transformation, the
XSLT engine will generally build a tree in memory. This will usually be much
more efficient than a DOM tree, but it will still be a tree.

> I'm trying to transform a 80M 
> file using an XSL stylesheet to an output stream.

80M is feasible but challenging. You should allocate at least 512M of memory
to the java VM (use -Xmx512M).

> I'm assuming that the JAXP API uses a DOM object to do this 
> transformation.

The JAXP API is just an API. What actually happens internally depends on
which implementation of the JAXP API you are using, e.g. Xalan, Saxon,
Oracle.

Michael Kay
http://www.saxonica.com/