Currently links between APIs refer to the
http://www.netbeans.org/download/dev/javadoc/
website. This is fine when browsing them online to
begin with, but annoying when browsing them
offline (from within NB using the
apisupport/apidocs module). Possibilities:

1. Have apisupport/apidocs register a URLMapper to
convert URLs with the prefix
http://www.netbeans.org/download/dev/javadoc/ to
the proper nbfs: URLs. Problems: (a) URLMapper
provides no URL -> URL facility. (b) Not at all
clear how to make this work w/ externalbrowser +
httpserver - would need to convert to and from
internal representation? Very ugly.

2. Make the javadoc module add an optional
URL-valued "mirror" to its javadoc DTD, indicating
what online prefix this doc set mirrors. Then in
its environment provider for the DTD, filter the
returned filesystem (MFS can be a filter) as
follows: for any file object whose content type is
text/html, override the length and input stream
methods to get the real content, then go through
the HTML and rewrite links. Any links (just <a
href="..."> I guess) which pointed to a URL with a
prefix in the mirror list of a mounted javadoc FS,
would be written to point to the file object in
that other FS (using URLMapper.EXTERNAL I guess).
This is a cleaner solution but probably more work.