Haven't used it but using java.util.jar seems better than
java.util.zip.  It can write jar files, I think you can use this to
read entries, delete entries, and then add entries, which you may be
able to encapsulate as an update:

http://java.sun.com/j2se/1.4.2/docs/api/java/util/jar/package-summary.html

Good luck

Opalinski
opalpa@gmail.com
http://www.geocities.com/opalpaweb/

See the code at http://mindprod.com/jgloss/zip.html

You can certainly add a new entry. So perhaps the way you are supposed
to do it is by deleting the old entry and writing a new one.

On my curiosity list is to discover how clever java is about writing
zips..  In the worst scenario, every time you update an element it
would rewrite the whole file, or collapse and created hole in place.
If it were smart it could just tack on the end, and when you did the
file close, it could copy the whole thing to a new file, leaving out
the holes. In the happy case there were no holes, it could bypass the
final copy/rename.

Anyone old enough to remember IBM 360 and the endless in-place
compressing of partitioned datasets, shaking the drives? Univac byte
machines had the same problem. I wrote a compressor that found another
physical drive to copy to, leaving the holes behind,  then copied
back.  It was much faster and put less wear on the drives,  since
neither head had to move more than a track at a time.

I have wondered how long until we get high performance drives with
multiple sets of arms then a smart algorithm in the drive  reduce head
motions.  Their existence could be transparent to the OS, though the
system might work better if the two co-operated.

Similarly, I still have not seen any marthaing hard drives. I proposed
this idea way back in the early 80s.  See
http://mindprod.com/jgloss/martha.html

Jes, there is indeed no such API in Java. You would have to unpack the
zip/jar and repack it with your new file.

Not doing it. Why do you need to have that xml file in the jar file?

Or you could try going native. One particular form of a Zip file has its
directory at the file end. Maybe (I have never tried or studied it in
detail), it is possible to use this to manipulate the contents without
the need to repack the complete file. Maybe not.

/Thomas