I've always believed that there is no way to delete or replace an entry from
a zip or a jar. As far as I knew, you have to create a new jar that has the
desired files in it.

However, according to the Ant user manual,
http://ant.apache.org/manual/index.html, the Jar and Zip tasks have an
'update' flag. Apparently:

---
The update parameter controls what happens if the ZIP file already exists.
When set to yes, the ZIP file is updated with the files specified. (New
files are added; old files are replaced with the new versions.) When set to
no (the default) the ZIP file is overwritten if any of the files that would
be added to the archive are newer than the entries inside the archive.

[The update parameter is supposed to behave the same for the Jar task as for
the Zip task.]
---

Unfortunately, the examples in the articles for the Zip and Jar tasks are
not very helpful. Perhaps the original poster should ask on the ant user
mailing list? That is a fairly active list and some very experienced Ant
users hang out there to help. This page - http://ant.apache.org/mail.html -
gives instructions on joining the mailing list. A search of the Ant archives
for the 'user' list might also turn up useful information. You can access
that archive at the same page and search the archive for keywords.

Or maybe someone will come along who has used the update flag in a Jar task
who can explain what the original poster is doing wrong.

The only possibility that occurs to me is based on this sentence from both
the Jar and Zip tasks:

---
Please note that ZIP files store file modification times with a granularity
of two seconds. If a file is less than two seconds newer than the entry in
the archive, Ant will not consider it newer.
---

Is it possible that the new versions of the files that are to be replaced in
the jar have two seconds or less difference in their timestamps?

Rhino