Re: update jar file with java code

"Chris Uppal" <>
Tue, 1 Aug 2006 10:23:17 +0100
Mark Space wrote:

This is good information, but it still all falls under the heading of
"doing something tricky" to a jar, yes? I mean there's no explicit API
that allows one to determine if a given zip or jar file can be modified
with out re-writing the whole archive.

Correct. But then, there is no API for modifying JAR/ZIP files at all....

A sensible API /might/ include the ability to append new entries to a ZIP file,
which is always possible as an in-place operation, but the Sun stuff currently
does not.

More general replacements would probably be more effort than they are worth --
the API would be complicated, confusing, and error prone, both for the user of
the API and for its implementer. A far better choice would be to expose the
incremental/streaming nature of the ZIP format at the API level, and let the
client programmer work directly with that. The Java implementation is
noticeably less than full-featured, but it does include the basics needed to do
this yourself if you wish.

And even if there was, can a jar be modified and then streamed in place?
  I think you still have to make a temp file, which can be kinda a pain
on local resources if the jar is very large.

The ZIP file format is designed for streaming/incremental reading or writing --
e.g you can create a ZIP file, writing it to a tape device or network
connection, without ever seeking backwards. Similarly you can read the
contents of a ZIP file sequentially with no backward seeks (though you do have
to scan the entire file; obviously you cannot make use of the table of contents
to do random-access in this scenario).

The only non-constant space aspect to creating a ZIP file is maintaining an
internal copy of the table of contents which will eventually be written to the
end of the output stream. Theoretically that could be built incrementally by
creating a temporary file on-disk, but in practise there's no reason not just
to keep the data in memory.

Then you still got to be able to stream the modified file without
closing it, which I have no idea how to do....

I can't be bothered to check the details, but the following pseudo-code loop
should copy a ZIP file making modifications as it goes. Note that neither the
entire input, nor the entire output, is held in memory at any one time, and
also note that we never seek backwards in either the input or output streams.
Buffering, error checking, etc are all omitted.

    InputStream input = ...
    OutputStream output = ...
    ZIpInputStream inZip = new ZipInputStream(input);
    ZIpOuputStream outZip = new ZipOuputStream(output);

    ZipEntry in;
    while ((in = inZip.nextEntry()) != null)
        ZipEntry out = self.replacementFor(in);
        if (out == null)
            // just copy "in"
            self.copyEntryContents(inZip, outZip);
            // write replacement
            self.writeReplacementContents(in, outZip);


See the JavaDocs for the details. If you are going to do this kind of thing in
a production environment, then it would probably be a good idea to acquire at
least a basic understanding of the ZIP file format -- in particular the way
that the size, crc, etc, of an entry can be written either before or after the
data itself.

    -- chris

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