Updating existing JAR files

This is mostly a note to myself, because every time I have to alter a JAR, it takes me way longer than it should. In essence, adding or removing files to/from an existing Java archive is not possible. The official way to go is extracting the files, changing them as needed, and re-packaging the final set. This is cumbersome and error-prone – suddenly you end-up with a different manifest, unintentionally packaged hidden system files (like .DS_Store on the Mac), or loose track of the folder hierarchies.

There is one good thing about JAR files: They are also ZIP files. Thus you can rename existingArchive.jar to existingArchive.zip and suddenly have all the zip command line tool goodness on your side:

Adding a file to a ZIP archive:

ls foo/bar/new.txt | zip -@ existingArchive.zip

Removing a file from a ZIP archive:

zip -d existingArchive.zip foo/bar/old.txt

If you just need to alter a single archive, using one of the various ZIP freeware GUIs might be even easier.