JAR to ZIP
Nothing you choose here leaves your browser, and nothing is downloaded to make it work — not a library, not a compression module, nothing. Your browser already knows how to deflate and gzip; this page writes the ZIP around it.
A .jar IS a .zip. This renames it and tells you what is inside.
A Java archive is a ZIP with a META-INF/MANIFEST.MF in it, and nothing else about the format is different — same central directory, same deflate. Anything that opens a ZIP can open a JAR already; the extension is what stops Windows offering to. The classes come across untouched, because they are just files.
How this works, and why nothing is fetched
A JAR is a ZIP, so there is nothing to decompress that your browser cannot already do: it needs deflate for web pages and this uses the same thing through DecompressionStream. Writing the archive round it is a byte layout rather than an algorithm.
What it will not do
It will not open a RAR, and nothing ever writes one: RARLAB's licence forbids using their code to re-create the RAR algorithm, which is proprietary. That is a legal restriction and not a difficulty. It will not open a 7z, bzip2 or xz either — those need compressors your browser does not have, so supporting them would mean shipping one, which is the thing this page avoids. It says which of these it is looking at rather than just refusing.
It does not open password-protected archives, and it says so per file rather than failing silently. It reads plain ZIPs rather than ZIP64, so an archive over 4 GB or with more than 65,535 files is refused with that reason given.
Where to go next
- JAR to TAR — uncompressed, and it keeps Unix permissions
- JAR to TAR.GZ — the Unix default, small and lossless
- JSON to CSV — for the data inside them