Jar-files are in fact exactly the same as standard zip files. That means that you can compress your jar files using any zip compressor that is compatible with the original zip deflate algorithm. So, by recompressing your jar file using a better zip compressor than the standard “jar” utility, you can shave off some precious extra bytes.

The best free standards-compliant zip compressor I’ve found is KZip, made by Ken Silverman. It basically always produces a few percent smaller zip files than by using for example winzip with “high compression” enabled, and it’s always more efficient than your standard IDE “jar”-compressor.

To make it even more easier for you to recompress your jar files, I’ve made a small utility called KJar. It simply uncompresses your jar file and recompresses it using KZip. Check it out.