How can I read the entries in a Java .jar file from Ruby?

I tried the standard library Zlib, but that doesn't work ... I think  
because .jar files use the zip format instead of the gzip format.

I tried the rubyzip library at http://rubyzip.sourceforge.net/. For  
some reason that didn't get to all the entries in the file, only  
"META-INF/" and "META-INF/MANIFEST.MF".