<p>I have an exe that I know was written in java. I understand that java programs can be made into an exe and there are tools to convert jar files to exe but is it possible to convert back? AFAIK jar files can be run on any platform that can run java and I would like to use a windows compiled java program on mac without using any extra virtualisation (wine,vmware)</p>

It depends how the exe has been built:
If it has simply wrapped, with a tool like
JSmooth
, the same tool can
extract the jar
If it it has been compiled, with for instance gcj (as illustrated by
this question
),... I am not sure.
If it has been compiled by a static compiler (or
AOT - Ahead-Of-Time
-), I believe it is not possible to extract the orignial jars.