I was interested in your question because some time ago I did an NFe pilot project using Ruby and the information is actually harder to find.
Looking at the site of the Secretary of Finance of SP, has an issuer of NFe, made in Java. link
If you take a look at the jnlp manifest that it downloads, there are several libraries (jars) that might be useful.
As it apparently does not work on Mac, I have not been able to investigate further. I do not know if I would download these jars if they have documentation and what kind of license it would take.
It's just a pointer for you to investigate.