This is a lib I wrote a couple of years ago. It is a native binding to the gpgme library from Werner Koch. It uses JNI.

gpgpme is the standard library for developing third-party apps on top of GnuPG.

I wrote it for 32-Bit Intel GNU/Linux platforms. Some of my colleagues added 64-Bit Intel GNU/Linux support. It should run on other Unix-platforms, too. But this is not tested. Also I am not aware if it runs with the current version of gpgme. It should be easy to add Windows support by compiling a DLL, adding this to the jar and extend the loading mechanism to load a DLL on a Windows platform instead of loading a .so lib.