Shaj (Simple Host Authentication for Java) is a simple library that allows your Java app to verify users with the underlying operating system. Shaj also allows you to check group membership. Shaj is not a competitor for full featured authentication APIs but rather a complimentary way to piggyback on system accounts on any platforms.
Shaj is used in FishEye for local account authentication, hence it is in use on most flavours of Windows and *NIX.
Shaj currently supports Windows and Unix (PAM), and comes with pre-compiled JNI libraries for win32, Linux, Mac OS-X and Solaris. Shaj should work on Java 1.2+ JVMs. Shaj is written in C and Java.