(1) You can write a Java class containing native methods that follow the
JNI convention. From those methods you can call any existing functions
(2) you can purchase a commercial library that provides the wrappers for
you, already made, debugged, and tested. One example is xFunction ( http://www.excelsior-usa.com/xfunction.html
), but there are several others.
(3) you can find a free implementation, such as SWIG ( http://www.swig.org
and especially http://www.swig.org/Doc1.3/Java.html#n2
). You may even find that Windows API functions that you need are already wrapped inside WinPack (a free addition to JNIWrapper)