This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
AD has an LDAP interface to it, so that is probably the easiest way to do it. If you haven't used LDAP through Java before, this is probably a good place to start.
The alternative is to write the code that accesses AD in C/C++ and then write a JNI wrapper around that code. This is definitely the less easy option but will give you greater flexibility when accessing AD.