You can't use a DLL in java directly. You have to use something called the "native interface", which means you have to create a wrapper DLL around the DLL you want to use so Java can map the DLL functions to methods in a class. This is not easy and requires that you use a native compiler (like GCC for windows or Visual C++).
also be aware that a DLL is windows specific, so you won't be able to run your servlets on any other platform.