As I wrote earlier, you need to write your own DLL that follows the
JNI rules and naming conventions. For example, consider this Java
class:

 public class MyClass {
   public native int foo(int a);
   [ ...and other methods...]
 }

Note that MyClass has a native method foo(), which is found in a DLL
that has been written expressly for the purpose of implementing one or
more of the methods of MyClass:

 JNIEXPORT jint JNICALL Java_MyClass_foo(JNIEnv *env, jobject this_foo, jint a)
 {
   return some_other_function(a);
 }

From native methods like this one, any other functions can be invoked,
including those in your existing DLL.

You need to write a set of native methods like foo() that bridge
between your Java application and the library functions you want to
use.

Start reading here:
http://java.sun.com/j2se/1.5.0/docs/guide/jni/index.html
http://java.sun.com/docs/books/jni/index.html

/gordon