You can use the Java Native Interface to call native code written in C++ directly from Java - and vice versa, which is less often used. When calling Java from C/C++, a virtual machine has to be launched before calling classes, using the so-called "Invocation Interface". A simple example can be found here.

It is important that you do not use underscores in the Java class names when implementing this scenario - JNI won't find your classes then, as happened to me using Sun's Java 6 JRE. Unnecessary to mention that it took Thomas and me about on hour to find the problem...