Using Java to wrap over C++

I have a project written in C++ and I'm looking to write a Java GUI as an interface to it. The choice of Java is fixed so I'd need to learn how to be able to call the C++ code from Java. Rewriting the C++ code is not an option. I'd like input on:

1. What tools can I use to achieve this wrapping.
2. How much of the C++ code would I have to necessarily modify, if any.
3. Any other insights/follow up questions that you'd have.

Thanks.