Is there an easy way to call C++ code from Java and the other way around?

The JunC++ion tool by Codemesh (http://www.codemesh.com) does just that.

JunC++ion has a code generator that generates C++ proxy classes for Java classes. These C++ proxy classes are usable like the underlying Java classes. All communications between components in the two different languages are performed under the hood through JNI.

This allows you to write C++ code like:

  #include "java_lang_System.h"
  #include "java_lang_String.h"

  using namespace java::lang;

  int main()
  {
    String         test( "test" );

    System::out.println( "This is a " + test );
  }