Instantiate Java objects from C++ and the ability for callbacks

Question

I've been reading up on JNI and I've been able to
1) load a C++ DLL and call C++ functions from Java and
2) call Java functions from a C++ DLL.
However, I haven't been able to find a way to pass a C++ interface class to a Java method (from a C++ DLL), where the Java method will call functions in that interface. (That is; I need to initiate the C++ DLL before any Java code is called.) I'll illustrate below if I'm unclear.
// Java
public class javaClass
     public void doStuff(WrapperInterface wi)
          wi.foobar();
// C++
class WrapperInterface
     virtual void foobar() = 0;
class cppClass : public WrapperInterface
     virtual void foobar()
          printf("helloworld");
int main()
     // initiate JVM, environment etc
     javaClass javaInstance;
     WrapperInterface* cppInstance = new cppClass();
     javaInstance.doStuff(cppInstance);
}How can I accomplish this?
I can't seem to instantiate a class in C++, that have been compiled as C++ with the javah command, so I can't instantiate it and set its initial values. (The functions are not declared as static in my Java class at least...) I can on the other hand instantiate it as a Java class. However, nothing happens when I call (from C++) the functions of the instantiated Java class.
Also, it seems according to my tests and the documentation that I could find that you are required to use a System.LoadLibrary() from Java. Will not that make the DLL it loads load again, thus creating two instantiations of that DLL?
As a final thought, if this is not possible with SUN's JNI, will it be possible to do with other native interfaces for Java?

Answer 1

You can pass a pointer back to java by using a java long data type. You can pass that to other java native (JNI) methods and write code that casts the long back to the C++ class and then calls a class method for that class.