How to re-establish a C++ object through a loop of C++/Java/C++ ?

Question

Hi, there:
My problem is a bit awkward. My application has two JNI components,
handling different aspects of low level computation. Each of the JNI
wraps up a C++ library with the java version APIs. The java code
calls one JNI to establish an object corresponding to a C++ class.
Then after some manipulation, I need to go back to C/C++ level to
use this object to do more low level work using the second JNI.
How can I put the jobject back to a C++ object, so that I can call
the C++ APIs of the first C++ library directly from my second JNI
and avoid go through the wrapped java APIs?
Hopefully, I didn't ask something obvious. Thanks.
Ligg

Answer 1

I wouldn't know about using JVM data copying operations to solve this problem. Because of my ignorance, it sounds just plain scary.
However:
o You are calling your first dll from java. That tells me that you have somehow defined a subroutine with C linkage. That subroutine ought to be able to grab the C++ object in question, and store the address in a globally-defined pointer.
o It sounds like the second dll knows about the C++ object definition from the first dll. (Otherwise it would make no sense trying to pass to it an object of the sort you mention.) So it sounds like the second dll ought to be able to declare that an external pointer exists to this desireable C++ object, and the loadLibrary call on the second dll ought to fill in the appropriate linkage for you - resolving the addresses.
I say "ought to". I haven't tried this, although in C I have certainly used external linkages before and let loaders do the fixup.