Using Java objects from C++

Mon May 3 19:50:00 GMT 2004

On Mon, 2004年05月03日 at 15:03, Bryce McKinlay wrote:
> Dave Menendez wrote:
>> >I do have one nagging problem, though, and although I've searched this
> >mailing list several ways, I can't seem to find a definitive answer to
> >this question: can I instantiate a Java object from (pure) C++ and
> >somehow indicate to the JVM that I have retained a reference to it
> >(i.e., I want to store a pointer to the Java object in a C struct or as
> >a C++ instance member, but I don't want the GC to get rid of the object
> >until I say I'm done with it)?
> > 
> >
>> A very good question. Currently we are lacking an API to register a 
> reference from CNI. I think we need one. How about something like the 
> following?
>> JvNewGlobalRef(jobject)
> JvDeleteGlobalRef(jobject)

> I'll go ahead and implement something like this if others agree its a 
> good idea.
>
I would be ecstatic if something like this were available. I know it
places just a bit more burden on me, the programmer, to make sure I call
JvDeleteGlobalRef(jobject) when I'm done with it, but it's no more
painful than having to remember to delete C++ objects or free malloc'ed
memory.
I'm not familiar with the release cycle of GCC. Would we have to wait
for the next formal GCC release for this added functionality, or since
we have the source code, could a patch be posted whenever this is
ready? Just curious - I'm not trying to make demands of anyone's time.
> >A co-worker suggested perhaps creating a very simple Java object that
> >simply stores references to other Java objects, and instantiating one
> >static, global instance of this object in C++. That would mean if I
> >wanted to keep a Java object around in C++, I'd just have to add it to
> >this static Java object, and then remove it when I'm done with it. It's
> >kludgy, but might it work?
> > 
> >
>> This will work, since static data areas currently are scanned by the GC. 
> We've talked about changing this behaviour, however, and requiring 
> explicit registration of static roots, in order to improve performance 
> and the possibility of porting to different collectors. So it might not 
> be a good idea to rely on this behaviour for new CNI code.
>
I see. Is explicit registration of static roots something that is
already available or something you would be planning to add in? I may
just have to dig around the header files some more if it's already
something I should be doing with the 3.4.0 GCC release.
> >P.S. One more thing, I was a bit mystified when I read in the mailing
> >list that even though I use "new" to allocate a Java object from C++,
> >I'm not supposed to call "delete" to get rid of it, but that it somehow
> >automagically gets collected by the GC. If someone could provide a
> >brief description of how the GC decides what is eligible for collection
> >or point me to a document to help me understand the logic, that would
> >also be much appreciated.
> > 
> >
>> "new" is overloaded by java::lang::Object. So when you allocate a Java 
> object, "new" actually calls the GC allocator, not the standard C++ one.

Maybe I need to read up on allocators in general - I understand that the
GC allocator gets used instead, but I wasn't sure how the GC detects
that one particular object I instantiated from C++ is no longer in use
if I didn't add it to some other Java object. Is it automatically
"dereferenced" somehow when the block containing the variable holding it
ends?