Question

I know that finalize() is called whenever a class instance is collected by the garbage collector. However, I am a little bit confused when passing an instance of a class to another thread via a queue.

Let’s say this is a skeleton of Thread1:

for(i=0; i<1000; i++) {
   Packet pkt = new Packet();  // instance of class
   pkt.id = i;
   thread2.queue.put(pkt);
}

Then, thread 2 will remove the packet from the queue and perform lengthy operations. Does this second thread get a “copy” of the packet, or is it by some form of reference? The importance is that, if it is by copy, the finalize() on the instance created in thread 1 can be called before thread 2 is done with the packet. If it is by reference, I am guaranteed that finalize() is only called once for the information in the packet.

This basic example may not show the importance, but I am storing a C-pointer (from JNI) in the packet to destroy some memory when I am done with the object. If it is passed by copy, the memory may get destroyed before the second thread is done with it. If it is passed by reference, then it should only be destroyed once the GC sees it is no longer in use by BOTH threads (my desired behavior). If this latter scenario is not guaranteed, I will not use finalize() and use something else but it will be more complex.

Answer 1

The second thread receives the same actual object instance. You’re safe from premature finalization.

It receives a copy of the object reference, if you want to think of it that way.

In addition, finalize is not necessarily run when the garbage collector finds that the object has become garbage – the VM is free to run it at any later time, and to actually reclaim the memory some time after that. You really can’t rely on when finalize will be run. However, since what you care about is knowing that finalize won’t be called before the second thread finishes with the object, that’s immaterial. But worth knowing!