How to programmatically determine if an object is ready to be destroyed?

Question

Dear All,
How can I programmatically determine that whether an object is ready to be garbage collected? I know that the garbage collection process is not guaranteed to occur until the JVM is close to running out of memory. Yet, it is also said that an object is ready for garbage collection if there are no more references to it. Therefore, I am wondering if we have means to know whether an object is ready to be garbage collected, even though that may not happen yet.
For example, if I define such a simple class structure:
public class A
B b = new B();
public void methodA()
b.printB();
public static void main(String[] args)
A a = new A();
a.methodA();
class B {
public void printB() {
System.out.println("from B");
Here I am assuming the object b (of type B) is ready for garbage collection BEFORE the object a (of type A), because the object b is declared and created as a field within object a. Is this assumption true? If yes, how can I verify that during runtime?
Thank you very much for any help!
-- SunnyDay

Answer 1

1. It seems that the System.gc() should be called to make sure garbage collection is performed so that all those WeakReference objects are put into the ReferenceQueue (suppose no strong references to those
objects to which the weak references are pointing). Is all of this guaranteed to happen? That is, is it guaranteed that those weakly referenced objects are being garbage collected and their WeakReference objects are put into the queue, whenever the System.gc() is called?
No, it is not guaranteed. Though it seems to be true for current implementation of JVM, where System.gc() usually does Full GC. However, you should not rely on this and it can be in fact changed in future implemenations.
2. The WeakReference description says the garbage collector may enqueue the newly-cleared weak reference "AT SOME TIME AFTER" the weak reference is cleared. If this happens to be the case, how can I know which
object is deleted first in my example (e.g., if the weak reference of object A is put into the queue before that of B, it may falsely indicate object A is destroyed beofore B)?
Yes, it is quite possible for weak reference to be enqueued in different order.
3. Suppose System.gc() is called and an object's weak reference is put into the queue, can I still know which object is being destroyed at this moment?
You can't. In fact, java threads may be stopped while GC is running. GC may free memory for multiple objects at once (which in fact happens with Mark-Sweep algorithm). So, "destroyed at the moment" does not make sense from this perspective.
If can gave us more details on what you are originally trying to do, probably we could find another solution.