监视Java对象销毁

Question

I have some objects that are created/destroyed very often and that can exist in many lists at the same time. To ensure I have no references left to them the objects have a flag isDestroyed, and if this is set, each list is responsible for removing the object from the list.

However this is ofcourse a growing ground for memory leaks. What if I forget to remove objects from one of the lists? To visually monitor that the program behaves correctly, I override finalize and increase a global variable to track destructions (not a formal test, only to get an idea). However as I have no control over the GC, I could in theory wait forever until something is destroyed.

So the question is two-fold: When having objects that are in multiple lists, is a "isDestroyed" considered a good way to control the object lifetime? It forces everyone who uses the object to take care to remove it from their lists, which seems bad.

And, is there any good way to see when the reference count reaches zero on an object, ie when its scheduled for destruction?

EDIT: To be more specific, in my case I my objects represent physical entities in a room. And I have one manager class that draws each object, therefore it is in one list. Another list contains all the objects that are clickable, so there I have another list. Having all objects in one list and using polymorphism or instance of is not an option in this case. When a object is "destroyed", it should neither be shown or clickable in any way, therefore I want to remove it from both lists.

Answer 1

You should have a look at the java.lang.ref Package.

And, is there any good way to see when the reference count reaches zero on an object, ie when its scheduled for destruction?

You can use the ReferenceQueue Object

From JavaDoc of java.lang.ref.ReferenceQueue

Reference queues, to which registered reference objects are appended by the garbage collector after the appropriate reachability changes are detected.

Answer 2

I think this what WeakReference and ReferenceQueue is for - you create a WeakReference for the object you are tracking and associate it with a ReferenceQueue. Then you have another thread that processes WeakReference(s) as it is returned from ReferenceQueue.remove(). WeakReference's are added to ReferenceQueue when the referenced objects is GC'd. But can you give an example on what these lists you are trying to clean up when the referenced objects are dead?

Answer 3

The way this is usually handled is through the Observer pattern. Each list attaches a destroy-listener that gets notified upon destruction. How this meshes with you architecture, I have no details to judge from.

Answer 4

If you want to be notified I'm almost sure you need PhantomReference, read here:

http://weblogs.java.net/blog/2006/05/04/understanding-weak-references