What if a finalizer makes an object reachable?

问题:

In Java, finalize is called on an object (that overrides it) when it's about to be garbage collectioned, so when it's unreachable. But what if the finalizer makes the object reachable again, what happens then?

回答1:

The object will not be collected until it gets unreachable again.

According to the JavaDoc, finalize() will not be called again.

回答2:

Then the object doesn't get garbage collected, basically. This is called object resurrection. Perform a search for that term, and you should get a bunch of interesting articles. As Jim mentioned, one important point is that the finalizer will only be run once.

回答3:

If you read the API description carefully, you'll see that the finalizer can make the object reachable again. The object won't be discarded until it is unreachable (again), but finalize() won't be called more than once.

回答4:

Yeah, this is why you don't use finalizers (Well, one of the many reasons).

There is a reference collection that is made to do this stuff. I'll look it up and post it here in a sec, but I think it's PhantomReference.

Yep, PhantomReference:

Phantom reference objects, which are enqueued after the collector determines that their referents may otherwise be reclaimed. Phantom references are most often used for scheduling pre-mortem cleanup actions in a more flexible way than is possible with the Java finalization mechanism.

回答5:

It actually does another pass to check and make sure there are no more references to the object. Since it will fail that test on its second pass, you'll end up not freeing the memory for the object.

Because finalize is only called a single time for any given object, the next time through when it has no references, it will just free the memory without calling finalize. Some good information here on finalization.