RFR: 8071507: (ref) Clear phantom reference as soft and weak references do
mandy.chung at oracle.com
Sat Dec 5 16:28:27 UTC 2015
> On Dec 5, 2015, at 3:12 AM, Peter Levart <peter.levart at gmail.com> wrote:
> On 12/04/2015 10:58 PM, Mandy Chung wrote:
>> "It is possible to create a phantom reference with a null queue, but such a reference is completely useless: Its get method will always return null and, since it does not have a queue, it will never be enqueued.”
>> The puzzling part to me is why PhantomReference accepts null ReferenceQueue. I can’t evaluate how high of the source incompatibility risk if we fixed it but I may propose that in a future release until I have cycle.
> Well, it is not completely useless for PhantomReference to accept null ReferenceQueue. It's sometimes useful to have a common subtype of PhantomReference where most of instances perform a function of PhantomReference, but some instances are just there to provide the "glue" in the data structure. See the implementation of java.lang.ref.Cleaner and it's "root" nodes of a doubly-linked list ;-)
There may likely be some reason why it takes the null ReferenceQueue as noted in this comment in sun.misc.Cleaner:
// Dummy reference queue, needed because the PhantomReference constructor
// insists that we pass a queue.
More information about the hotspot-gc-dev