RFR: 8071507: (ref) Clear phantom reference as soft and weak references do

[Mandy Chung]

> 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.
>> 
>> Mandy
>> 
> 
> 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 ;-)

That’s right.

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.

Mandy