About the finalization queue and reference queue

[Bernd Eckenfels]

Hello,

what is interesting to know is, that each finalizeable object which is
tracked is wrapped/tracked with an instance of j.lang.ref.Finalizer
(which is a FinalizerReference<Obj> subclass i.e. a final reference).

Generally for references to work you need to keep alive the
Reference instance. The finalizer does this with a built-in linked
list in the Finalizer instance (static unfinalized points to the head
of the list and each Finalizer object has a next/prev pointer. When
the VM tracks a finalizeable object it calls the Finalize(Object)
constructor which makes sure to add it.

So if you have thousands of finalized objects there are all indirectly
referenced from this long linear list.

When the GC does its thing and a instance becomes unreachable, it will
add it to the ReferenceQueue of the finalizer. The FinalizerThread will
consume it from there and make sure to finally also remove the
Finalizer reference for that object from the double linked list.

If you work with heap-dumps and analyse memeory leak informations you
need to ignore the memeory consumotion under "Finalizer.unfinalized"
with the (possibly long if the Finalizer thread is blocked and unwanted)
ReferenceQueue Finalizer#queue#head single linked list.

Gruss
Bernd

Am Tue, 11 Oct 2016 18:25:29 +0000
schrieb Jun Zhuang <jun.zhuang at hobsons.com>:

> Hi,
> 
> While reading about re-defining the finalize() method explicitly in a
> class I came across some statements and like to get some
> clarification from the experts.
> 
> On http://www.fasterj.com/articles/finalizer2.shtml, the author
> states that "the GC adds each of those Finalizer objects to the
> reference queue at java.lang.ref.Finalizer.ReferenceQueue.". Based on
> this the Finalizer object associated with the finalizeable object
> goes on the reference queue.
> 
> On page 311 of book Service-Oriented Computing - ICSOC 2011
> Workshops<https://books.google.com/books?id=hIa5BQAAQBAJ&pg=PA311&lpg=PA311&dq=java,+finalization+queue&source=bl&ots=LLGiYGWh0L&sig=Glvf0kn0zKHrdWoPzM6y6wtsr_M&hl=en&sa=X&ved=0ahUKEwjHrsL2pdPPAhUk3IMKHdXRCtc4ChDoAQgbMAA#v=onepage&q=finalization%20queue&f=false>
> "... all those objects that have a finalize () method and are found
> to be unreachable(dead) by garbage collector, are pushed into a
> finalization queue.". So the finalizeable object goes on the
> finalization queue.
> 
> Then this site, https://yourkit.com/forum/viewtopic.php?f=3&t=4672,
> states that "Objects of all classes with redefined finalize() method
> are added to a queue at the moment of creation. The queue head is
> referenced from a static field in java.lang.ref.Finalizer. An
> instance of Finalizer is created for each "finalizeable" object and
> is stored in that queue, which is in fact a linked list of
> Finalizers.", so both the finalizeable object and the associated
> Finalizer object are stored in the same queue?
> 
> So my questions are: Are there one or two queues involved? Exactly
> how object finalization works?
> 
> 
> Appreciate your input,
> Jun
> 
> Jun Zhuang
> Sr. Performance QA Engineer |
> Hobsons<https://www.hobsons.com/?utm_source=outlook&utm_medium=email&utm_campaign=banner_02.12.16_general>
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