RFR: 8266936: Add a finalization JFR event [v2]

[Erik Gahlin]

On Tue, 18 May 2021 22:41:06 GMT, Brent Christian <bchristi at openjdk.org> wrote:

>> Please review this enhancement to add a new JFR event, generated whenever a finalizer is run.
>> (The makeup is similar to the Deserialization event, [JDK-8261160](https://bugs.openjdk.java.net/browse/JDK-8261160).)
>> 
>> The event's only datum (beyond those common to all jfr events) is the class of the object that was finalized.
>> 
>> The Category for the event:
>> `"Java Virtual Machine" / "GC" / "Finalization"`
>> is what made sense to me, even though the event is generated from library code.
>> 
>> Along with the new regtest, I added a run mode to the basic finalizer test to enable jfr.
>> Automated testing looks good so far.
>> 
>> Thanks,
>> -Brent
>
> Brent Christian has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Test flag should be volatile

> I was also thinking if this event should be implemented in the VM in order to avoid some performance overhead such as object allocation. Erik, what is the benefit of implementing in in the VM?

No startup cost, no allocation and there are callbacks when a class gets unloaded, so it's probably easier to clear any table where the statistics is held.

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PR: https://git.openjdk.java.net/jdk/pull/4101