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 wonder if there needs to be one event per finalized object?

Perhaps a counter per class would be as useful, i.e. jdk.FinalizationStatistics, and if it could be implemented in the VM, without other implications, that would be great. 

Such an event could be enabled by default and provide much greater value than an event that users would need to know about and configure themselves, which 99,99% of all user will not do.

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