RFR: 8298783: java/lang/ref/FinalizerHistogramTest.java failed with "RuntimeException: MyObject is not found in test output" [v3]

[Brent Christian]

On Thu, 27 Mar 2025 02:10:40 GMT, Jaikiran Pai <jpai at openjdk.org> wrote:

>> `ForceGC` is already waiting for `trappedCount > 0`. And I would point out that `ForceGC` will be making several `System.gc()` calls, compared to just one in the original version of the test.
>> 
>> But if there's concern that we won't reach the needed state during `ForceGC`'s default timeout, I can call `ForceGC.waitFor()` with a longer timeout. `60s` ought to be plenty. 
>> 
>> What do you think?
>
>> But if there's concern that we won't reach the needed state during ForceGC's default timeout, I can call ForceGC.waitFor() with a longer timeout. 60s ought to be plenty.
> 
> I think using 60 seconds as a timeout for `ForceGC` would be good. Also just for debugging, in case the test fails for some intermittent reason, I think it would be a good idea to print the return value of the `ForceGC` call when it returns `false` indicating that the condition didn't satisfy after the wait time.

Change updated.

It's a good thing you suggested to print out the ForceGC result. It turns out that the the ForceGC conditions weren't met, but the test was still passing!

`PhantomReference` is the wrong type of reference for this purpose. From `java.lang.ref` package doc:
>An object is phantom reachable if it is neither strongly, softly, nor weakly reachable, **_it has been finalized_**, and....

The `PhantomReference`s were never being cleared, because nothing could get finalized.

`WeakReference`s are what's needed ("When the weak references to a weakly-reachable object are cleared, **_the object becomes eligible for finalization_**.").

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PR Review Comment: https://git.openjdk.org/jdk/pull/24143#discussion_r2017778051