RFR: 8364588: Export the NPE backtracking functionality to general null-checking APIs [v4]

[Jaikiran Pai]

On Tue, 5 Aug 2025 16:04:08 GMT, Chen Liang <liach at openjdk.org> wrote:

>> Provide a general facility for our null check APIs like Objects::requireNonNull or future Checks::nullCheck (void), converting the existing infrastructure to start tracking from a given stack site (depth offset) and a given stack slot (offset value).
>> 
>> This is a necessary prerequisite for https://bugs.openjdk.org/browse/JDK-8233268, which proposes enhanced null messages to `Objects::requireNonNull`.
>
> Chen Liang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update NPE per roger review

Hello Chen, I had a look at the changes, but I'm missing some broader context of this change.

The JBS issue and the PR description states that this PR is in preparation (to upcoming additional work?) to support better `NullPointerException` exception messages when some code uses `Objects.requireNonNull()`. It also talks about a `java.lang.runtime.Checks::nullCheck` method, but there's no such `java.lang.runtime.Checks` class in mainline and it appears to be a proposed class in Valhalla project (https://bugs.openjdk.org/browse/JDK-8357936). But there too it's non-existent for now. So I'll leave out the `Checks` class from this discussion.

If this change were to be integrated into mainline, would there be any change to NullPointerException exception messages in any code paths? If not, then would it better to do these changes along with the `Objects.requireNonNull()` change itself, as part of https://bugs.openjdk.org/browse/JDK-8233268? In JDK-8233268, there's also a comment from Goetz proposing how the new exception message should look like. If the current changes were to be done, would it allow for that proposal to be implemented?

I did have a brief look at the new jtreg test cases in this PR, but it wasn't immediately clear to me which APIs would start seeing this new proposed exception messages.

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PR Comment: https://git.openjdk.org/jdk/pull/26600#issuecomment-3284904586