RFR: 8302899: Executors.newSingleThreadExecutor can use Cleaner to shutdown executor [v5]

[Martin Buchholz]

On Fri, 24 Feb 2023 13:44:40 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> Executors.newSingleThreadExecutor returns a delegating ExecutorService that has finalizer to shutdown the underlying TPE when the wrapper is finalizable. It goes back to JDK 6 and JDK-6399443. This is the last non-empty finalizer in java.base. Removing it will likely lead to bug reports/complaints as the current behavior goes back to 2006. So the proposal is to just replace it with a Cleaner, trivially done in this case. As part of this change, I've added a RF to DelegatedExecutorService.shutdown to be consistent with the other methods - this is a benign change and avoids needing to puzzle over why it wasn't there previously. I've replaced the existing test with a more modern test that exercises more scenarios.
>
> Alan Bateman has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Rename cleaner, fix typo in test comments

Marked as reviewed by martin (Reviewer).

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PR: https://git.openjdk.org/jdk/pull/12675