RFR: 8302899: Executors.newSingleThreadExecutor can use Cleaner to shutdown executor [v2]

[Martin Buchholz]

On Tue, 21 Feb 2023 18:24:05 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 the changes, 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:
> 
>   Fix typo in comment, remove blank line

Allow me to be your top cheerleader for upgrading jdk test infrastructure to junit 5.  It's enough to make me want to write some tests!

test/jdk/java/util/concurrent/Executors/AutoShutdown.java line 57:

> 55:     }
> 56: 
> 57:     private static Stream<Arguments> executorAndQueuedTaskCounts() {

I've long wondered about "cartesian products" of Streams, so off to stackoverflow, which (TIL) advises a chain of flatmaps except at the end where you just put a non-flat map.  So better style seems

    private static Stream<Arguments> executorAndQueuedTaskCounts() {
        int[] queuedTaskCounts = { 0, 1, 2 };
        return executors()
            .flatMap(s -> IntStream.of(queuedTaskCounts)
                     .mapToObj(i -> Arguments.of(s, i)));
    }

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