RFR: 8294437: java/nio/channels/FileChannel tests slow on Windows [v2]

[Daniel Jeliński]

On Thu, 29 Sep 2022 14:30:21 GMT, Daniel Jeliński <djelinski at openjdk.org> wrote:

>> Please review this test-only change that improves the execution speed of a few FileChannel tests:
>> - Transfer2GPlus, Transfer4GBFile, and TransferTo6GBFile were modified to use sparse files. Their speed is now consistent across platforms, provided that the platform / filesystem supports sparse files.
>> - LargeMapTest was rewritten to use sparse files, and to more precisely target the issue it was written to detect. In my tests it still crashed with EXCEPTION_ACCESS_VIOLATION when JDK-8286637 was reverted.
>> - MapTest: repetition was removed from `testForce` method; I'm not sure what issues it was supposed to catch, but at far as I can tell, it was only triggering timeouts, see [JDK-8289526](https://bugs.openjdk.org/browse/JDK-8289526), [JDK-8224480](https://bugs.openjdk.org/browse/JDK-8224480)
>
> Daniel Jeliński has updated the pull request incrementally with seven additional commits since the last revision:
> 
>  - Remove intermittently failing deletes
>  - reintroduce deleteOnExit
>  - rewrite testToWritableByteChannel without using file output
>  - Migrate Transfer4GBFile to nio
>  - Implement createSparseTempFile
>  - Use RandomFactory to generate random bytes
>  - Update copyright year

I went with adding `FileUtils.createSparseTempFile()`; the method implementation is still ugly, but at least it's not visible when looking at the tests.

I added one more modification to `Transfer2GPlus`; noticed that `testToWritableByteChannel` doesn't need to write to disk, and modified the code to take advantage of that; it's much faster now.

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PR: https://git.openjdk.org/jdk/pull/10464