RFR: 8287168: (fs) WatchService leaves thread running if not closed

[Daniel Jeliński]

On Thu, 2 Jun 2022 02:52:55 GMT, Brian Burkhalter <bpb at openjdk.org> wrote:

> Add a `Cleaner` to each `WatchService` implementation.

The cleaners will never be invoked.

`Poller` class is kept alive by a thread, and while the class is static, it holds a hard reference to the enclosing WatchService. See [Windows](https://github.com/openjdk/jdk/blob/8323d9a8e0fa99947bb2a34b6b76bfecb0861b67/src/java.base/windows/classes/sun/nio/fs/WindowsWatchService.java#L320) / [Linux](https://github.com/openjdk/jdk/blob/8323d9a8e0fa99947bb2a34b6b76bfecb0861b67/src/java.base/linux/classes/sun/nio/fs/LinuxWatchService.java#L207)

A test checking if `WeakReference`s to `WatchService`s are cleared during GC would probably catch that.

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PR: https://git.openjdk.java.net/jdk/pull/8986