RFR: 8276700: Improve java.lang.ref.Cleaner javadocs

[Brent Christian]

On Mon, 8 Nov 2021 20:18:17 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

>> You have a point.
>> 
>> BUT, at least it's a working example and not some pseudo code. We do want to move to working example code long term, don't we?
>> 
>> When I see <cleaner>, I'm just wondering what those <> type operators are good for here...
>
> A cleaner can/should be shared within some scope and purpose, in this case the example class .
> Each cleaner has a dedicated Thread so its not a lightweight object and should not be proliferated.
> (Loom may be able to use Virtual Threads).
> 
> The reasons to not share are a Cleaner are based on possible interference between the cleanup callbacks.
> If they share a thread and are non-trivial, it might slow other cleaners. If the cleaner is created and shared
> for a particular purpose it will share the thread resource and still be efficient.

The new example Cleaner instance _is_ shared, though on a pretty small scale (just among instances of CleaningExample).  A demonstration of larger scale sharing of a Cleaner instance would be out of scope for this example.

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PR: https://git.openjdk.java.net/jdk/pull/6076