RFR: 8254275: Development to revise "value-based class" & apply to wrappers [v2]

[Dan Smith]

On Wed, 14 Oct 2020 20:08:32 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

>> Dan Smith has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Revise definition for more flexible ==. Apply revised boilerplate to wrappers and existing references.
>
> src/java.base/share/classes/java/lang/doc-files/ValueBased.html line 70:
> 
>> 68: <p>Synchronization on instances of value-based classes is strongly discouraged,
>> 69:     because the programmer cannot usually guarantee unique ownership of the
>> 70:     associated lock.</p>
> 
> I don't think the phrase:
> `because the programmer cannot usually guarantee unique ownership of the associated lock.`
> adds anything and is obscure.  Until the semantics of object change, there is no change in the synchronization behavior.
> For Valhalla value-based instances, there is explicitly no (implicitly) associated lock, so synchronization is illegal.

The point is that if you can't prove you have a unique identity (per the factory method clause), then you can't
guarantee that you have exclusive control over the monitor, so synchronization is dangerous. A variant of the text
discouraging synchronization was there before any design discussions about primitive object semantics.

I added "usually" because of the possibility that someone can prove that a field value they provide is unique
(according to 'equals') and thus can prove that the value-based class instance is also unique. But when I think about
actual examples of value-based classes, that seems quite unlikely, and it would probably be clearer to remove the
"usually".

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PR: https://git.openjdk.java.net/valhalla/pull/222