RFR: 8266846: Add java.time.InstantSource [v2]

[Stephen Colebourne]

On Mon, 17 May 2021 14:30:20 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

>> Stephen Colebourne has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   8266846: Add java.time.InstantSource
>
> src/java.base/share/classes/java/time/Clock.java line 128:
> 
>> 126:  * Implementations should implement {@code Serializable} wherever possible and must
>> 127:  * document whether or not they do support serialization.
>> 128:  *
> 
> The ImplSpec needs to say how it is implemented.
> The 'implements InstantSource' can not mandate any particular implementation. Its just an interface the real behavior comes from its implementations.  In this case Clock.  Referring to the static methods of InstantSource behavior may be sufficient because that behavior is concrete.

There are plenty of examples of interfaces in `java.time` and elsewhere that apply restrictions to implementations. Nevertheless, for simplicity and expediency I have reinstated the `implSpec` on `Clock`

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PR: https://git.openjdk.java.net/jdk/pull/4016