On Tue, 1 Jul 2025 14:39:11 GMT, Jaikiran Pai <j...@openjdk.org> wrote:
>> Kieran Farrell has refreshed the contents of this pull request, and previous
>> commits have been removed. The incremental views will show differences
>> compared to the previous content of the PR. The pull request contains one
>> new commit since the last revision:
>>
>> update spec
>
> src/java.base/share/classes/java/util/UUID.java line 107:
>
>> 105:
>> 106: private static long monotonicMS() {
>> 107: return ORIGIN_MS + (System.nanoTime() - ORIGIN_NS) / 1_000_000;
>
> Hello Kieran, the `System.nanoTime()` specifies:
>
>> This method provides nanosecond precision, but not necessarily
> nanosecond resolution (that is, how frequently the value changes) - no
> guarantees are made except that the resolution is at least as
> good as that of {@link #currentTimeMillis()}.
>
> Then `System.currentTimeMillis()` specifies:
>
>> Note that while the unit of time of the return value is a millisecond,
> the granularity of the value depends on the underlying
> operating system and may be larger. For example, many
> operating systems measure time in units of tens of
> milliseconds.
>
> Would that then mean that there's no guarantee here on this line that
> multiple sequential calls to `System.nanoTime()` will not return the same
> value and thus the breaking the monotonic guarantee of this method?
The uniqueness comes not just from the timestamp but also from the random data
in the other bytes that are generated for each new UUID.
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PR Review Comment: https://git.openjdk.org/jdk/pull/25303#discussion_r2177815716
