RFR: 8334015: Add Support for UUID Version 7 (UUIDv7) defined in RFC 9562 [v14]

[Roger Riggs]

On Wed, 10 Sep 2025 19:46:12 GMT, Kieran Farrell <kfarrell at openjdk.org> wrote:

>> With the recent approval of UUIDv7 (https://datatracker.ietf.org/doc/rfc9562/), this PR aims to add a new static method UUID.timestampUUID() which constructs and returns a UUID in support of the new time generated UUID version. 
>> 
>> The specification requires embedding the current timestamp in milliseconds into the first bits 0–47. The version number in bits 48–51, bits 52–63 are available for sub-millisecond precision or for pseudorandom data. The variant is set in bits 64–65. The remaining bits 66–127 are free to use for more pseudorandom data or to employ a counter based approach for increased time percision (https://www.rfc-editor.org/rfc/rfc9562.html#name-uuid-version-7).
>> 
>> The choice of implementation comes down to balancing the sensitivity level of being able to distingush UUIDs created below <1ms apart with performance. A test simulating a high-concurrency environment with 4 threads generating 10000 UUIDv7 values in parallel to measure the collision rate of each implementation (the amount of times the time based portion of the UUID was not unique and entries could not distinguished by time) yeilded the following results for each implemtation:
>> 
>> 
>> - random-byte-only - 99.8%
>> - higher-precision - 3.5%
>> - counter-based - 0%
>> 
>> 
>> Performance tests show a decrease in performance as expected with the counter based implementation due to the introduction of synchronization:
>> 
>> - random-byte-only   143.487 ± 10.932  ns/op
>> - higher-precision      149.651 ±  8.438 ns/op
>> - counter-based         245.036 ±  2.943  ns/op
>> 
>> The best balance here might be to employ a higher-precision implementation as the large increase in time sensitivity comes at a very slight performance cost.
>
> Kieran Farrell has updated the pull request incrementally with one additional commit since the last revision:
> 
>   update method name

By dropping the no-arg `epochMillis()` we avoid the concerns about how the epoch millis maintains the requirements for monoticity.

We can leave the computation of the epoch millis to the application. A obvious convenient value is from `System.currentTimeMillis()`. There's a risk that they will ignore lack of a guarantee of monoticity of that source and only occasionally suffer from it.
The current API note on `public static UUID epochMillis(long timestamp)` links to the RFC to cover the requirements of its argument.  
It might be useful to add a cautionary sentence mentioning that `System.currentTimeMillis()` does not meet all of the requirements of the RFC; but that could turn in to a longer paragraph.

So Yes, we can reduce the functionality to be a carrier of a V7 UUID (in this PR) and separately consider the higher level semantics.

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PR Comment: https://git.openjdk.org/jdk/pull/25303#issuecomment-3336106242