RFR: 8334015: Add Support for UUID Version 7 (UUIDv7) defined in RFC 9562 [v18]

[Lars Bruun-Hansen]

On Wed, 1 Oct 2025 17:22:39 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:
> 
>   remove old test case reference

public long epochMilliTimestamp() { ... }

Honestly, such methods do not belong in the UUID class. A UUID is an _opaque object_. With the exception of `version()` and `variant()` there is nothing you should be able to extract from it. The whole idea of an UUID is that it does not carry any information.

I fully recognize that methods `node()`, `clockSequence()` and `timestamp()` already exists in the class. But I believe that is a historical mistake. If anything, those methods should be deprecated. 

The JDK should not encourage this by adding yet another method. Also, it will be an endless game if the JDK tries to keep up with all sorts of things that potentially _could_ be extracted from an UUID  (but shouldn't)

Just my 2c.

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PR Comment: https://git.openjdk.org/jdk/pull/25303#issuecomment-3360690499