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

[Roger Riggs]

On Mon, 19 May 2025 13:33:51 GMT, kieran-farrell <duke 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.

src/java.base/share/classes/java/util/UUID.java line 195:

> 193:      *
> 194:      * @return A {@code UUID} generated from the current system time
> 195:      */

Seems like there should be a reference to the RFC somewhere here using the @spec javadoc tag.

And also in the class javadoc.

src/java.base/share/classes/java/util/UUID.java line 219:

> 217:         randomBytes[8] |= (byte) 0x80;
> 218: 
> 219:         return new UUID(randomBytes);

This could remove the allocation by composing the high and low longs using shifts and binary operations and ng.next().
Can the sub-microsecond value just be truncated and avoid the expensive divide operation?

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/25303#discussion_r2096254869
PR Review Comment: https://git.openjdk.org/jdk/pull/25303#discussion_r2096265946