RFR: 8334015: Add Support for UUID Version 7 (UUIDv7) defined in RFC 9562 [v16]

[Jaikiran Pai]

On Mon, 29 Sep 2025 15:08:14 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:
> 
>   missing semicolon

test/jdk/java/util/UUID/UUIDTest.java line 211:

> 209:         try {
> 210:             long now = System.currentTimeMillis();
> 211:             UUID u = UUID.epochMillis(now);

Please also add a assert for non-null `u`.

test/jdk/java/util/UUID/UUIDTest.java line 213:

> 211:             UUID u = UUID.epochMillis(now);
> 212:         } catch (Exception e) {
> 213:             throw new AssertionError("Unexpected exception with valid timestamp: " + e);

Please also include the timestamp value in the exception message to help debugging if at all this ever fails. Also at a few other places in the newly introduced code in this test where we throw any exception due to test failure.

test/jdk/java/util/UUID/UUIDTest.java line 224:

> 222:         // Should throw for timestamp > 48 bits
> 223:         try {
> 224:             UUID.epochMillis(1L << 48);

Can you also include a test for the exact 48 bit (allowed) value and assert that it is accepted by this method and a non-null UUID instance is returned?

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PR Review Comment: https://git.openjdk.org/jdk/pull/25303#discussion_r2390006514
PR Review Comment: https://git.openjdk.org/jdk/pull/25303#discussion_r2390007505
PR Review Comment: https://git.openjdk.org/jdk/pull/25303#discussion_r2390005257