RFR: 8334015: Add Support for UUID Version 7 (UUIDv7) defined in RFC 9562 [v3]

[Philippe Marschall]

On Tue, 20 May 2025 17:12:08 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.
>
> kieran-farrell has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update src/java.base/share/classes/java/util/UUID.java
>   
>   Co-authored-by: Andrey Turbanov <turbanoff at gmail.com>

src/java.base/share/classes/java/util/UUID.java line 200:

> 198:     public static UUID timestampUUID() {
> 199:         long msTime = System.currentTimeMillis();
> 200:         long nsTime = System.nanoTime();

I is my understanding that `System#nanoTime()` does not provide the nanosecond fraction and is not guaranteed to have nanosecond resolution. You'd have to look at `VM#getNanoTimeAdjustment(long)` to get the nanotime adjustment. However the resolution of this is OS dependent, usually microseconds, likely 10 microseconds on Windows.

src/java.base/share/classes/java/util/UUID.java line 203:

> 201:         SecureRandom ng = Holder.numberGenerator;
> 202:         byte[] randomBytes = new byte[16];
> 203:         ng.nextBytes(randomBytes);

It's a bit unfortunate that `SecureRandom#nextBytes(byte[])` does not allow us to pass an offset and length. Here we generate 16 bytes of random data but throw the first 8 bytes away.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25303#discussion_r2098704776
PR Review Comment: https://git.openjdk.org/jdk/pull/25303#discussion_r2098708696