UUID v7 in Java

ofEpochMillis rejects a timestamp that does not fit in 48 bits. If you need several values inside one millisecond to increase, pass a timestamp that you already made monotonic. The method will not do that itself.

The older snippet reuses one SecureRandom. Creating a new SecureRandom on every call spends extra time seeding. The bit operations match the layout used by this site: random rand_a and rand_b, not a counter.

JDK 26

import java.time.Instant;
import java.util.UUID;

UUID id = UUID.ofEpochMillis(Instant.now().toEpochMilli());

Earlier JDKs

import java.security.SecureRandom;
import java.util.UUID;

final class UuidV7 {
  private static final SecureRandom RANDOM = new SecureRandom();

  static UUID create() {
    byte[] bytes = new byte[16];
    RANDOM.nextBytes(bytes);
    long ts = System.currentTimeMillis();
    bytes[0] = (byte) (ts >>> 40);
    bytes[1] = (byte) (ts >>> 32);
    bytes[2] = (byte) (ts >>> 24);
    bytes[3] = (byte) (ts >>> 16);
    bytes[4] = (byte) (ts >>> 8);
    bytes[5] = (byte) ts;
    bytes[6] = (byte) ((bytes[6] & 0x0f) | 0x70);
    bytes[8] = (byte) ((bytes[8] & 0x3f) | 0x80);
    long msb = 0;
    long lsb = 0;
    for (int i = 0; i < 8; i++) msb = (msb << 8) | (bytes[i] & 0xffL);
    for (int i = 8; i < 16; i++) lsb = (lsb << 8) | (bytes[i] & 0xffL);
    return new UUID(msb, lsb);
  }
}