UUID in Java

What is a UUID in Java

UUID.randomUUID() has been in java.util since Java 5. It returns a variant-2 version-4 UUID built from a cryptographically strong generator, and it is the correct default for request ids, entity keys and correlation ids in logs. Note the variant: the JDK emits the IETF variant, not the older Microsoft layout, so the 17th hex digit is always in 8–b.

For a value that must be reproducible, UUID.nameUUIDFromBytes(bytes) hashes arbitrary bytes with MD5 into a version 3 UUID — the same input gives the same identifier in every JVM. There is no built-in v5 (SHA-1) or v7 in the JDK, so if you want time-ordered keys that keep B-tree inserts sequential you need a library such as uuid-creator, whose UuidCreator.getTimeOrderedEpoch() produces v7 values your database can index in order.

How to

1. Call UUID.randomUUID() for a random v4 identifier.
2. Use UUID.nameUUIDFromBytes(byte[]) when the identifier must be derived from a natural key.
3. Add uuid-creator when you need v7 and want inserts to stay ordered.

Use cases

• Entity and request ids in Spring and Jakarta EE applications, where a random v4 avoids a database round trip.
• Deterministic dedupe keys: hashing a composite business key gives an id that is stable across services.
• Correlation ids in logs and tracing headers, where a compact hexadecimal id is easier to grep than a verbose alternative.

Compare UUID in Java with other formats

Code examples

Random v4

import java.util.UUID;

UUID id = UUID.randomUUID();
System.out.println(id);          // 550e8400-e29b-41d4-a716-446655440000
System.out.println(id.version());  // 4
System.out.println(id.variant());  // 2

Deterministic v3

import java.util.UUID;
import java.nio.charset.StandardCharsets;

UUID stable = UUID.nameUUIDFromBytes(
    "user:42".getBytes(StandardCharsets.UTF_8));
// same input -> same UUID in every JVM

Time-ordered v7 (library)

// implementation 'com.github.f4b6a3:uuid-creator:6.0.0'
import com.github.f4b6a3.uuid.UuidCreator;

UUID v7 = UuidCreator.getTimeOrderedEpoch();
// 019535d9-3df7-79fb-b466-fa907fa17f9e

// Values created later sort later, which keeps index inserts sequential

Parsing and validating

import java.util.UUID;

try {
    UUID parsed = UUID.fromString(input);
    // well-formed **and** legal hex
} catch (IllegalArgumentException notAUuid) {
    // reject the input
}

Frequently asked questions