UUIDs (Universally unique identifiers; Java Class) are 128 bit long numbers that are used by Minecraft to distinguish between separate instances.
Representation
- Hyphenated hexadecimal: A hexadecimal representation of the UUID, with hyphens separating the different sections into individual numbers.
The hyphen are set to split the UUID into numbers of the format8-4-4-4-12with each number marking the number of hexadecimal digits fitting into the corresponding section. Further detail follows in the Technical aspects section.
An example of this representation would bef81d4fae-7dec-11d0-a765-00a0c91e6bf6.- Because every part is evaluated as an individual number, empty digits at the start of a section can be ignored.
For example,00000001-0002-0003-0004-000000000005can be evaluated as the same as1-2-3-4-5.
- Because every part is evaluated as an individual number, empty digits at the start of a section can be ignored.
- Hexadecimal: The same as the hyphenated hexadecimal representation, but without the separation of the different sections.
An example for this representation would be00000001000200030004000000000005, where it is impossible for the majority of empty digits to be removed as opposed to the hyphenated hexadecimal representation. - Most/Least: A separation of the 64 most significant bits from the 64 least significant bits. Each of the two numbers is stored separately and uses the
Longdata type in the game.
This format is deprecated as it was used before 1.16 and was entirely replaced by the Int-array format.
An example of this representation would beUUIDMost:-568210367123287600paired withUUIDLeast:-6384696206158828554. - Int-array: A separation into four 32 bit numbers. Each part is stored in an integer array ordered from most significant to least significant.
An example of this representation would be[I;-132296786,2112623056,-1486552928,-920753162]
Occurences
The following table represents the situations in which UUIDs are used in places accessible by a player:
| Location (general) | Location (path) | Format | Use |
|---|---|---|---|
| All entities (NBT) | UUID |
int-array | Own UUID |
| Attribute modifiers (NBT) | UUID |
int-array | Own UUID. Used when equipping and unequipping the item to identify which modifier to add or remove from the entity. |
| Attribute modifiers (Loot table) | id |
Hyphenated hexadecimal | Set UUID of modifier in set_attributes function
|
/attribute |
- | Hyphenated hexadecimal | Used to identify modifier |
| Tameable mobs (NBT) | Owner |
int-array | Owner |
| Projectiles (NBT) | Owner |
int-array | Entity that shot the projectile |
| Item entities (NBT) | Owner |
int-array | Target player for /give command
|
| Item entities (NBT) | Thrower |
int-array | Player who dropped the item |
| Shulker bullets (NBT) | Target |
int-array | Entity targeted for attack |
| Conduits (NBT) | Target |
int-array | Entity targeted for attack |
| Player heads (NBT) | SkullOwner.Id |
int-array | Owner of the skin |
| Player heads (NBT) | SkullOwner.Properties.textures[].Value.ProfileId |
hexadecimal | Owner of the skin |
| Mobs (NBT) | Leash.UUID |
int-array | Entity that leashed the mob |
| Breedable mobs (NBT) | LoveCause |
int-array | Player who fed the mob |
| Wardens (NBT) | anger.suspects[].uuid |
int-array | Target associated with the anger level |
| Zombie Villagers (NBT) | ConversionPlayer |
int-array | Player who is converting the Zombie Villager |
| Neutral mobs except piglins (NBT) | AngryAt |
int-array | Entity that hurt the neutral mob |
| Villagers (NBT) | Gossips[].Target |
int-array | Player who caused the gossip |
| Players (NBT) | RootVehicle.Attach |
int-array | Identifier of the entity that the player is riding |
| Target selectors (Commands) | - | Hyphenated hexadecimal | Selected entity |
Technical aspects
Value range
A UUID can be expressed as a 128 bit number, which means that it supports all integer values from -(2^127) to (2^127)-1. It is important to note that this evaluation does not refer to the range of values generated by the game but instead focuses on the capacity of the format.
Hyphenated hexadecimal format section names
Hyphenated hexadecimal format UUIDs follow the pattern xxxxxxxx-xxxx-Axxx-Bxxx-xxxxxxxxxxxx.
| Section name | Bit range | |||
|---|---|---|---|---|
| From | To | Size | Characters | |
| time-low | 96 | 127 | 32 | 8 |
| time-mid | 80 | 95 | 16 | 4 |
| time-high-and-version | 54 | 79 | 16 | 4 |
| clock-seq-and-reserved | 36 | 53 | 8 | 2 |
| clock-seq-low | 48 | 45 | 8 | 2 |
| node | 0 | 47 | 48 | 12 |
(The names make sense only for version 1 and 2 UUIDs, but it has been passed onto all forms of UUIDs used.)
Versions and variants
There have been five versions of UUIDs. Versions 1 and 2 are based on current time and MAC address; versions 3 and 5 are based on hashing a string; version 4 is based on completely random generation. The "version" value occupies four bits, and is located in the UUID string format at the position indicated by an "A".
The "B" position is the variant (format) of the UUID. Depending on the specific variant, it may occupy the high 1 to 3 bits:
- 0xx is the "variant 0" of Apollo NCS 1.5, an old type of UUID.
- 10x is the "variant 1" of IETF RFC 4122 (Leach-Salz). It's referred to as "variant 2" by Java.
- 110 is the "variant 2" of old Microsoft products. It's referred to as "variant 6" by Java.
- 111 is the "reserved variant". It's referred to as "variant 7" by Java.
In Minecraft
Minecraft uses version 4, variant 1 (RFC) UUIDs, which means that the entire number with the exception of the bits used for metadata (version and variant) is randomly generated. Java's UUID.randomUUID() function performs this exact task.
A table with the locations and values of the metadata: