1 UUID version
version 1
NIC mac address time +
Version 2
This version can be ignored
Version 3
Calculate md5 unique name space, such as url, itself is unique.
Version 4
Randomly generated
How to ensure that the world's only 2 UUID
version 1
mac address of the card is the world's only, unless cloning, while the clock is ticking, so self-generated UUID is unique.
version 3
Namespace is unique, and then calculate md5
verison 4
Randomly generated in a large space, the collision probability is very small.
3 UUID probability of collision
Small
3.1 themselves and their collision
3.2 Collision own and others'
4 java version of the UUID
java the UUID of randomUUID generated version of the UUID 4.