格式
UUID(universally unique identifier) 长度是128 bit，也就是由32个16进制数值组成。其中 M 表示 version，N 表示 Variants
xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx。例如：5aadc328-8d5e-11ec-8a00-acde48001122
版本
- Version 1 (date-time and MAC address)
- Version 2 (date-time and MAC address, DCE security version)
- Versions 3 and 5 (namespace name-based)
- Version 4 (random)
Version 1 (date-time and MAC address)
version1 的 UUID 是基于时间戳和 Mac 地址的。先用 ifconfig -v en9查看一下 Mac地址为：acde48001122
en9: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500 index 4
eflags=21000080<TXSTART,ECN_ENABLE,DIRECTLINK>
ether ac:de:48:00:11:22
inet6 fe80::aede:48ff:fe00:1122%en9 prefixlen 64 scopeid 0x4
nd6 options=201<PERFORMNUD,DAD>
media: autoselect (100baseTX <full-duplex>)
status: active
type: Ethernet
link quality: 100 (good)
state availability: 0 (true)
scheduler: FQ_CODEL
link rate: 100.00 Mbps
qosmarking enabled: no mode: none
用 Python 生成的 UUID
>>> import uuid;
>>> uuid.uuid1();
UUID('5aadc328-8d5e-11ec-8a00-acde48001122')
Java
在 JDK 的工具包中，默认只提供了 V3 和 V4 版本的UUID实现。
V3
public static UUID nameUUIDFromBytes(byte[] name) {
MessageDigest md;
try {
md = MessageDigest.getInstance("MD5");
} catch (NoSuchAlgorithmException nsae) {
throw new InternalError("MD5 not supported", nsae);
}
byte[] md5Bytes = md.digest(name);
md5Bytes[6] &= 0x0f; /* clear version */
md5Bytes[6] |= 0x30; /* set to version 3 */
md5Bytes[8] &= 0x3f; /* clear variant */
md5Bytes[8] |= 0x80; /* set to IETF variant */
return new UUID(md5Bytes);
}
V4
public static UUID randomUUID() {
SecureRandom ng = Holder.numberGenerator;
byte[] randomBytes = new byte[16];
ng.nextBytes(randomBytes);
randomBytes[6] &= 0x0f; /* clear version */
randomBytes[6] |= 0x40; /* set to version 4 */
randomBytes[8] &= 0x3f; /* clear variant */
randomBytes[8] |= 0x80; /* set to IETF variant */
return new UUID(randomBytes);
}
我们依赖一个 Java 库来看一下生成的 V1 版本的 UUID，依赖如下：
<dependency>
<groupId>com.fasterxml.uuid</groupId>
<artifactId>java-uuid-generator</artifactId>
<version>4.0.1</version>
</dependency>
生成 V1 版本的 UUID。
java-uuid-generator
UUID uuid = Generators.timeBasedGenerator().generate();
991c146f-8f07-11ec-93eb-3d5453c2d114
可以看到 Python 生成的 UUID 和 Java 生成的 UUID 的 Mac 地址并不相同。Python 是真实的 Mac 地址，如果我们去看 Java 的生成代码会发现，它的 Mac 地址是随机生成的。
我们分析一下 5aadc328-8d5e-11ec-8a00-acde48001122的结构
| 名称 | 实例 | 长度(hex digits) | 说明 |
|---|---|---|---|
| time_low | 5aadc328 | 8 | |
| time_mid | 8d5e | 4 | |
| time_hi_and_version | 11ec | 4 | 1(version)+1ec(time_hi) |
| variant and clock_sequence | 8a00 | 4 | 1 to 3-bit "variant" in the most significant bits, followed by the 13 to 15-bit clock sequence （from wikipedia） |
This field is composed of a varying number of bits.
0 - - Reserved for NCS backward compatibility
1 0 - The IETF aka Leach-Salz variant (used by this class)
1 1 0 Reserved, Microsoft backward compatibility
1 1 1 Reserved for future definition.
（from Java comments）
clock sequence 目的是为了避免重复，理解为随机数据就好 |
| node | acde48001122 | 12 | Mac地址 |
根据 time_low、time_mid、time_hi 我们计算一下 UUID 的生成时间。
首先拼接出完成时间 time_hi+time_mid+time_low 即 1ec+8d5e+5aadc328 转换成十进制
然后在通过 Java 获取一下时间戳，可以发现是一致的。
UUID uuid = UUID.fromString("5aadc328-8d5e-11ec-8a00-acde48001122");
System.out.println(uuid.timestamp());
138641124929422120
看看时间戳是怎么来的，它是从 **1582年10月15日 **开始每隔100纳秒加1。
being the number of 100-nanosecond intervals since midnight 15 October 1582 Coordinated Universal Time (UTC), the date on which the Gregorian calendar was first adopted
最后再来转成当前的时间：
UUID uuid = UUID.fromString("5aadc328-8d5e-11ec-8a00-acde48001122");
Calendar uuidEpoch = Calendar.getInstance(TimeZone.getTimeZone("UTC"));
uuidEpoch.clear();
uuidEpoch.set(1582, 9, 15, 0, 0, 0); // 9 = October
long epochMillis = uuidEpoch.getTime().getTime();
long time = (uuid.timestamp() / 10000L) + epochMillis;
System.out.println(time);
1644819692942
Version 2 (date-time and MAC address, DCE security version)
UUID Version 2 与 Version 1 类似，Version 2 的 clock_sequence 的低 8 位和 timestamp 的最低 32 位进行了修改，关于 Version 2，RFC 4122 提供的信息也比较少，有的语言也没有实现 UUID Version 2。
Python 2.7.18
>>> import uuid;
>>> uuid.uuid1();
UUID('5aadc328-8d5e-11ec-8a00-acde48001122')
>>> >>> uuid.uuid2();
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'uuid2'
Versions 3 and 5 (namespace name-based)
v3 和 v5 都是基于 namespace 的
v3 = MD5(namespace)
Java v3 实现
public static UUID nameUUIDFromBytes(byte[] name) {
MessageDigest md;
try {
md = MessageDigest.getInstance("MD5");
} catch (NoSuchAlgorithmException nsae) {
throw new InternalError("MD5 not supported", nsae);
}
byte[] md5Bytes = md.digest(name);
md5Bytes[6] &= 0x0f; /* clear version */
md5Bytes[6] |= 0x30; /* set to version 3 */
md5Bytes[8] &= 0x3f; /* clear variant */
md5Bytes[8] |= 0x80; /* set to IETF variant */
return new UUID(md5Bytes);
}
v5 = SHA1(namespace)
Version 4 (random)
Java v4 实现
public static UUID randomUUID() {
SecureRandom ng = Holder.numberGenerator;
byte[] randomBytes = new byte[16];
ng.nextBytes(randomBytes);
randomBytes[6] &= 0x0f; /* clear version */
randomBytes[6] |= 0x40; /* set to version 4 */
randomBytes[8] &= 0x3f; /* clear variant */
randomBytes[8] |= 0x80; /* set to IETF variant */
return new UUID(randomBytes);
}
Nil UUID
00000000-0000-0000-0000-000000000000
参考
https://en.wikipedia.org/wiki/Universally_unique_identifier
https://www.ietf.org/rfc/rfc4122.txt
https://stackoverflow.com/questions/13070674/get-the-unix-timestamp-from-uuid-version-1
https://python.iitter.com/other/194094.html
