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Analysis of the implementation principle of UUID from the perspective of source code

2022-08-10 11:52:00 【51CTO】

这是 《水煮 JDK 源码》系列 的第 11 篇文章,计划撰写100篇关于JDK源码相关的文章

UUID 的全称是 universally unique identifier,表示通用唯一标识符,UUID 类位于 java.util 包下,自 JDK 1.5 版本新增的,它是一个 final 类,不能被继承,在平常的开发中,通常会使用 UUID Class to generate a unique identifier,比如下面的代码：

运行后,输出的结果如下：

The results above show is the default UUID 字符串,它是由数字、字母和 - 组成,The numbers and letters together32个字符,- 占4个字符,总共36个字符,当我们运行上面的程序时,实际上会调用 UUID 哪些方法呢？具体如下：

那么 UUID 是如何生成的呢？32What does each character mean?？Let's take a look at the specific source code below.

1、UUID 类定义

UUID 类实现了 Serializable 和 Comparable 接口,其定义如下：

Serializable ：它是一个标记接口,no method definition,用于对象的序列化;
Comparable ：该接口只有一个 compareTo 方法,Usually used to compare the size of objects of the class that implements it;

2、成员变量

UUID 类中定义了2个成员变量,respectively represent the most effective64bit and least significant64位,如下：

mostSigBits 和 leastSigBits 它们是构成 UUID An important part of the identifier.

3、构造函数

UUID 类提供了 2 个构造函数,其定义如下：

/** 私有的构造方法 */
private UUID(byte[] data) {
    long msb = 0;
    long lsb = 0;
    // 通过 assert assertions to determine parameters data 长度是否为 16
    assert data.length == 16 : "data must be 16 bytes in length";
    for (int i=0; i<8; i++)
        msb = (msb << 8) | (data[i] & 0xff);
    for (int i=8; i<16; i++)
        lsb = (lsb << 8) | (data[i] & 0xff);
    this.mostSigBits = msb;
    this.leastSigBits = lsb;
}

public UUID(long mostSigBits, long leastSigBits) {
    this.mostSigBits = mostSigBits;
    this.leastSigBits = leastSigBits;
}

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在构造函数中,Mainly for member variables mostSigBits 和 leastSigBits 赋值,虽然 UUID 提供了 public 构造函数,但是在平时开发中,may rarely be created directly through the constructor UUID 对象,更多的是使用 randomUUID() 方法,Breakpoint way below look at some of the calculated the private constructor mostSigBits 和 leastSigBits 具体值是多少,如下：

从源码角度分析UUID的实现原理_UUID

4、方法

UUID The methods in a class are mainly divided into static methods and instance methods,Among them, static methods are mainly used to create UUID 实例的,Instance methods are mainly used to obtain UUID 中的一些基本信息,比如版本号、时间戳、clock sequence etc..

4.1 静态方法 - randomUUID()

public static UUID randomUUID() {
    // Via inner static class Holder 获取 SecureRandom 对象
    SecureRandom ng = Holder.numberGenerator;
	// 定义一个长度为 16 的字节数组
    byte[] randomBytes = new byte[16];
    // 通过 SecureRandom.nextBytes() 方法随机生成 16 个字节数字,and populate the array
    ng.nextBytes(randomBytes);
    // 索引为 6 The number of bytes represents the UUID 的版本号
    // First clear the version number,Then set the version number to 4
    randomBytes[6]  &= 0x0f;  /* clear version */
    randomBytes[6]  |= 0x40;  /* set to version 4 */
    // 索引为 8 The number of bytes represents the UUID A variant of number
    // Clear the number first,然后设置为 IETF 变种,其值 为 2
    randomBytes[8]  &= 0x3f;  /* clear variant */
    randomBytes[8]  |= 0x80;  /* set to IETF variant */
    // Call the private constructor to create UUID 对象实例
    return new UUID(randomBytes);
}

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randomUUID() The method is probably the most used one way,从实现可以看出,First it through internal static class Holder got a random SecureRandom 对象,主要用于产生随机数,Holder 类定义如下：

4.2 静态方法 - nameUUIDFromBytes()

public static UUID nameUUIDFromBytes(byte[] name) {
    MessageDigest md;
    try {
        // 获取 MD5 信息摘要算法
        md = MessageDigest.getInstance("MD5");
    } catch (NoSuchAlgorithmException nsae) {
        throw new InternalError("MD5 not supported", nsae);
    }
    // 通过 MD5算法 对传入的 name[] byte array to convert
    // After conversion, the length is 16 的 md5Bytes 新数组
    byte[] md5Bytes = md.digest(name);
    // Set the version number to 3
    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);
}

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nameUUIDFromBytes() methods can be created from a byte array UUID,首先会通过 MD5 Digest algorithm cryptographically transforms byte arrays,得到一个长度为 16 的新字节数组,Then build with the new byte array UUID,This method will set the version number to 3,而通过 randomUUID() The version number set by the method is 4.

4.3 静态方法 - fromString()

public static UUID fromString(String name) {
    // 传入的 name The format must be and UUID.toString() The obtained string is consistent
    // 通过 - 对字符串进行分割,Need to get the length of 5 的字符串数组
    String[] components = name.split("-");
    if (components.length != 5)
        throw new IllegalArgumentException("Invalid UUID string: "+name);
    for (int i=0; i<5; i++)
        components[i] = "0x"+components[i];

    // Calculate the most effective64位的值
    long mostSigBits = Long.decode(components[0]).longValue();
    mostSigBits <<= 16;
    mostSigBits |= Long.decode(components[1]).longValue();
    mostSigBits <<= 16;
    mostSigBits |= Long.decode(components[2]).longValue();

    // Calculate the least effective64位的值
    long leastSigBits = Long.decode(components[3]).longValue();
    leastSigBits <<= 48;
    leastSigBits |= Long.decode(components[4]).longValue();

    return new UUID(mostSigBits, leastSigBits);
}

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In addition to the above which can be constructed from byte arrays UUID 外,You can also build directly from strings,But not just any string,but requires and UUID.toString() The string format obtained by the method is consistent,也就是使用 - For at the time of the partitioning,must be of length 5 的字符串数组,否则就会抛出 IllegalArgumentException 异常,Then according to the divided value to calculate mostSigBits 和 leastSigBits 的值,最后再通过 mostSigBits 和 leastSigBits 构建 UUID 对象.

4.4 实例方法

UUID 类提供的实例方法,主要有以下这些：

version()：获取当前 UUID 的版本信息;
variant()：获取当前 UUID variant number of;
timestamp()：获取当前 UUID 的时间戳;
clockSequence()：获取当前 UUID The clock sequence value of;
node()：获取当前 UUID 的节点值;
toString()：将 UUID 对象转换为字符串,In this way, you can understand UUID 的构成;
hashCode()：获取当前 UUID 的哈希值;
equals(Object obj)：用于比较两个 UUID 对象是否相同;
compareTo(UUID val)：比较两个 UUID the size of the object value;

（1）Basic Information Method

public int version() {
    // Version is bits masked by 0x000000000000F000 in MS long
    return (int)((mostSigBits >> 12) & 0x0f);
}

public int variant() {
    // 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.
    return (int) ((leastSigBits >>> (64 - (leastSigBits >>> 62)))
                  & (leastSigBits >> 63));
} 

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For version number and variant number information,是所有 UUID 通用的,For time-based UUID,then you can call the following 3 methods to obtain time-related information,如下：

public long timestamp() {
    // 如果 version 版本号不为 1,则直接抛出异常
    // must be time-based UUID
    if (version() != 1) {
        throw new UnsupportedOperationException("Not a time-based UUID");
    }

    // Convert by high-order value to get timestamp
    return (mostSigBits & 0x0FFFL) << 48
        | ((mostSigBits >> 16) & 0x0FFFFL) << 32
        | mostSigBits >>> 32;
}

public int clockSequence() {
    // 如果 version 版本号不为 1,则直接抛出异常
    // must be time-based UUID
    if (version() != 1) {
        throw new UnsupportedOperationException("Not a time-based UUID");
    }

    // Convert by low-order value to get clock sequence value
    return (int)((leastSigBits & 0x3FFF000000000000L) >>> 48);
}

public long node() {
    // 如果 version 版本号不为 1,则直接抛出异常
    // must be time-based UUID
    if (version() != 1) {
        throw new UnsupportedOperationException("Not a time-based UUID");
    }

    // Get the node value by converting the low-order value
    return leastSigBits & 0x0000FFFFFFFFFFFFL;
}

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（2）toString() 方法

public String toString() {
    return (digits(mostSigBits >> 32, 8) + "-" +
            digits(mostSigBits >> 16, 4) + "-" +
            digits(mostSigBits, 4) + "-" +
            digits(leastSigBits >> 48, 4) + "-" +
            digits(leastSigBits, 12));
}

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通过 toString() 方法,可以得知 UUID The specific composition of the string,主要是通过 mostSigBits 和 leastSigBits obtained by transforming,digits() 方法定义如下：

private static String digits(long val, int digits) {
    long hi = 1L << (digits * 4);
    return Long.toHexString(hi | (val & (hi - 1))).substring(1);
}

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UUID The composition is mainly divided into the following：

* UUID                   = <time_low> "-" <time_mid> "-"
*                          <time_high_and_version> "-"
*                          <variant_and_sequence> "-"
*                          <node>
* time_low               = 4*<hexOctet>
* time_mid               = 2*<hexOctet>
* time_high_and_version  = 2*<hexOctet>
* variant_and_sequence   = 2*<hexOctet>
* node                   = 6*<hexOctet>
* hexOctet               = <hexDigit><hexDigit>
* hexDigit               =
*       "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"
*       | "a" | "b" | "c" | "d" | "e" | "f"
*       | "A" | "B" | "C" | "D" | "E" | "F"

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通过上面的信息可以看出,UUID 由 时间低位 + time median + Time high and version number + Variant number and serial number + 节点值 五个部分组成的,can see a specific value

（3）其他方法

public int hashCode() {
    long hilo = mostSigBits ^ leastSigBits;
    return ((int)(hilo >> 32)) ^ (int) hilo;
}

public boolean equals(Object obj) {
    if ((null == obj) || (obj.getClass() != UUID.class))
        return false;
    UUID id = (UUID)obj;
    // 比较两个 UUID 是否相同,主要比较 mostSigBits 和 leastSigBits 是否相等
    return (mostSigBits == id.mostSigBits &&
            leastSigBits == id.leastSigBits);
}

public int compareTo(UUID val) {
    // 比较两个 UUID 的大小,也是比较 mostSigBits 和 leastSigBits 的大小
    return (this.mostSigBits < val.mostSigBits ? -1 :
            (this.mostSigBits > val.mostSigBits ? 1 :
             (this.leastSigBits < val.leastSigBits ? -1 :
              (this.leastSigBits > val.leastSigBits ? 1 :
               0))));
}

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For comparing two UUID Is it the same or is it equal,are direct comparisons mostSigBits 和 leastSigBits 的值.

5、测试验证

下面通过 3 种不同的方式来创建 UUID 实例,Simultaneously output basic information.

package com.magic.test;

import java.util.UUID;

public class UUIDTest {

    public static void main(String[] args) {
        UUID uuid = UUID.randomUUID();
        System.out.println(uuid);
        System.out.println("version : " + uuid.version());
        System.out.println("variant : " + uuid.variant());

        UUID uuid1 = UUID.nameUUIDFromBytes("TEST".getBytes());
        System.out.println(uuid1);
        System.out.println("version : " + uuid1.version());
        System.out.println("variant : " + uuid1.variant());

        UUID uuid2 = UUID.fromString("033bd94b-1168-37e4-b0d6-44c3c95e35bf");
        System.out.println(uuid2);
        System.out.println("version : " + uuid2.version());
        System.out.println("variant : " + uuid2.variant());
    }
}

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运行程序,输出信息如下：

d3f83827-8ccd-4ff7-8118-5155db6dbf4b
version : 4
variant : 2
033bd94b-1168-37e4-b0d6-44c3c95e35bf
version : 3
variant : 2
033bd94b-1168-37e4-b0d6-44c3c95e35bf
version : 3
variant : 2

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If directly to the creation of the above UUID Invoke time-dependent methods,则会抛出 UnsupportedOperationException 异常,测试代码如下：

The output error message is as follows：

Exception in thread "main" java.lang.UnsupportedOperationException: Not a time-based UUID
	at java.util.UUID.timestamp(UUID.java:293)
	at com.magic.test.UUIDTest.main(UUIDTest.java:26)

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It can be known from the previous source code analysis,timestamp()、clockSequence()、node() Methods must be time-based UUID 才能调用,基于时间的 UUID 的版本号为 1,而上面的 uuid、uuid1、uuid2 Neither is time-based,所以会抛出异常.

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