public final class UUID implements java.io.Serializable, Comparable<UUID> {
    private static final long serialVersionUID = -4856846361193249489L;
    // uuid 的最高有效位为64位
    private final long mostSigBits;
    // uuid 的最低有效位为64位
    private final long leastSigBits;

    // 此类使用的随机数生成器来创建基于随机的UUID。
    private static class Holder {
        static final SecureRandom numberGenerator = new SecureRandom();
    }

    // 私有构造函数，它使用字节数组构造新的UUID。
    private UUID(byte[] data) {
        long msb = 0;
        long lsb = 0;
        // 断言长度为16位
        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;
    }

    // 有参构造
    public UUID(long mostSigBits, long leastSigBits) {
        this.mostSigBits = mostSigBits;
        this.leastSigBits = leastSigBits;
    }

    // 静态方法 使用最多的方法
    public static UUID randomUUID() {
        // 使用SecureRandom生成随机数，替换version和variant就是 UUID
        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);
    }

    // 静态工厂检索类型3（基于名称的） UUID的基于指定的字节数组
    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);
    }

    // 创建一个UUID如在所描述的字符串标准表示toStrin
    public static UUID fromString(String name) {
        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];

        long mostSigBits = Long.decode(components[0]).longValue();
        mostSigBits <<= 16;
        mostSigBits |= Long.decode(components[1]).longValue();
        mostSigBits <<= 16;
        mostSigBits |= Long.decode(components[2]).longValue();

        long leastSigBits = Long.decode(components[3]).longValue();
        leastSigBits <<= 48;
        leastSigBits |= Long.decode(components[4]).longValue();

        return new UUID(mostSigBits, leastSigBits);
    }

    // 获取低位个数
    public long getLeastSignificantBits() {
        return leastSigBits;
    }

    // 获取最长的个数
    public long getMostSignificantBits() {
        return mostSigBits;
    }
    // 获得版本号
    // 与此相关联的版本号UUID 。 版本号描述这怎么UUID生成。 版本号具有以下含义：
    // 基于时间的UUID 1
    // 2 DCE安全UUID
    // 基于名字的3 UUID
    // 4随机生成UUID
    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));
    }

    // 生成时间戳
    public long timestamp() {
        if (version() != 1) {
            throw new UnsupportedOperationException("Not a time-based UUID");
        }

        return (mostSigBits & 0x0FFFL) << 48
            | ((mostSigBits >> 16) & 0x0FFFFL) << 32
            | mostSigBits >>> 32;
    }

    // 与此UUID相关联的时钟序列值
    public int clockSequence() {
        if (version() != 1) {
            throw new UnsupportedOperationException("Not a time-based UUID");
        }

        return (int)((leastSigBits & 0x3FFF000000000000L) >>> 48);
    }

    // 与此UUID相关联的节点的值
    public long node() {
        if (version() != 1) {
            throw new UnsupportedOperationException("Not a time-based UUID");
        }

        return leastSigBits & 0x0000FFFFFFFFFFFFL;
    }

    // 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));
    }

    // 返回VAL由十六进制数字指定的数来表示。
    private static String digits(long val, int digits) {
        long hi = 1L << (digits * 4);
        return Long.toHexString(hi | (val & (hi - 1))).substring(1);
    }

    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;
        return (mostSigBits == id.mostSigBits &&
                leastSigBits == id.leastSigBits);
    }
    
    public int compareTo(UUID val) {
        return (this.mostSigBits < val.mostSigBits ? -1 :
                (this.mostSigBits > val.mostSigBits ? 1 :
                 (this.leastSigBits < val.leastSigBits ? -1 :
                  (this.leastSigBits > val.leastSigBits ? 1 :
                   0))));
    }
}