UUID

基本实现

@Override
public String genId() {
    return UUID.randomUUID().toString()
            .replaceAll(PunctuationConst.MIDDLE_LINE, PunctuationConst.EMPTY);
}

测试代码

System.out.println(new UUIDId().genId())

日志信息

379a1e5df3534885a94001373467f33e

源码解析

toString()

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

digits()

private static String digits(long val, int digits) {
    long hi = 1L >>= shift;
    } while (i != 0);
    return new String(buf, charPos, (64 - charPos));
}

其中 Integer.digits 是最基本的常量

/**
 * All possible chars for representing a number as a String
 */
final static char[] digits = {
    '0' , '1' , '2' , '3' , '4' , '5' ,
    '6' , '7' , '8' , '9' , 'a' , 'b' ,
    'c' , 'd' , 'e' , 'f' , 'g' , 'h' ,
    'i' , 'j' , 'k' , 'l' , 'm' , 'n' ,
    'o' , 'p' , 'q' , 'r' , 's' , 't' ,
    'u' , 'v' , 'w' , 'x' , 'y' , 'z'
};

random()

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);
}

SecureRandom 单例初始化

其中 Holder.numberGenerator 创建单例 SecureRandom

private static class Holder {
    static final SecureRandom numberGenerator = new SecureRandom();
}

UUID(bytes)

构造器，内容如下

private UUID(byte[] data) {
    long msb = 0;
    long lsb = 0;
    assert data.length == 16 : "data must be 16 bytes in length";
    for (int i=0; i uuidUnits = uuidUnits();
        StringBuilder stringBuilder = new StringBuilder();
        for (String unit : uuidUnits) {
            int x = Integer.parseInt(unit, 16);
            stringBuilder.append(CHARS[x % 62]);
        }
        return stringBuilder.toString();
    }

    /**
     * 获取 uuid 分段后的内容
     * @return 分段后的内容
     */
    private List uuidUnits() {
        final String uuid32 = new UUID32().genId();
        final int size = 8;

        List units = Guavas.newArrayList(size);
        for(int i = 0; i  
/// Generate a new  using the comb algorithm. 
///  
private Guid GenerateComb()
{
    byte[] guidArray = Guid.NewGuid().ToByteArray();
 
    DateTime baseDate = new DateTime(1900, 1, 1);
    DateTime now = DateTime.Now;
 
    // Get the days and milliseconds which will be used to build    
    //the byte string    
    TimeSpan days = new TimeSpan(now.Ticks - baseDate.Ticks);
    TimeSpan msecs = now.TimeOfDay;
 
    // Convert to a byte array        
    // Note that SQL Server is accurate to 1/300th of a    
    // millisecond so we divide by 3.333333    
    byte[] daysArray = BitConverter.GetBytes(days.Days);
    byte[] msecsArray = BitConverter.GetBytes((long)
      (msecs.TotalMilliseconds / 3.333333));
 
    // Reverse the bytes to match SQL Servers ordering    
    Array.Reverse(daysArray);
    Array.Reverse(msecsArray);
 
    // Copy the bytes into the guid    
    Array.Copy(daysArray, daysArray.Length - 2, guidArray,
      guidArray.Length - 6, 2);
    Array.Copy(msecsArray, msecsArray.Length - 4, guidArray,
      guidArray.Length - 4, 4);
 
    return new Guid(guidArray);
}

个人收获

uuid

uuid 从实现上看起来很简单，一个方法即可。

但是其中的原理是一样的。

组合

组合是一种非常好的方式。

其实后面的随机序列也是一种组合。

时间戳+随机数的组合

前者保证顺序性，后者保证唯一性。