推特的雪花id算法加单例模式封装

• • • 雪花算法snowflake介绍以及单例模封装
    • • 雪花算法的由来
      • 雪花算法的结构
      • • • 组成
          • 作用
      • 结合静态单例模式的实现

1. Twitter使用scala语言开源了一种分布式 id 生成算法——SnowFlake算法，被翻译成了雪花算法。
2. 雪花属于六方晶系，它具有四个结晶轴，其中三个辅轴在一个基面上，互相以60度的角度相交，第四轴(主晶轴)与三个辅轴所形成的基面垂直.
3. 雪花的基本形状是六角形，但是大自然中却几乎找不出两朵完全相同的雪花，每一个雪花都拥有自己的独有图案，就象地球上找不出两个完全相同的人一样。许多学者用显微镜观测过成千上万朵雪花，这些研究最后表明，形状、大小完全一样和各部分完全对称的雪花，在自然界中是无法形成的

雪花算法生成的ID是一个64 bit的long型的数字且按时间趋势递增。大致由首位无效符、时间戳差值、机器编码，序列号四部分组成。

雪花算法可以保证:

• 生成的所有的id都是随着时间递增
• 分布式系统内不会产生重复的id（因为有机器位做区分）

所以雪花算法可以作为我们生成数据库主键id 甚至是在某些情况下生成唯一序列码的首要选择之一

import java.lang.management.ManagementFactory;
import java.net.InetAddress;
import java.net.NetworkInterface;


public class IdWorker {
    
    private final static long START = 1288834974657L;
    
    private final static long WORKER_ID_BITS = 5L;

    
    private final static long DATACENTER_ID_BITS = 5L;

    
    private final static long MAX_WORKER_ID = ~(-1L << WORKER_ID_BITS);

    
    private final static long MAX_DATACENTER_ID = ~(-1L << DATACENTER_ID_BITS);

    
    private final static long SEQUENCE_BITS = 12L;

    
    private final static long WORKER_ID_SHIFT = SEQUENCE_BITS;

    
    private final static long DATACENTER_ID_SHIFT = SEQUENCE_BITS + WORKER_ID_BITS;

    
    private final static long TIMESTAMP_LEFT_SHIFT = SEQUENCE_BITS + WORKER_ID_BITS + DATACENTER_ID_BITS;

    private final static long SEQUENCE_MASK = ~(-1L << SEQUENCE_BITS);

    
    private static long lastTimestamp = -1L;

    
    private long sequence = 0L;

    private final long workerId;

    
    private final long datacenterId;

    private final static IdWorker ID_WORKER = new IdWorker();

    private IdWorker(){
        this.datacenterId = getDatacenterId();
        this.workerId = getMaxWorkerId(datacenterId);
    }

    public static IdWorker getInstance(){
        return ID_WORKER;
    }

    
    public synchronized long nextId() {
        long timestamp = timeGen();
        if (timestamp < lastTimestamp) {
            throw new RuntimeException(String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
        }

        if (lastTimestamp == timestamp) {
            // 当前毫秒内，则+1
            sequence = (sequence + 1) & SEQUENCE_MASK;
            if (sequence == 0) {
                // 当前毫秒内计数满了，则等待下一秒
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0L;
        }
        lastTimestamp = timestamp;
        // ID偏移组合生成最终的ID，并返回ID

        return ((timestamp - START) << TIMESTAMP_LEFT_SHIFT)
                | (datacenterId << DATACENTER_ID_SHIFT)
                | (workerId << WORKER_ID_SHIFT) | sequence;
    }

    private long tilNextMillis(final long lastTimestamp) {
        long timestamp = this.timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = this.timeGen();
        }
        return timestamp;
    }

    private long timeGen() {
        return System.currentTimeMillis();
    }

    
    protected static long getMaxWorkerId(long datacenterId) {
        StringBuilder mid = new StringBuilder();
        mid.append(datacenterId);
        String name = ManagementFactory.getRuntimeMXBean().getName();
        if (!name.isEmpty()) {
         
            mid.append(name.split("@")[0]);
        }
      
        return (mid.toString().hashCode() & 0xffff) % (IdWorker.MAX_WORKER_ID + 1);
    }

    
    protected static long getDatacenterId() {
        long id = 0L;
        try {
            InetAddress ip = InetAddress.getLocalHost();
            NetworkInterface network = NetworkInterface.getByInetAddress(ip);
            if (network == null) {
                id = 1L;
            } else {
                byte[] mac = network.getHardwareAddress();
                id = ((0x000000FF & (long) mac[mac.length - 1])
                        | (0x0000FF00 & (((long) mac[mac.length - 2]) << 8))) >> 6;
                id = id % (IdWorker.MAX_DATACENTER_ID + 1);
            }
        } catch (Exception e) {
            System.out.println(" getDatacenterId: " + e.getMessage());
        }
        return id;
    }
}