手写分布式雪花 (Snowflake) 算法生成 ID

import java.util.HashSet;

import java.util.concurrent.atomic.AtomicLong;

public class SnowFlake {

    // 时间 41 位

    private static long lastTime = System.currentTimeMillis();

    // 数据中心 ID 5 位 (默认 0-31)

    private long datacenterId = 0;

    private long datacenterIdShift = 5;

    // 机房机器 ID 5 位 (默认 0-31)

    private long workerId = 0;

    private long workerIdShift = 5;

    // 随机数 12 位 (默认 0~4095)

    private AtomicLong random = new AtomicLong();

    private long randomShift = 12;

    // 随机数的最大值

    private long maxRandom = (long) Math.pow(2, randomShift);

    public SnowFlake() {

    }

    public SnowFlake(long workerIdShift, long datacenterIdShift){

        if (workerIdShift < 0 ||

                datacenterIdShift < 0 ||

                workerIdShift + datacenterIdShift > 22) {

            throw new IllegalArgumentException("参数不匹配");

        }

        this.workerIdShift = workerIdShift;

        this.datacenterIdShift = datacenterIdShift;

        this.randomShift = 22 - datacenterIdShift - workerIdShift;

        this.maxRandom = (long) Math.pow(2, randomShift);

    }

    // 获取雪花的 ID

    private long getId() {

        return lastTime << (workerIdShift + datacenterIdShift + randomShift) |

                workerId << (datacenterIdShift + randomShift) |

                datacenterId << randomShift |

                random.get();

    }

    // 生成一个新的 ID

    public synchronized long nextId() {

        long now = System.currentTimeMillis();

        // 如果当前时间和上一次时间不在同一毫秒内，直接返回

        if (now > lastTime) {

            lastTime = now;

            random.set(0);

            return getId();

        }

	// 将最后的随机数，进行 + 1 操作

        if (random.incrementAndGet() < maxRandom) {

            return getId();

        }

        // 自选等待下一毫秒

        while (now <= lastTime) {

            now = System.currentTimeMillis();

        }

        lastTime = now;

        random.set(0);

        return getId();

    }

    // 测试

    public static void main(String[] args) {

        SnowFlake snowFlake = new SnowFlake();

        HashSet<Long> set = new HashSet<>();

        for (int i = 0; i < 10000; i++) {

            set.add(snowFlake.nextId());

        }

        System.out.println(set.size());

    }

}