SnowFlake.java

public class SnowFlake {

  private long workerId; // 这个就是代表了机器id
  private long datacenterId; // 这个就是代表了机房id
  private long sequence; // 这个就是代表了一毫秒内生成的多个id的最新序号

  private SnowFlake(long workerId, long datacenterId, long sequence) {
    // sanity check for workerId
    // 这儿不就检查了一下，要求就是你传递进来的机房id和机器id不能超过32，不能小于0
    // 这个是二进制运算，就是5 bit最多只能有31个数字，也就是说机器id最多只能是32以内
    long maxWorkerId = ~(-1L << workerIdBits);
    if (workerId > maxWorkerId || workerId < 0) {

      throw new IllegalArgumentException(
          String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
    }

    // 这个是一个意思，就是5 bit最多只能有31个数字，机房id最多只能是32以内
    long maxDatacenterId = ~(-1L << datacenterIdBits);
    if (datacenterId > maxDatacenterId || datacenterId < 0) {

      throw new IllegalArgumentException(
          String.format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId));
    }
    this.workerId = workerId;
    this.datacenterId = datacenterId;
    this.sequence = sequence;
  }

  private long workerIdBits = 5L;
  private long datacenterIdBits = 5L;

  private long sequenceBits = 12L;
  private long workerIdShift = sequenceBits;
  private long datacenterIdShift = sequenceBits + workerIdBits;
  private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
  private long sequenceMask = ~(-1L << sequenceBits);
  private long lastTimestamp = -1L;

  public long getWorkerId() {
    return workerId;
  }

  public long getDatacenterId() {
    return datacenterId;
  }

  public long getTimestamp() {
    return System.currentTimeMillis();
  }

  // 这个是核心方法，通过调用nextId()方法，让当前这台机器上的snowflake算法程序生成一个全局唯一的id
  private synchronized long nextId() {
    // 这儿就是获取当前时间戳，单位是毫秒
    long timestamp = timeGen();
    if (timestamp < lastTimestamp) {
      System.err.printf(
          "clock is moving backwards. Rejecting requests until %d.", lastTimestamp);
      throw new RuntimeException(
          String.format("Clock moved backwards. Refusing to generate id for %d milliseconds",
              lastTimestamp - timestamp));
    }

    // 下面是说假设在同一个毫秒内，又发送了一个请求生成一个id
    // 这个时候就得把seqence序号给递增1，最多就是4096
    if (lastTimestamp == timestamp) {

      // 这个意思是说一个毫秒内最多只能有4096个数字，无论你传递多少进来，
      //这个位运算保证始终就是在4096这个范围内，避免你自己传递个sequence超过了4096这个范围
      sequence = (sequence + 1) & sequenceMask;
      if (sequence == 0) {
        timestamp = tilNextMillis(lastTimestamp);
      }

    } else {
      sequence = 0;
    }
    // 这儿记录一下最近一次生成id的时间戳，单位是毫秒
    lastTimestamp = timestamp;
    // 这儿就是最核心的二进制位运算操作，生成一个64bit的id
    // 先将当前时间戳左移，放到41 bit那儿；将机房id左移放到5 bit那儿；将机器id左移放到5 bit那儿；将序号放最后12 bit
    // 最后拼接起来成一个64 bit的二进制数字，转换成10进制就是个long型
    long twepoch = 1288834974657L;
    return ((timestamp - twepoch) << timestampLeftShift) |
        (datacenterId << datacenterIdShift) |
        (workerId << workerIdShift) | sequence;
  }

  private long tilNextMillis(long lastTimestamp) {

    long timestamp = timeGen();

    while (timestamp <= lastTimestamp) {
      timestamp = timeGen();
    }
    return timestamp;
  }

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

  //---------------测试---------------
  public static void main(String[] args) {

    SnowFlake worker = new SnowFlake(1, 1, 1);

    for (int i = 0; i < 30; i++) {
      System.out.println(worker.nextId());
    }
  }
}