Snowflake算法

具体详解请看：http://www.lanindex.com/twitter-snowflake%EF%BC%8C64%E4%BD%8D%E8%87%AA%E5%A2%9Eid%E7%AE%97%E6%B3%95%E8%AF%A6%E8%A7%A3/

代码来源于网络：

public class IdWorker {
 
	
	//http://www.lanindex.com/twitter-snowflake%EF%BC%8C64%E4%BD%8D%E8%87%AA%E5%A2%9Eid%E7%AE%97%E6%B3%95%E8%AF%A6%E8%A7%A3/
	
	    /*snowflake的结构如下(每部分用-分开):

		0 - 0000000000 0000000000 0000000000 0000000000 0 - 00000 - 00000 - 000000000000

		第一位为未使用，接下来的41位为毫秒级时间(41位的长度可以使用69年)，然后是5位datacenterId和5位workerId(10位的长度最多支持部署1024个节点） ，最后12位是毫秒内的计数（12位的计数顺序号支持每个节点每毫秒产生4096个ID序号）

		一共加起来刚好64位，为一个Long型。(转换成字符串长度为18)

		snowflake生成的ID整体上按照时间自增排序，并且整个分布式系统内不会产生ID碰撞（由datacenter和workerId作区分），并且效率较高。据说：snowflake每秒能够产生26万个ID。*/
	
	
  private final long twepoch = 1288834974657L;
  private final long workerIdBits = 5L;
  private final long datacenterIdBits = 5L;
  private final long maxWorkerId = -1L ^ (-1L << workerIdBits);
  private final long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
  private final long sequenceBits = 12L;
  private final long workerIdShift = sequenceBits;
  private final long datacenterIdShift = sequenceBits + workerIdBits;
  private final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
  private final long sequenceMask = -1L ^ (-1L << sequenceBits);
 
  private long workerId;
  private long datacenterId;
  private long sequence = 0L;
  private long lastTimestamp = -1L;
 
  public IdWorker(long workerId, long datacenterId) {
    if (workerId > maxWorkerId || workerId < 0) {
      throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
    }
    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;
  }
 
  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) {
      sequence = (sequence + 1) & sequenceMask;
      if (sequence == 0) {
        timestamp = tilNextMillis(lastTimestamp);
      }
    } else {
      sequence = 0L;
    }
 
    lastTimestamp = timestamp;
 
    return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift) | (workerId << workerIdShift) | sequence;
  }
 
  protected long tilNextMillis(long lastTimestamp) {
    long timestamp = timeGen();
    while (timestamp <= lastTimestamp) {
      timestamp = timeGen();
    }
    return timestamp;
  }
 
  protected long timeGen() {
    return System.currentTimeMillis();
  }
 
  public static void main(String[] args) {
    IdWorker idWorker = new IdWorker(0, 0);
    for (int i = 0; i < 1000; i++) {
      long id = idWorker.nextId();
      System.out.println(id);
    }
  }
}