分布式数据库主键生成方式之一（twitter）

public class Idwork {

	private final long workerId;
	private final static long twepoch = 1288834974657L;
	private long sequence = 0L;
	private final static long workerIdBits = 4L;
	public final long maxWorkerId = -1L ^ -1L << workerIdBits;
	private final static long sequenceBits = 10L;
	private final long workerIdShift = sequenceBits;
	private final static long timestampLeftShift = sequenceBits + workerIdBits;
	public final long sequenceMask = -1L ^ -1L << sequenceBits;
	private long lastTimestamp = -1L;

	public Idwork(final long workerId) {
		super();
		if (workerId > this.maxWorkerId || workerId < 0) {
			throw new IllegalArgumentException(String.format(
					"worker Id can't be greater than %d or less than 0",
					this.maxWorkerId));
		}
		this.workerId = workerId;
	}

	public synchronized long nextId() {
		long timestamp = this.timeGen();
		if (this.lastTimestamp == timestamp) {
			this.sequence = (this.sequence + 1) & this.sequenceMask;
			if (this.sequence == 0) {
				//System.out.println("###########" + sequenceMask);
				timestamp = this.tilNextMillis(this.lastTimestamp);
			}
		} else {
			this.sequence = 0;
		}
		if (timestamp < this.lastTimestamp) {
			try {
				throw new Exception(
						String.format(
								"Clock moved backwards. Refusing to generate id for %d milliseconds",
								this.lastTimestamp - timestamp));
			} catch (Exception e) {
				e.printStackTrace();
			}
		}

		this.lastTimestamp = timestamp;
		long nextId = ((timestamp - twepoch << timestampLeftShift))
				| (this.workerId << this.workerIdShift) | (this.sequence);
		/*
		System.out.println("timestamp:" + timestamp + ",timestampLeftShift:"
				+ timestampLeftShift + ",nextId:" + nextId + ",workerId:"
				+ workerId + ",sequence:" + sequence);
		*/
		return nextId;
	}

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

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

	public static void main(String[] args) {
		Idwork worker2 = new Idwork(2);
		for(int i = 0 ; i < 100 ;i++){
			System.out.println(worker2.nextId());
		}
	}

}