集群情况下产生一个唯一编号

http://www.blogjava.net/bolo/archive/2015/07/13/426200.html

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * 集群情况下产生一个唯一编号
 */
public class IdGeneratorUtil {

	private long workerId;
	private long datacenterId;
	private long sequence = 0L;
	private long twepoch = 1481176952230L; // 8 Dec 2016 06:04:39 GMT
	private long workerIdBits = 5L; // 节点ID长度
	private long datacenterIdBits = 5L; // 数据中心ID长度
	private long maxWorkerId = -1L ^ (-1L << workerIdBits); // 最大支持机器节点数0~31，一共32个
	private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits); // 最大支持数据中心节点数0~31，一共32个
	private long sequenceBits = 12L; // 序列号12位
	private long workerIdShift = sequenceBits; // 机器节点左移12位
	private long datacenterIdShift = sequenceBits + workerIdBits; // 数据中心节点左移17位
	private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits; // 时间毫秒数左移22位
	private long sequenceMask = -1L ^ (-1L << sequenceBits); // 4095
	private long lastTimestamp = -1L;

	private static class IdGenHolder {
		private static final IdGeneratorUtil INSTANCE = new IdGeneratorUtil();
	}

	public static IdGeneratorUtil get() {
		return IdGenHolder.INSTANCE;
	}

	/**
	 * 默认构造器
	 */
	public IdGeneratorUtil() {
		this(1L, 1L);
	}

	/**
	 * 待参数的构造器
	 * @param workerId
	 * @param datacenterId
	 */
	public IdGeneratorUtil(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;
	}

	/**
	 * 生成序列号
	 * @return
	 */
	public synchronized Long generatorId() {
		long timestamp = timeGen(); // 获取当前毫秒数
		// 如果服务器时间有问题(时钟后退) 报错。
		if (timestamp < lastTimestamp) {
			throw new RuntimeException(String.format("Clock moved backwards.Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
		}
		// 如果上次生成时间和当前时间相同,在同一毫秒内
		if (lastTimestamp == timestamp) {
			// sequence自增，因为sequence只有12bit，所以和sequenceMask相与一下，去掉高位
			sequence = (sequence + 1) & sequenceMask;
			// 判断是否溢出,也就是每毫秒内超过4095，当为4096时，与sequenceMask相与，sequence就等于0
			if (sequence == 0) {
				timestamp = tilNextMillis(lastTimestamp); // 自旋等待到下一毫秒
			}
		} else {
			sequence = 0L; // 如果和上次生成时间不同,重置sequence，就是下一毫秒开始，sequence计数重新从0开始累加
		}
		lastTimestamp = timestamp;
		// 最后按照规则拼出ID。
		// 000000000000000000000000000000000000000000 00000 00000 000000000000
		// time datacenterId workerId sequence
		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();
	}

	/**
	 * 生成业务编号
	 * @return
	 */
	public synchronized String genBizNo() {
	    // 生成唯一ID
	    Long id = get().generatorId();
	    if (null == id) {
	        id = get().generatorId();
	    }
	    if (null == id) {
	        return null;
	    }
	    String code = id.toString();
	    DateFormat format = new SimpleDateFormat("yyyyMMdd");
	    String date = format.format(new Date());
	    return date + code.substring(code.length() - 10);
	}
	
}