关于雪花算法全是偶数的问题处理

1. 问题描述

生成的id全是偶数的

2. 问题发现

由于跨毫秒后，最后的sequence累加就会清零，导入末位为偶数。如果id生成不频繁，则生成的就是全是偶数

3. 解决

主要的根本就在跨毫秒清零，如果在跨毫秒时候sequence不清零，实际上每次生成id时，时间毫秒在增加，sequence也在增加，也是不会重复的。所以，索性去掉这一块

同时由于sequence = (sequence + 1) & MAX_SEQUENCE;sequence累加到最大值后，下一次依然是0，所以只保留这句话就行了。

 

完整代码：

class Snowflake {

	/**
	 * 起始的时间戳
	 */
	private final static long START_STMP = 1480166465631L;

	/**
	 * 每一部分占用的位数
	 */
	private final static long SEQUENCE_BIT = 12; // 序列号占用的位数
	private final static long MACHINE_BIT = 5; // 机器标识占用的位数
	private final static long DATACENTER_BIT = 5;// 数据中心占用的位数

	/**
	 * 每一部分的最大值
	 */
	private final static long MAX_DATACENTER_NUM = -1L ^ (-1L << DATACENTER_BIT);
	private final static long MAX_MACHINE_NUM = -1L ^ (-1L << MACHINE_BIT);
	private final static long MAX_SEQUENCE = -1L ^ (-1L << SEQUENCE_BIT);

	/**
	 * 每一部分向左的位移
	 */
	private final static long MACHINE_LEFT = SEQUENCE_BIT;
	private final static long DATACENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
	private final static long TIMESTMP_LEFT = DATACENTER_LEFT + DATACENTER_BIT;

	private long datacenterId; // 数据中心
	private long machineId; // 机器标识
	private long sequence = 0L; // 序列号
	private long lastStmp = -1L;// 上一次时间戳

	public Snowflake(long datacenterId, long machineId) {
		if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0) {
			throw new IllegalArgumentException("datacenterId can't be greater than MAX_DATACENTER_NUM or less than 0");
		}
		if (machineId > MAX_MACHINE_NUM || machineId < 0) {
			throw new IllegalArgumentException("machineId can't be greater than MAX_MACHINE_NUM or less than 0");
		}
		this.datacenterId = datacenterId;
		this.machineId = machineId;
	}

	/**
	 * 产生下一个ID
	 *
	 * @return
	 */
	public synchronized long createId() {
		long currStmp = getNewstmp();
		if (currStmp < lastStmp) {
			throw new RuntimeException("Clock moved backwards.  Refusing to generate id");
		}

		sequence = (sequence + 1) & MAX_SEQUENCE;
//下方的原代码，全部注释，值保留上面一行，解决跨毫秒全为偶数问题
//        if (currStmp == lastStmp) {
//            //相同毫秒内，序列号自增
//            sequence = (sequence + 1) & MAX_SEQUENCE;
//            //同一毫秒的序列数已经达到最大
//            if (sequence == 0L) {
//                currStmp = getNextMill();
//            }
//        } else {
//            //不同毫秒内，序列号置为0
//            sequence = 0L;
//        }

		lastStmp = currStmp;

		return (currStmp - START_STMP) << TIMESTMP_LEFT // 时间戳部分
				| datacenterId << DATACENTER_LEFT // 数据中心部分
				| machineId << MACHINE_LEFT // 机器标识部分
				| sequence; // 序列号部分
	}

	private long getNextMill() {
		long mill = getNewstmp();
		while (mill <= lastStmp) {
			mill = getNewstmp();
		}
		return mill;
	}

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

	public static void main(String[] args) {
		Snowflake snowFlake = new Snowflake(2, 3);

		long start = System.currentTimeMillis();
		for (int i = 0; i < 1000000; i++) {
			System.out.println(snowFlake.createId());
		}

		System.out.println(System.currentTimeMillis() - start);

	}
}