雪花算法

好久沒更新了，剛剛 換了個公司，繼續做各種Mark。

今天來一份雪花算法，主要的解決主鍵的問題，大夥可在這個上做各種拓展

public class SnowFlakeUtils {
	
	private long workerId;
    private long datacenterId;
    private long sequence = 0L;
    private long twepoch = 1288834974657L;
     //Thu, 04 Nov 2010 01:42:54 GMT
    private long workerIdBits = 0L;
     //節點ID長度
    private long datacenterIdBits = 0L;
     //數據中心ID長度
    private long maxWorkerId = -1L ^ (-1L << workerIdBits);
     //最大支持機器節點數0~31，一共32個
    private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
     //最大支持數據中心節點數0~31，一共32個
    private long sequenceBits = 1L;
     //序列號12位
    private long workerIdShift = sequenceBits;
     //機器節點左移12位
    private long datacenterIdShift = sequenceBits + workerIdBits;
     //數據中心節點左移17位
    private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
     //時間毫秒數左移22位
    private long sequenceMask = -1L ^ (-1L << sequenceBits);
     //最大爲4095
    private long lastTimestamp = -1L;
    
    private static class SnowFlakeHolder {
        private static final SnowFlakeUtils instance = new SnowFlakeUtils();
    }
    
    public static SnowFlakeUtils get(){
        return SnowFlakeHolder.instance;
    }

    public SnowFlakeUtils() {
        this(0L, 0L);
    }

    public SnowFlakeUtils(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 String getContractNo(){
    	String dateFormat = DateUtil.getCurDate("YYYYMMdd");
    	return dateFormat + nextId();
    }
    
    public synchronized long nextId() {
        long timestamp = timeGen();
        //獲取當前毫秒數
        //如果服務器時間有問題(時鐘後退) 報錯。
        if (timestamp < lastTimestamp) {
            throw new RuntimeException(String.format(
                    "Clock moved backwards.  Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
        }
        //如果上次生成時間和當前時間相同,在同一毫秒內
        if (lastTimestamp == timestamp) {
            //sequence自增，因爲sequence只有12bit，所以和sequenceMask相與一下，去掉高位
            sequence = (sequence + 1) & sequenceMask;
            //判斷是否溢出,也就是每毫秒內超過4095，當爲4096時，與sequenceMask相與，sequence就等於0
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
                 //自旋等待到下一毫秒
            }
        } else {
            sequence = 0L;
             //如果和上次生成時間不同,重置sequence，就是下一毫秒開始，sequence計數重新從0開始累加
        }
        lastTimestamp = timestamp;
        // 最後按照規則拼出ID。
        // 000000000000000000000000000000000000000000  00000            00000       000000000000
        // time                                      datacenterId      workerId     sequence
         // return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift)
         //        | (workerId << workerIdShift) | sequence;
         
         long longStr= ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift) | (workerId << workerIdShift) | sequence;
         // System.out.println(longStr);
         return longStr;
    }

    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) {
		for(int i=0; i< 5; i++){
			System.out.println(SnowFlakeUtils.get().getContractNo());
		}
	}
	
}

網上應該也很多的，我本人也只是做個筆記，如有雷同，完全是意外。