探讨面试常见问题雪花算法、时钟回拨问题，java中优雅的实现方式

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| 1 位符号位 | 41 位时间戳 | 10 位机器ID | 12 位序列号 |

java复制代码
public class SnowflakeIdGenerator {  
// 起始时间戳（2020-01-01 00:00:00 的 Unix 时间戳）  
private final long twepoch = 1577836800000L;  
// 机器ID所占的bit数  
private final long workerIdBits = 10L;  
// 数据中心ID所占的bit数  
private final long datacenterIdBits = 10L;  
// 支持的最大机器ID数量，结果为1024 (这个位数的机器ID最多1024个（0-1023）)  
private final long maxWorkerId = -1L ^ (-1L << workerIdBits);  
// 支持的最大数据中心ID数为1024  
private final long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);  
// 序列在ID中占的位数  
private final long sequenceBits = 12L;  
// 机器ID向左移12位  
private final long workerIdShift = sequenceBits;  
// 数据中心ID向左移22位  
private final long datacenterIdShift = sequenceBits + workerIdBits;  
// 时间戳向左移22位  
private final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;  
// 生成序列的掩码，这里位运算保证只取12位  
private final long sequenceMask = -1L ^ (-1L << sequenceBits);  
private long workerId;  
private long datacenterId;  
private long sequence = 0L;  
private long lastTimestamp = -1L;  
public SnowflakeIdGenerator(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;  
    }  
// 产生下一个ID  
public synchronized long nextId() {  
long currentTimestamp = timeGen();  
if (currentTimestamp < lastTimestamp) {  
throw new RuntimeException(String.format("Clock moved backwards. Refusing to generate id for %d milliseconds", lastTimestamp - currentTimestamp));  
        }  
if (currentTimestamp == lastTimestamp) {  
// 如果在同一毫秒内  
            sequence = (sequence + 1) & sequenceMask;  
if (sequence == 0) {  
// 阻塞到下一个毫秒  
                currentTimestamp = tilNextMillis(lastTimestamp);  
            }  
        } else {  
            sequence = 0L;  
        }  
        lastTimestamp = currentTimestamp;  
return ((currentTimestamp - twepoch) << timestampLeftShift) |  
                (datacenterId << datacenterIdShift) |  
                (workerId << workerIdShift) |  
                sequence;  
    }  
// 阻塞到下一个毫秒，直到获得新的时间戳  
private long tilNextMillis(long lastTimestamp) {  
long timestamp = timeGen();  
while (timestamp <= lastTimestamp) {  
            timestamp = timeGen();  
        }  
return timestamp;  
    }  
// 获取当前时间戳  
private long timeGen() {  
return System.currentTimeMillis();  
    }  
public static void main(String[] args) {  
SnowflakeIdGenerator idWorker = new SnowflakeIdGenerator(1, 1);  
for (int i = 0; i < 10; i++) {  
long id = idWorker.nextId();  
            System.out.println(id);  
        }  
    }  
}