分布式的Id生成器

package com.cqfc.id;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
* tweeter的snowflake 移植到Java:
*   (a) id构成: 42位的时间前缀 + 10位的节点标识 + 12位的sequence避免并发的数字(12位不够用时强制得到新的时间前缀)
*       注意这里进行了小改动: snowkflake是5位的datacenter加5位的机器id; 这里变成使用10位的机器id
*   (b) 对系统时间的依赖性非常强，需关闭ntp的时间同步功能。当检测到ntp时间调整后，将会拒绝分配id
*/

public class IdWorker {

private final static Logger logger = LoggerFactory.getLogger(IdWorker.class);

private final long workerId;
private final long epoch = 1403854494756L;   // 时间起始标记点，作为基准，一般取系统的最近时间
private final long workerIdBits = 10L;      // 机器标识位数
private final long maxWorkerId = -1L ^ -1L << this.workerIdBits;// 机器ID最大值: 1023
private long sequence = 0L;                   // 0，并发控制
private final long sequenceBits = 12L;      //毫秒内自增位

private final long workerIdShift = this.sequenceBits;                             // 12
private final long timestampLeftShift = this.sequenceBits + this.workerIdBits;// 22
private final long sequenceMask = -1L ^ -1L << this.sequenceBits;                 // 4095,111111111111,12位
private long lastTimestamp = -1L;

private IdWorker(long workerId) {
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() throws Exception {
long timestamp = this.timeGen();
if (this.lastTimestamp == timestamp) { // 如果上一个timestamp与新产生的相等，则sequence加一(0-4095循环); 对新的timestamp，sequence从0开始
this.sequence = this.sequence + 1 & this.sequenceMask;
if (this.sequence == 0) {
timestamp = this.tilNextMillis(this.lastTimestamp);// 重新生成timestamp
}
} else {
this.sequence = 0;
}

if (timestamp < this.lastTimestamp) {
logger.error(String.format("clock moved backwards.Refusing to generate id for %d milliseconds", (this.lastTimestamp - timestamp)));
throw new Exception(String.format("clock moved backwards.Refusing to generate id for %d milliseconds", (this.lastTimestamp - timestamp)));
}

this.lastTimestamp = timestamp;
return timestamp - this.epoch << this.timestampLeftShift | this.workerId << this.workerIdShift | this.sequence;
}

private static IdWorker flowIdWorker = new IdWorker(1);
public static IdWorker getFlowIdWorkerInstance() {
return flowIdWorker;
}

/**
* 等待下一个毫秒的到来, 保证返回的毫秒数在参数lastTimestamp之后
*/
private long tilNextMillis(long lastTimestamp) {
long timestamp = this.timeGen();
while (timestamp <= lastTimestamp) {
timestamp = this.timeGen();
}
return timestamp;
}

/**
* 获得系统当前毫秒数
*/
private static long timeGen() {
return System.currentTimeMillis();
}

public static void main(String[] args) throws Exception {
System.out.println(timeGen());

IdWorker idWorker = IdWorker.getFlowIdWorkerInstance();
// System.out.println(Long.toBinaryString(idWorker.nextId()));
System.out.println(idWorker.nextId());
System.out.println(idWorker.nextId());
}

}