分布式唯一ID的几种生成方案，一次性全掌握！

public class IdWorker {

   private long workerId; // 这个就是代表了机器id
   private long datacenterId; // 这个就是代表了机房id
   private long sequence; // 这个就是代表了一毫秒内生成的多个id的最新序号

   public IdWorker(long workerId, long datacenterId, long sequence){

       // sanity check for workerId
       // 这儿不就检查了一下，要求就是你传递进来的机房id和机器id不能超过32，不能小于0
       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;
       this.sequence = sequence;
   }

   private long twepoch = 1288834974657L;

   private long workerIdBits = 5L;
   private long datacenterIdBits = 5L;
   
   // 这个是二进制运算，就是5 bit最多只能有31个数字，也就是说机器id最多只能是32以内
   private long maxWorkerId = -1L ^ (-1L << workerIdBits);

   // 这个是一个意思，就是5 bit最多只能有31个数字，机房id最多只能是32以内
   private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
   private long sequenceBits = 12L;

   private long workerIdShift = sequenceBits;
   private long datacenterIdShift = sequenceBits + workerIdBits;
   private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
   private long sequenceMask = -1L ^ (-1L << sequenceBits);

   private long lastTimestamp = -1L;

   public long getWorkerId(){
       return workerId;
   }

   public long getDatacenterId(){
       return datacenterId;
   }

   public long getTimestamp(){
       return System.currentTimeMillis();
   }

   // 这个是核心方法，通过调用nextId()方法，让当前这台机器上的snowflake算法程序生成一个全局唯一的id
   public synchronized long nextId(){

       // 这儿就是获取当前时间戳，单位是毫秒
       long timestamp = timeGen();

       if (timestamp < lastTimestamp) {
           System.err.printf(
               "clock is moving backwards. Rejecting requests until %d.", lastTimestamp);
           throw new RuntimeException(
               String.format("Clock moved backwards. Refusing to generate id for %d milliseconds",
                             lastTimestamp - timestamp));
       }      
       // 下面是说假设在同一个毫秒内，又发送了一个请求生成一个id
       // 这个时候就得把seqence序号给递增1，最多就是4096
       if (lastTimestamp == timestamp) {
       
           // 这个意思是说一个毫秒内最多只能有4096个数字，无论你传递多少进来，
           //这个位运算保证始终就是在4096这个范围内，避免你自己传递个sequence超过了4096这个范围
           sequence = (sequence + 1) & sequenceMask;

           if (sequence == 0) {
               timestamp = tilNextMillis(lastTimestamp);
           }
       
       } else {
           sequence = 0;
       }

       // 这儿记录一下最近一次生成id的时间戳，单位是毫秒
       lastTimestamp = timestamp;

       // 这儿就是最核心的二进制位运算操作，生成一个64bit的id
       // 先将当前时间戳左移，放到41 bit那儿；将机房id左移放到5 bit那儿；将机器id左移放到5 bit那儿；将序号放最后12 bit
       // 最后拼接起来成一个64 bit的二进制数字，转换成10进制就是个long型
       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();
   }
   //---------------测试---------------
   public static void main(String[] args){
       
       IdWorker worker = new IdWorker(1,1,1);
       
       for (int i = 0; i < 30; i++) {
           System.out.println(worker.nextId());
       }
   }
}