Java 通过手写分布式雪花SnowFlake生成ID方法详解

import java.util.HashSet;

import java.util.concurrent.atomic.AtomicLong;

public class SnowFlake {

     //时间 41位

     private static long lastTime = System.currentTimeMillis();

     //数据中心ID 5位(默认0-31)

     private long datacenterId = 0 ;

     private long datacenterIdShift = 5 ;

     //机房机器ID 5位(默认0-31)

     private long workerId = 0 ;

     private long workerIdShift = 5 ;

     //随机数 12位(默认0~4095)

     private AtomicLong random = new AtomicLong();

     private long randomShift = 12 ;

     //随机数的最大值

     private long maxRandom = ( long ) Math.pow( 2 , randomShift);

     public SnowFlake() {

     }

     public SnowFlake( long workerIdShift, long datacenterIdShift){

         if (workerIdShift < 0 ||

                 datacenterIdShift < 0 ||

                 workerIdShift + datacenterIdShift > 22 ) {

             throw new IllegalArgumentException( "参数不匹配" );

         }

         this .workerIdShift = workerIdShift;

         this .datacenterIdShift = datacenterIdShift;

         this .randomShift = 22 - datacenterIdShift - workerIdShift;

         this .maxRandom = ( long ) Math.pow( 2 , randomShift);

     }

     //获取雪花的ID

     private long getId() {

         return lastTime << (workerIdShift + datacenterIdShift + randomShift) |

                 workerId << (datacenterIdShift + randomShift) |

                 datacenterId << randomShift |

                 random.get();

     }

     //生成一个新的ID

     public synchronized long nextId() {

         long now = System.currentTimeMillis();

         //如果当前时间和上一次时间不在同一毫秒内，直接返回

         if (now > lastTime) {

             lastTime = now;

             random.set( 0 );

             return getId();

         }

     //将最后的随机数，进行+1操作

         if (random.incrementAndGet() < maxRandom) {

             return getId();

         }

         //自选等待下一毫秒

         while (now <= lastTime) {

             now = System.currentTimeMillis();

         }

         lastTime = now;

         random.set( 0 );

         return getId();

     }

     //测试

     public static void main(String[] args) {

         SnowFlake snowFlake = new SnowFlake();

         HashSet<Long> set = new HashSet<>();

         for ( int i = 0 ; i < 10000 ; i++) {

             set.add(snowFlake.nextId());

         }

         System.out.println(set.size());

     }

}