简介

本文基于Twitter的分布式ID生成策略，并做了一定的改动，以适应业务需要。
分布式ID生成的关键点：

1. 全局唯一性；
2. 生成速度；
3. ID 所表达的含义；

功能：

分布式ID生成服务

• nextId() 新 ID ，线程安全

源码：

接口

/**
 * <p>Function: ID 生成服务. </p>
 * @author Dotions 2016年5月24日下午5:58:36
 */
public interface IdGenService {
    public long nextId() throws Exception;
}

实现类

import java.util.HashSet;
import java.util.Set;

/**
 * <p>
 * Function: Twitter 的 ID 生成策略.<br/>
 * 此实现中，简化了Twitter的生成策略。主要是取消了datacenter的标记，并将workerId值扩展到了1023。<br/>
 * </p>
 * 
 * @author Wang Yunfei 2016年5月24日下午6:00:31
 */
public class TwitterIdGenServiceImpl implements IdGenService {

    private long workerId;
    
    // 不用datacenter Id
//    private long datacenterId;
    
    private long sequence = 0L;

    private long twepoch = 1288834974657L; // 起始标记点，作为基准
    
//    private long workerIdBits = 5L;
//    private long datacenterIdBits = 5L;
    
    // 不需要datacenterId，只用workerId，扩展到10位
    private long workerIdBits = 10L; // 只允许workid的范围为：0-1023
    
    private long maxWorkerId = -1L ^ (-1L << workerIdBits);
    
//    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 timestampLeftShift = sequenceBits + workerIdBits;
    
    private long sequenceMask = -1L ^ (-1L << sequenceBits);
    private long lastTimestamp = -1L;
    
    public TwitterIdGenServiceImpl(long workerId) {
        super();
        
        // sanity check for workerId
        // 只允许workId的范围为：0-1023
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
        }
        this.workerId = workerId;
    }
    
    /**
     *  下一个ID
     * */
    public long synchronized nextId() throws Exception {
        
        long timestamp = timeGen();

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

        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0;
        }
        
        lastTimestamp = timestamp;
        
        return ((timestamp - twepoch) << timestampLeftShift) |
//                  (datacenterId << datacenterIdShift) |
                  (workerId << workerIdShift) | sequence;
    }

    /**
     * 保证返回的毫秒数在参数之后
     * 
     * @param lastTimestamp
     * @return
     */
    private static long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }

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

用法：

public static void main(String[] args) {
        IdGenService idWorkder = new TwitterIdGenServiceImpl(0);
        try {
            
            Set<Long> set = new HashSet<Long>(100000);
            
            for (int i = 0; i < 10000000; i++) {
                set.add(idWorkder.nextId());
            }
            
            System.out.println(set.size());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }