snowflake算法

1、snowflake算法ID生成器介绍

snowflake 是 twitter 开源的一个分布式ID 生成器

2、为什么使用snowflake

(1) 主键自增弊端：不是全局id，当多表合并、构建数据仓库、进行数据分析、会导致主键冲突

(2) uuid或guid弊端：数据量过大

(3)全局redis生成弊端：内存开销大、网络开销大

(4)snowflake解决上述问题、效率高、经测试每秒可产生26万个id

3、生成原理

（1）采用64位二进制数结构

（2）第1位不用，保留

（3）后41位为时间戳、可支持69年跨度

（4）后5位为机器id

（5）后5位为业务id

（6）后12为毫秒内计算器产生的序列数（每毫秒产生2的12方个id）

4、使用方式

（1）拷入源码（工具类）

package com.test.snowflake;

import java.lang.management.ManagementFactory;
import java.net.InetAddress;
import java.net.NetworkInterface;

//雪花算法代码实现
public class IdWorker {
    // 时间起始标记点，作为基准，一般取系统的最近时间（一旦确定不能变动）
    private final static long twepoch = 1288834974657L;
    // 机器标识位数
    private final static long workerIdBits = 5L;
    // 数据中心标识位数
    private final static long datacenterIdBits = 5L;
    // 机器ID最大值
    private final static long maxWorkerId = -1L ^ (-1L << workerIdBits);
    // 数据中心ID最大值
    private final static long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
    // 毫秒内自增位
    private final static long sequenceBits = 12L;
    // 机器ID偏左移12位
    private final static long workerIdShift = sequenceBits;
    // 数据中心ID左移17位
    private final static long datacenterIdShift = sequenceBits + workerIdBits;
    // 时间毫秒左移22位
    private final static long timestampLeftShift = sequenceBits + workerIdBits +
    datacenterIdBits;
    private final static long sequenceMask = -1L ^ (-1L << sequenceBits);
    /* 上次生产id时间戳 */
    private static long lastTimestamp = -1L;
    // 0，并发控制
    private long sequence = 0L;
    private final long workerId;
    // 数据标识id部分
    private final long datacenterId;
    public IdWorker(){
        this.datacenterId = getDatacenterId(maxDatacenterId);
        this.workerId = getMaxWorkerId(datacenterId, maxWorkerId);
    }
    /**
    * @param workerId
    * 工作机器ID
    * @param datacenterId
    * 序列号
    */
    public IdWorker(long workerId, long datacenterId) {
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException(String.format("worker Id can't begreater 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;
    }
    /**
    * 获取下一个ID
    *
    * @return
    */
    public synchronized long nextId() {
        long timestamp = timeGen();
        if (timestamp < lastTimestamp) {
            throw new RuntimeException(String.format("Clock moved backwards. Refusingto generate id for %d milliseconds", lastTimestamp - timestamp));
        }
        if (lastTimestamp == timestamp) {
            // 当前毫秒内，则+1
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                // 当前毫秒内计数满了，则等待下一秒
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0L;
        }
        lastTimestamp = timestamp;
        // ID偏移组合生成最终的ID，并返回ID
        long nextId = ((timestamp - twepoch) << timestampLeftShift)
        | (datacenterId << datacenterIdShift)
        | (workerId << workerIdShift) | sequence;
        return nextId;
    }
    private long tilNextMillis(final long lastTimestamp) {
        long timestamp = this.timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = this.timeGen();
        }
        return timestamp;
    }
    private long timeGen() {
        return System.currentTimeMillis();
    }
    /**
    * <p>
    * 获取 maxWorkerId
    * </p>
    */
    protected static long getMaxWorkerId(long datacenterId, long maxWorkerId) {
        StringBuffer mpid = new StringBuffer();
        mpid.append(datacenterId);
        String name = ManagementFactory.getRuntimeMXBean().getName();
        if (!name.isEmpty()) {
            /*
            * GET jvmPid
            */
            mpid.append(name.split("@")[0]);
        }
        /*
        * MAC + PID 的 hashcode 获取16个低位
        */
        return (mpid.toString().hashCode() & 0xffff) % (maxWorkerId + 1);
    }
    /**
    * <p>
    * 数据标识id部分
    * </p>
    */
    protected static long getDatacenterId(long maxDatacenterId) {
        long id = 0L;
        try {
            InetAddress ip = InetAddress.getLocalHost();
            NetworkInterface network = NetworkInterface.getByInetAddress(ip);
            if (network == null) {
                id = 1L;
            } else {
                byte[] mac = network.getHardwareAddress();
                id = ((0x000000FF & (long) mac[mac.length - 1])
                | (0x0000FF00 & (((long) mac[mac.length - 2]) << 8))) >> 6;
                id = id % (maxDatacenterId + 1);
            }
        } catch (Exception e) {
            System.out.println(" getDatacenterId: " + e.getMessage());
        }
        return id;
    }
}

(2)测试

项目中work对象建议托管spring容器

IdWorker work = new IdWorker();
long id = work.nextId();
System.out.println(id);