1、snowflake算法ID生成器介紹

snowflake 是 twitter 開源的一個分布式ID 生成器

2、為什麽使用snowflake

(1) 主鍵自增弊端：不是全局id，當多表合並、構建數據倉庫、進行數據分析、會導致主鍵沖突

(2) uuid或guid弊端：數據量過大

(3)全局redis生成弊端：內存開銷大、網絡開銷大

(4)snowflake解決上述問題、效率高、經測試每秒可產生26萬個id

3、生成原理

（1）采用64位二進制數結構

（2）第1位不用，保留

（3）後41位為時間戳、可支持69年跨度

（4）後5位為機器id

（5）後5位為業務id

（6）後12為毫秒內計算器產生的序列數（每毫秒產生2的12方個id）

4、使用方式

（1）拷入源碼（工具類）

package com.test.snowflake;

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

//雪花算法代碼實現
public class IdWorker {
    // 時間起始標記點，作為基準，一般取系統的最近時間（一旦確定不能變動）
    private final static long twepoch = 1288834974657L;
    // 機器標識位數
 

    private final static long workerIdBits = 5L;
    // 數據中心標識位數
    private final static long datacenterIdBits = 5L;
    // 機器ID最大值
    private final static long maxWorkerId = -1L ^ (-1L << workerIdBits);
    // 數據中心ID最大值
    private final static long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
    
 
// 毫秒內自增位
    private final static long sequenceBits = 12L;
    // 機器ID偏左移12位
    private final static long workerIdShift = sequenceBits;
    // 數據中心ID左移17位
    private final static long datacenterIdShift = sequenceBits + workerIdBits;
    // 時間毫秒左移22位
    private final static long timestampLeftShift = sequenceBits + workerIdBits +
    datacenterIdBits;
    private final static long sequenceMask = -1L ^ (-1L << sequenceBits);
    /* 上次生產id時間戳 */
    private static long lastTimestamp = -1L;
    // 0，並發控制
    private long sequence = 0L;
    private final long workerId;
    // 數據標識id部分
    private final long datacenterId;
    public IdWorker(){
        this.datacenterId = getDatacenterId(maxDatacenterId);
        this.workerId = getMaxWorkerId(datacenterId, maxWorkerId);
    }
    /**
    * @param workerId
    * 工作機器ID
    * @param datacenterId
    * 序列號
    */
    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;
    }
    /**
    * 獲取下一個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) {
            // 當前毫秒內，則+1
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                // 當前毫秒內計數滿了，則等待下一秒
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0L;
        }
        lastTimestamp = timestamp;
        // ID偏移組合生成最終的ID，並返回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>
    * 獲取 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 獲取16個低位
        */
        return (mpid.toString().hashCode() & 0xffff) % (maxWorkerId + 1);
    }
    /**
    * <p>
    * 數據標識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)測試

項目中work對象建議托管spring容器

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

snowflake算法