資料庫主鍵id使用自動增長的壞處
1. 程式後臺新增測試
2. 舊資料匯入
3. 資料庫遷移
4. 快取 (如，兩個庫中id都為2時，快取的時候會覆蓋)以下為程式碼塊
結論：一般在實際生產環境中，一般用程式來確保id的唯一性。
以下為UUID的生成工具類：

package com.cnksi.utils;

import java.net.InetAddress;

/**
 * 在分散式系統中，需要生成全域性UID的場合還是比較多的，twitter的snowflake解決了這種需求，
 * 實現也還是很簡單的，除去配置資訊，核心程式碼就是毫秒級時間41位+機器ID 10位+毫秒內序列12位。
 * 該專案地址為：https://github.com/twitter/snowflake是用Scala實現的。
 * python版詳見開源專案https://github.com/erans/pysnowflake。
 *
 * @author
 
 xiqiao
 * @Date 2017-12-19
 */
public class IdWorker {
    //根據具體機器環境提供
    private final long workerId;
    //濾波器,使時間變小,生成的總位數變小,一旦確定不能變動
    private final static long twepoch = 1361753741828L;
    private long sequence = 0L;
    private final static long workerIdBits = 10L;
    private final static long maxWorkerId = -1
 
L ^ -1L << workerIdBits;
    private final static long sequenceBits = 12L;

    private final static long workerIdShift = sequenceBits;
    private final static long timestampLeftShift = sequenceBits + workerIdBits;
    private final static long sequenceMask = -1L ^ -1L << sequenceBits;

    private
 
 long lastTimestamp = -1L;
    //根據主機id獲取機器碼
    private static IdWorker worker = new IdWorker();

    /**
     * 建立 IdWorker物件.
     *
     * @param workerId
     * @Deprecated 請呼叫靜態方法getId()
     */
    @Deprecated
    public IdWorker(final long workerId) {
        if (workerId > IdWorker.maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0",
                    IdWorker.maxWorkerId));
        }
        this.workerId = workerId;
    }

    public IdWorker() {
        this.workerId = getAddress() % (IdWorker.maxWorkerId + 1);
    }

    public static long getId() {
        return worker.nextId();
    }

    public synchronized long nextId() {
        long timestamp = this.timeGen();
        if (this.lastTimestamp == timestamp) {
            this.sequence = (this.sequence + 1) & IdWorker.sequenceMask;
            if (this.sequence == 0) {
                //System.out.println("###########" + sequenceMask);//等待下一毫秒
                timestamp = this.tilNextMillis(this.lastTimestamp);
            }
        } else {
            this.sequence = 0;
        }
        if (timestamp < this.lastTimestamp) {
            try {
                throw new Exception(String.format(
                        "Clock moved backwards.  Refusing to generate id for %d milliseconds", this.lastTimestamp
                                - timestamp));
            } catch (Exception e) {
                e.printStackTrace();
            }
        }

        this.lastTimestamp = timestamp;
        long nextId = ((timestamp - twepoch << timestampLeftShift)) | (this.workerId << IdWorker.workerIdShift)
                | (this.sequence);
        //  System.out.println("timestamp:" + timestamp + ",timestampLeftShift:"
        //      + timestampLeftShift + ",nextId:" + nextId + ",workerId:"
        //      + workerId + ",sequence:" + sequence);
        return nextId;
    }

    private long tilNextMillis(final long lastTimestamp1) {
        long timestamp = this.timeGen();
        while (timestamp <= lastTimestamp1) {
            timestamp = this.timeGen();
        }
        return timestamp;
    }

    private static long getAddress() {
        try {
            String currentIpAddress = InetAddress.getLocalHost().getHostAddress();
            String[] str = currentIpAddress.split("\\.");
            StringBuilder hardware = new StringBuilder();
            for (int i = 0; i < str.length; i++) {
                hardware.append(str[i]);
            }
            return Long.parseLong(hardware.toString());
        } catch (Exception e) {
            e.printStackTrace();
        }

        return 2L;
    }

    private long timeGen() {
        return System.currentTimeMillis();
    }

    public static void main(final String[] args) {
        //IdWorker worker2 = new IdWorker(0);
        //System.out.println(worker2.nextId());
        //long ll = getAddress() % 16;
        //System.out.println(ll);
        //  long start = System.currentTimeMillis();
        for (int i = 0; i < 10; i++) {
            System.out.println(getId());
        }
        // long end = System.currentTimeMillis();
        // System.out.println((10 / (end - start)) + "個/ms");
        // System.out.println(getId());
    }

}