1、前言

簡(jiǎn)單的說(shuō)串述，mybatis插件就是對(duì)ParameterHandler执解、ResultSetHandler、StatementHandler纲酗、Executor這四個(gè)接口上的方法進(jìn)行攔截衰腌，利用JDK動(dòng)態(tài)代理機(jī)制，為這些接口的實(shí)現(xiàn)類(lèi)創(chuàng)建代理對(duì)象觅赊，在執(zhí)行方法時(shí)右蕊，先去執(zhí)行代理對(duì)象的方法，從而執(zhí)行自己編寫(xiě)的攔截邏輯吮螺，所以真正要用好mybatis插件尤泽，主要還是要熟悉這四個(gè)接口的方法以及這些方法上的參數(shù)的含義；

另外规脸，如果配置了多個(gè)攔截器的話坯约，會(huì)出現(xiàn)層層代理的情況，即代理對(duì)象代理了另外一個(gè)代理對(duì)象莫鸭，形成一個(gè)代理鏈條闹丐，執(zhí)行的時(shí)候，也是層層執(zhí)行被因；

關(guān)于mybatis插件涉及到的設(shè)計(jì)模式和軟件思想如下：

設(shè)計(jì)模式：代理模式卿拴、責(zé)任鏈模式；
軟件思想：AOP編程思想梨与，降低模塊間的耦合度堕花，使業(yè)務(wù)模塊更加獨(dú)立；
一些注意事項(xiàng)：

不要定義過(guò)多的插件粥鞋，代理嵌套過(guò)多缘挽，執(zhí)行方法的時(shí)候，比較耗性能；
攔截器實(shí)現(xiàn)類(lèi)的intercept方法里最后不要忘了執(zhí)行invocation.proceed()方法壕曼，否則多個(gè)攔截器情況下苏研，執(zhí)行鏈條會(huì)斷掉；

2腮郊、Springboot 編寫(xiě) mybatis 插件

編寫(xiě) mybatis 插件很簡(jiǎn)單摹蘑，首先定義要攔截的是上面說(shuō)到的哪個(gè)類(lèi)，攔截哪個(gè)方法轧飞，參數(shù)是啥衅鹿，然后配置一下即可

package com.snowflake1.test.config;

import org.apache.ibatis.executor.statement.StatementHandler;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.plugin.*;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.sql.Connection;
import java.util.Properties;

@Intercepts({
        @Signature(type = StatementHandler.class, method = "prepare", args = {Connection.class, Integer.class})
})
public class MyPlugin implements Interceptor {

    private Logger logger = LoggerFactory.getLogger(MyPlugin.class);

    private long time;


    //方法攔截
    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        //通過(guò)StatementHandler獲取執(zhí)行的sql
        StatementHandler statementHandler = (StatementHandler) invocation.getTarget();
        BoundSql boundSql = statementHandler.getBoundSql();
        String sql = boundSql.getSql();

        long start = System.currentTimeMillis();
        Object proceed = invocation.proceed();
        long end = System.currentTimeMillis();
        if ((end - start) > time) {
            logger.info("本次數(shù)據(jù)庫(kù)操作是慢查詢(xún)，sql是:" + sql);
        }
        return proceed;
    }

    //獲取到攔截的對(duì)象过咬，底層也是通過(guò)代理實(shí)現(xiàn)的大渤，實(shí)際上是拿到一個(gè)目標(biāo)代理對(duì)象
    @Override
    public Object plugin(Object target) {
        return Plugin.wrap(target, this);
    }

    //獲取設(shè)置的閾值等參數(shù)
    @Override
    public void setProperties(Properties properties) {
        this.time = Long.parseLong(properties.getProperty("time"));
    }
}

在 springboot 那配置一下（我用的是 mybatisplus）

package com.snowflake1.test.config;

import com.baomidou.mybatisplus.autoconfigure.ConfigurationCustomizer;
import com.baomidou.mybatisplus.core.MybatisConfiguration;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

import java.util.Properties;

@Configuration
public class MapperConfig {
    //將插件加入到mybatis插件攔截鏈中
    @Bean
    public ConfigurationCustomizer configurationCustomizer() {
        return new ConfigurationCustomizer() {
            @Override
            public void customize(MybatisConfiguration configuration) {
                //插件攔截鏈采用了責(zé)任鏈模式，執(zhí)行順序和加入連接鏈的順序有關(guān)
                MyPlugin myPlugin = new MyPlugin();
                //設(shè)置參數(shù)援奢，比如閾值等兼犯，可以在配置文件中配置忍捡，這里直接寫(xiě)死便于測(cè)試
                Properties properties = new Properties();
                //這里設(shè)置慢查詢(xún)閾值為1毫秒集漾，便于測(cè)試
                properties.setProperty("time", "1");
                myPlugin.setProperties(properties);
                configuration.addInterceptor(myPlugin);
            }
        };
    }
}

或者

import com.baomidou.mybatisplus.core.MybatisConfiguration;
import com.baomidou.mybatisplus.extension.plugins.PaginationInterceptor;
import com.xdd.mybatis.CatMybatisPlugin;
import org.apache.ibatis.session.SqlSessionFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Configuration;

import javax.annotation.PostConstruct;

@Configuration
public class MybatisInterceptorConfig {

    @Autowired
    private SqlSessionFactory sqlSessionFactory;

    @Autowired
    private SnowflakeIdGenerator snowflakeIdGenerator;

    @PostConstruct
    public void addInterceptor() {
        this.sqlSessionFactory.getConfiguration().addInterceptor(new PaginationInterceptor());
        this.sqlSessionFactory.getConfiguration().addInterceptor(new CatMybatisPlugin());

        MybatisConfiguration mybatisConfiguration = (MybatisConfiguration) this.sqlSessionFactory.getConfiguration();
        mybatisConfiguration.getGlobalConfig().setIdentifierGenerator(snowflakeIdGenerator);
    }
}

SnowflakeIdGenerator：

import cn.com.kemai.open.util.SnowflakeIdUtils;
import com.baomidou.mybatisplus.core.incrementer.IKeyGenerator;
import com.baomidou.mybatisplus.core.incrementer.IdentifierGenerator;
import org.springframework.stereotype.Component;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Random;

@Component
public class SnowflakeIdGenerator implements IKeyGenerator, IdentifierGenerator {

    @Override
    public Number nextId(Object entity) {
        return SnowflakeIdUtils.generateId();
    }

    /**
     * @param incrementerName
     * 增量器名稱(chēng)，@KeySequence中的value之會(huì)傳入該參數(shù)中
     * @return
     */
    @Override
    public String executeSql(String incrementerName) {
        return "select " + getId() + " from dual";
    }

    public static String getId() {
        //獲取當(dāng)前時(shí)間戳
        String str = String.valueOf(System.currentTimeMillis());
        List list = new ArrayList();
        //將時(shí)間戳放入到List中
        for (Character s : str.toCharArray()) {
            list.add(s.toString());
        }
        //隨機(jī)打亂
        Collections.shuffle(list);
        //拼接字符串砸脊，并添加2(自定義)位隨機(jī)數(shù)
        return String.join("", list) + randomNumber(2);
    }

    /**
     * 生成指定長(zhǎng)度的一個(gè)數(shù)字字符串
     *
     * @param num
     * @return
     */
    public static String randomNumber(int num) {
        if (num < 1) {
            num = 1;
        }
        Random random = new Random();
        StringBuilder str = new StringBuilder();
        for (int i = 0; i < num; i++) {
            str.append(random.nextInt(10));
        }
        return str.toString();
    }

}

SnowflakeIdUtils:

import org.apache.commons.lang3.RandomUtils;
import org.apache.commons.lang3.StringUtils;
import org.apache.commons.lang3.SystemUtils;

import java.net.Inet4Address;
import java.net.UnknownHostException;
import java.util.Date;

/**
 * Twitter_Snowflake<br>
 * SnowFlake的結(jié)構(gòu)如下(每部分用-分開(kāi)):<br>
 * 0 - 0000000000 0000000000 0000000000 0000000000 0 - 00000 - 00000 - 000000000000 <br>
 * 1位標(biāo)識(shí)具篇，由于long基本類(lèi)型在Java中是帶符號(hào)的，最高位是符號(hào)位凌埂，正數(shù)是0驱显，負(fù)數(shù)是1，所以id一般是正數(shù)瞳抓，最高位是0<br>
 * 41位時(shí)間截(毫秒級(jí))埃疫，注意，41位時(shí)間截不是存儲(chǔ)當(dāng)前時(shí)間的時(shí)間戳孩哑，而是存儲(chǔ)時(shí)間戳的差值（當(dāng)前時(shí)間戳 - 開(kāi)始時(shí)間戳)
 * 得到的值）栓霜，這里的的開(kāi)始時(shí)間戳，一般是我們的id生成器開(kāi)始使用的時(shí)間横蜒，由我們程序來(lái)指定的（如下下面程序IdWorker類(lèi)的startTime屬性）胳蛮。41位的時(shí)間戳，可以使用69年丛晌，年T = (1L << 41) / (1000L * 60 * 60 * 24 * 365) = 69<br>
 * 10位的數(shù)據(jù)機(jī)器位仅炊，可以部署在1024個(gè)節(jié)點(diǎn)，包括5位datacenterId和5位workerId<br>
 * 12位序列澎蛛，毫秒內(nèi)的計(jì)數(shù)抚垄，12位的計(jì)數(shù)順序號(hào)支持每個(gè)節(jié)點(diǎn)每毫秒(同一機(jī)器，同一時(shí)間截)產(chǎn)生4096個(gè)ID序號(hào)<br>
 * 加起來(lái)剛好64位，為一個(gè)Long型督勺。<br>
 * SnowFlake的優(yōu)點(diǎn)是渠羞，整體上按照時(shí)間自增排序，并且整個(gè)分布式系統(tǒng)內(nèi)不會(huì)產(chǎn)生ID碰撞(由數(shù)據(jù)中心ID和機(jī)器ID作區(qū)分)智哀，并且效率較高次询，經(jīng)測(cè)試，SnowFlake每秒能夠產(chǎn)生26萬(wàn)ID左右瓷叫。
 */
public class SnowflakeIdUtils {

    /** 開(kāi)始時(shí)間截 (2021-01-01) */
    private final long twepoch = 1609430400000L;

    /** 機(jī)器id所占的位數(shù) */
    private final long workerIdBits = 5L;

    /** 數(shù)據(jù)標(biāo)識(shí)id所占的位數(shù) */
    private final long dataCenterIdBits = 5L;

    /** 支持的最大機(jī)器id屯吊，結(jié)果是31 (這個(gè)移位算法可以很快的計(jì)算出幾位二進(jìn)制數(shù)所能表示的最大十進(jìn)制數(shù)) */
    private final long maxWorkerId = -1L ^ (-1L << workerIdBits);

    /** 支持的最大數(shù)據(jù)標(biāo)識(shí)id，結(jié)果是31 */
    private final long maxDataCenterId = -1L ^ (-1L << dataCenterIdBits);

    /** 序列在id中占的位數(shù) */
    private final long sequenceBits = 12L;

    /** 機(jī)器ID向左移12位 */
    private final long workerIdShift = sequenceBits;

    /** 數(shù)據(jù)標(biāo)識(shí)id向左移17位(12+5) */
    private final long dataCenterIdShift = sequenceBits + workerIdBits;

    /** 時(shí)間截向左移22位(5+5+12) */
    private final long timestampLeftShift = sequenceBits + workerIdBits + dataCenterIdBits;

    /** 生成序列的掩碼摹菠，這里為4095 (0b111111111111=0xfff=4095) */
    private final long sequenceMask = -1L ^ (-1L << sequenceBits);

    /** 工作機(jī)器ID(0~31) */
    private long workerId;

    /** 數(shù)據(jù)中心ID(0~31) */
    private long dataCenterId;

    /** 毫秒內(nèi)序列(0~4095) */
    private long sequence = 0L;

    /** 上次生成ID的時(shí)間截 */
    private long lastTimestamp = -1L;

    private static SnowflakeIdUtils idWorker;

    static {
        idWorker = new SnowflakeIdUtils(getWorkId(), getDataCenterId());
    }

    /**
     * 構(gòu)造函數(shù)
     * @param workerId 工作ID (0~31)
     * @param dataCenterId 數(shù)據(jù)中心ID (0~31)
     */
    public SnowflakeIdUtils(long workerId, long dataCenterId) {
        if (workerId > maxWorkerId || workerId < 0) {
            throw new IllegalArgumentException(String.format("workerId can't be greater than %d or less than 0", maxWorkerId));
        }
        if (dataCenterId > maxDataCenterId || dataCenterId < 0) {
            throw new IllegalArgumentException(String.format("dataCenterId can't be greater than %d or less than 0", maxDataCenterId));
        }
        this.workerId = workerId;
        this.dataCenterId = dataCenterId;
    }

    /**
     * 獲得下一個(gè)ID (該方法是線程安全的)
     * @return SnowflakeId
     */
    public synchronized long nextId() {
        long timestamp = timeGen();

        //如果當(dāng)前時(shí)間小于上一次ID生成的時(shí)間戳盒卸，說(shuō)明系統(tǒng)時(shí)鐘回退過(guò)這個(gè)時(shí)候應(yīng)當(dāng)拋出異常
        if (timestamp < lastTimestamp) {
            throw new RuntimeException(String.format("Clock moved backwards. Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
        }

        //如果是同一時(shí)間生成的，則進(jìn)行毫秒內(nèi)序列
        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & sequenceMask;
            //毫秒內(nèi)序列溢出
            if (sequence == 0) {
                //阻塞到下一個(gè)毫秒,獲得新的時(shí)間戳
                timestamp = tilNextMillis(lastTimestamp);
            }
        }
        //時(shí)間戳改變次氨，毫秒內(nèi)序列重置
        else {
            sequence = 0L;
        }

        //上次生成ID的時(shí)間截
        lastTimestamp = timestamp;

        //移位并通過(guò)或運(yùn)算拼到一起組成64位的ID
        return ((timestamp - twepoch) << timestampLeftShift)
                | (dataCenterId << dataCenterIdShift)
                | (workerId << workerIdShift)
                | sequence;
    }

    /**
     * 阻塞到下一個(gè)毫秒蔽介，直到獲得新的時(shí)間戳
     * @param lastTimestamp 上次生成ID的時(shí)間截
     * @return 當(dāng)前時(shí)間戳
     */
    protected long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }

    /**
     * 返回以毫秒為單位的當(dāng)前時(shí)間
     * @return 當(dāng)前時(shí)間(毫秒)
     */
    protected long timeGen() {
        return System.currentTimeMillis();
    }

    private static Long getWorkId(){
        try {
            String hostAddress = Inet4Address.getLocalHost().getHostAddress();
            int[] ints = StringUtils.toCodePoints(hostAddress);
            int sums = 0;
            for(int b : ints) {
                sums += b;
            }
            return (long)(sums % 32);
        } catch (UnknownHostException e) {
            // 如果獲取失敗，則使用隨機(jī)數(shù)備用
            return RandomUtils.nextLong(0,31);
        }
    }

    private static Long getDataCenterId(){
        int[] ints = StringUtils.toCodePoints(SystemUtils.getHostName());
        int sums = 0;
        for (int i: ints) {
            sums += i;
        }
        return (long)(sums % 32);
    }

    /**
     * 靜態(tài)工具類(lèi)
     *
     * @return
     */
    public static synchronized Long generateId(){
        long id = idWorker.nextId();
        return id;
    }

    /** 測(cè)試 */
    public static void main(String[] args) {
        System.out.println(System.currentTimeMillis());
        long startTime = System.nanoTime();
        for (int i = 0; i < 5; i++) {
            long id = SnowflakeIdUtils.generateId();
            System.out.println(id);
        }
        System.out.println((System.nanoTime()-startTime)/1000000+"ms");

        Date date = DateUtils.stringToDate("2021-01-01 00:00:00");
        System.out.println(date.getTime());
    }

}