1. 引言

讀寫分離要做的事情就是對于一條SQL該選擇哪個數(shù)據(jù)庫去執(zhí)行，至于誰來做選擇數(shù)據(jù)庫這件事兒，無非兩個，要么中間件幫我們做，要么程序自己做。因此，一般來講，讀寫分離有兩種實(shí)現(xiàn)方式。第一種是依靠中間件(比如：MyCat)，也就是說應(yīng)用程序連接到中間件，中間件幫我們做SQL分離；第二種是應(yīng)用程序自己去做分離。這里我們選擇程序自己來做，主要是利用Spring提供的路由數(shù)據(jù)源，以及AOP。

然而，應(yīng)用程序?qū)用嫒プ鲎x寫分離最大的弱點(diǎn)(不足之處)在于無法動態(tài)增加數(shù)據(jù)庫節(jié)點(diǎn)，因?yàn)閿?shù)據(jù)源配置都是寫在配置中的，新增數(shù)據(jù)庫意味著新加一個數(shù)據(jù)源，必然改配置，并重啟應(yīng)用。當(dāng)然，好處就是相對簡單。

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2. AbstractRoutingDataSource

基于特定的查找key路由到特定的數(shù)據(jù)源。它內(nèi)部維護(hù)了一組目標(biāo)數(shù)據(jù)源，并且做了路由key與目標(biāo)數(shù)據(jù)源之間的映射，提供基于key查找數(shù)據(jù)源的方法。

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3. 實(shí)踐

關(guān)于配置請參考：

https://www.cnblogs.com/cjsblog/p/9706370.html

3.1. maven依賴

<?xml version="1.0" encoding="UTF-8"?>4.0.0com.cjs.example cjs-datasource-demo 0.0.1-SNAPSHOTjarcjs-datasource-demoorg.springframework.boot spring-boot-starter-parent 2.0.5.RELEASEUTF-8UTF-81.8org.springframework.boot spring-boot-starter-aop org.springframework.boot spring-boot-starter-jdbc org.springframework.boot spring-boot-starter-web org.mybatis.spring.boot mybatis-spring-boot-starter 1.3.2org.apache.commons commons-lang3 3.8mysql mysql-connector-java runtimeorg.springframework.boot spring-boot-starter-test testorg.springframework.boot spring-boot-maven-plugin

3.2. 數(shù)據(jù)源配置

application.yml

spring: datasource: master: jdbc-url: jdbc:mysql://192.168.102.31:3306/test username: root password: 123456 driver-class-name: com.mysql.jdbc.Driver slave1: jdbc-url: jdbc:mysql://192.168.102.56:3306/test username: pig # 只讀賬戶 password: 123456 driver-class-name: com.mysql.jdbc.Driver slave2: jdbc-url: jdbc:mysql://192.168.102.36:3306/test username: pig # 只讀賬戶 password: 123456 driver-class-name: com.mysql.jdbc.Driver

多數(shù)據(jù)源配置

package com.cjs.example.config;import com.cjs.example.bean.MyRoutingDataSource;import com.cjs.example.enums.DBTypeEnum;import org.springframework.beans.factory.annotation.Qualifier;import org.springframework.boot.context.properties.ConfigurationProperties;import org.springframework.boot.jdbc.DataSourceBuilder;import org.springframework.context.annotation.Bean;import org.springframework.context.annotation.Configuration;import javax.sql.DataSource;import java.util.HashMap;import java.util.Map;/** * 關(guān)于數(shù)據(jù)源配置，參考SpringBoot官方文檔第79章《Data Access》 * 79. Data Access * 79.1 Configure a Custom DataSource * 79.2 Configure Two DataSources */@Configurationpublic class DataSourceConfig { @Bean @ConfigurationProperties("spring.datasource.master") public DataSource masterDataSource() { return DataSourceBuilder.create().build(); } @Bean @ConfigurationProperties("spring.datasource.slave1") public DataSource slave1DataSource() { return DataSourceBuilder.create().build(); } @Bean @ConfigurationProperties("spring.datasource.slave2") public DataSource slave2DataSource() { return DataSourceBuilder.create().build(); } @Bean public DataSource myRoutingDataSource(@Qualifier("masterDataSource") DataSource masterDataSource, @Qualifier("slave1DataSource") DataSource slave1DataSource, @Qualifier("slave2DataSource") DataSource slave2DataSource) { Map targetDataSources = new HashMap<>(); targetDataSources.put(DBTypeEnum.MASTER, masterDataSource); targetDataSources.put(DBTypeEnum.SLAVE1, slave1DataSource); targetDataSources.put(DBTypeEnum.SLAVE2, slave2DataSource); MyRoutingDataSource myRoutingDataSource = new MyRoutingDataSource(); myRoutingDataSource.setDefaultTargetDataSource(masterDataSource); myRoutingDataSource.setTargetDataSources(targetDataSources); return myRoutingDataSource; }}

這里，我們配置了4個數(shù)據(jù)源，1個master，2兩個slave，1個路由數(shù)據(jù)源。前3個數(shù)據(jù)源都是為了生成第4個數(shù)據(jù)源，而且后續(xù)我們只用這最后一個路由數(shù)據(jù)源。

MyBatis配置

package com.cjs.example.config;import org.apache.ibatis.session.SqlSessionFactory;import org.mybatis.spring.SqlSessionFactoryBean;import org.springframework.context.annotation.Bean;import org.springframework.context.annotation.Configuration;import org.springframework.core.io.support.PathMatchingResourcePatternResolver;import org.springframework.jdbc.datasource.DataSourceTransactionManager;import org.springframework.transaction.PlatformTransactionManager;import org.springframework.transaction.annotation.EnableTransactionManagement;import javax.annotation.Resource;import javax.sql.DataSource;@EnableTransactionManagement@Configurationpublic class MyBatisConfig { @Resource(name = "myRoutingDataSource") private DataSource myRoutingDataSource; @Bean public SqlSessionFactory sqlSessionFactory() throws Exception { SqlSessionFactoryBean sqlSessionFactoryBean = new SqlSessionFactoryBean(); sqlSessionFactoryBean.setDataSource(myRoutingDataSource); sqlSessionFactoryBean.setMapperLocations(new PathMatchingResourcePatternResolver().getResources("classpath:mapper/*.xml")); return sqlSessionFactoryBean.getObject(); } @Bean public PlatformTransactionManager platformTransactionManager() { return new DataSourceTransactionManager(myRoutingDataSource); }}

由于Spring容器中現(xiàn)在有4個數(shù)據(jù)源，所以我們需要為事務(wù)管理器和MyBatis手動指定一個明確的數(shù)據(jù)源。

3.3. 設(shè)置路由key / 查找數(shù)據(jù)源

目標(biāo)數(shù)據(jù)源就是那前3個這個我們是知道的，但是使用的時候是如果查找數(shù)據(jù)源的呢？

首先，我們定義一個枚舉來代表這三個數(shù)據(jù)源

package com.cjs.example.enums;public enum DBTypeEnum { MASTER, SLAVE1, SLAVE2;}

接下來，通過ThreadLocal將數(shù)據(jù)源設(shè)置到每個線程上下文中

package com.cjs.example.bean;import com.cjs.example.enums.DBTypeEnum;import java.util.concurrent.atomic.AtomicInteger;public class DBContextHolder { private static final ThreadLocal contextHolder = new ThreadLocal<>(); private static final AtomicInteger counter = new AtomicInteger(-1); public static void set(DBTypeEnum dbType) { contextHolder.set(dbType); } public static DBTypeEnum get() { return contextHolder.get(); } public static void master() { set(DBTypeEnum.MASTER); System.out.println("切換到master"); } public static void slave() { // 輪詢 int index = counter.getAndIncrement() % 2; if (counter.get() > 9999) { counter.set(-1); } if (index == 0) { set(DBTypeEnum.SLAVE1); System.out.println("切換到slave1"); }else { set(DBTypeEnum.SLAVE2); System.out.println("切換到slave2"); } }}

獲取路由key

package com.cjs.example.bean;import org.springframework.jdbc.datasource.lookup.AbstractRoutingDataSource;import org.springframework.lang.Nullable;public class MyRoutingDataSource extends AbstractRoutingDataSource { @Nullable @Override protected Object determineCurrentLookupKey() { return DBContextHolder.get(); }}

設(shè)置路由key

默認(rèn)情況下，所有的查詢都走從庫，插入/修改/刪除走主庫。我們通過方法名來區(qū)分操作類型(CRUD)

package com.cjs.example.aop;import com.cjs.example.bean.DBContextHolder;import org.apache.commons.lang3.StringUtils;import org.aspectj.lang.JoinPoint;import org.aspectj.lang.annotation.Aspect;import org.aspectj.lang.annotation.Before;import org.aspectj.lang.annotation.Pointcut;import org.springframework.stereotype.Component;@Aspect@Componentpublic class DataSourceAop { @Pointcut("!@annotation(com.cjs.example.annotation.Master) " + "&& (execution(* com.cjs.example.service..*.select*(..)) " + "|| execution(* com.cjs.example.service..*.get*(..)))") public void readPointcut() { } @Pointcut("@annotation(com.cjs.example.annotation.Master) " + "|| execution(* com.cjs.example.service..*.insert*(..)) " + "|| execution(* com.cjs.example.service..*.add*(..)) " + "|| execution(* com.cjs.example.service..*.update*(..)) " + "|| execution(* com.cjs.example.service..*.edit*(..)) " + "|| execution(* com.cjs.example.service..*.delete*(..)) " + "|| execution(* com.cjs.example.service..*.remove*(..))") public void writePointcut() { } @Before("readPointcut()") public void read() { DBContextHolder.slave(); } @Before("writePointcut()") public void write() { DBContextHolder.master(); } /** * 另一種寫法：if...else... 判斷哪些需要讀從數(shù)據(jù)庫，其余的走主數(shù)據(jù)庫 */// @Before("execution(* com.cjs.example.service.impl.*.*(..))")// public void before(JoinPoint jp) {// String methodName = jp.getSignature().getName(); if (StringUtils.startsWithAny(methodName, "get

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