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)，因為數(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。

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。

3. 實(shí)踐

3.1. maven依賴

xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">

4.0.0

com.cjs.example

cjs-datasource-demo

0.0.1-SNAPSHOT

jar

cjs-datasource-demo

org.springframework.boot

spring-boot-starter-parent

2.0.5.RELEASE

UTF-8

UTF-8

1.8

org.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

1.3.2

org.apache.commons

commons-lang3

3.8

mysql

mysql-connector-java

runtime

org.springframework.boot

spring-boot-starter-test

test

org.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

*/

@Configuration

public 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

@Configuration

public 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

@Component

public 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", "select", "find")) {

// DBContextHolder.slave();

// }else {

// DBContextHolder.master();

// }

// }

}

有一般情況就有特殊情況，特殊情況是某些情況下我們需要強(qiáng)制讀主庫，針對這種情況，我們定義一個主鍵，用該注解標(biāo)注的就讀主庫

package com.cjs.example.annotation;

public @interface Master {

}

例如，假設(shè)我們有一張表member

package com.cjs.example.service.impl;

import com.cjs.example.annotation.Master;

import com.cjs.example.entity.Member;

import com.cjs.example.entity.MemberExample;

import com.cjs.example.mapper.MemberMapper;

import com.cjs.example.service.MemberService;

import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.stereotype.Service;

import org.springframework.transaction.annotation.Transactional;

import java.util.List;

@Service

public class MemberServiceImpl implements MemberService {

@Autowired

private MemberMapper memberMapper;

@Transactional

@Override

public int insert(Member member) {

return memberMapper.insert(member);

}

@Master

@Override

public int save(Member member) {

return memberMapper.insert(member);

}

@Override

public List selectAll() {

return memberMapper.selectByExample(new MemberExample());

}

@Master

@Override

public String getToken(String appId) {

// 有些讀操作必須讀主數(shù)據(jù)庫

// 比如，獲取微信access_token，因為高峰時期主從同步可能延遲

// 這種情況下就必須強(qiáng)制從主數(shù)據(jù)讀

return null;

}

}

4. 測試

package com.cjs.example;

import com.cjs.example.entity.Member;

import com.cjs.example.service.MemberService;

import org.junit.Test;

import org.junit.runner.RunWith;

import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.boot.test.context.SpringBootTest;

import org.springframework.test.context.junit4.SpringRunner;

@RunWith(SpringRunner.class)

@SpringBootTest

public class CjsDatasourceDemoApplicationTests {

@Autowired

private MemberService memberService;

@Test

public void testWrite() {

Member member = new Member();

member.setName("zhangsan");

memberService.insert(member);

}

@Test

public void testRead() {

for (int i = 0; i < 4; i++) {

memberService.selectAll();

}

}

@Test

public void testSave() {

Member member = new Member();

member.setName("wangwu");

memberService.save(member);

}

@Test

public void testReadFromMaster() {

memberService.getToken("1234");

}

}

查看控制臺

5. 工程結(jié)構(gòu)

6. 參考

https://www.jianshu.com/p/f2f4256a2310

http://www.cnblogs.com/gl-developer/p/6170423.html

https://www.cnblogs.com/huangjuncong/p/8576935.html

https://blog.csdn.net/liu976180578/article/details/77684583

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