1. 引言
讀寫分離要做的事情就是對於一條SQL該選擇哪個數據庫去執行，至於誰來做選擇數據庫這件事兒，無非兩個，要麼中間件幫我們做，要麼程序自己做。因此，一般來講，讀寫分離有兩種實現方式。第一種是依靠中間件（比如：MyCat），也就是說應用程序連接到中間件，中間件幫我們做SQL分離；第二種是應用程序自己去做分離。這裏我們選擇程序自己來做，主要是利用Spring提供的路由數據源，以及AOP
然而，應用程序層面去做讀寫分離最大的弱點（不足之處）在於無法動態增加數據庫節點，因爲數據源配置都是寫在配置中的，新增數據庫意味着新加一個數據源，必然改配置，並重啓應用。當然，好處就是相對簡單。
2. AbstractRoutingDataSource
基於特定的查找key路由到特定的數據源。它內部維護了一組目標數據源，並且做了路由key與目標數據源之間的映射，提供基於key查找數據源的方法。
3. 實踐
關於配置請參考《 MySQL主從複製配置 》
3.1. maven依賴
<?xml version="1.0" encoding="UTF-8"?><project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.cjs.example</groupId>
<artifactId>cjs-datasource-demo</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging>
<name>cjs-datasource-demo</name>
<description></description>
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.0.5.RELEASE</version>
<relativePath/> <!-- lookup parent from repository -->
</parent>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
<java.version>1.8</java.version>
</properties>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-aop</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-jdbc</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
<groupId>org.mybatis.spring.boot</groupId>
<artifactId>mybatis-spring-boot-starter</artifactId>
<version>1.3.2</version>
</dependency>
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-lang3</artifactId>
<version>3.8</version>
</dependency>
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<scope>runtime</scope>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-test</artifactId>
<scope>test</scope>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
<!--<plugin>
<groupId>org.mybatis.generator</groupId>
<artifactId>mybatis-generator-maven-plugin</artifactId>
<version>1.3.5</version>
<dependencies>
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.46</version>
</dependency>
</dependencies>
<configuration>
<configurationFile>${basedir}/src/main/resources/myBatisGeneratorConfig.xml</configurationFile>
<overwrite>true</overwrite>
</configuration>
<executions>
<execution>
<id>Generate MyBatis Artifacts</id>
<goals>
<goal>generate</goal>
</goals>
</execution>
</executions>
</plugin>-->
</plugins>
</build></project>3.2. 數據源配置
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
多數據源配置
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;/**
* 關於數據源配置，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<Object, Object> 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;
}
}這裏，我們配置了4個數據源，1個master，2兩個slave，1個路由數據源。前3個數據源都是爲了生成第4個數據源，而且後續我們只用這最後一個路由數據源。
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);
}
}由於Spring容器中現在有4個數據源，所以我們需要爲事務管理器和MyBatis手動指定一個明確的數據源。
3.3. 設置路由key / 查找數據源
目標數據源就是那前3個這個我們是知道的，但是使用的時候是如果查找數據源的呢？
首先，我們定義一個枚舉來代表這三個數據源
package com.cjs.example.enums;public enum DBTypeEnum {
MASTER, SLAVE1, SLAVE2;
}接下來，通過ThreadLocal將數據源設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<DBTypeEnum> 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();
}
}設置路由key
默認情況下，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據庫，其餘的走主數據庫
*/// @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制讀主庫，針對這種情況，我們定義一個主鍵，用該註解標註的就讀主庫
package com.cjs.example.annotation;public @interface Master {
}例如，假設我們有一張表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;@Servicepublic 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<Member> selectAll() { return memberMapper.selectByExample(new MemberExample());
} @Master
@Override
public String getToken(String appId) { // 有些讀操作必須讀主數據庫
// 比如，獲取微信access_token，因爲高峯時期主從同步可能延遲
// 這種情況下就必須強制從主數據讀
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)@SpringBootTestpublic 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");
}
}查看控制檯
5. 工程結構
