1. 引言

读写分离要做的事情就是对于一条SQL该选择哪个数据库去执行，至于谁来做选择数据库这件事儿，无非两个，要么中间件帮我们做，要么程序自己做。因此，一般来讲，读写分离有两种实现方式。第一种是依靠中间件（比如：MyCat），也就是说应用程序连接到中间件，中间件帮我们做SQL分离；第二种是应用程序自己去做分离。这里我们选择程序自己来做，主要是利用Spring提供的路由数据源，以及AOP

然而，应用程序层面去做读写分离最大的弱点（不足之处）在于无法动态增加数据库节点，因为数据源配置都是写在配置中的，新增数据库意味着新加一个数据源，必然改配置，并重启应用。当然，好处就是相对简单。

2. AbstractRoutingDataSource

基于特定的查找key路由到特定的数据源。它内部维护了一组目标数据源，并且做了路由key与目标数据源之间的映射，提供基于key查找数据源的方法。

3. 实践

MySQL主从复制配置()》

3.1. maven依赖

<?xml version="1.0" encoding="UTF-8"?><project xmlns="; xmlns:xsi="; xsi:schemaLocation=" ;>    <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>

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   # 只读账户      password: 123456      driver-class-name: com.mysql.jdbc.Driver    slave2:      jdbc-url: jdbc:mysql://192.168.102.36:3306/test      username: pig   # 只读账户      password: 123456      driver-class-name: com.mysql.jdbc.Driver

多数据源配置

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; /** * 关于数据源配置，参考SpringBoot官方文档第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<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;    }}

这里，我们配置了4个数据源，1个master，2两个slave，1个路由数据源。前3个数据源都是为了生成第4个数据源，而且后续我们只用这最后一个路由数据源。

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容器中现在有4个数据源，所以我们需要为事务管理器和MyBatis手动指定一个明确的数据源。

3.3. 设置路由key / 查找数据源

目标数据源就是那前3个这个我们是知道的，但是使用的时候是如果查找数据源的呢？

首先，我们定义一个枚举来代表这三个数据源

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

接下来，通过ThreadLocal将数据源设置到每个线程上下文中

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("切换到master");    } public static void slave() { // 轮询        int index = counter.getAndIncrement() % 2; if (counter.get() > 9999) {            counter.set(-1);        } if (index == 0) {            set(DBTypeEnum.SLAVE1);            System.out.println("切换到slave1");        }else {            set(DBTypeEnum.SLAVE2);            System.out.println("切换到slave2");        }    }}

获取路由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();    }}

设置路由key

默认情况下，所有的查询都走从库，插入/修改/删除走主库。我们通过方法名来区分操作类型（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();    } /** * 另一种写法：if...else...  判断哪些需要读从数据库，其余的走主数据库 */// @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(); // } // }}

有一般情况就有特殊情况，特殊情况是某些情况下我们需要强制读主库，针对这种情况，我们定义一个主键，用该注解标注的就读主库

 package com.cjs.example.annotation; public @interface Master {}

例如，假设我们有一张表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<Member> selectAll() { return memberMapper.selectByExample(new MemberExample());    }    @Master    @Override public String getToken(String appId) { // 有些读操作必须读主数据库 // 比如，获取微信access_token，因为高峰时期主从同步可能延迟 // 这种情况下就必须强制从主数据读        return null;    }}

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");    }}

查看控制台

5. 工程结构