SpringBoot + Mybatis 实现多数据源原来如此简单

1、为什么需要整合多数据源

在开发的过程中，我们可能会遇到一个工程使用多个数据源的情况，总体而言分为以下几个原因

a、数据隔离：将不同的数据存储在不同的数据库中，如多租户场景

b、性能优化：将数据分散到多个数据库中，提高系统的性能。常见的如读写分离，将读操作分散到读库中，减轻主数据库的负载，提高读取操作的性能

c、业务场景：某些业务场景可能需要使用其他数据库中的数据，这种场景也可以通过调用第三方 rpc 接口获取数据

2、实现多数据源过程

a、maven依赖

<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-aop</artifactId>
</dependency>
<dependency><groupId>com.baomidou</groupId><artifactId>mybatis-plus-boot-starter</artifactId><version>3.0.7.1</version>
</dependency>
<dependency><groupId>org.springframework</groupId><artifactId>spring-jdbc</artifactId><version>5.1.7.RELEASE</version>
</dependency>
<dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><scope>runtime</scope>
</dependency>

b、创建动态数据源对象

// 多数据源持有对象
public class DBContextHolder {public static final String DB_PRIMARY = "primaryDataSource";public static final String DB_SECOND = "secondDataSource";private static ThreadLocal<String> contextHolder = new ThreadLocal();public static String getDB() {return contextHolder.get();}public static void setDB(String dbName) {DBContextHolder.contextHolder.set(dbName);}public static void cleanDB() {contextHolder.remove();}
}// 决定使用那个数据源
public class DynamicDataSource extends AbstractRoutingDataSource {@Nullable@Overrideprotected Object determineCurrentLookupKey() {return DBContextHolder.getDB();}
}

c、在mybatis配置 sqlSessionFactory 中指定动态数据源

@Bean("sqlSessionFactory")
public SqlSessionFactory sqlSessionFactory(DynamicDataSource dynamicDataSource) throws Exception {MybatisSqlSessionFactoryBean sqlSessionFactory = new MybatisSqlSessionFactoryBean();//设置数据源sqlSessionFactory.setDataSource(dynamicDataSource);sqlSessionFactory.setTypeAliasesPackage("com.jyt.service.testdb.entity");sqlSessionFactory.setGlobalConfig(globalConfiguration());sqlSessionFactory.setPlugins(new Interceptor[]{ //OptimisticLockerInterceptor(),performanceInterceptor()paginationInterceptor()});sqlSessionFactory.setConfiguration(mybatisConfiguration());return sqlSessionFactory.getObject();
}

d、通过 aop 动态指定 DBContextHolder 中的 dbName

// 设置默认数据源，不指定时使用
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.TYPE})
public @interface DB {String name() default DBContextHolder.DB_PRIMARY;
}@Slf4j
@Aspect
@Component
public class DynamicAop implements Ordered {// 此处也可以按自己的想法实现按目录区分@Around("@annotation(db)")public void around(ProceedingJoinPoint joinPoint, DB db) throws Throwable {try {DBContextHolder.setDB(db.name());log.info("setDB {}", DBContextHolder.getDB());joinPoint.proceed();} finally {log.info("threadLocal cleanDB {}", DBContextHolder.getDB());DBContextHolder.cleanDB();}}/**aop要在spring事务开启之前设置*/@Overridepublic int getOrder() {return 1;}
}

e、准备数据源配置信息

spring:datasource:druid:primary:driver-class-name: com.mysql.jdbc.Driverurl: jdbc:mysql://localhost:3306/basefun?useUnicode=true&characterEncoding=utf8&allowMultiQueries=true&serverTimezone=GMT%2B8username: rootpassword: rootsecond:driver-class-name: com.mysql.jdbc.Driverurl: jdbc:mysql://localhost:3306/basefun2?useUnicode=true&characterEncoding=utf8&allowMultiQueries=true&serverTimezone=GMT%2B8username: rootpassword: root

@Configuration
public class DatabaseConfig {@Bean(DBContextHolder.DB_PRIMARY)@ConfigurationProperties("spring.datasource.druid.primary")public DruidDataSource primaryDataSource() {return new DruidDataSource();}@Bean(DBContextHolder.DB_SECOND)@ConfigurationProperties("spring.datasource.druid.second")public DataSource secondDataSource() {return new DruidDataSource();}@Beanpublic DynamicDataSource dynamicDataSource() {DynamicDataSource dynamicDataSource = new DynamicDataSource();// 维护了所有的数据源列表dynamicDataSource.setTargetDataSources(ImmutableMap.of(DBContextHolder.DB_PRIMARY, primaryDataSource(), DBContextHolder.DB_SECOND, secondDataSource()));// 设置默认使用的数据源dynamicDataSource.setDefaultTargetDataSource(primaryDataSource());return dynamicDataSource;}
}

至此配置工作已经完成，启动既可以验证多数据源了

@Service
public class TestDBService {@Resourceprivate StudentDao studentDao;@DB@Transactional(propagation = Propagation.REQUIRED, rollbackFor = Exception.class)public void testDB() {studentDao.insert(new Student().setAge(10).setName("张三"));}@DB(name = DBContextHolder.DB_SECOND)@Transactional(propagation = Propagation.REQUIRED, rollbackFor = Exception.class)public void testDB1() throws Exception {studentDao.insert(new Student().setAge(11).setName("里斯"));//int i = 1 / 0; 回滚 保存失败，上面执行成功}
}

3、分析下 spring 是如何帮我们实现多数据源的？

首先我们看下 DynamicDataSource#determineCurrentLookupKey 何时会被调用

如图所示，sqlSessionFactory.getObject() 初始化时会调用 afterPropertiesSet() 方法，在这个方法中集中初始化，点进去查看源码，我们发现在MybatisSqlSessionFactoryBean#buildSqlSessionFactory 中会我们调用我们指定数据源的 getConnection 方法