使用gradle和springboot进行多模块开发，如果每个模块都有各自的mybatis的mapper，但是springboot容器内有两个数据源，且多模块中只有1个主模块有main方法，那么怎么指定从模块的mapper映射对应的数据源呢？

在多数据源的情况下，需要通过配置多个SqlSessionFactory和DataSource来实现，然后通过在每个Mapper上使用@Qualifier注解指定要使用的数据源。

假设我们有两个数据源，分别为主数据源和从数据源，每个模块都有对应的Mapper映射，我们可以在每个模块的build.gradle文件中添加对应的依赖：

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-jdbc'
    implementation 'com.alibaba:druid-spring-boot-starter:1.2.6'
    implementation 'org.mybatis.spring.boot:mybatis-spring-boot-starter:2.1.4'
    //其他依赖
}

然后在每个模块的application.yml配置文件中，需要配置对应的数据源和SqlSessionFactory：

#主数据源配置
spring:
  datasource:
    master:
      url: jdbc:mysql://localhost:3306/master?useUnicode=true&characterEncoding=utf-8&useSSL=false&serverTimezone=Asia/Shanghai
      username: root
      password: root
      driver-class-name: com.mysql.cj.jdbc.Driver

mybatis:
  configuration:
    map-underscore-to-camel-case: true
  mapper-locations: classpath:mapper/*.xml
  type-aliases-package: com.example.demo.entity

master:
  datasource:
    type: com.alibaba.druid.pool.DruidDataSource
    driver-class-name: com.mysql.cj.jdbc.Driver
    url: jdbc:mysql://localhost:3306/master?useUnicode=true&characterEncoding=utf-8&useSSL=false&serverTimezone=Asia/Shanghai
    username: root
    password: root
  mapper-locations: classpath:mapper/*.xml
  type-aliases-package: com.example.demo.entity

#从数据源配置
slave:
  datasource:
    type: com.alibaba.druid.pool.DruidDataSource
    driver-class-name: com.mysql.cj.jdbc.Driver
    url: jdbc:mysql://localhost:3306/slave?useUnicode=true&characterEncoding=utf-8&useSSL=false&serverTimezone=Asia/Shanghai
    username: root
    password: root
  mapper-locations: classpath:mapper/*.xml
  type-aliases-package: com.example.demo.entity

在主模块的启动类中，需要配置多个SqlSessionFactory和DataSource，并使用@Qualifier注解指定要使用的数据源：

@SpringBootApplication
@EnableAutoConfiguration
public class MainApp {
    @Bean(name = "masterDataSource")
    @ConfigurationProperties(prefix = "master.datasource")
    public DataSource masterDataSource() {
        return DruidDataSourceBuilder.create().build();
    }

    @Bean(name = "slaveDataSource")
    @ConfigurationProperties(prefix = "slave.datasource")
    public DataSource slaveDataSource() {
        return DruidDataSourceBuilder.create().build();
    }

    @Bean(name = "masterSqlSessionFactory")
    public SqlSessionFactory masterSqlSessionFactory(@Qualifier("masterDataSource") DataSource masterDataSource) throws Exception {
        SqlSessionFactoryBean bean = new SqlSessionFactoryBean();
        bean.setDataSource(masterDataSource);
        Resource[] resources = new PathMatchingResourcePatternResolver().getResources("classpath*:mapper/*.xml");
        bean.setMapperLocations(resources);
        return bean.getObject();
    }

    @Bean(name = "slaveSqlSessionFactory")
    public SqlSessionFactory slaveSqlSessionFactory(@Qualifier("slaveDataSource") DataSource slaveDataSource) throws Exception {
        SqlSessionFactoryBean bean = new SqlSessionFactoryBean();
        bean.setDataSource(slaveDataSource);
        Resource[] resources = new PathMatchingResourcePatternResolver().getResources("classpath*:mapper/*.xml");
        bean.setMapperLocations(resources);
        return bean.getObject();
    }

    @Bean(name = "masterSqlSessionTemplate")
    public SqlSessionTemplate masterSqlSessionTemplate(@Qualifier("masterSqlSessionFactory") SqlSessionFactory masterSqlSessionFactory) {
        return new SqlSessionTemplate(masterSqlSessionFactory);
    }

    @Bean(name = "slaveSqlSessionTemplate")
    public SqlSessionTemplate slaveSqlSessionTemplate(@Qualifier("slaveSqlSessionFactory") SqlSessionFactory slaveSqlSessionFactory) {
        return new SqlSessionTemplate(slaveSqlSessionFactory);
    }

    public static void main(String[] args) {
        SpringApplication.run(MainApp.class, args);
    }
}

在从模块的Mapper接口中，可以在@Mapper注解上添加@Qualifier注解指定要使用的数据源：

@Mapper
@Qualifier("slaveSqlSessionFactory")
public interface SlaveMapper {
    //从数据源查询用户
    User selectUserById(Integer id);
}

这样，每个Mapper就可以指定要使用的数据源，实现了多数据源的配置。