使用gradle和springboot进行多模块开发，只有主模块有Application.class的main方法，且主模块有2个DataSource数据源，每个模块都有各自的mybatis的mapper文件，怎么指定子模块的mapper文件使用固定的第2个数据源？

可以在每个子模块的配置文件中指定使用哪一个数据源。

首先，在主模块的配置文件中定义两个数据源，例如：

spring.datasource.primary.url=jdbc:mysql://localhost:3306/db1
spring.datasource.primary.username=root
spring.datasource.primary.password=root

spring.datasource.secondary.url=jdbc:mysql://localhost:3306/db2
spring.datasource.secondary.username=root
spring.datasource.secondary.password=root

接着，在主模块中定义两个数据源的bean：

@Configuration
public class DataSourceConfig {

    @Primary
    @Bean(name = "primaryDataSource")
    @ConfigurationProperties(prefix = "spring.datasource.primary")
    public DataSource primaryDataSource() {
        return DataSourceBuilder.create().build();
    }

    @Bean(name = "secondaryDataSource")
    @ConfigurationProperties(prefix = "spring.datasource.secondary")
    public DataSource secondaryDataSource() {
        return DataSourceBuilder.create().build();
    }
}

然后，在每个子模块的配置文件中指定使用哪一个数据源和mapper文件位置，例如：

mybatis.mapper-locations=classpath:mapper/module1/*.xml
mybatis.configuration.map-underscore-to-camel-case=true

spring.datasource.name=secondaryDataSource

mybatis.mapper-locations=classpath:mapper/module2/*.xml
mybatis.configuration.map-underscore-to-camel-case=true

spring.datasource.name=primaryDataSource

最后，在每个子模块中定义对应的SqlSessionFactoryBean和MapperScannerConfigurer：

@Configuration
public class Module1Config {

    @Autowired
    @Qualifier("secondaryDataSource")
    private DataSource dataSource;

    @Bean(name = "module1SqlSessionFactory")
    public SqlSessionFactoryBean module1SqlSessionFactory() throws Exception {
        SqlSessionFactoryBean sessionFactory = new SqlSessionFactoryBean();
        sessionFactory.setDataSource(dataSource);
        sessionFactory.setMapperLocations(new PathMatchingResourcePatternResolver().getResources("classpath:mapper/module1/*.xml"));
        return sessionFactory;
    }

    @Bean(name = "module1MapperScannerConfigurer")
    public MapperScannerConfigurer module1MapperScannerConfigurer() {
        MapperScannerConfigurer scannerConfigurer = new MapperScannerConfigurer();
        scannerConfigurer.setBasePackage("com.example.module1.mapper");
        scannerConfigurer.setSqlSessionFactoryBeanName("module1SqlSessionFactory");
        return scannerConfigurer;
    }
}

@Configuration
public class Module2Config {

    @Autowired
    @Qualifier("primaryDataSource")
    private DataSource dataSource;

    @Bean(name = "module2SqlSessionFactory")
    public SqlSessionFactoryBean module2SqlSessionFactory() throws Exception {
        SqlSessionFactoryBean sessionFactory = new SqlSessionFactoryBean();
        sessionFactory.setDataSource(dataSource);
        sessionFactory.setMapperLocations(new PathMatchingResourcePatternResolver().getResources("classpath:mapper/module2/*.xml"));
        return sessionFactory;
    }

    @Bean(name = "module2MapperScannerConfigurer")
    public MapperScannerConfigurer module2MapperScannerConfigurer() {
        MapperScannerConfigurer scannerConfigurer = new MapperScannerConfigurer();
        scannerConfigurer.setBasePackage("com.example.module2.mapper");
        scannerConfigurer.setSqlSessionFactoryBeanName("module2SqlSessionFactory");
        return scannerConfigurer;
    }
}

这样就可以实现多数据源和多模块的mybatis配置，并且指定子模块的mapper文件使用固定的第2个数据源了。