首先看一下MapperScannerConfigurer的结构：

该类实现了BeanDefinitionRegistryPostProcessor结果。

在Spring中的核心逻辑如下：

// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();

从上图不难看出，MapperScannerConfigurer在很早就行Bean的实例化了。

<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
    <property name="basePackage" value="org.mybatis.spring.sample.mapper" />
    <!-- optional unless there are multiple session factories defined -->
    <property name="sqlSessionFactoryBeanName" value="sqlSessionFactory" />
</bean>

在Spring时代，该类是通过以上方式在xml文件中配置的，现在在SpringBoot中都是在applicationContext.properties（或yml）中进行配置的，但是仔细查看，并没有关于basePackage的设置，就算有有这个配置，那是如何设置进去的呢？
在SpringBoot中引用MyBatis自动注入，主要的逻辑在如下类实现的

org.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration

源码如下：

public static class AutoConfiguredMapperScannerRegistrar implements BeanFactoryAware, ImportBeanDefinitionRegistrar {

  private BeanFactory beanFactory;

  @Override
  public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
    if (!AutoConfigurationPackages.has(this.beanFactory)) {
      logger.debug("Could not determine auto-configuration package, automatic mapper scanning disabled.");
      return;
    }
    logger.debug("Searching for mappers annotated with @Mapper");
	// 在此处添加一个断点
    List<String> packages = AutoConfigurationPackages.get(this.beanFactory);
    if (logger.isDebugEnabled()) {
      packages.forEach(pkg -> logger.debug("Using auto-configuration base package '{}'", pkg));
    }

    BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class);
    builder.addPropertyValue("processPropertyPlaceHolders", true);
    builder.addPropertyValue("annotationClass", Mapper.class);
    builder.addPropertyValue("basePackage", StringUtils.collectionToCommaDelimitedString(packages));
    BeanWrapper beanWrapper = new BeanWrapperImpl(MapperScannerConfigurer.class);
    Stream.of(beanWrapper.getPropertyDescriptors())
        // Need to mybatis-spring 2.0.2+
        .filter(x -> x.getName().equals("lazyInitialization")).findAny()
        .ifPresent(x -> builder.addPropertyValue("lazyInitialization", "${mybatis.lazy-initialization:false}"));
    registry.registerBeanDefinition(MapperScannerConfigurer.class.getName(), builder.getBeanDefinition());
  }

  @Override
  public void setBeanFactory(BeanFactory beanFactory) {
    this.beanFactory = beanFactory;
  }
}

从上图中可以看出现在的basePackage不需要进行设置，而是自动去的SpringBoot启动主类的路径，然后扫描该努力下所有注解有@org.apache.ibatis.annotations.Mapper的接口类。
着实被忽悠了一把！！！！