MapperScannerConfigurer: 本身是一个BeanFactoryPostProcessor，beanfacotryPostProcessor的作用就是可以更改bean的类型。相当于扩展点

 private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
        Iterator var3 = beanDefinitions.iterator();

        while(var3.hasNext()) {
            BeanDefinitionHolder holder = (BeanDefinitionHolder)var3.next();
            GenericBeanDefinition definition = (GenericBeanDefinition)holder.getBeanDefinition();
            String beanClassName = definition.getBeanClassName();
            LOGGER.debug(() -> {
                return "Creating MapperFactoryBean with name '" + holder.getBeanName() + "' and '" + beanClassName + "' mapperInterface";
            });
            definition.getConstructorArgumentValues().addGenericArgumentValue(beanClassName);
            "更改bean的类型为facotrybean"
            definition.setBeanClass(this.mapperFactoryBean.getClass());
            ...
    }

代码中可以看到，统一调整为mapperFactoryBean。
MapperFactoryBean：这是一个工厂类，在实例化时，会调用getObject方法得到对象

    public T getObject() throws Exception {
        return this.getSqlSession().getMapper(this.mapperInterface);
    }

MapperRegistry：

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

MapperProxyFactory:代理对象工厂类

  protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

最终产生的对象就是MapperProxy