流程：
- @mapperScan(“路径”，SqlSessionFactory)
- @Import(MapperScannerRegistrar.class)
- 初始化MapperScannerConfigurer并设置ClassPathMapperScanner相关属性
- ClassPathMapperScanner.doscan();
- ClassPathMapperScanner.processBeanDefinitions()，设置GenericBeanDefinition的类属性为MapperFactoryBean
- MapperFactoryBean实现接口FactoryBean，getObject（）生成代理类
流程图：
