Mybatis Mapper Bean 生成源码分析一
问题
我们的Mapper接口，用的是@Mapper，而非@Service等会被Spring扫描的注解，那么它最后怎么会生成一个Bean呢。
重点方法一：ClassPathMapperScanner#doScan
org.mybatis.spring.mapper.ClassPathMapperScanner继承自org.springframework.context.annotation.ClassPathBeanDefinitionScanner,重载了其#doScan方法，调用其父类进行包下的扫描，再对扫描出来的BeanDefinitionHolder beanDefinitions 进行处理，代码如下：
/** * Calls the parent search that will search and register all the candidates. Then the registered objects are post * processed to set them as MapperFactoryBeans */ @Override public Set<BeanDefinitionHolder> doScan(String... basePackages) { // 让Spring的父类负责扫描 Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages); if (beanDefinitions.isEmpty()) { LOGGER.warn(() -> "No MyBatis mapper was found in '" + Arrays.toString(basePackages) + "' package. Please check your configuration."); } else { // mybatis 定制，修改了bean 定义 processBeanDefinitions(beanDefinitions); } return beanDefinitions; }
重点方法二：ClassPathMapperScanner#processBeanDefinitions
private Class<? extends MapperFactoryBean> mapperFactoryBeanClass = MapperFactoryBean.class; private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) { GenericBeanDefinition definition; for (BeanDefinitionHolder holder : beanDefinitions) { definition = (GenericBeanDefinition) holder.getBeanDefinition(); String beanClassName = definition.getBeanClassName(); // the mapper interface is the original class of the bean // but, the actual class of the bean is MapperFactoryBean definition.getConstructorArgumentValues().addGenericArgumentValue(beanClassName); // 替换成了MapperFactoryBean，一个FactoryBean，在Bean真正产生时用来创建Bean的工厂 definition.setBeanClass(this.mapperFactoryBeanClass); } }
总结
mybatis通过扩展doScan方法，将扫描到的Mapper的BeanDefinitionHolder中的beanClass从接口，改成了MapperFactoryBean，完成了这波偷天换日的第一步，之后Bean的初始化时，最终将通过MapperFactoryBean扩展的FactoryBean 的 getObject方法返回真正的Mapper(JDK 代理)。
