Mybatis——@MapperScan原理

@MapperScan配置在@Configuration注解的类上会导入MapperScannerRegistrar类。

而MapperScannerRegistrar实现了ImportBeanDefinitionRegistrar接口，可以向BeanDefinitionRegistry注册BeanDefinition。

public class MapperScannerRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware {
  private ResourceLoader resourceLoader;
  @Override
  public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
    AnnotationAttributes annoAttrs = AnnotationAttributes.fromMap(importingClassMetadata.getAnnotationAttributes(MapperScan.class.getName()));
    ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
    //略。。。
    List<String> basePackages = new ArrayList<String>();
    for (String pkg : annoAttrs.getStringArray("value")) {
      if (StringUtils.hasText(pkg)) {
        basePackages.add(pkg);
      }
    }
    for (String pkg : annoAttrs.getStringArray("basePackages")) {
      if (StringUtils.hasText(pkg)) {
        basePackages.add(pkg);
      }
    }
    for (Class<?> clazz : annoAttrs.getClassArray("basePackageClasses")) {
      basePackages.add(ClassUtils.getPackageName(clazz));
    }
    scanner.registerFilters();
    scanner.doScan(StringUtils.toStringArray(basePackages));
  }
}

从上述源码可以看到MapperScannerRegistrar的registerBeanDefinitions，从mapperScan注解获取到要扫描的包，将扫描路径提交给ClassPathMapperScanner的doScan方法。

ClassPathMapperScanner继承于ClassPathBeanDefinitionScanner，并覆盖doScan方法，通过父类获取到basePackages路径下的类的beanDefinition，然后修改beanDefinition的beanClass为MapperFactoryBean。

public class ClassPathMapperScanner extends ClassPathBeanDefinitionScanner {
  @Override
  public Set<BeanDefinitionHolder> doScan(String... basePackages) {
    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 {
      processBeanDefinitions(beanDefinitions);
    }
    return beanDefinitions;
  }
  private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
    GenericBeanDefinition definition;
    for (BeanDefinitionHolder holder : beanDefinitions) {
      definition = (GenericBeanDefinition) holder.getBeanDefinition();
      if (logger.isDebugEnabled()) {
        logger.debug("Creating MapperFactoryBean with name '" + holder.getBeanName()
          + "' and '" + definition.getBeanClassName() + "' mapperInterface");
      }
      //修改构造方法参数为mapper的class
      definition.getConstructorArgumentValues()
            .addGenericArgumentValue(definition.getBeanClassName());
      //修改beanClass为mapperFactoryBean
      definition.setBeanClass(this.mapperFactoryBean.getClass());
      definition.getPropertyValues().add("addToConfig", this.addToConfig);
      //注入sqlSessionFactory/sqlSessionTemplate
      boolean explicitFactoryUsed = false;
      if (StringUtils.hasText(this.sqlSessionFactoryBeanName)) {
        definition.getPropertyValues().add("sqlSessionFactory", new RuntimeBeanReference(this.sqlSessionFactoryBeanName));
        explicitFactoryUsed = true;
      } else if (this.sqlSessionFactory != null) {
        definition.getPropertyValues().add("sqlSessionFactory", this.sqlSessionFactory);
        explicitFactoryUsed = true;
      }
      if (StringUtils.hasText(this.sqlSessionTemplateBeanName)) {
        if (explicitFactoryUsed) {
          logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
        }
        definition.getPropertyValues().add("sqlSessionTemplate", new RuntimeBeanReference(this.sqlSessionTemplateBeanName));
        explicitFactoryUsed = true;
      } else if (this.sqlSessionTemplate != null) {
        if (explicitFactoryUsed) {
          logger.warn("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
        }
        definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate);
        explicitFactoryUsed = true;
      }
      if (!explicitFactoryUsed) {
        if (logger.isDebugEnabled()) {
          logger.debug("Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.");
        }
        definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
      }
    }
  }
  @Override
    //覆盖父类的isCandidateComponent，判断bean为接口且独立
  protected boolean isCandidateComponent(AnnotatedBeanDefinition beanDefinition) {
    return beanDefinition.getMetadata().isInterface() && beanDefinition.getMetadata().isIndependent();
  }
}

最后通过BeanFactory实例化MapperFactoryBean时，因为MapperFactoryBean实现了FactoryBean，会调用getObject生成bean，注册到beanFactory。

public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T> {
  private Class<T> mapperInterface;
  private boolean addToConfig = true;
  public MapperFactoryBean() {
    //intentionally empty
  }
  public MapperFactoryBean(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }
  @Override
    //将mapper注册到Configuration中
  protected void checkDaoConfig() {
    super.checkDaoConfig();
    notNull(this.mapperInterface, "Property 'mapperInterface' is required");
    Configuration configuration = getSqlSession().getConfiguration();
    if (this.addToConfig && !configuration.hasMapper(this.mapperInterface)) {
      try {
        configuration.addMapper(this.mapperInterface);
      } catch (Exception e) {
        logger.error("Error while adding the mapper '" + this.mapperInterface + "' to configuration.", e);
        throw new IllegalArgumentException(e);
      } finally {
        ErrorContext.instance().reset();
      }
    }
  }
  @Override
    //通过SqlSession获取Mapper代理
  public T getObject() throws Exception {
    return getSqlSession().getMapper(this.mapperInterface);
  }
}