Mybatis Annotation使用小结

之前一直有看过mybatis的注解使用方式，但没有去看过它的原理。今天看使用的时候，debug了下看了它的加载过程。

1. 先编写一个示例接口

   @Mapper // 标志为 Mybatis 的 Mapperpublic interface CityDao {    /**     * 根据城市名称，查询城市信息     *     * @param cityName 城市名     */    @Select("SELECT * FROM city")    // 返回 Map 结果集    @Results({            @Result(property = "id", column = "id"),            @Result(property = "provinceId", column = "province_id"),            @Result(property = "cityName", column = "city_name"),            @Result(property = "description", column = "description"),    })    City findByName(@Param("cityName") String cityName);}

2. springboot引入mybatis-spring-boot-starter依赖

                     
          
           org.mybatis.spring.boot
                      
          
           mybatis-spring-boot-starter
                      
          
           ${mybatis-spring-boot}
          
         

   它会自动注入MybatisAutoConfiguration这个bean然后解析执行AutoConfiguredMapperScannerRegistrar这个ImportBeanDefinitionRegistrar，它的registerBeanDefinitions方法会扫描包把带有Mapper注解的接口注入到spring容器，并且在内部修改BeanDefinition的class为MapperFactoryBean，并且设置它的自动注入模式为AUTOWIRE_BY_TYPE，实现了SqlSessionFactory的自动注入。

   @Override    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {      logger.debug("Searching for mappers annotated with @Mapper");      ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);      try {        if (this.resourceLoader != null) {          scanner.setResourceLoader(this.resourceLoader);        }        List
         
           packages = AutoConfigurationPackages.get(this.beanFactory);        if (logger.isDebugEnabled()) {          for (String pkg : packages) {            logger.debug("Using auto-configuration base package '{}'", pkg);          }        }        scanner.setAnnotationClass(Mapper.class);        scanner.registerFilters();        scanner.doScan(StringUtils.toStringArray(packages));      } catch (IllegalStateException ex) {        logger.debug("Could not determine auto-configuration package, automatic mapper scanning disabled.", ex);      }    }
         

3. MapperFactoryBean的实例化

   MapperFactoryBean继承了SqlSessionDaoSupport，SqlSessionDaoSupport继承了DaoSupport，DaoSupport它实现了InitializingBean，所以实例化MapperFactoryBean的时候会调用afterPropertiesSet方法。

   @Override        public final void afterPropertiesSet() throws IllegalArgumentException, BeanInitializationException {            // Let abstract subclasses check their configuration.            checkDaoConfig();            // Let concrete implementations initialize themselves.            try {                initDao();            }            catch (Exception ex) {                throw new BeanInitializationException("Initialization of DAO failed", ex);            }        }

   这里会调用MapperFactoryBean的重写方法checkDaoConfig

   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();      }    }  }

   因为配置了方法注解所以我们在不配置xml的时候configuration里并没有对应的mapperRegistry，所以会调用configuration的addMapper方法，内部会调用mapperRegistry的addMapper方法

   public 
         
           void addMapper(Class
          
            type) {    if (type.isInterface()) {      if (hasMapper(type)) {        throw new BindingException("Type " + type + " is already known to the MapperRegistry.");      }      boolean loadCompleted = false;      try {        knownMappers.put(type, new MapperProxyFactory
           
            (type));        // It's important that the type is added before the parser is run        // otherwise the binding may automatically be attempted by the        // mapper parser. If the type is already known, it won't try.        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);        parser.parse();        loadCompleted = true;      } finally {        if (!loadCompleted) {          knownMappers.remove(type);        }      }    }  }
           
          
         

   从以上代码可知通过MapperAnnotationBuilder这个类去解析当前接口的方法上的配置信息转化成MappedStatement并设置到全局的Configuration里面。这样就和直接解析xml配置达到了一样的效果。

4. MapperFactoryBean生成的动态代理bean的调用

   MapperFactoryBean生成的动态代理bean，调用方法的过程最终会委派给MapperProxy的invoke方法，后续的调用过程就和xml解析出来的配置一致了，也就是说不管是注解配置还是xml配置只是解析配置的方式不同，最终都会设置到Configuration里面。然而调用过程一致。

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代码下载参考