Mybatis Annotation使用小结

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springboot-mybatis-annotation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 Mapper
   public 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

   <dependency>
   			<groupId>org.mybatis.spring.boot</groupId>
   			<artifactId>mybatis-spring-boot-starter</artifactId>
   			<version>${mybatis-spring-boot}</version>
   </dependency>
   

   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<String> 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 <T> void addMapper(Class<T> 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<T>(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[springboot-learning-example](https://github.com/yaojf/springboot-learning-example)