引言

在spring-mybatis.xml中还有一条配置：

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<!-- 配置 MapperScannerConfigurer -->
   <bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
       <!-- 配置 Dao 接口所在的包 -->
       <property name="basePackage" value="com.minivision.test.mybatis.mapper"/>
   </bean>

作用是递归地从基本包开始搜索接口，并将它们注册为MapperFactoryBean。注意，只有具有至少一个方法的接口才会被注册，具体类将被忽略。通过这一步，将MyBatis与Spring联系起来了。

1、MyBatis与Spring关联

MapperScannerConfigurer实现了BeanDefinitionRegistryPostProcessor接口，这个接口可以动态的往springIOC容器中注入BeanDefinition,在这实现Mybatis的所有接口托管于spring。

接下来看MapperScannerConfigurer核心入口方法：

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@Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
        //有些值是否需要替换
        if (this.processPropertyPlaceHolders) {
            processPropertyPlaceHolders();
        }

        //初始化一些操作，比如Component注解过滤设置
        ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
        scanner.setAddToConfig(this.addToConfig);
        scanner.setAnnotationClass(this.annotationClass);
        scanner.setMarkerInterface(this.markerInterface);
        scanner.setSqlSessionFactory(this.sqlSessionFactory);
        scanner.setSqlSessionTemplate(this.sqlSessionTemplate);
        scanner.setSqlSessionFactoryBeanName(this.sqlSessionFactoryBeanName);
        scanner.setSqlSessionTemplateBeanName(this.sqlSessionTemplateBeanName);
        scanner.setResourceLoader(this.applicationContext);
        scanner.setBeanNameGenerator(this.nameGenerator);
        scanner.registerFilters();
        //核心，包名可以配置多个，以分号或逗号分割
        scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
    }

ClassPathMapperScanner实现了spring-context包中的ClassPathBeanDefinitionScanner。
ClassPathMapperScanner.png
在指定的基本包中执行扫描。

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public int scan(String... basePackages) {
	int beanCountAtScanStart = this.registry.getBeanDefinitionCount();
//核心
	doScan(basePackages);
	// 如果需要，注册注释配置处理器。
	if (this.includeAnnotationConfig) {
	    AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
	}
	return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
}

ClassPathBeanDefinitionScanner中的doScan由其子类ClassPathMapperScanner实现：

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@Override
public Set<BeanDefinitionHolder> doScan(String... basePackages) {
  //首先调用父类doScan
    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 {
	  //处理beanDefinition
      processBeanDefinitions(beanDefinitions);
    }
    return beanDefinitions;
  }

调用父类ClassPathBeanDefinitionScanner中的doScan：
在指定的基本包中执行扫描，返回已注册的bean定义。此方法不注册注释配置处理器，而是将此任务留给调用者。

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protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
	Assert.notEmpty(basePackages, "At least one base package must be specified");
	Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
	for (String basePackage : basePackages) {
		//扫描类路径，寻找候选组件。
		Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
		for (BeanDefinition candidate : candidates) {
			ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
			candidate.setScope(scopeMetadata.getScopeName());
			String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
			if (candidate instanceof AbstractBeanDefinition) {
				postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
			}
			if (candidate instanceof AnnotatedBeanDefinition) {
				AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
			}
			if (checkCandidate(beanName, candidate)) {
				BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
				definitionHolder =
						AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
				beanDefinitions.add(definitionHolder);
				registerBeanDefinition(definitionHolder, this.registry);
			}
		}
	}
	return beanDefinitions;
}

处理beanDefinition:

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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");
      }
      // the mapper interface is the original class of the bean
      // but, the actual class of the bean is MapperFactoryBean
      definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName()); // issue #59
      definition.setBeanClass(this.mapperFactoryBean.getClass());

      definition.getPropertyValues().add("addToConfig", this.addToConfig);

      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);
      }
    }
  }

假设当前处理的是com.demo.mybatis.mapper.PeopleMapper。
那么：

  • beanName为：peopleMapper；
  • baseClass为：MapperFactoryBean<?>
  • 因为设置了sqlSessionFactory属性，所以这个bean属性填充的时候会调用setSqlSessionFactory()。

但是因为没有指定sqlSessionFactory，所以definition设置了按类型注入，所以在spring实例化definition注入属性的时候，会按类型注入的方式找到类型为SqlSessionFactory实例的进行注入，这一点很重要。

2、Mapper代理类生成

当从spring中获取mapper的实现类时，获取的是动态代理生成的代理类。
下面分析获取mapper代理类：

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ApplicationContext context = new ClassPathXmlApplicationContext("spring-mybatis.xml");
PeopleMapper peopleMapper = context.getBean(PeopleMapper.class);

当执行getBean方法的时候，深入调用，最后会调用到MapperFactoryBean(继承FactoryBean)的getObject()方法：

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@Override
public T getObject() throws Exception {
    //首先获取SqlSession，这里为SqlSessionTemplate
    //获取mapper
  return getSqlSession().getMapper(this.mapperInterface);
}

这里的getSqlSession()方法直接返回了SqlSession对象，这个对象在MapperFactoryBean的父类SqlSessionDaoSupport中维护，当MapperBean属性填充的时候，会调用到SqlSessionDaoSupport的setSqlSessionFactory(SqlSessionFactory sqlSessionFactory)方法：

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public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {
    if (!this.externalSqlSession) {
      this.sqlSession = new SqlSessionTemplate(sqlSessionFactory);
    }
  }

即当前的sqlSession的实现为SqlSessionTemplate。再深入看SqlSessionTemplate中的getMapper方法：

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  @Override
  public <T> T getMapper(Class<T> type) {
    return getConfiguration().getMapper(type, this);
  }
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

再深入MapperProxyFactory中：

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public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }
 protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

最终使用了JDK的动态代理技术，当调用这个mapper的方法的时候，就会调用MapperProxy的invoke方法。

简单整理一下：

  • 首先解析mybatis配置文件，生成SqlSessionFactoryBean
  • 之后通过spring扫描mapper接口,生成BeanDefinition。
  • 当获取mapper实例的时候，会执行属性填充，那时候将SqlSessionFactory注入。
  • 最后通过JDK动态代理返回一个代理的mapper。

代理对象：
mapper.png

tencent.jpg