Mybatis的mapper接口在Spring中實例化過程

在spring中使用mybatis時一般有下面的配置

    <bean id="mapperScannerConfigurer" class="org.mybatis.spring.mapper.MapperScannerConfigurer">
        <property name="basePackage" value="org.xuan.springmvc.mapper,org.xuan.springxxx.mapper"/>
    </bean>

查看注入的MapperScannerConfigurer實現

public class MapperScannerConfigurer implements BeanDefinitionRegistryPostProcessor, InitializingBean, ApplicationContextAware, BeanNameAware {
......
}

發現繼承了BeanDefinitionRegistryPostProcessor，

注：BeanDefinitionRegistryPostProcessor繼承自BeanFactoryPostProcessor，其中有兩個接口，postProcessBeanDefinitionRegistry是BeanDefinitionRegistryPostProcessor自帶的，postProcessBeanFactory是從BeanFactoryPostProcessor繼承過來的。postProcessBeanDefinitionRegistry是在所有Bean定義信息將要被加載，Bean實例還未創建的時候執行，優先postProcessBeanFactory執行。可以在BeanDefinitionRegistryPostProcessor的實現類中增加或者修改bean定義。

所以在初始化spring容器的時候會調用MapperScannerConfigurer#postProcessBeanDefinitionRegistry

  @Override
  public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
    if (this.processPropertyPlaceHolders) {
      processPropertyPlaceHolders();
    }

    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();
    scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
  }

跟蹤ClassPathMapperScanner#scan

  @Override
  public Set<BeanDefinitionHolder> doScan(String... basePackages) {
    //掃描相關包下面的接口
    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 {
      //配置相關參數
      processBeanDefinitions(beanDefinitions);
    }

    return beanDefinitions;
  }

 

在掃描完接口后，會配置一下接口的參數

  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
      //配置參數，在實例化mapperFactoryBean的時候傳入，也是接口的類型
      definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName()); // issue #59
      //定義BeanClass,mapperFactoryBean是一個FactoryBean，（對於FactoryBean類型的Bean,在注入對象的時候是調用的getObject創建對象）
      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);
      }
    }
  }

注意紅色的部分，就是spring管理mybatis的關鍵，可以去了解FactoryBean的用法（可以簡單理解為產生對象的bean，在注入這種類型的對象的時候是調用自己實現的getObject來創建的）。

跟蹤MapperFactoryBean

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) {
    //把前面配置的對應的mapper接口傳進來
    this.mapperInterface = mapperInterface;
  }

  /**
   * {@inheritDoc}
   */
  @Override
  protected void checkDaoConfig() {
    super.checkDaoConfig();

    notNull(this.mapperInterface, "Property 'mapperInterface' is required");

    //創建完成后，看是否在configuration的mapperRegistry中，沒有就添加
    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();
      }
    }
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public T getObject() throws Exception {
    //通過SqlSession創建實例對象
    return getSqlSession().getMapper(this.mapperInterface);
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public Class<T> getObjectType() {
    //在實例化對象的時候判斷配置是哪一個mapper接口
    return this.mapperInterface;
  }
  
  ......
}

這樣就把創建接口對象又返回到mybatis中進行管理了

跟蹤getSqlSession().getMapper(this.mapperInterface)；最后進入了MapperRegistry#getMapper

  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);
    }
  }

繼續MapperProxyFactory#newInstance

  protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

所以其實我們最后得到的對象其實是MapperProxy的代理對象。在代碼中查看mapper對象

 

 發現和分析的一直。