Mapper(DAO層)接口如何實例化

上篇mybatis 映射文件加載是分析了一下我們所有的xml映射文件如何加載的，但在我學習ssm的過程中，發現dao層的接口在service層會注入一個實例化對象，直接可以使用，但我們並沒有做dao層的實現類，

很好奇的去查了查資料，下面就分析一下

mybatis配置文件

 

mybatis是通過org.mybatis.spring.mapper.MapperScannerConfigurer類來實現dao層掃描的 我們進入MapperScannerConfigurer這個類，有一個postProcessBeanDefinitionRegistry方法

  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();
        //掃描basepackage包下的所有dao接口
scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
  }

進入doscan方法

//由父類去找到符合條件的interface類，並轉化為bean類
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 {
//處理找到的interface bean類
  processBeanDefinitions(beanDefinitions);
}

return beanDefinitions;

然后看processBeanDefinitions方法

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

      //設置beanclass類型為mapperFactoryBean
      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);
      }
    }
  }

 

最終會把dao層接口包裝成MapperFactoryBean

也就是說我們會把所有的dao層文件，封裝成和以下配置文件一樣的效果，是不是很熟悉了

<bean id="userMapper" class="org.mybatis.spring.mapper.MapperFactoryBean">
        <property name="mapperInterface" value="com.zsh.dao.UserMapper"></property>
        <property name="sqlSessionFactory" ref="sqlSessionFactory" />
</bean>

以上是我們掃描dao層的原理，下面我們繼續分析如何自動實例化接口

再說自動實例化接口之前我們了解一下什么是動態代理 Java 動態代理機制分析及擴展 

上面是通過工廠方法創建Bean 
首先我們定位到類 org.mybatis.spring.mapper.MapperFactoryBean

public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements
        FactoryBean<T> {
    private Class<T> mapperInterface;
    private boolean addToConfig;

可以看到該類實現了org.springframework.beans.factory.FactoryBean接口,通過調用 
org.mybatis.spring.mapper.MapperFactoryBean.getObject()方法來獲得Bean

    public T getObject() throws Exception {
        return getSqlSession().getMapper(this.mapperInterface);
    }

我們通過分析以上的getSqlSession()可以看出getMapper的實現類是SqlSessionTemplate，我們跳轉到SqlSessionTemplate看一下getMapper方法

    public <T> T getMapper(Class<T> type) {
        return getConfiguration().getMapper(type, this);
    }

可以看到，它首先調用了自身的getConfiguration()方法返回一個Configuration對象，然后再調用Configuration對象的getMapper方法

我們直接定位到Configuration的getMapper方法

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

直接定位到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);
    }
  }

在這個方法里，首先獲得一個MapperProxyFactory的對象mapperProxyFactory，然后調用該對象的newInstance方法創建我們需要的bean，定位到newInstance方法

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

最后我們進入newInstance這個重載函數

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

在這里用jdk的動態代理創建了一個代理對象，也就是說，我們最終是對mapper的所有接口分別創建了各自的代理對象，通過代理對象來執行我們的sql

參考並感謝

https://www.cnblogs.com/question-sky/p/6654101.html

http://blog.csdn.net/tanqidong1992/article/details/48026491

https://www.jianshu.com/p/3e619786dd18