摘要：聲明解決方案是基于源碼，進(jìn)行二次開發(fā)實現(xiàn)。其是基于層面，不存在任何的接口實現(xiàn)類。因而在實現(xiàn)的過程中，首先要解決的是如何動態(tài)實現(xiàn)接口的實例化。其次是如何將使接口根據(jù)注解實現(xiàn)相應(yīng)的功能。

聲明解決方案是基于Mybatis源碼，進(jìn)行二次開發(fā)實現(xiàn)。

問題領(lǐng)導(dǎo)最近跟我提了一個需求，是有關(guān)于實現(xiàn)類Mybatis的@Select、@Insert注解的功能。其是基于interface層面，不存在任何的接口實現(xiàn)類。因而在實現(xiàn)的過程中，首先要解決的是如何動態(tài)實現(xiàn)接口的實例化。其次是如何將使接口根據(jù)注解實現(xiàn)相應(yīng)的功能。

我們先來看看Mybatis是如何實現(xiàn)Dao類的掃描的。

MapperScannerConfigurer.java

public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
    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));
  }

ClassPathMapperScanner是Mybatis繼承ClassPathBeanDefinitionScanner類而來的。這里對于ClassPathMapperScanner的配置參數(shù)來源于我們在使用Mybatis時的配置而來，是不是還記得在使用Mybatis的時候要配置basePackage的參數(shù)呢？

接著我們就順著scanner.scan()方法，進(jìn)入查看一下里面的實現(xiàn)。

ClassPathBeanDefinitionScanner.java

public int scan(String... basePackages) {
   int beanCountAtScanStart = this.registry.getBeanDefinitionCount();

   doScan(basePackages);

   // Register annotation config processors, if necessary.
   if (this.includeAnnotationConfig) {
      AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
   }

   return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
}

這里關(guān)鍵的代碼是doScan(basePackages);，那么我們在進(jìn)去看一下。可能你會看到的是Spring源碼的實現(xiàn)方法，但這里Mybatis也實現(xiàn)了自己的一套，我們看一下Mybatis的實現(xiàn)。

ClassPathMapperScanner.java

public Set doScan(String... basePackages) {
  Set beanDefinitions = super.doScan(basePackages);

  if (beanDefinitions.isEmpty()) {
    logger.warn("No MyBatis mapper was found in "" + Arrays.toString(basePackages) + "" package. Please check your configuration.");
  } else {
    for (BeanDefinitionHolder holder : beanDefinitions) {
      GenericBeanDefinition 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.getPropertyValues().add("mapperInterface", definition.getBeanClassName());
      definition.setBeanClass(MapperFactoryBean.class);

      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);
      }
    }
  }

  return beanDefinitions;
}

definition.setBeanClass(MapperFactoryBean.class);這行代碼是非常關(guān)鍵的一句，由于在Spring中存在兩種自動實例化的方式，一種是我們常用的本身的接口實例化類進(jìn)行接口實例化，還有一種就是這里的自定義實例化。而這里的setBeanClass方法就是在BeanDefinitionHolder中進(jìn)行配置。在Spring進(jìn)行實例化的時候進(jìn)行處理。

那么我們在看一下MapperFactoryBean.class

MapperFactoryBean.java

public class MapperFactoryBean extends SqlSessionDaoSupport implements FactoryBean {

  private Class mapperInterface;

  private boolean addToConfig = true;

  /**
   * Sets the mapper interface of the MyBatis mapper
   *
   * @param mapperInterface class of the interface
   */
  public void setMapperInterface(Class mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  /**
   * If addToConfig is false the mapper will not be added to MyBatis. This means
   * it must have been included in mybatis-config.xml.
   * 

   * If it is true, the mapper will be added to MyBatis in the case it is not already
   * registered.
   * 

   * By default addToCofig is true.
   *
   * @param addToConfig
   */
  public void setAddToConfig(boolean addToConfig) {
    this.addToConfig = addToConfig;
  }

  /**
   * {@inheritDoc}
   */
  @Override
  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 (Throwable t) {
        logger.error("Error while adding the mapper "" + this.mapperInterface + "" to configuration.", t);
        throw new IllegalArgumentException(t);
      } finally {
        ErrorContext.instance().reset();
      }
    }
  }

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

  /**
   * {@inheritDoc}
   */
  public Class getObjectType() {
    return this.mapperInterface;
  }

  /**
   * {@inheritDoc}
   */
  public boolean isSingleton() {
    return true;
  }

在該類中其實現(xiàn)了FactoryBean接口，看過Spring源碼的人，我相信對其都有很深的印象，其在Bean的實例化中起著很重要的作用。在該類中我們要關(guān)注的是getObject方法，我們之后將動態(tài)實例化的接口對象放到Spring實例化列表中，這里就是入口，也是我們的起點。不過要特別說明的是mapperInterface的值是如何被賦值的，可能會有疑問，我們再來看看上面的ClassPathMapperScanner.java我們在配置MapperFactoryBean.class的上面存在一行 definition.getPropertyValues().add("mapperInterface", definition.getBeanClassName());其在之后在Spring的PostProcessorRegistrationDelegate類的populateBean方法中進(jìn)行屬性配置，會將其依靠反射的方式將其注入到MapperFactoryBean.class中。

而且definition.getPropertyValues().add中添加的值是注入到MapperFactoryBean對象中去的。這一點需要說明一下。

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