接口动态实现解决方案

声明解决方案是基于Mybatis源码，进行二次开发实现。

问题领导最近跟我提了一个需求，是有关于实现类Mybatis的@Select、@Insert注解的功能。其是基于interface层面，不存在任何的接口实现类。因而在实现的过程中，首先要解决的是如何动态实现接口的实例化。其次是如何将使接口根据注解实现相应的功能。

我们先来看看Mybatis是如何实现Dao类的扫描的。

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继承ClassPathBeanDefinitionScanner类而来的。这里对于ClassPathMapperScanner的配置参数来源于我们在使用Mybatis时的配置而来，是不是还记得在使用Mybatis的时候要配置basePackage的参数呢？

接着我们就顺着scanner.scan()方法，进入查看一下里面的实现。

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);
}

这里关键的代码是doScan(basePackages);，那么我们在进去看一下。可能你会看到的是Spring源码的实现方法，但这里Mybatis也实现了自己的一套，我们看一下Mybatis的实现。

ClassPathMapperScanner.java

public Set<BeanDefinitionHolder> doScan(String... basePackages) {
  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 {
    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);这行代码是非常关键的一句，由于在Spring中存在两种自动实例化的方式，一种是我们常用的本身的接口实例化类进行接口实例化，还有一种就是这里的自定义实例化。而这里的setBeanClass方法就是在BeanDefinitionHolder中进行配置。在Spring进行实例化的时候进行处理。

那么我们在看一下MapperFactoryBean.class

MapperFactoryBean.java

public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T> {

  private Class<T> mapperInterface;

  private boolean addToConfig = true;

  /**
   * Sets the mapper interface of the MyBatis mapper
   *
   * @param mapperInterface class of the interface
   */
  public void setMapperInterface(Class<T> 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.
   * <p>
   * If it is true, the mapper will be added to MyBatis in the case it is not already
   * registered.
   * <p>
   * 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<T> getObjectType() {
    return this.mapperInterface;
  }

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

在该类中其实现了FactoryBean接口，看过Spring源码的人，我相信对其都有很深的印象，其在Bean的实例化中起着很重要的作用。在该类中我们要关注的是getObject方法，我们之后将动态实例化的接口对象放到Spring实例化列表中，这里就是入口，也是我们的起点。不过要特别说明的是mapperInterface的值是如何被赋值的，可能会有疑问，我们再来看看上面的ClassPathMapperScanner.java我们在配置MapperFactoryBean.class的上面存在一行 definition.getPropertyValues().add("mapperInterface", definition.getBeanClassName());其在之后在Spring的PostProcessorRegistrationDelegate类的populateBean方法中进行属性配置，会将其依靠反射的方式将其注入到MapperFactoryBean.class中。

而且definition.getPropertyValues().add中添加的值是注入到MapperFactoryBean对象中去的。这一点需要说明一下。