Mybatis Mapper 源码分析

天天都在用的Mybatis，为啥调用一个Mapper接口就能执行SQL，你有没有想过这个问题？

这一切都得从 @MapperScan 这个注解开始说起。打开这个注解定义可以看到：

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
@Documented
@Import({MapperScannerRegistrar.class})

上面的元注解中重点关注@Import(MapperScannerRegistrar.class)，这是Spring的一个注解，允许导入@Configuration类、 ImportSelector和ImportBeanDefinitionRegistrar实现。很明显MapperScannerRegistrar.class 应该是ImportBeanDefinitionRegistrar的实现：

public class MapperScannerRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware {

  private ResourceLoader resourceLoader;

  /**
   * {@inheritDoc}
   */
  @Override
  public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {

    AnnotationAttributes annoAttrs = AnnotationAttributes.fromMap(importingClassMetadata.getAnnotationAttributes(MapperScan.class.getName()));
    ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);

    // this check is needed in Spring 3.1
    if (resourceLoader != null) {
      scanner.setResourceLoader(resourceLoader);
    }

    Class<? extends Annotation> annotationClass = annoAttrs.getClass("annotationClass");
    if (!Annotation.class.equals(annotationClass)) {
      scanner.setAnnotationClass(annotationClass);
    }

    Class<?> markerInterface = annoAttrs.getClass("markerInterface");
    if (!Class.class.equals(markerInterface)) {
      scanner.setMarkerInterface(markerInterface);
    }

    Class<? extends BeanNameGenerator> generatorClass = annoAttrs.getClass("nameGenerator");
    if (!BeanNameGenerator.class.equals(generatorClass)) {
      scanner.setBeanNameGenerator(BeanUtils.instantiateClass(generatorClass));
    }

    Class<? extends MapperFactoryBean> mapperFactoryBeanClass = annoAttrs.getClass("factoryBean");
    if (!MapperFactoryBean.class.equals(mapperFactoryBeanClass)) {
      scanner.setMapperFactoryBean(BeanUtils.instantiateClass(mapperFactoryBeanClass));
    }

    scanner.setSqlSessionTemplateBeanName(annoAttrs.getString("sqlSessionTemplateRef"));
    scanner.setSqlSessionFactoryBeanName(annoAttrs.getString("sqlSessionFactoryRef"));

    List<String> basePackages = new ArrayList<String>();
    for (String pkg : annoAttrs.getStringArray("value")) {
      if (StringUtils.hasText(pkg)) {
        basePackages.add(pkg);
      }
    }
    for (String pkg : annoAttrs.getStringArray("basePackages")) {
      if (StringUtils.hasText(pkg)) {
        basePackages.add(pkg);
      }
    }
    for (Class<?> clazz : annoAttrs.getClassArray("basePackageClasses")) {
      basePackages.add(ClassUtils.getPackageName(clazz));
    }
    scanner.registerFilters();
    scanner.doScan(StringUtils.toStringArray(basePackages));
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public void setResourceLoader(ResourceLoader resourceLoader) {
    this.resourceLoader = resourceLoader;
  }

}

可以看到上面代码在做一件事，就是初始化ClassPathMapperScanner，最后调用了doScan：

public class ClassPathMapperScanner extends ClassPathBeanDefinitionScanner {

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 {
      processBeanDefinitions(beanDefinitions);
    }

    return beanDefinitions;
  }

首先调用了父类的doScan，里面的主要逻辑是查找指定的basePackages下的所有Components，即声明了@Component以及将@Component作为元注解的注解，例如@Repository，
感兴趣的可以看看：

• org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider#findCandidateComponents
• org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider#isCandidateComponent(org.springframework.core.type.classreading.MetadataReader)
• org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider#registerDefaultFilters
• org.springframework.core.type.filter.AnnotationTypeFilter#hasAnnotation
• org.springframework.core.annotation.AnnotationUtils#getAnnotation(java.lang.reflect.AnnotatedElement, java.lang.Class)

接下来调用了processBeanDefinitions:

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);
      }
    }
  }

重点看看这两行代码：

definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName()); 
definition.setBeanClass(this.mapperFactoryBean.getClass());

似乎已经可以看出点蛛丝马迹，这里将Mapper的BeanDefinition的BeanClass替换为了this.mapperFactoryBean.getClass():

private MapperFactoryBean<?> mapperFactoryBean = new MapperFactoryBean<Object>();

并且将原本的BeanClass添加为了构造函数参数：

definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName()); 

熟悉Spring的应该都知道，Spring加载完BeanDefinition后会通过BeanClass来去实例化Bean，这里的BeanClass被替换为了一个FactoryBean：

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) {
    this.mapperInterface = mapperInterface;
  }

  /**
   * {@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 (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 {
    return getSqlSession().getMapper(this.mapperInterface);
  }

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

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

  //------------- mutators --------------

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

  /**
   * Return the mapper interface of the MyBatis mapper
   *
   * @return class of the interface
   */
  public Class<T> getMapperInterface() {
    return 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;
  }

  /**
   * Return the flag for addition into MyBatis config.
   *
   * @return true if the mapper will be added to MyBatis in the case it is not already
   * registered.
   */
  public boolean isAddToConfig() {
    return addToConfig;
  }
}

可以看到之前那个被注册的构造函数参数应该就是在这被使用的：

 public MapperFactoryBean(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
}

Spring 关于这个Bean构造函数的处理：

  org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#autowireConstructor
// Need to determine the constructor...
		Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
		if (ctors != null ||
				mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
				mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
			return autowireConstructor(beanName, mbd, ctors, args);
		}
    

总之，Spring通过构造函数创建了一个 MapperFactoryBean 对象。接下来Spring去初始化这个Bean，Spring对FactoryBean有特殊处理，实际会调用getObject方法返回具体的Bean。
感兴趣的可以看：

org.springframework.beans.factory.support.FactoryBeanRegistrySupport#doGetObjectFromFactoryBean

这下又回到了MapperFactoryBean中，来看看getObject：

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

调用了父类getSqlSession()方法获得SqlSession，然后调用getMapper，传入了当前实际的Mapper。通过：

• org.apache.ibatis.session.SqlSessionFactoryBuilder#build(org.apache.ibatis.session.Configuration)
• org.apache.ibatis.session.defaults.DefaultSqlSessionFactory#openSessionFromDataSource

可以知道SqlSession实际的类型应该是DefaultSqlSession，看看getMapper：

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

又调用了configuration的getMapper，configuration初始化可以看看org.mybatis.spring.SqlSessionFactoryBean#buildSqlSessionFactory，
我们写的SQL就是在这个里面被解析的。到这里已经基本很清晰了。接着看：

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

//MapperRegistry中的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);
    }
}

关键就是 MapperProxyFactory了，这个是在 org.apache.ibatis.binding.MapperRegistry#addMapper 时会为每个Mapper绑定一个。
看newInstance：

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);
  }

也就是说最终Mapper注册到BeanFactory中的是一个代理类MapperProxy，我们知道被代理的方法最终都会被转发到代理类的invoke方法:

@Override
 public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
   try {
     if (Object.class.equals(method.getDeclaringClass())) {
       return method.invoke(this, args);
     } else if (isDefaultMethod(method)) {
       return invokeDefaultMethod(proxy, method, args);
     }
   } catch (Throwable t) {
     throw ExceptionUtil.unwrapThrowable(t);
   }
   final MapperMethod mapperMethod = cachedMapperMethod(method);
   return mapperMethod.execute(sqlSession, args);
 }

如果不是Object类的方法，也不是Mapper接口的default（默认）方法，那么调用的是mapperMethod.execute(sqlSession, args):

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    switch (command.getType()) {
      case INSERT: {
      Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.insert(command.getName(), param));
        break;
      }
      case UPDATE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.update(command.getName(), param));
        break;
      }
      case DELETE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.delete(command.getName(), param));
        break;
      }
      case SELECT:
        if (method.returnsVoid() && method.hasResultHandler()) {
          executeWithResultHandler(sqlSession, args);
          result = null;
        } else if (method.returnsMany()) {
          result = executeForMany(sqlSession, args);
        } else if (method.returnsMap()) {
          result = executeForMap(sqlSession, args);
        } else if (method.returnsCursor()) {
          result = executeForCursor(sqlSession, args);
        } else {
          Object param = method.convertArgsToSqlCommandParam(args);
          result = sqlSession.selectOne(command.getName(), param);
        }
        break;
      case FLUSH:
        result = sqlSession.flushStatements();
        break;
      default:
        throw new BindingException("Unknown execution method for: " + command.getName());
    }
    if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
      throw new BindingException("Mapper method '" + command.getName() 
          + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
    }
    return result;
  }

到这就差不多了吧？后面已经没啥好说了，基本就是通过statement name找到MappedStatement然后去执行sql了。

另外一个小知识，Mybatis是怎么在插入之后返回自增主键的？答案就在：

• org.apache.ibatis.executor.statement.PreparedStatementHandler#update
• org.apache.ibatis.executor.keygen.KeyGenerator#processAfter

写在最后

Spring代码实在太难看了，一环套一环，各种递归，必须debug才能看清。