Mybatis源码分析——如何为dao类生成代理对象

Mybatis入门，一个简单的demo

在上面demo中，我们定义了一个dao接口，并没有实现类，然后直接调用接口的方法，就可以执行mapper里的sql。这是怎么做到的呢。

Mybatis源码分析——mapper.xml解析

在这篇文章中，我们分析到：mapper.xml对应的dao类会添加到configuration的MapperRegistry的Map, MapperProxyFactory> knownMappers里，key是类的全名，value是对应的代理对象。

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入口就在这里，在xml配置文件中，我们会有以下配置，启动扫描配置路径下的dao类

我们来看看MapperScannerConfigurer类，它实现了BeanDefinitionRegistryPostProcessor，我们找到对应的postProcessBeanDefinitionRegistry方法，如下

  @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));
  }

processPropertyPlaceHolders();方法是做配置解析的。重点在ClassPathMapperScanner.scan方法。ClassPathMapperScanner继承于ClassPathBeanDefinitionScanner。来看scan方法，这是ClassPathBeanDefinitionScanner.scan，如下

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);
}

再看doScan，这里调用的是ClassPathMapperScanner.doScan，如下

  @Override
  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 {
      processBeanDefinitions(beanDefinitions);
    }

    return beanDefinitions;
  }

然后它首先调用了父类也就是ClassPathBeanDefinitionScanner.doScan方法，如下

protected Set doScan(String... basePackages) {
		Assert.notEmpty(basePackages, "At least one base package must be specified");
		Set beanDefinitions = new LinkedHashSet();
		for (String basePackage : basePackages) {
			Set candidates = findCandidateComponents(basePackage);
			for (BeanDefinition candidate : candidates) {
				ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
				candidate.setScope(scopeMetadata.getScopeName());
				String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
				if (candidate instanceof AbstractBeanDefinition) {
					postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
				}
				if (candidate instanceof AnnotatedBeanDefinition) {
					AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
				}
				if (checkCandidate(beanName, candidate)) {
					BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
					definitionHolder =
							AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
					beanDefinitions.add(definitionHolder);
					registerBeanDefinition(definitionHolder, this.registry);
				}
			}
		}
		return beanDefinitions;
	}

这里通过findCandidateComponents(basePackage);找到配置包路径下的类，然后封装成BeanDefinition，再次封装形成BeanDefinitionHolder，再通过registerBeanDefinition(definitionHolder, this.registry);注册到容器里，如下

	protected void registerBeanDefinition(BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry) {
		BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, registry);
	}

	public static void registerBeanDefinition(
			BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
			throws BeanDefinitionStoreException {

		// Register bean definition under primary name.
		String beanName = definitionHolder.getBeanName();
		registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

		// Register aliases for bean name, if any.
		String[] aliases = definitionHolder.getAliases();
		if (aliases != null) {
			for (String alias : aliases) {
				registry.registerAlias(beanName, alias);
			}
		}
	}

注册完成后，返回Set，回到ClassPathMapperScanner.doScan方法，通过processBeanDefinitions(beanDefinitions);对返回的beanDefinitions做加工处理，如下

private void processBeanDefinitions(Set 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);
      }
    }
  }

在这里，通过definition.setBeanClass(this.mapperFactoryBean.getClass());绑定了bean的实际对象为MapperFactoryBean。通过definition.getPropertyValues().add("sqlSessionFactory", this.sqlSessionFactory);绑定了sqlSessionFactory。如果我们配置了sqlSessionTemplate，这里也会绑定：definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate);。通过definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);设置了查找bean的方式。

 

好了，到这里，我们可以知道，dao类其实绑定的是mapperFactoryBean。那我们来看看mapperFactoryBean是怎么产生一个代理对象的吧。

MapperFactoryBean实现了FactoryBean，来看看getObject()方法，如下

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

  public SqlSession getSqlSession() {
    return this.sqlSession;
  }

 public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {
    if (!this.externalSqlSession) {
      this.sqlSession = new SqlSessionTemplate(sqlSessionFactory);
    }
  }

  public void setSqlSessionTemplate(SqlSessionTemplate sqlSessionTemplate) {
    this.sqlSession = sqlSessionTemplate;
    this.externalSqlSession = true;
  }

getSqlSession()来自于它的父类SqlSessionDaoSupport，可以看到返回的是一个SqlSessionTemplate实例。getMapper方法如下

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

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

  @SuppressWarnings("unchecked")
  public  T getMapper(Class type, SqlSession sqlSession) {
    final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) 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);
    }
  }

最后来到MapperRegistry的getMapper，这里从knownMappers里取出代理对象，mapperProxyFactory.newInstance(sqlSession);如下

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

  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

好了，到这里已经很明显了，这里用了java的动态代理产生一个实例对象。我们再来看看MapperProxy，它继承了InvocationHandler，如下

  @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);
  }

  private MapperMethod cachedMapperMethod(Method method) {
    MapperMethod mapperMethod = methodCache.get(method);
    if (mapperMethod == null) {
      mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
      methodCache.put(method, mapperMethod);
    }
    return mapperMethod;
  }

可以看到，这里封装了MapperMethod，并且做了缓存，最终调用MapperMethod.execute方法来执行，如下

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;
  }

好了，这里要结尾了。非常明显地看到，这里所有的操作最终都是交给SqlSession来执行，这里的SqlSession实例是SqlSessionTemplate。

下一篇我们再分析SqlSession是如何执行sql的。

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