SpringMVC 6+源码分析（四）DispatcherServlet实例化流程 3--（HandlerAdapter初始化）

一、概述

HandlerAdapter 是 Spring MVC 框架中的一个核心组件，它在 DispatcherServlet 和处理程序（handler）之间扮演适配器的角色。DispatcherServlet 接收到 HTTP 请求后，需要调用对应的 handler 来处理请求（如控制器方法），但不同 handler 的调用方式可能不同，例如：基于注解的控制器 或者 实现特定接口的控制器。HandlerAdapter 通过标准化接口屏蔽这些差异，确保 DispatcherServlet 能以统一方式调用任何 handler，从而提升框架的灵活性和扩展性。
框架默认的HandlerAdapter有四个，分别为：

• RequestMappingHandlerAdapter：这是最常用的实现类，支持基于注解的控制器（如 @Controller 和@RequestMapping）。它通过反射调用注解方法，并处理参数绑定、返回值转换等。
• SimpleControllerHandlerAdapter：支持实现 Controller 接口的handler。该接口要求handler 实现 ModelAndView handleRequest(HttpServletRequest request,HttpServletResponse response) 方法。
• HttpRequestHandlerAdapter：适配实现HttpRequestHandler 接口的 handler，常用于处理原始HTTP 请求（如资源处理）。
• SimpleServletHandlerAdapter：支持 Servlet 类型的 handler，允许直接使用 ServletAPI处理请求。

二、初始化流程

系统在加载handlerAdapters 时，容器中是否已经存在对应的HandlerAdapter。如果不存在就加载默认的的四个HandlerAdapter。接下来 我们将以 RequestMappingHandlerAdapter为例，讲解整个初始化流程。

private void initHandlerAdapters(ApplicationContext context) {
		this.handlerAdapters = null;

		if (this.detectAllHandlerAdapters) {
			// Find all HandlerAdapters in the ApplicationContext, including ancestor contexts.
			Map<String, HandlerAdapter> matchingBeans =
					BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false);
			if (!matchingBeans.isEmpty()) {
				this.handlerAdapters = new ArrayList<>(matchingBeans.values());
				// We keep HandlerAdapters in sorted order.
				AnnotationAwareOrderComparator.sort(this.handlerAdapters);
			}
		}
		else {
			try {
				HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class);
				this.handlerAdapters = Collections.singletonList(ha);
			}
			catch (NoSuchBeanDefinitionException ex) {
				// Ignore, we'll add a default HandlerAdapter later.
			}
		}

		// Ensure we have at least some HandlerAdapters, by registering
		// default HandlerAdapters if no other adapters are found.
		if (this.handlerAdapters == null) {
			this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);
			if (logger.isTraceEnabled()) {
				logger.trace("No HandlerAdapters declared for servlet '" + getServletName() +
						"': using default strategies from DispatcherServlet.properties");
			}
		}
	}

可以看到RequestMappingHandlerAdapter 继承了AbstractHandlerMethodAdapter 实现InitializingBean和BeanFactoryAware接口，所以对象被容器创建以后则将spring上下文设置到对应的实现方法中。

从源码中可以知道afterPropertiesSet() 第一个初始化的是ControllerAdvice（Controller 增强器），其主要作用是将扫描到的bean分类处理。

	@Override
	public void setBeanFactory(BeanFactory beanFactory) {
		if (beanFactory instanceof ConfigurableBeanFactory cbf) {
			this.beanFactory = cbf;
		}
	}

@Override
	public void afterPropertiesSet() {
		// Do this first, it may add ResponseBody advice beans
		initControllerAdviceCache();
		initMessageConverters();

		if (this.argumentResolvers == null) {
			List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
			this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
		}
		if (this.initBinderArgumentResolvers == null) {
			List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();
			this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
		}
		if (this.returnValueHandlers == null) {
			List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
			this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
		}
	}

先扫描过滤带有ControllerAdvice 注解的类，然后对对应的增强器进行分类并进行缓存。
如：参数增强器，数据绑定增强等。

private void initControllerAdviceCache() {
		if (getApplicationContext() == null) {
			return;
		}

		List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());

		List<Object> requestResponseBodyAdviceBeans = new ArrayList<>();

		for (ControllerAdviceBean adviceBean : adviceBeans) {
			Class<?> beanType = adviceBean.getBeanType();
			if (beanType == null) {
				throw new IllegalStateException("Unresolvable type for ControllerAdviceBean: " + adviceBean);
			}
			Set<Method> attrMethods = MethodIntrospector.selectMethods(beanType, MODEL_ATTRIBUTE_METHODS);
			if (!attrMethods.isEmpty()) {
				this.modelAttributeAdviceCache.put(adviceBean, attrMethods);
			}
			Set<Method> binderMethods = MethodIntrospector.selectMethods(beanType, INIT_BINDER_METHODS);
			if (!binderMethods.isEmpty()) {
				this.initBinderAdviceCache.put(adviceBean, binderMethods);
			}
			if (RequestBodyAdvice.class.isAssignableFrom(beanType) || ResponseBodyAdvice.class.isAssignableFrom(beanType)) {
				requestResponseBodyAdviceBeans.add(adviceBean);
			}
		}

		if (!requestResponseBodyAdviceBeans.isEmpty()) {
			this.requestResponseBodyAdvice.addAll(0, requestResponseBodyAdviceBeans);
		}

		if (logger.isDebugEnabled()) {
			int modelSize = this.modelAttributeAdviceCache.size();
			int binderSize = this.initBinderAdviceCache.size();
			int reqCount = getBodyAdviceCount(RequestBodyAdvice.class);
			int resCount = getBodyAdviceCount(ResponseBodyAdvice.class);
			if (modelSize == 0 && binderSize == 0 && reqCount == 0 && resCount == 0) {
				logger.debug("ControllerAdvice beans: none");
			}
			else {
				logger.debug("ControllerAdvice beans: " + modelSize + " @ModelAttribute, " + binderSize +
						" @InitBinder, " + reqCount + " RequestBodyAdvice, " + resCount + " ResponseBodyAdvice");
			}
		}
	}

接下来就是 初始化 消息转换器，它的左右就是将 http报文转化成json或者java对象。默认转化器有 ByteArrayHttpMessageConverter、StringHttpMessageConverter、AllEncompassingFormHttpMessageConverter。

	private void initMessageConverters() {
		if (!this.messageConverters.isEmpty()) {
			return;
		}
		this.messageConverters.add(new ByteArrayHttpMessageConverter());
		this.messageConverters.add(new StringHttpMessageConverter());

		this.messageConverters.add(new AllEncompassingFormHttpMessageConverter());
	}

最后将初始化各类的Resolvers，有参数处理的Resolvers、数据绑定Resolvers，返回参数Resolvers。它们会根据不同的请求类型，数据类型，选择对应的处理器，对每个请求或者数据经行处理。这些Resolvers将对整个请求流程起到重要作用。

getDefaultArgumentResolvers（）方法将加载默认的所有ArgumentResolvers。

private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() {
		List<HandlerMethodArgumentResolver> resolvers = new ArrayList<>(30);

		// Annotation-based argument resolution
		resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), false));
		resolvers.add(new RequestParamMapMethodArgumentResolver());
		resolvers.add(new PathVariableMethodArgumentResolver());
		resolvers.add(new PathVariableMapMethodArgumentResolver());
		resolvers.add(new MatrixVariableMethodArgumentResolver());
		resolvers.add(new MatrixVariableMapMethodArgumentResolver());
		resolvers.add(new ServletModelAttributeMethodProcessor(false));
		resolvers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));
		resolvers.add(new RequestPartMethodArgumentResolver(getMessageConverters(), this.requestResponseBodyAdvice));
		resolvers.add(new RequestHeaderMethodArgumentResolver(getBeanFactory()));
		resolvers.add(new RequestHeaderMapMethodArgumentResolver());
		resolvers.add(new ServletCookieValueMethodArgumentResolver(getBeanFactory()));
		resolvers.add(new ExpressionValueMethodArgumentResolver(getBeanFactory()));
		resolvers.add(new SessionAttributeMethodArgumentResolver());
		resolvers.add(new RequestAttributeMethodArgumentResolver());

		// Type-based argument resolution
		resolvers.add(new ServletRequestMethodArgumentResolver());
		resolvers.add(new ServletResponseMethodArgumentResolver());
		resolvers.add(new HttpEntityMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));
		resolvers.add(new RedirectAttributesMethodArgumentResolver());
		resolvers.add(new ModelMethodProcessor());
		resolvers.add(new MapMethodProcessor());
		resolvers.add(new ErrorsMethodArgumentResolver());
		resolvers.add(new SessionStatusMethodArgumentResolver());
		resolvers.add(new UriComponentsBuilderMethodArgumentResolver());
		if (KotlinDetector.isKotlinPresent()) {
			resolvers.add(new ContinuationHandlerMethodArgumentResolver());
		}

		// Custom arguments
		if (getCustomArgumentResolvers() != null) {
			resolvers.addAll(getCustomArgumentResolvers());
		}

		// Catch-all
		resolvers.add(new PrincipalMethodArgumentResolver());
		resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));
		resolvers.add(new ServletModelAttributeMethodProcessor(true));

		return resolvers;
	}

getDefaultInitBinderArgumentResolvers()初始化默认参数数据绑定的 Resolver。

	private List<HandlerMethodArgumentResolver> getDefaultInitBinderArgumentResolvers() {
		List<HandlerMethodArgumentResolver> resolvers = new ArrayList<>(20);

		// Annotation-based argument resolution
		resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), false));
		resolvers.add(new RequestParamMapMethodArgumentResolver());
		resolvers.add(new PathVariableMethodArgumentResolver());
		resolvers.add(new PathVariableMapMethodArgumentResolver());
		resolvers.add(new MatrixVariableMethodArgumentResolver());
		resolvers.add(new MatrixVariableMapMethodArgumentResolver());
		resolvers.add(new ExpressionValueMethodArgumentResolver(getBeanFactory()));
		resolvers.add(new SessionAttributeMethodArgumentResolver());
		resolvers.add(new RequestAttributeMethodArgumentResolver());

		// Type-based argument resolution
		resolvers.add(new ServletRequestMethodArgumentResolver());
		resolvers.add(new ServletResponseMethodArgumentResolver());

		// Custom arguments
		if (getCustomArgumentResolvers() != null) {
			resolvers.addAll(getCustomArgumentResolvers());
		}

		// Catch-all
		resolvers.add(new PrincipalMethodArgumentResolver());
		resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));

		return resolvers;
	}

getDefaultReturnValueHandlers 方法初始化 返回值处理的Handler。

/**
	 * Return the list of return value handlers to use including built-in and
	 * custom handlers provided via {@link #setReturnValueHandlers}.
	 */
	private List<HandlerMethodReturnValueHandler> getDefaultReturnValueHandlers() {
		List<HandlerMethodReturnValueHandler> handlers = new ArrayList<>(20);

		// Single-purpose return value types
		handlers.add(new ModelAndViewMethodReturnValueHandler());
		handlers.add(new ModelMethodProcessor());
		handlers.add(new ViewMethodReturnValueHandler());
		handlers.add(new ResponseBodyEmitterReturnValueHandler(getMessageConverters(),
				this.reactiveAdapterRegistry, this.taskExecutor, this.contentNegotiationManager));
		handlers.add(new StreamingResponseBodyReturnValueHandler());
		handlers.add(new HttpEntityMethodProcessor(getMessageConverters(),
				this.contentNegotiationManager, this.requestResponseBodyAdvice));
		handlers.add(new HttpHeadersReturnValueHandler());
		handlers.add(new CallableMethodReturnValueHandler());
		handlers.add(new DeferredResultMethodReturnValueHandler());
		handlers.add(new AsyncTaskMethodReturnValueHandler(this.beanFactory));

		// Annotation-based return value types
		handlers.add(new ServletModelAttributeMethodProcessor(false));
		handlers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(),
				this.contentNegotiationManager, this.requestResponseBodyAdvice));

		// Multi-purpose return value types
		handlers.add(new ViewNameMethodReturnValueHandler());
		handlers.add(new MapMethodProcessor());

		// Custom return value types
		if (getCustomReturnValueHandlers() != null) {
			handlers.addAll(getCustomReturnValueHandlers());
		}

		// Catch-all
		if (!CollectionUtils.isEmpty(getModelAndViewResolvers())) {
			handlers.add(new ModelAndViewResolverMethodReturnValueHandler(getModelAndViewResolvers()));
		}
		else {
			handlers.add(new ServletModelAttributeMethodProcessor(true));
		}

		return handlers;
	}

三、总结

到此整个初始化的大致流程已经就结束了，还有很多细节上的地方没有讲解到。后续在执行流程中，涉及到的在补充说明。在此过程中如存在纰漏或者错误，麻烦请支持，相互探讨相互学习。