/**
 * 最核心的控制器
 * 源碼位置：org.springframework.web.servlet.DispatcherServlet.doDispatch(HttpServletRequest, HttpServletResponse)
 */
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
	HttpServletRequest processedRequest = request;
	HandlerExecutionChain mappedHandler = null;
	boolean multipartRequestParsed = false;

	WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

	try {
		ModelAndView mv = null;
		Exception dispatchException = null;

		try {
			// 驗證是不是上傳的請求，上傳的請求會轉(zhuǎn)化為MultipartHttpServletRequest
			processedRequest = checkMultipart(request);
			multipartRequestParsed = (processedRequest != request);

			// 進行映射
			mappedHandler = getHandler(processedRequest);
			if (mappedHandler == null) {
				noHandlerFound(processedRequest, response);
				return;
			}

			// 找到最合適的HandlerAdapter，按照順序，那個先解析到就是那個
			HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

			// Process last-modified header, if supported by the handler.  HTTP緩存相關(guān)
			String method = request.getMethod();
			boolean isGet = HttpMethod.GET.matches(method);
			if (isGet || HttpMethod.HEAD.matches(method)) {
				long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
				if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
					return;
				}
			}
			// 前置攔截器
			if (!mappedHandler.applyPreHandle(processedRequest, response)) {
				// 返回false就不進行后續(xù)處理了
				return;
			}

			// Actually invoke the handler.
			// 獲取參數(shù)，執(zhí)行方法
			mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

			if (asyncManager.isConcurrentHandlingStarted()) {
				return;
			}
			// 如果mv有  視圖沒有，給你設(shè)置默認視圖
			applyDefaultViewName(processedRequest, mv);
			//后置攔截器
			mappedHandler.applyPostHandle(processedRequest, response, mv);
		}
		catch (Exception ex) {
			dispatchException = ex;
		}
		catch (Throwable err) {
			// As of 4.3, we're processing Errors thrown from handler methods as well,
			// making them available for @ExceptionHandler methods and other scenarios.
			dispatchException = new NestedServletException("Handler dispatch failed", err);
		}
		// 渲染視圖
		processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
	}
	catch (Exception ex) {
		triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
	}
	catch (Throwable err) {
		triggerAfterCompletion(processedRequest, response, mappedHandler,
				new NestedServletException("Handler processing failed", err));
	}
	finally {
		if (asyncManager.isConcurrentHandlingStarted()) {
			// Instead of postHandle and afterCompletion
			if (mappedHandler != null) {
				mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
			}
		}
		else {
			// Clean up any resources used by a multipart request.
			if (multipartRequestParsed) {
				cleanupMultipart(processedRequest);
			}
		}
	}
}

/**
 * 獲取處理器映射器
 * 源碼位置：org.springframework.web.servlet.DispatcherServlet.getHandler(HttpServletRequest)
 */
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
	if (this.handlerMappings != null) {
		/** 拿到所有handlerMappings （容器啟動階段初始化：拿到所有實現(xiàn)了HandlerMapping的Bean）
		 * @see DispatcherServlet#initHandlerMappings
		 * 測試發(fā)現(xiàn)： 不同的HandlerMapping可以有相同path, 誰先解析到就用哪個
		 * */
		for (HandlerMapping mapping : this.handlerMappings) {
			HandlerExecutionChain handler = mapping.getHandler(request);
			if (handler != null) {
				return handler;
			}
		}
	}
	return null;
}

/**
 * 獲取處理器適配器
 * 源碼位置：org.springframework.web.servlet.DispatcherServlet.getHandlerAdapter(Object)
 */
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
	if (this.handlerAdapters != null) {
		// 按照配置的順序，誰先解析到就用那個
		for (HandlerAdapter adapter : this.handlerAdapters) {
			if (adapter.supports(handler)) {
				return adapter;
			}
		}
	}
	throw new ServletException("No adapter for handler [" + handler +
			"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
}

/**
 * 獲取參數(shù)，執(zhí)行方法最外層的調(diào)用
 * 源碼位置：org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(HttpServletRequest, HttpServletResponse, Object)
 */
public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
		throws Exception {
	// 直接調(diào)用這個方法
	return handleInternal(request, response, (HandlerMethod) handler);
}

/**
 * 獲取參數(shù)，執(zhí)行方法內(nèi)部的調(diào)用邏輯
 * 源碼位置：org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(HttpServletRequest, HttpServletResponse, HandlerMethod)
 */
protected ModelAndView handleInternal(HttpServletRequest request,
		HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

	ModelAndView mav;
	// 檢查當(dāng)前請求的method是否為支持的method(默認Null,可通過繼承AbstractController設(shè)置supportedMethods)
	// 檢查當(dāng)前請求是否必須session  (默認false,可通過繼承AbstractController設(shè)置requireSession)
	checkRequest(request);

	/**
	 * 判斷當(dāng)前是否需要支持在同一個session中只能線性地處理請求：一個session同時只能處理一個線程
	 * 因為鎖是通過 synchronized 是 JVM 進程級，所以在分布式環(huán)境下，
	 * 無法達到同步相同 Session 的功能。默認情況下，synchronizeOnSession 為 false
	 */
	if (this.synchronizeOnSession) {
		// 獲取當(dāng)前請求的session對象
		HttpSession session = request.getSession(false);
		if (session != null) {
			// 為當(dāng)前session生成一個唯一的可以用于鎖定的key
			Object mutex = WebUtils.getSessionMutex(session);
			synchronized (mutex) {
				// 對HandlerMethod進行參數(shù)等的適配處理，并調(diào)用目標(biāo)handler
				mav = invokeHandlerMethod(request, response, handlerMethod);
			}
		}
		else {
			// 如果當(dāng)前不存在session，則直接對HandlerMethod進行適配
			mav = invokeHandlerMethod(request, response, handlerMethod);
		}
	}
	else {
		// *如果當(dāng)前不需要對session進行同步處理，則直接對HandlerMethod進行適配
		mav = invokeHandlerMethod(request, response, handlerMethod);
	}


	//判斷當(dāng)前請求頭中是否包含Cache-Control請求頭，如果不包含，則對當(dāng)前response進行處理
	if (!response.containsHeader(HEADER_CACHE_CONTROL)) {
		// 如果當(dāng)前SessionAttribute中存在配置的attributes，則為其設(shè)置過期時間。
		// 這里SessionAttribute主要是通過@SessionAttribute注解生成的
		if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
			applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
		}
		else {
			// 如果當(dāng)前不存在SessionAttributes，則判斷當(dāng)前是否存在Cache-Control設(shè)置，
			// 如果存在，則按照該設(shè)置進行response處理，如果不存在，則設(shè)置response中的
			// Cache的過期時間為-1，即立即失效
			prepareResponse(response);
		}
	}

	return mav;
}

/**
 * 獲取參數(shù)，執(zhí)行方法前的準(zhǔn)備邏輯
 * 源碼位置：org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(HttpServletRequest, HttpServletResponse, HandlerMethod)
 */
protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
		HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
	// 把我們的請求req resp包裝成 ServletWebRequest
	ServletWebRequest webRequest = new ServletWebRequest(request, response);
	try {
		// 獲取容器中全局配置的InitBinder和當(dāng)前HandlerMethod所對應(yīng)的Controller中
		// 配置的InitBinder，用于進行參數(shù)的綁定
		WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);

		// 獲取容器中全局配置的ModelAttribute和當(dāng)前HandlerMethod所對應(yīng)的Controller 中配置的ModelAttribute，
		// 這些配置的方法將會在目標(biāo)方法調(diào)用之前進行調(diào)用
		ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);

		// 封裝handlerMethod，會在調(diào)用前解析參數(shù)、調(diào)用后對返回值進行處理
		ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
		if (this.argumentResolvers != null) {
			// 讓invocableMethod擁有參數(shù)解析能力
			invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
		}
		if (this.returnValueHandlers != null) {
			// 讓invocableMethod擁有返回值處理能力
			invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
		}
		// 讓invocableMethod擁有InitBinder解析能力
		invocableMethod.setDataBinderFactory(binderFactory);
		// 設(shè)置ParameterNameDiscoverer，該對象將按照一定的規(guī)則獲取當(dāng)前參數(shù)的名稱
		invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);
		// ModelAndView處理容器
		ModelAndViewContainer mavContainer = new ModelAndViewContainer();
		// 將request的Attribute復(fù)制一份到ModelMap
		mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
		// *調(diào)用我們標(biāo)注了@ModelAttribute的方法,主要是為我們的目標(biāo)方法預(yù)加載
		modelFactory.initModel(webRequest, mavContainer, invocableMethod);
		// 重定向的時候，忽略model中的數(shù)據(jù) 默認false
		mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);

		// 獲取當(dāng)前的AsyncWebRequest，這里AsyncWebRequest的主要作用是用于判斷目標(biāo)
		// handler的返回值是否為WebAsyncTask或DeferredResult，如果是這兩種中的一種，
		// 則說明當(dāng)前請求的處理應(yīng)該是異步的。所謂的異步，指的是當(dāng)前請求會將Controller中
		// 封裝的業(yè)務(wù)邏輯放到一個線程池中進行調(diào)用，待該調(diào)用有返回結(jié)果之后再返回到response中。
		// 這種處理的優(yōu)點在于用于請求分發(fā)的線程能夠解放出來，從而處理更多的請求,提高吞吐。
		// 只有待目標(biāo)任務(wù)完成之后才會回來將該異步任務(wù)的結(jié)果返回。
		AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
		asyncWebRequest.setTimeout(this.asyncRequestTimeout);
		// 封裝異步任務(wù)的線程池、request、interceptors到WebAsyncManager中
		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
		asyncManager.setTaskExecutor(this.taskExecutor);
		asyncManager.setAsyncWebRequest(asyncWebRequest);
		asyncManager.registerCallableInterceptors(this.callableInterceptors);
		asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);

		// 這里就是用于判斷當(dāng)前請求是否有異步任務(wù)結(jié)果的，如果存在，則對異步任務(wù)結(jié)果進行封裝
		if (asyncManager.hasConcurrentResult()) {
			Object result = asyncManager.getConcurrentResult();
			mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
			asyncManager.clearConcurrentResult();
			LogFormatUtils.traceDebug(logger, traceOn -> {
				String formatted = LogFormatUtils.formatValue(result, !traceOn);
				return "Resume with async result [" + formatted + "]";
			});
			invocableMethod = invocableMethod.wrapConcurrentResult(result);
		}
		// *對請求參數(shù)進行處理，調(diào)用目標(biāo)HandlerMethod，并且將返回值封裝為一個ModelAndView對象
		invocableMethod.invokeAndHandle(webRequest, mavContainer);
		if (asyncManager.isConcurrentHandlingStarted()) {
			return null;
		}

		// 對封裝的ModelAndView進行處理，主要是判斷當(dāng)前請求是否進行了重定向，如果進行了重定向，
		// 還會判斷是否需要將FlashAttributes封裝到新的請求中
		return getModelAndView(mavContainer, modelFactory, webRequest);
	}
	finally {
		webRequest.requestCompleted();
	}
}

/**
 * 獲取參數(shù)，執(zhí)行方法
 * 源碼位置：org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletWebRequest, ModelAndViewContainer, Object...)
 */
public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
		Object... providedArgs) throws Exception {

	/*真正的調(diào)用我們的目標(biāo)對象 很重要 很重要*/
	Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
	// 設(shè)置相關(guān)的返回狀態(tài)
	setResponseStatus(webRequest);
	// 如果請求處理完成，則設(shè)置requestHandled屬性
	if (returnValue == null) {
		if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
			disableContentCachingIfNecessary(webRequest);
			mavContainer.setRequestHandled(true);
			return;
		}
	}
	// 如果請求失敗，但是有錯誤原因，那么也會設(shè)置requestHandled屬性
	else if (StringUtils.hasText(getResponseStatusReason())) {
		mavContainer.setRequestHandled(true);
		return;
	}

	mavContainer.setRequestHandled(false);
	Assert.state(this.returnValueHandlers != null, "No return value handlers");
	try {
		// 遍歷當(dāng)前容器中所有ReturnValueHandler，判斷哪種handler支持當(dāng)前返回值的處理，
		// 如果支持，則使用該handler處理該返回值
		this.returnValueHandlers.handleReturnValue(
				returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
	}
	catch (Exception ex) {
		if (logger.isTraceEnabled()) {
			logger.trace(formatErrorForReturnValue(returnValue), ex);
		}
		throw ex;
	}
}

/**
 * 渲染視圖邏輯
 * 源碼位置：org.springframework.web.servlet.DispatcherServlet.processDispatchResult(HttpServletRequest, HttpServletResponse, HandlerExecutionChain, ModelAndView, Exception)
 */
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
		@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
		@Nullable Exception exception) throws Exception {

	boolean errorView = false;

	// 異常視圖
	if (exception != null) {
		if (exception instanceof ModelAndViewDefiningException) {
			logger.debug("ModelAndViewDefiningException encountered", exception);
			mv = ((ModelAndViewDefiningException) exception).getModelAndView();
		}
		else {
			Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
			mv = processHandlerException(request, response, handler, exception);
			errorView = (mv != null);
		}
	}

	// Did the handler return a view to render?
	if (mv != null && !mv.wasCleared()) {
		// 解析、渲染視圖：解析視圖名，拼接前后綴
		render(mv, request, response);
		if (errorView) {
			WebUtils.clearErrorRequestAttributes(request);
		}
	}
	else {
		if (logger.isTraceEnabled()) {
			logger.trace("No view rendering, null ModelAndView returned.");
		}
	}

	if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
		// Concurrent handling started during a forward
		return;
	}

	if (mappedHandler != null) {
		// Exception (if any) is already handled..   攔截器：AfterCompletion
		mappedHandler.triggerAfterCompletion(request, response, null);
	}
}