Spring MVC筆記（三） -- DispatcherServlet請求處理流程

從doGet、doPost方法入手，在父類FrameworkServlet中

protected final void doGet(HttpServletRequest request, HttpServletResponse response)
		throws ServletException, IOException {
	processRequest(request, response);
}

@Override
protected final void doPost(HttpServletRequest request, HttpServletResponse response)
		throws ServletException, IOException {
	processRequest(request, response);
}

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
		throws ServletException, IOException {

	long startTime = System.currentTimeMillis();
	Throwable failureCause = null;

	LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
	LocaleContext localeContext = buildLocaleContext(request);

	RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
	ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

	WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
	asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
	//綁定當前線程
	initContextHolders(request, localeContext, requestAttributes);
	try {
		//進一步處理
		doService(request, response);
	}
	catch ...
	finally {
		//恢復線程到原始狀態
		resetContextHolders(request, previousLocaleContext, previousAttributes);
		if (requestAttributes != null) {
			requestAttributes.requestCompleted();
		}
		logResult(request, response, failureCause, asyncManager);
		//發佈事件通知
		publishRequestHandledEvent(request, response, startTime, failureCause);
	}
}

doGet、doPost方法都是調用processRequest方法，做了一些準備工作，然後交由doService方法進一步處理

DispatcherServlet重寫了doService方法

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
	...
	Map<String, Object> attributesSnapshot = null;
	if (WebUtils.isIncludeRequest(request)) {
		attributesSnapshot = new HashMap<>();
		Enumeration<?> attrNames = request.getAttributeNames();
		while (attrNames.hasMoreElements()) {
			String attrName = (String) attrNames.nextElement();
			if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
				attributesSnapshot.put(attrName, request.getAttribute(attrName));
			}
		}
	}
	// 設置變量
	request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
	request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
	request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
	request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
	if (this.flashMapManager != null) {
		FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
		if (inputFlashMap != null) {
			request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
		}
		request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
		request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
	}
	try {
		doDispatch(request, response);
	}
	finally {
		if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
			// Restore the original attribute snapshot, in case of an include.
			if (attributesSnapshot != null) {
				restoreAttributesAfterInclude(request, attributesSnapshot);
			}
		}
	}
}

做好準備工作將已經初始化的功能輔助工具變量，如localeResolver、themeResolver等設置到request屬性中。doDispatch才真正進行處理請求。

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 {
			//檢測是否是文件上傳
			processedRequest = checkMultipart(request);
			multipartRequestParsed = (processedRequest != request);

			//【標記1】 從handlerMappings獲取相應的handler
			mappedHandler = getHandler(processedRequest);
			if (mappedHandler == null) {
				noHandlerFound(processedRequest, response);
				return;
			}
			//【標記2】 從handlerAdapters獲取相應的handlerAdapter
			HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
			// 是否支持last-modified
			String method = request.getMethod();
			boolean isGet = "GET".equals(method);
			if (isGet || "HEAD".equals(method)) {
				long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
				if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
					return;
				}
			}
			//調用攔截器的前置方法
			if (!mappedHandler.applyPreHandle(processedRequest, response)) {
				return;
			}
			// 真正調用handler進行請求的處理
			mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
			if (asyncManager.isConcurrentHandlingStarted()) {
				return;
			}
			//如果沒返回視圖，則使用默認的視圖
			applyDefaultViewName(processedRequest, mv);
			//調用攔截器的後置方法
			mappedHandler.applyPostHandle(processedRequest, response, mv);
		}
		catch (Exception ex) {
			dispatchException = ex;
		}
		catch (Throwable err) {
			dispatchException = new NestedServletException("Handler dispatch failed", err);
		}
		//【標記3】處理handler返回的結果
		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);
			}
		}
	}
}

【標記1】獲取handler

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
	if (this.handlerMappings != null) {
		for (HandlerMapping mapping : this.handlerMappings) {
			HandlerExecutionChain handler = mapping.getHandler(request);
			if (handler != null) {
				return handler;
			}
		}
	}
	return null;
}

getHandler方法主要是從handlerMappings獲取相應的handler,然後封裝成HandlerExecutionChain返回。

【標記2】 獲取HandlerAdapter

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
	if (this.handlerAdapters != null) {
		for (HandlerAdapter adapter : this.handlerAdapters) {
			if (adapter.supports(handler)) {
				return adapter;
			}
		}
	}
	throw...
}

getHandlerAdapter方法主要是從handlerAdapters獲取相應的adapter返回。

【標記3】 視圖渲染

protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
	Locale locale =
			(this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
	response.setLocale(locale);
	View view;
	String viewName = mv.getViewName();
	if (viewName != null) {
		// 【標記4】通過視圖名獲取View.
		view = resolveViewName(viewName, mv.getModelInternal(), locale, request);
		if (view == null) {
			throw...
		}
	}
	else {
		// 檢驗是否存在視圖
		view = mv.getView();
		if (view == null) {
			throw...
		}
	}
	try {
		if (mv.getStatus() != null) {
			response.setStatus(mv.getStatus().value());
		}
		//【標記5】
		view.render(mv.getModelInternal(), request, response);
	}
	catch...
}

protected View resolveViewName(String viewName, @Nullable Map<String, Object> model,
		Locale locale, HttpServletRequest request) throws Exception {
	if (this.viewResolvers != null) {
		for (ViewResolver viewResolver : this.viewResolvers) {
			View view = viewResolver.resolveViewName(viewName, locale);
			if (view != null) {
				return view;
			}
		}
	}
	return null;
}

【標記4】獲取View對象，以InternalResourceViewResolver爲例，resolveViewName在其父類AbstractCachingViewResolver中

public View resolveViewName(String viewName, Locale locale) throws Exception {
	if (!isCache()) {//沒緩存就創建
		return createView(viewName, locale);
	}
	else {
		Object cacheKey = getCacheKey(viewName, locale);
		View view = this.viewAccessCache.get(cacheKey);
		if (view == null) {
			synchronized (this.viewCreationCache) {
				view = this.viewCreationCache.get(cacheKey);
				if (view == null) {
					// 子類進行創建
					view = createView(viewName, locale);
					if (view == null && this.cacheUnresolved) {
						view = UNRESOLVED_VIEW;
					}
					if (view != null) {
						this.viewAccessCache.put(cacheKey, view);
						this.viewCreationCache.put(cacheKey, view);
					}
				}
			}
		}
		return (view != UNRESOLVED_VIEW ? view : null);
	}
}

UrlBasedViewResolver類

protected View createView(String viewName, Locale locale) throws Exception {
	// 檢查是否支持該視圖
	if (!canHandle(viewName, locale)) {
		return null;
	}
	// 重定向處理
	if (viewName.startsWith(REDIRECT_URL_PREFIX)) {
		String redirectUrl = viewName.substring(REDIRECT_URL_PREFIX.length());
		RedirectView view = new RedirectView(redirectUrl,
				isRedirectContextRelative(), isRedirectHttp10Compatible());
		String[] hosts = getRedirectHosts();
		if (hosts != null) {
			view.setHosts(hosts);
		}
		return applyLifecycleMethods(REDIRECT_URL_PREFIX, view);
	}
	// 轉發處理
	if (viewName.startsWith(FORWARD_URL_PREFIX)) {
		String forwardUrl = viewName.substring(FORWARD_URL_PREFIX.length());
		InternalResourceView view = new InternalResourceView(forwardUrl);
		return applyLifecycleMethods(FORWARD_URL_PREFIX, view);
	}

	// 回到父類的createView進行調用loadView
	return super.createView(viewName, locale);
}

protected View createView(String viewName, Locale locale) throws Exception {
	return loadView(viewName, locale);
}
protected abstract View loadView(String viewName, Locale locale) throws Exception;

protected View loadView(String viewName, Locale locale) throws Exception {
	//真正創建View
	AbstractUrlBasedView view = buildView(viewName);
	//交由spring管理聲明週期
	View result = applyLifecycleMethods(viewName, view);
	return (view.checkResource(locale) ? result : null);
}

真正創建View在buildView方法

protected AbstractUrlBasedView buildView(String viewName) throws Exception {
	Class<?> viewClass = getViewClass();
	//創建
	AbstractUrlBasedView view = (AbstractUrlBasedView) BeanUtils.instantiateClass(viewClass);
	//設置前綴、後綴
	view.setUrl(getPrefix() + viewName + getSuffix());
	//類型設置
	String contentType = getContentType();
	if (contentType != null) {
		view.setContentType(contentType);
	}
	...
	return view;
}

【標記5】頁面跳轉

AbstractView類

public void render(@Nullable Map<String, ?> model, HttpServletRequest request,
		HttpServletResponse response) throws Exception {
	...
	//獲取整合一些屬性
	Map<String, Object> mergedModel = createMergedOutputModel(model, request, response);
	prepareResponse(request, response);
	//處理頁面跳轉
	renderMergedOutputModel(mergedModel, getRequestToExpose(request), response);
}
InternalResourceView類

protected void renderMergedOutputModel(
		Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {

	// 把屬性設置到request中
	exposeModelAsRequestAttributes(model, request);
	// Expose helpers as request attributes, if any.
	exposeHelpers(request);
	// 獲取路徑
	String dispatcherPath = prepareForRendering(request, response);
	//拿到請求派發器
	RequestDispatcher rd = getRequestDispatcher(request, dispatcherPath);
	if (rd == null) {
		throw...
	}
	if (useInclude(request, response)) {
		response.setContentType(getContentType());
		rd.include(request, response);
	}

	else {
		...
		rd.forward(request, response);
	}
}

總結參考：SpringMVC請求流程詳解