spring mvc 的工做原理（1）DispatcherServlet 請求處理流程

類關係

DispatcherServlet 繼承 FrameworkServlet 繼承 HttpServletBean，HttpServletBean 是一個HttpServlet和EnvironmentCapable, EnvironmentAware接口的實現。

本章關注點在DispatcherServlet對請求的分派調用流程上，這些類暫不關注。

成員變量

一、靜態變量主要是一些bean名稱，以及request attribute 的key。如

/** Well-known name for the MultipartResolver object in the bean factory for this namespace. */
	public static final String MULTIPART_RESOLVER_BEAN_NAME = "multipartResolver";

	/** Well-known name for the LocaleResolver object in the bean factory for this namespace. */
	public static final String LOCALE_RESOLVER_BEAN_NAME = "localeResolver";

	/** Well-known name for the ThemeResolver object in the bean factory for this namespace. */
	public static final String THEME_RESOLVER_BEAN_NAME = "themeResolver";

二、具體的特殊的bean，上一章中提到的特殊bean。如

/** MultipartResolver used by this servlet */
	private MultipartResolver multipartResolver;

	/** LocaleResolver used by this servlet */
	private LocaleResolver localeResolver;

	/** ThemeResolver used by this servlet */
	private ThemeResolver themeResolver;

	/** List of HandlerMappings used by this servlet */
	private List<HandlerMapping> handlerMappings;

方法

方法重點關注doService，doDispatch這兩個方法，其餘的方法是get/set 和初始化方法，不是本章的關注對象。下面的這兩個方法源碼和註釋。

//重寫HttpServlet 的請求處理方法
	@Override
	protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
		if (logger.isDebugEnabled()) {
			String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
			logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +
					" processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");
		}

		// Keep a snapshot of the request attributes in case of an include,
		// to be able to restore the original attributes after the include.
		//保留一個原請求屬性的備份。
		Map<String, Object> attributesSnapshot = null;
		if (WebUtils.isIncludeRequest(request)) {
		    //是內部發出的請求 forward 
			attributesSnapshot = new HashMap<String, Object>();
			Enumeration<?> attrNames = request.getAttributeNames();
			while (attrNames.hasMoreElements()) {
				String attrName = (String) attrNames.nextElement();
				//若是是DispatcherServlet cleanupAfterInclude(是include 請求就清理)屬性爲true，或者request的屬性是DispatcherServlet附加的，保存備份
				if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
					attributesSnapshot.put(attrName, request.getAttribute(attrName));
				}
			}
		}

		//附加webApplicationContext 的特殊bean
		// Make framework objects available to handlers and view objects.
		//web application context
		request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
		//國際化
		request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
		//主題
		request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
		//？？？
		request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

		//閃存，重定向時能夠不攜帶參數
		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 {
		    //分派到handler，進行業務邏輯處理
			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);
				}
			}
		}
	}

	
	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 {
			    //檢查是否 multipart request 
				processedRequest = checkMultipart(request);
				//request 不等於 processedRequest 表示檢查到multi ，並構建返回 multi request
				multipartRequestParsed = (processedRequest != request);

				// Determine handler for the current request.
				//決定、判斷、獲取 handler
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null || mappedHandler.getHandler() == null) {
				    //找不到相應的handler ，返回並提示 noHandlerFound
					noHandlerFound(processedRequest, response);
					return;
				}

				// Determine handler adapter for the current request.
				//查找handler 的適配器（不一樣的方法細節多樣、參數等，經過adapter 封裝）
				HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

				// Process last-modified header, if supported by the handler.
				//判斷是否支持last-modified
				String method = request.getMethod();
				boolean isGet = "GET".equals(method);
				if (isGet || "HEAD".equals(method)) {
					long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
					if (logger.isDebugEnabled()) {
						logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
					}
					if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
						return;
					}
				}

				//執行前攔截器
				if (!mappedHandler.applyPreHandle(processedRequest, response)) {
					return;
				}

				// Actually invoke the handler.
				//調用實際業務方法邏輯
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

				if (asyncManager.isConcurrentHandlingStarted()) {
					return;
				}
				//解析modelview 中的名稱映射對應的視圖模板。
				applyDefaultViewName(processedRequest, mv);
				//執行後攔截器
				mappedHandler.applyPostHandle(processedRequest, response, mv);
			}
			catch (Exception ex) {
				dispatchException = ex;
			}
			//處理執行的結果，異常的話，跳轉處理異常的代碼。
			processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
		}
		catch (Exception ex) {
			triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
		}
		catch (Error err) {
			triggerAfterCompletionWithError(processedRequest, response, mappedHandler, 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);
				}
			}
		}
	}

註釋不正確的地方，請指出。

從源碼咱們能夠看出，基本上請求的出來流程正如第一篇中所描述的。

第一步是綁定各類各樣的web context bean。

第二步是搜索獲取對應請求的處理handler，以及對應的adapter。

第三步是執行找出的handler，前攔截器，handler， 後攔截器。

第四步是將結果或異常進行處理，如將string 映射到具體的view 或者輸出json等。捕獲異常的調用Exception handler 處理。