springMVC源碼分析--DispatcherServlet請求獲取及處理

在之前的博客 springMVC源碼分析--容器初始化（二）DispatcherServlet中我們介紹過DispatcherServlet，是在容器初始化過程中出現的，我們之前也說過DispatcherServlet其實就是一個HttpServlet，其實他是HttpServlet的子類，所以它和普通的HttpServlet有同樣的配置：

<span style="font-size:14px;"><span style="white-space:pre">    </span><servlet>
        <servlet-name>springmvc</servlet-name>
        <servlet-class>
            org.springframework.web.servlet.DispatcherServlet
        </servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>classpath:springmvc-config.xml</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet-mapping>
        <servlet-name>springmvc</servlet-name>
        <url-pattern>*.action</url-pattern>
    </servlet-mapping></span>

僅僅把DispatcherServlet當做一個Servlet的話，上面配置的含義就是這個Servlet會被所有的*.action的請求所調用。

既然DispatcherServlet是一個HttpServlet那麼它應該會實現HttpServlet提供的如下方法：

當然這些所有的方法的實現是DispatcherServlet的父類FrameworkServlet中實現的。

當然這些實現方法中的默認實現其實是如下的

FrameworkServlet類中

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

        processRequest(request, response);
    }

@Override
    protected final void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        processRequest(request, response);
    }

processRequest的實現是在FrameworkServlet中，此方法中最主要的操作就是調用doService方法

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());

        initContextHolders(request, localeContext, requestAttributes);

        try {
            doService(request, response);
        }
        catch (ServletException ex) {
            failureCause = ex;
            throw ex;
        }
        catch (IOException ex) {
            failureCause = ex;
            throw ex;
        }
        catch (Throwable ex) {
            failureCause = ex;
            throw new NestedServletException("Request processing failed", ex);
        }

        finally {
            resetContextHolders(request, previousLocaleContext, previousAttributes);
            if (requestAttributes != null) {
                requestAttributes.requestCompleted();
            }

            if (logger.isDebugEnabled()) {
                if (failureCause != null) {
                    this.logger.debug("Could not complete request", failureCause);
                }
                else {
                    if (asyncManager.isConcurrentHandlingStarted()) {
                        logger.debug("Leaving response open for concurrent processing");
                    }
                    else {
                        this.logger.debug("Successfully completed request");
                    }
                }
            }

            publishRequestHandledEvent(request, response, startTime, failureCause);
        }
    }

doService方法的最終實現是在DispatcherServlet中，這樣所有的Http請求（GET、POST、PUT和DELETE等）的最終操作就DispatcherServlet中實現了。

DispatcherServlet中doService的實現如下，對Request設置了一些全局屬性，最終接下來的操作是在doDispatcher函數中實現了。

//獲取請求，設置一些request的參數，然後分發給doDispatch
    @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.
        Map<String, Object> attributesSnapshot = null;
        if (WebUtils.isIncludeRequest(request)) {
            attributesSnapshot = new HashMap<String, Object>();
            Enumeration<?> attrNames = request.getAttributeNames();
            while (attrNames.hasMoreElements()) {
                String attrName = (String) attrNames.nextElement();
                if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
                    attributesSnapshot.put(attrName, request.getAttribute(attrName));
                }
            }
        }

        // Make framework objects available to handlers and view objects.
        /* 設置web應用上下文**/
        request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
        /* 國際化本地**/
        request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
        /* 樣式**/
        request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
        //設置樣式資源
        request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
        //請求刷新時保存屬性
        FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
        if (inputFlashMap != null) {
            request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
        }
        //Flash attributes 在對請求的重定向生效之前被臨時存儲（通常是在session)中，並且在重定向之後被立即移除
        request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
        //FlashMap 被用來管理 flash attributes 而 FlashMapManager 則被用來存儲，獲取和管理 FlashMap 實體.
        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);
                }
            }
        }
    }

doDispatch函數中完成了對一個請求的所有操作，包含的內容還是比較多的，我們就不做詳細分解，接下來我們會一步一步的分析一個請求調用Controller的完整過程。

/**
     *將Handler進行分發，handler會被handlerMapping有序的獲得
     *通過查詢servlet安裝的HandlerAdapters來獲得HandlerAdapters來查找第一個支持handler的類
     *所有的HTTP的方法都會被這個方法掌控。取決於HandlerAdapters 或者handlers 他們自己去決定哪些方法是可用
     *@param request current HTTP request
     *@param response current HTTP response
     */
    protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        /* 當前HTTP請求**/
        HttpServletRequest processedRequest = request;
        HandlerExecutionChain mappedHandler = null;
        boolean multipartRequestParsed = false;

        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

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

            try {
                //判斷是否有文件上傳
                processedRequest = checkMultipart(request);
                multipartRequestParsed = (processedRequest != request);

                // 獲得HandlerExecutionChain，其包含HandlerIntercrptor和HandlerMethod
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null || mappedHandler.getHandler() == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }


                //獲得HandlerAdapter
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                //獲得HTTP請求方法
                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;
                    }
                }
                //如果有攔截器的話，會執行攔截器的preHandler方法
                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }

                //返回ModelAndView
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }
                //當view爲空時，，根據request設置默認view
                applyDefaultViewName(processedRequest, mv);
                //執行攔截器的postHandle
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) {
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Error err) {
            triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
        }
        finally {
            //判斷是否是異步請求
            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.
                //刪除上傳資源
                if (multipartRequestParsed) {
                    cleanupMultipart(processedRequest);
                }
            }
        }
    }

調用完doDispatch之後就完成了一個請求的訪問，其會將渲染後的頁面或者數據返回給請求發起者。