Spring框架系列(14)

前文我們有了IOC的原始碼基礎以及SpringMVC的基礎，我們便可以進一步深入理解SpringMVC主要實現原理，包含DispatcherServlet的初始化過程和DispatcherServlet處理請求的過程的原始碼解析。本文是第二篇：DispatcherServlet處理請求的過程的原始碼解析。@pdai

Spring框架系列(14) - SpringMVC實現原理之DispatcherServlet處理請求的過程
- DispatcherServlet處理請求的過程？
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DispatcherServlet處理請求的過程？

一個請求發出，經過DispatcherServlet進行了什麼樣的處理，最後將內容返回的呢？

回顧整理處理流程

首先讓我們整體看一下Spring Web MVC 處理請求的流程：

核心架構的具體流程步驟如下：

首先使用者傳送請求——>DispatcherServlet，前端控制器收到請求後自己不進行處理，而是委託給其他的解析器進行
處理，作為統一存取點，進行全域性的流程控制；
DispatcherServlet——>HandlerMapping， HandlerMapping 將會把請求對映為 HandlerExecutionChain 物件（包含一
個Handler 處理器（頁面控制器）物件、多個HandlerInterceptor 攔截器）物件，通過這種策略模式，很容易新增新
的對映策略；
DispatcherServlet——>HandlerAdapter，HandlerAdapter 將會把處理器包裝為介面卡，從而支援多種型別的處理器，
即介面卡設計模式的應用，從而很容易支援很多型別的處理器；
HandlerAdapter——>處理器功能處理方法的呼叫，HandlerAdapter 將會根據適配的結果呼叫真正的處理器的功能處
理方法，完成功能處理；並返回一個ModelAndView 物件（包含模型資料、邏輯檢視名）；
ModelAndView 的邏輯檢視名——> ViewResolver，ViewResolver 將把邏輯檢視名解析為具體的View，通過這種策
略模式，很容易更換其他檢視技術；
View——>渲染，View 會根據傳進來的Model 模型資料進行渲染，此處的Model 實際是一個Map 資料結構，因此
很容易支援其他檢視技術；
返回控制權給DispatcherServlet，由DispatcherServlet 返回響應給使用者，到此一個流程結束。

doGet入口

我們以上個demo中這個GET請求為例，請求URL是http://localhost:8080/011_spring_framework_demo_springmvc_war_exploded/user

我們知道servlet處理get請求是doGet方法，所以我們去找DispatcherServlet類結構中的doGet方法。

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

  processRequest(request, response);
}

processRequest處理請求的方法如下：

/**
  * Process this request, publishing an event regardless of the outcome.
  * <p>The actual event handling is performed by the abstract
  * {@link #doService} template method.
  */
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {

  // 計算處理請求的時間
  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());

  // 初始化context
  initContextHolders(request, localeContext, requestAttributes);

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

  finally {
    // 重置context
    resetContextHolders(request, previousLocaleContext, previousAttributes);
    if (requestAttributes != null) {
      requestAttributes.requestCompleted();
    }
    logResult(request, response, failureCause, asyncManager);
    publishRequestHandledEvent(request, response, startTime, failureCause);
  }
}

本質上就是呼叫doService方法，由DispatchServlet類實現

/**
  * Exposes the DispatcherServlet-specific request attributes and delegates to {@link #doDispatch}
  * for the actual dispatching.
  */
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
  logRequest(request);

  // 儲存下請求之前的引數.
  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));
      }
    }
  }

  // 方便後續 handlers 和 view 要使用它們.
  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);
  }

  RequestPath previousRequestPath = null;
  if (this.parseRequestPath) {
    previousRequestPath = (RequestPath) request.getAttribute(ServletRequestPathUtils.PATH_ATTRIBUTE);
    ServletRequestPathUtils.parseAndCache(request);
  }

  try {
    // 看這裡，終於將這個請求分發出去了
    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);
      }
    }
    if (this.parseRequestPath) {
      ServletRequestPathUtils.setParsedRequestPath(previousRequestPath, request);
    }
  }
}

請求分發

doDispatch方法是真正處理請求的核心方法

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 {
      // 判斷是不是檔案上傳型別的request
      processedRequest = checkMultipart(request);
      multipartRequestParsed = (processedRequest != request);

      // 根據request獲取匹配的handler.
      mappedHandler = getHandler(processedRequest);
      if (mappedHandler == null) {
        noHandlerFound(processedRequest, response);
        return;
      }

      // 根據handler獲取匹配的handlerAdapter
      HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

      // 如果handler支援last-modified頭處理
      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;
        }
      }

      if (!mappedHandler.applyPreHandle(processedRequest, response)) {
        return;
      }

      // 真正handle處理，並返回modelAndView
      mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

      if (asyncManager.isConcurrentHandlingStarted()) {
        return;
      }

      // 通過檢視的prefix和postfix獲取完整的檢視名
      applyDefaultViewName(processedRequest, mv);

      // 應用後置的攔截器
      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);
    }

    // 處理handler處理的結果，顯然就是對ModelAndView 或者 出現的Excpetion處理
    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);
      }
    }
  }
}

對映和介面卡處理

對於真正的handle方法，我們看下其處理流程

/**
  * This implementation expects the handler to be an {@link HandlerMethod}.
  */
@Override
@Nullable
public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
    throws Exception {

  return handleInternal(request, response, (HandlerMethod) handler);
}

交給handleInternal方法處理，以RequestMappingHandlerAdapter這個HandlerAdapter中的處理方法為例

@Override
protected ModelAndView handleInternal(HttpServletRequest request,
    HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

  ModelAndView mav;
  checkRequest(request);

  // Execute invokeHandlerMethod in synchronized block if required.
  if (this.synchronizeOnSession) {
    HttpSession session = request.getSession(false);
    if (session != null) {
      Object mutex = WebUtils.getSessionMutex(session);
      synchronized (mutex) {
        mav = invokeHandlerMethod(request, response, handlerMethod);
      }
    }
    else {
      // No HttpSession available -> no mutex necessary
      mav = invokeHandlerMethod(request, response, handlerMethod);
    }
  }
  else {
    // No synchronization on session demanded at all...
    mav = invokeHandlerMethod(request, response, handlerMethod);
  }

  if (!response.containsHeader(HEADER_CACHE_CONTROL)) {
    if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
      applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
    }
    else {
      prepareResponse(response);
    }
  }

  return mav;
}

然後執行invokeHandlerMethod這個方法，用來對RequestMapping（usercontroller中的list方法）進行處理

/**
  * Invoke the {@link RequestMapping} handler method preparing a {@link ModelAndView}
  * if view resolution is required.
  * @since 4.2
  * @see #createInvocableHandlerMethod(HandlerMethod)
  */
@Nullable
protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
    HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

  ServletWebRequest webRequest = new ServletWebRequest(request, response);
  try {
    
    WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
    ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);

    // 重要：設定handler(controller#list)方法上的引數，返回值處理，繫結databinder等
    ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
    if (this.argumentResolvers != null) {
      invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
    }
    if (this.returnValueHandlers != null) {
      invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
    }
    invocableMethod.setDataBinderFactory(binderFactory);
    invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);

    ModelAndViewContainer mavContainer = new ModelAndViewContainer();
    mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
    modelFactory.initModel(webRequest, mavContainer, invocableMethod);
    mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);

    
    AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
    asyncWebRequest.setTimeout(this.asyncRequestTimeout);

    WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    asyncManager.setTaskExecutor(this.taskExecutor);
    asyncManager.setAsyncWebRequest(asyncWebRequest);
    asyncManager.registerCallableInterceptors(this.callableInterceptors);
    asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);

    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);
    }

    // 執行controller中方法
    invocableMethod.invokeAndHandle(webRequest, mavContainer);
    if (asyncManager.isConcurrentHandlingStarted()) {
      return null;
    }

    return getModelAndView(mavContainer, modelFactory, webRequest);
  }
  finally {
    webRequest.requestCompleted();
  }
}

invokeAndHandle交給UserController中具體執行list方法執行

後續invoke執行的方法，直接看整個請求流程的呼叫鏈即可

執行後獲得檢視和Model

檢視渲染

接下來繼續執行processDispatchResult方法，對檢視和model（如果有異常則對例外處理）進行處理（顯然就是渲染頁面了）

/**
  * Handle the result of handler selection and handler invocation, which is
  * either a ModelAndView or an Exception to be resolved to a ModelAndView.
  */
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
    @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
    @Nullable Exception exception) throws Exception {

  boolean errorView = false;

  // 如果處理過程有異常，則例外處理
  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);
    }
  }

  // 是否需要渲染檢視
  if (mv != null && !mv.wasCleared()) {
    render(mv, request, response); // 渲染檢視
    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..
    mappedHandler.triggerAfterCompletion(request, response, null);
  }
}

接下來顯然就是渲染檢視了, spring在initStrategies方法中初始化的元件（LocaleResovler等）就派上用場了。

/**
  * Render the given ModelAndView.
  * <p>This is the last stage in handling a request. It may involve resolving the view by name.
  * @param mv the ModelAndView to render
  * @param request current HTTP servlet request
  * @param response current HTTP servlet response
  * @throws ServletException if view is missing or cannot be resolved
  * @throws Exception if there's a problem rendering the view
  */
protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
  // Determine locale for request and apply it to the response.
  Locale locale =
      (this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
  response.setLocale(locale);

  View view;
  String viewName = mv.getViewName();
  if (viewName != null) {
    // We need to resolve the view name.
    view = resolveViewName(viewName, mv.getModelInternal(), locale, request);
    if (view == null) {
      throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
          "' in servlet with name '" + getServletName() + "'");
    }
  }
  else {
    // No need to lookup: the ModelAndView object contains the actual View object.
    view = mv.getView();
    if (view == null) {
      throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +
          "View object in servlet with name '" + getServletName() + "'");
    }
  }

  // Delegate to the View object for rendering.
  if (logger.isTraceEnabled()) {
    logger.trace("Rendering view [" + view + "] ");
  }
  try {
    if (mv.getStatus() != null) {
      response.setStatus(mv.getStatus().value());
    }
    view.render(mv.getModelInternal(), request, response);
  }
  catch (Exception ex) {
    if (logger.isDebugEnabled()) {
      logger.debug("Error rendering view [" + view + "]", ex);
    }
    throw ex;
  }
}

後續就是通過viewResolver進行解析了，這裡就不再繼續看程式碼了，上述流程基本上夠幫助你構建相關的認知了。

最後無非是返回控制權給DispatcherServlet，由DispatcherServlet 返回響應給使用者。

最後的最後我們看下請求的紀錄檔：