Spring框架系列(14) SpringMVC实现原理之DispatcherServlet处理请求的过程

前文我们有了IOC的源码基础以及SpringMVC的基础，我们便可以进一步深入理解SpringMVC主要实现原理，包含DispatcherServlet的初始化过程和DispatcherServlet处理请求的过程的源码解析。本文是第二篇：DispatcherServlet处理请求的过程的源码解析。@pdai

Spring框架系列(14) - SpringMVC实现原理之DispatcherServlet处理请求的过程
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DispatcherServlet处理请求的过程？

一个请求发出，经过DispatcherServlet进行了什么样的处理，最后将内容返回的呢？

回顾整理处理流程

首先让我们整体看一下Spring Web MVC 处理请求的流程：

核心架构的具体流程步骤如下：

首先用户发送请求——>DispatcherServlet，前端控制器收到请求后自己不进行处理，而是委托给其他的解析器进行
处理，作为统一访问点，进行全局的流程控制；
DispatcherServlet——>HandlerMapping， HandlerMapping 将会把请求映射为 HandlerExecutionChain 对象（包含一
个Handler 处理器（页面控制器）对象、多个HandlerInterceptor 拦截器）对象，通过这种策略模式，很容易添加新
的映射策略；
DispatcherServlet——>HandlerAdapter，HandlerAdapter 将会把处理器包装为适配器，从而支持多种类型的处理器，
即适配器设计模式的应用，从而很容易支持很多类型的处理器；
HandlerAdapter——>处理器功能处理方法的调用，HandlerAdapter 将会根据适配的结果调用真正的处理器的功能处
理方法，完成功能处理；并返回一个ModelAndView 对象（包含模型数据、逻辑视图名）；
ModelAndView 的逻辑视图名——> ViewResolver，ViewResolver 将把逻辑视图名解析为具体的View，通过这种策
略模式，很容易更换其他视图技术；
View——>渲染，View 会根据传进来的Model 模型数据进行渲染，此处的Model 实际是一个Map 数据结构，因此
很容易支持其他视图技术；
返回控制权给DispatcherServlet，由DispatcherServlet 返回响应给用户，到此一个流程结束。

doGet入口

我们以上个demo中这个GET请求为例，请求URL是http://localhost:8080/011_spring_framework_demo_springmvc_war_exploded/user

我们知道servlet处理get请求是doGet方法，所以我们去找DispatcherServlet类结构中的doGet方法。

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

  processRequest(request, response);
}

processRequest处理请求的方法如下：

/**
  * 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 {

  // 计算处理请求的时间
  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 {
    // 看这里
    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);
  }
}

本质上就是调用doService方法，由DispatchServlet类实现

/**
  * 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);

  // 保存下请求之前的参数.
  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));
      }
    }
  }

  // 方便后续 handlers 和 view 要使用它们.
  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 {
    // 看这里，终于将这个请求分发出去了
    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);
    }
  }
}

请求分发

doDispatch方法是真正处理请求的核心方法

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 {
      // 判断是不是文件上传类型的request
      processedRequest = checkMultipart(request);
      multipartRequestParsed = (processedRequest != request);

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

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

      // 如果handler支持last-modified头处理
      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处理，并返回modelAndView
      mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

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

      // 通过视图的prefix和postfix获取完整的视图名
      applyDefaultViewName(processedRequest, mv);

      // 应用后置的拦截器
      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);
    }

    // 处理handler处理的结果，显然就是对ModelAndView 或者 出现的Excpetion处理
    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);
      }
    }
  }
}

映射和适配器处理

对于真正的handle方法，我们看下其处理流程

/**
  * 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);
}

交给handleInternal方法处理，以RequestMappingHandlerAdapter这个HandlerAdapter中的处理方法为例

@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;
}

然后执行invokeHandlerMethod这个方法，用来对RequestMapping（usercontroller中的list方法）进行处理

/**
  * 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：设置handler(controller#list)方法上的参数，返回值处理，绑定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);
    }

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

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

invokeAndHandle交给UserController中具体执行list方法执行

后续invoke执行的方法，直接看整个请求流程的调用链即可

执行后获得视图和Model

视图渲染

接下来继续执行processDispatchResult方法，对视图和model（如果有异常则对异常处理）进行处理（显然就是渲染页面了）

/**
  * 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;

  // 如果处理过程有异常，则异常处理
  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);
    }
  }

  // 是否需要渲染视图
  if (mv != null && !mv.wasCleared()) {
    render(mv, request, response); // 渲染视图
    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);
  }
}

接下来显然就是渲染视图了, spring在initStrategies方法中初始化的组件（LocaleResovler等）就派上用场了。

/**
  * 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;
  }
}

后续就是通过viewResolver进行解析了，这里就不再继续看代码了，上述流程基本上够帮助你构建相关的认知了。

最后无非是返回控制权给DispatcherServlet，由DispatcherServlet 返回响应给用户。

最后的最后我们看下请求的日志：

21:45:53.390 [http-nio-8080-exec-6] DEBUG org.springframework.web.servlet.DispatcherServlet - GET "/011_spring_framework_demo_springmvc_war_exploded/user", parameters={}
21:45:53.400 [http-nio-8080-exec-6] DEBUG org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping - Mapped to tech.pdai.springframework.springmvc.controller.UserController#list(HttpServletRequest, HttpServletResponse)
22:51:14.504 [http-nio-8080-exec-6] DEBUG org.springframework.web.servlet.view.JstlView - View name 'userList', model {dateTime=Fri Apr 22 21:45:53 CST 2022, userList=[tech.pdai.springframework.springmvc.entity.User@7b8c8dc]}
22:51:14.550 [http-nio-8080-exec-6] DEBUG org.springframework.web.servlet.view.JstlView - Forwarding to [/WEB-INF/views/userList.jsp]
22:51:44.395 [http-nio-8080-exec-6] DEBUG org.springframework.web.servlet.DispatcherServlet - Completed 200 OK