Spring MVC请求处理过程不是两张流程图就能讲清楚的

/** 文件上传解析器 */
private MultipartResolver multipartResolver;

/** 区域解析器，用于国际化 */
private LocaleResolver localeResolver;

/** 主题解析器 */
private ThemeResolver themeResolver;

/** Handler映射信息 */
private List handlerMappings;

/** Handler适配器*/
private List handlerAdapters;

/** Handler执行异常解析器 */
private List handlerExceptionResolvers;

/** 请求到视图的转换器 */
private RequestToViewNameTranslator viewNameTranslator;

/** SpringMVC允许重定向时携带参数，存在session中，用完就销毁，所以叫FlashMap */
private FlashMapManager flashMapManager;

/** 视图解析器 */
private List viewResolvers;

@Controller
public class IndexController {

    @RequestMapping("/index/home")
    public String home(String id, Student student, @RequestParam("code") String code) {
        System.out.println(student.getName());
        return "index";
    }

    @ResponseBody
    @RequestMapping("/index/list")
    public String list() {
        return "success";
    }
}

public class Student {

    private String name;
    private Integer gender;

   // getter、setter
}

/**
 * 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);

 // 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<>();
  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));
   }
  }
 }

 // Make framework objects available to handlers and view objects.
 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 {
  // 真正执行的方法
  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);
   }
  }
 }
}

/**
 * 实际处理请求的方法
 */
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 {
   // 校验是否是文件上传请求
   processedRequest = checkMultipart(request);
   multipartRequestParsed = (processedRequest != request);

   // Determine handler for the current request.
   // 为当前请求找到一个合适的处理器（Handler）
   // 返回值是一个HandlerExecutionChain，也就是处理器执行链
   mappedHandler = getHandler(processedRequest);
   if (mappedHandler == null) {
    noHandlerFound(processedRequest, response);
    return;
   }

   // Determine handler adapter for the current request.
   // 根据HandlerExecutionChain携带的Handler找到合适的HandlerAdapter
   HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

   // Process last-modified header, if supported by the handler.
   // 处理GET请求的缓存
   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;
    }
   }

   // 执行拦截器的preHandle方法
   if (!mappedHandler.applyPreHandle(processedRequest, response)) {
    return;
   }

   // Actually invoke the handler.
   // 利用HandlerAdapter来执行Handler里对应的处理方法
   mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

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

   // 如果没有设置视图，则应用默认的视图名
   applyDefaultViewName(processedRequest, mv);
   // 执行拦截器的postHandle方法
   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);
  }
  // 根据ModelAndView对象解析视图
  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);
   }
  }
 }
}

/**
 * Return the HandlerExecutionChain for this request.
 * 为这个request返回一个HandlerExecutionChain
 */
@Nullable
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;
}

/**
  * Return the HandlerAdapter for this handler object.
  * @param handler the handler object to find an adapter for
  * @throws ServletException if no HandlerAdapter can be found for the handler. This is a fatal error.
  */
 protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
  if (this.handlerAdapters != null) {
   for (HandlerAdapter adapter : this.handlerAdapters) {
    if (adapter.supports(handler)) {
     return adapter;
    }
   }
  }
  throw new ServletException("No adapter for handler [" + handler +
    "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
 }

/**
 * 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;

 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);
  }
 }

 // Did the handler return a view to render?
 if (mv != null && !mv.wasCleared()) {
  // 主要调用该方法渲染视图
  render(mv, request, response);
  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);
 }
}

/**
 * Render the given ModelAndView.
 * 
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.
  // 根据给定的视图名称，解析获取View对象
  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;
 }
}

@Nullable
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;
}

<bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
 <property name="prefix" value="/WEB-INF/views/" />
 <property name="suffix" value=".html" />
bean>

// Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

/**
 * 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);
}

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

 ModelAndView mav;
 // 校验指定的请求以获取受支持的方法类型（GET、POST等）和所需的session
 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...
  // 真正执行handler的方法
  mav = invokeHandlerMethod(request, response, handlerMethod);
 }

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

 return mav;
}

/**
 * Invoke the {@link RequestMapping} handler method preparing a {@link ModelAndView}
 * if view resolution is required.
 * 执行@RequestMapping标注的handler方法，如果需要解析视图就准备一个ModelAndView
 */
@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);

  // HandlerMethod接口封装执行方法的信息，提供对方法参数，方法返回值，方法注释等的便捷访问。
  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可以看做ModelAndView的上下文容器，关联着Model和View的信息
  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);
  }

  // 真正执行Handler的方法
  invocableMethod.invokeAndHandle(webRequest, mavContainer);
  if (asyncManager.isConcurrentHandlingStarted()) {
   return null;
  }

  // 获取ModelAndeView对象
  return getModelAndView(mavContainer, modelFactory, webRequest);
 }
 finally {
  webRequest.requestCompleted();
 }
}

/**
 * Invoke the method and handle the return value through one of the
 * configured {@link HandlerMethodReturnValueHandler HandlerMethodReturnValueHandlers}.
 * @param webRequest the current request
 * @param mavContainer the ModelAndViewContainer for this request
 * @param providedArgs "given" arguments matched by type (not resolved)
 */
public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
  Object... providedArgs) throws Exception {

 // 执行handler的方法
 Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
 setResponseStatus(webRequest);

 if (returnValue == null) {
  if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
   disableContentCachingIfNecessary(webRequest);
   mavContainer.setRequestHandled(true);
   return;
  }
 }
 else if (StringUtils.hasText(getResponseStatusReason())) {
  mavContainer.setRequestHandled(true);
  return;
 }

 mavContainer.setRequestHandled(false);
 Assert.state(this.returnValueHandlers != null, "No return value handlers");
 try {
  this.returnValueHandlers.handleReturnValue(
    returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
 }
 catch (Exception ex) {
  if (logger.isTraceEnabled()) {
   logger.trace(formatErrorForReturnValue(returnValue), ex);
  }
  throw ex;
 }
}

/**
 * Invoke the method after resolving its argument values in the context of the given request.
 * 
Argument values are commonly resolved through
 * {@link HandlerMethodArgumentResolver HandlerMethodArgumentResolvers}.
 * The {@code providedArgs} parameter however may supply argument values to be used directly,
 * i.e. without argument resolution. Examples of provided argument values include a
 * {@link WebDataBinder}, a {@link SessionStatus}, or a thrown exception instance.
 * Provided argument values are checked before argument resolvers.
 * 
Delegates to {@link #getMethodArgumentValues} and calls {@link #doInvoke} with the
 * resolved arguments.
 * @param request the current request
 * @param mavContainer the ModelAndViewContainer for this request
 * @param providedArgs "given" arguments matched by type, not resolved
 * @return the raw value returned by the invoked method
 * @throws Exception raised if no suitable argument resolver can be found,
 * or if the method raised an exception
 * @see #getMethodArgumentValues
 * @see #doInvoke
 */
@Nullable
public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
  Object... providedArgs) throws Exception {

 // 获取参数
 Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
 if (logger.isTraceEnabled()) {
  logger.trace("Arguments: " + Arrays.toString(args));
 }
 // 执行
 return doInvoke(args);
}

/**
 * Get the method argument values for the current request, checking the provided
 * argument values and falling back to the configured argument resolvers.
 * 
The resulting array will be passed into {@link #doInvoke}.
 * @since 5.1.2
 */
protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
  Object... providedArgs) throws Exception {

 // 获取方法上所有的参数
 MethodParameter[] parameters = getMethodParameters();
 if (ObjectUtils.isEmpty(parameters)) {
  return EMPTY_ARGS;
 }

 Object[] args = new Object[parameters.length];
 for (int i = 0; i < parameters.length; i++) {
  MethodParameter parameter = parameters[i];
  parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
  args[i] = findProvidedArgument(parameter, providedArgs);
  if (args[i] != null) {
   continue;
  }
  if (!this.resolvers.supportsParameter(parameter)) {
   throw new IllegalStateException(formatArgumentError(parameter, "No suitable resolver"));
  }
  try {
   
   args[i] = this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);
  }
  catch (Exception ex) {
   // Leave stack trace for later, exception may actually be resolved and handled...
   if (logger.isDebugEnabled()) {
    String exMsg = ex.getMessage();
    if (exMsg != null && !exMsg.contains(parameter.getExecutable().toGenericString())) {
     logger.debug(formatArgumentError(parameter, exMsg));
    }
   }
   throw ex;
  }
 }
 return args;
}

/**
 * Iterate over registered
 * {@link HandlerMethodArgumentResolver HandlerMethodArgumentResolvers}
 * and invoke the one that supports it.
 * @throws IllegalArgumentException if no suitable argument resolver is found
 */
@Override
@Nullable
public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
  NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {

 // 获取合适的参数解析器
 HandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);
 if (resolver == null) {
  throw new IllegalArgumentException("Unsupported parameter type [" +
    parameter.getParameterType().getName() + "]. supportsParameter should be called first.");
 }
 // 执行参数绑定
 return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
}

/**
 * Find a registered {@link HandlerMethodArgumentResolver} that supports
 * the given method parameter.
 */
@Nullable
private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) {
 HandlerMethodArgumentResolver result = this.argumentResolverCache.get(parameter);
 if (result == null) {
  for (HandlerMethodArgumentResolver resolver : this.argumentResolvers) {
   if (resolver.supportsParameter(parameter)) {
    result = resolver;
    this.argumentResolverCache.put(parameter, result);
    break;
   }
  }
 }
 return result;
}

/**
 * Resolve the argument from the model or if not found instantiate it with
 * its default if it is available. The model attribute is then populated
 * with request values via data binding and optionally validated
 * if {@code @java.validation.Valid} is present on the argument.
 * @throws BindException if data binding and validation result in an error
 * and the next method parameter is not of type {@link Errors}
 * @throws Exception if WebDataBinder initialization fails
 */
@Override
@Nullable
public final Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
  NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {

 Assert.state(mavContainer != null, "ModelAttributeMethodProcessor requires ModelAndViewContainer");
 Assert.state(binderFactory != null, "ModelAttributeMethodProcessor requires WebDataBinderFactory");

 // 获取参数名
 String name = ModelFactory.getNameForParameter(parameter);
 // 获取参数上的ModelAttribute注解
 ModelAttribute ann = parameter.getParameterAnnotation(ModelAttribute.class);
 if (ann != null) {
  mavContainer.setBinding(name, ann.binding());
 }

 Object attribute = null;
 BindingResult bindingResult = null;

 if (mavContainer.containsAttribute(name)) {
  attribute = mavContainer.getModel().get(name);
 }
 else {
  // Create attribute instance
  try {
   // 创建参数类型的实例（未注入值），底层就是通过反射调用构造方法
   attribute = createAttribute(name, parameter, binderFactory, webRequest);
  }
  catch (BindException ex) {
   if (isBindExceptionRequired(parameter)) {
    // No BindingResult parameter -> fail with BindException
    throw ex;
   }
   // Otherwise, expose null/empty value and associated BindingResult
   if (parameter.getParameterType() == Optional.class) {
    attribute = Optional.empty();
   }
   bindingResult = ex.getBindingResult();
  }
 }

 if (bindingResult == null) {
  // Bean property binding and validation;
  // skipped in case of binding failure on construction.
  WebDataBinder binder = binderFactory.createBinder(webRequest, attribute, name);
  if (binder.getTarget() != null) {
   if (!mavContainer.isBindingDisabled(name)) {
    // 真正执行绑定（值注入）的方法
    bindRequestParameters(binder, webRequest);
   }
   validateIfApplicable(binder, parameter);
   if (binder.getBindingResult().hasErrors() && isBindExceptionRequired(binder, parameter)) {
    throw new BindException(binder.getBindingResult());
   }
  }
  // Value type adaptation, also covering java.util.Optional
  if (!parameter.getParameterType().isInstance(attribute)) {
   attribute = binder.convertIfNecessary(binder.getTarget(), parameter.getParameterType(), parameter);
  }
  bindingResult = binder.getBindingResult();
 }

 // Add resolved attribute and BindingResult at the end of the model
 Map<String, Object> bindingResultModel = bindingResult.getModel();
 mavContainer.removeAttributes(bindingResultModel);
 mavContainer.addAllAttributes(bindingResultModel);

 return attribute;
}

/**
 * This implementation downcasts {@link WebDataBinder} to
 * {@link ServletRequestDataBinder} before binding.
 * @see ServletRequestDataBinderFactory
 */
@Override
protected void bindRequestParameters(WebDataBinder binder, NativeWebRequest request) {
 ServletRequest servletRequest = request.getNativeRequest(ServletRequest.class);
 Assert.state(servletRequest != null, "No ServletRequest");
 ServletRequestDataBinder servletBinder = (ServletRequestDataBinder) binder;
 // 执行绑定的方法
 servletBinder.bind(servletRequest);
}

public void bind(ServletRequest request) {
 // 获取所有参数的键值对
 MutablePropertyValues mpvs = new ServletRequestParameterPropertyValues(request);
 // 处理文件上传请求
 MultipartRequest multipartRequest = WebUtils.getNativeRequest(request, MultipartRequest.class);
 if (multipartRequest != null) {
  bindMultipart(multipartRequest.getMultiFileMap(), mpvs);
 }
 // 把url中携带的参数也加入到MutablePropertyValues
 addBindValues(mpvs, request);
 // 执行绑定（注入值）
 doBind(mpvs);
}

/**
 * Invoke the method and handle the return value through one of the
 * configured {@link HandlerMethodReturnValueHandler HandlerMethodReturnValueHandlers}.
 * @param webRequest the current request
 * @param mavContainer the ModelAndViewContainer for this request
 * @param providedArgs "given" arguments matched by type (not resolved)
 */
public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
  Object... providedArgs) throws Exception {

 Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
 setResponseStatus(webRequest);

 if (returnValue == null) {
  if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
   disableContentCachingIfNecessary(webRequest);
   mavContainer.setRequestHandled(true);
   return;
  }
 }
 else if (StringUtils.hasText(getResponseStatusReason())) {
  mavContainer.setRequestHandled(true);
  return;
 }

 mavContainer.setRequestHandled(false);
 Assert.state(this.returnValueHandlers != null, "No return value handlers");
 try {
  // 真正处理不同类型返回值的方法
  this.returnValueHandlers.handleReturnValue(
    returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
 }
 catch (Exception ex) {
  if (logger.isTraceEnabled()) {
   logger.trace(formatErrorForReturnValue(returnValue), ex);
  }
  throw ex;
 }
}

/**
 * Iterate over registered {@link HandlerMethodReturnValueHandler HandlerMethodReturnValueHandlers} and invoke the one that supports it.
 * @throws IllegalStateException if no suitable {@link HandlerMethodReturnValueHandler} is found.
 */
@Override
public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
  ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {

 // 根据返回值个返回值类型选取合适的HandlerMethodReturnValueHandler
 HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
 if (handler == null) {
  throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
 }
 // 真正的处理返回值
 handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
}

@Nullable
private HandlerMethodReturnValueHandler selectHandler(@Nullable Object value, MethodParameter returnType) {
 boolean isAsyncValue = isAsyncReturnValue(value, returnType);
 for (HandlerMethodReturnValueHandler handler : this.returnValueHandlers) {
  if (isAsyncValue && !(handler instanceof AsyncHandlerMethodReturnValueHandler)) {
   continue;
  }
  if (handler.supportsReturnType(returnType)) {
   return handler;
  }
 }
 return null;
}

@Override
public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
  ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
  throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {

 // 设置该请求是否已在处理程序中完全处理，例如@ResponseBody方法不需要视图解析器，此处就可以设置为true。
 // 当控制器方法声明类型为ServletResponse或OutputStream的参数时，也可以设置此标志为true。
 // 这个属性设置成true之后，上层getModelAndView获取ModelAndView时会返回Null，因为不需要视图。
 // 默认值为false
 mavContainer.setRequestHandled(true);
 ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
 ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);

 // Try even with null return value. ResponseBodyAdvice could get involved.
 // 底层就是利用java.io.OutputStreamWriter类把返回值写到网络IO
 writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
}