关于后端:Spring-MVC请求处理过程你这样回答保证通过面试

本篇应用的 Spring 版本为 5.2.2.RELEASE

九大组件

SpringMVC 简直所有的性能都由九大组件来实现，所以明确九大组件的作用，对于学习 SpringMVC 来说十分重要。

/** 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<HandlerMapping> handlerMappings;

/** Handler 适配器 */
private List<HandlerAdapter> handlerAdapters;

/** Handler 执行异样解析器 */
private List<HandlerExceptionResolver> handlerExceptionResolvers;

/** 申请到视图的转换器 */
private RequestToViewNameTranslator viewNameTranslator;

/** SpringMVC 容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<ViewResolver> viewResolvers;
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• HandlerMapping：Handler 映射信息，依据申请携带的 url 信息查找处理器（Handler）。每个申请都须要对应的 Handler 解决。
• HandlerAdapter：Handler 适配器，SpringMVC 没有间接调用处理器（Handler），而是通过 HandlerAdapter 来调用，次要是为了对立 Handler 的调用形式
• ViewResolver：视图解析器，用来将字符串类型的视图名称解析为 View 类型的视图。ViewResolver 须要找到渲染所用的模板和所用的技术（也就是视图的类型）进行渲染，具体的渲染过程则交由不同的视图本人实现。
• MultipartResolver：文件上传解析器，次要用来解决文件上传申请
• HandlerExceptionResolver：Handler 执行异样解析器，用来对异样进行对立解决
• RequestToViewNameTranslator：申请到视图的转换器
• LocaleResolver：区域解析器，用于反对国际化
• FlashMapManager：SpringMVC 容许重定向时携带参数，存在 session 中，用完就销毁，所以叫 FlashMap
• ThemeResolver：主题解析器，用于反对不同的主题

九大组件中最重的的前三个，HandlerMapping、HandlerAdapter 和 ViewResolver，因为这是浏览源码时，避不开的三个组件。
《2020 最新 Java 根底精讲视频教程和学习路线！》

调试筹备

搭建一个根本的 Spring web 我的项目即可

Controller 局部

@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";}
}
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Entity 局部

public class Student {

    private String name;
    private Integer gender;

   // getter、setter
}
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还是那句话，Spring 源码十分宏大，不能只见树木不见森林，须要有针对性的浏览，所以本篇只须要关注主体流程即可。

外围办法

咱们都晓得，SpringMVC 有一个用来散发申请的前端控制器 DispatcherServlet，其中用来解决申请的办法就是 doService，该办法定义如下

doService

/**
 * 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 {
  // 真正执行的办法
  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);
   }
  }
 }
}
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doDispatch

doDispatch 是 doService 中真正用来解决申请的办法

/**
 * 理论解决申请的办法
 */
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申请
   processedRequest = checkMultipart(request);
   multipartRequestParsed = (processedRequest != request);

   // Determine handler for the current request.
   // 为以后申请找到一个适合的处理器（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.
   // 依据 HandlerExecutionChain 携带的 Handler 找到适合的 HandlerAdapter
   HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

   // Process last-modified header, if supported by the handler.
   // 解决 GET 申请的缓存
   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 办法
   if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}

   // Actually invoke the handler.
   // 利用 HandlerAdapter 来执行 Handler 里对应的解决办法
   mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

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

   // 如果没有设置视图，则利用默认的视图名
   applyDefaultViewName(processedRequest, mv);
   // 执行拦截器的 postHandle 办法
   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);
  }
  // 依据 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);
   }
  }
 }
}
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该办法就是 SpringMVC 解决申请的整体流程，其中波及到几个重要的办法。

getHandler

该办法定义如下

/**
 * 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;
}
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调试信息如下

依据调试信息能够看出，getHandler 办法次要是从 List handlerMappings 汇合中遍历查找一个适合的处理器（Handler），返回的后果是一个 HandlerExecutionChain。而后再依据 HandlerExecutionChain 里携带的 Handler 去获取 HandlerAdapter。

getHandlerAdapter

getHandlerAdapter 办法定义如下

/**
  * 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");
 }
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调试信息如下

同样 getHandlerAdapter 办法次要是从 List handlerAdapters 汇合中遍历查找一个适合的处理器适配器（HandlerAdapter），返回的后果是一个 HandlerAdapter。

能够看到此处 HandlerAdapter 真正的实现类是 RequestMappingHandlerAdapter。

processDispatchResult

processDispatchResult 办法次要依据办法执行实现后封装的 ModelAndView，转发到对应页面，定义如下

/**
 * 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()) {
  // 次要调用该办法渲染视图
  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);
 }
}
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render

render 办法定义如下

/**
 * 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.
  // 依据给定的视图名称，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;
 }
}
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resolveViewName

resolveViewName 办法定义如下

@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;
}
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调试信息如下

依据调试信息能够看到真正解析视图的 ViewResolver 的是 InternalResourceViewResolver 类，也就是咱们常常配置的一项类型

<!-- 定义视图文件解析 -->
<bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
 <property name="prefix" value="/WEB-INF/views/" />
 <property name="suffix" value=".html" />
</bean>
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至此咱们就失去了 SpringMVC 解决申请的残缺逻辑

SpringMVC 解决申请的整个流程曾经梳理分明了。

然而，有两个重要的问题没有解决，那就是：参数绑定和返回值解决。

因为在编写 Controller 外面的办法的时候，各种类型的参数都有，SpringMVC 是怎么解决不同类型的参数的呢？> SpringMVC 解决申请实现后，肯定会返回 ModelAndView 吗，如果加了 @ResponseBody 注解呢？

参数绑定

在整个流程中，还有一个最重要的办法，那就是真正执行 handler 的办法，参数的绑定和返回值的解决都在这个办法里，也就是

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

handle 办法的作用是依据申请参数，执行真正的解决办法，并且返回适合的 ModelAndView 对象，也有可能返回 null。该办法定义如下 在 AbstractHandlerMethodAdapter 类中

/**
 * 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);
}
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能够看到这个办法实现只有一行代码

handleInternal

持续深刻 handleInternal 办法

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

 ModelAndView mav;
 // 校验指定的申请以获取受反对的办法类型（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 的办法
  mav = invokeHandlerMethod(request, response, handlerMethod);
 }

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

 return mav;
}
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invokeHandlerMethod

持续深刻 invokeHandlerMethod 办法

/**
 * Invoke the {@link RequestMapping} handler method preparing a {@link ModelAndView}
 * if view resolution is required.
 * 执行 @RequestMapping 标注的 handler 办法，如果须要解析视图就筹备一个 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 接口封装执行办法的信息，提供对办法参数，办法返回值，办法正文等的便捷拜访。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 能够看做 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 的办法
  invocableMethod.invokeAndHandle(webRequest, mavContainer);
  if (asyncManager.isConcurrentHandlingStarted()) {return null;}

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

invokeAndHandle 办法的作用是执行并解决真正响应申请的办法，该办法定义如下

/**
 * 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 的办法
 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;
 }
}
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invokeForRequest

/**
 * Invoke the method after resolving its argument values in the context of the given request.
 * <p>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.
 * <p>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);
}
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真正的执行无非就是通过反射 invoke，所以更重要的是参数是如何绑定的，详情就在 getMethodArgumentValues 办法

getMethodArgumentValues

getMethodArgumentValues 办法用于从 request 申请中获取真正的参数，返回的是 Object 数组，该办法定义如下

/**
 * Get the method argument values for the current request, checking the provided
 * argument values and falling back to the configured argument resolvers.
 * <p>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 {

 // 获取办法上所有的参数
 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;
}
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依据调试信息能够看到，用来解决申请参数的类是 HandlerMethodArgumentResolver 接口的实现类 HandlerMethodArgumentResolverComposite，此时正在解决的参数是一个 Student 对象，并且曾经把值注绑定了，也就是说真正执行绑定的是办法 resolveArgument

resolveArgument

resolveArgument 是真正执行绑定的的办法

/**
 * 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 {

 // 获取适合的参数解析器
 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);
}
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getArgumentResolver

getArgumentResolver 该办法用于执行参数的绑定，定义如下

/**
 * 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;
}
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该办法的逻辑就是先从 argumentResolver 缓存中找到可能执行参数绑定的 HandlerMethodArgumentResolver，如果找不到就从 HandlerMethodArgumentResolver 找，SpringMVC 反对的 HandlerMethodArgumentResolver 一共有 26 种，用来解析各种类型的参数

依据博主的调试能够晓得

• RequestParamMethodArgumentResolver：解决一般参数（根本类型、包装类型、String），不论加不加 @RequestParam 注解
• ServletModelAttributeMethodProcessor：解决 POJO 类型的参数，比方自定义的 Student 对象
• RequestResponseBodyMethodProcessor：解决 @RequestBody 注解类型的参数

resolveArgument

因为不同类型的参数有不同的 HandlerMethodArgumentResolver 来解决，此处选取 POJO 类型参数的注入实现，对应的参数解析类是 ModelAttributeMethodProcessor，其中 resolveArgument 办法用来解析（绑定）参数办法定义如下

/**
 * 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 {
   // 创立参数类型的实例（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）的办法
    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;
}
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依据调试信息也能够看到 bindRequestParameters(binder, webRequest) 执行实现之后，POJO 类型的参数曾经实现了绑定。

bindRequestParameters

/**
 * 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;
 // 执行绑定的办法
 servletBinder.bind(servletRequest);
}
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bind

持续深刻 bind 办法

public void bind(ServletRequest request) {
 // 获取所有参数的键值对
 MutablePropertyValues mpvs = new ServletRequestParameterPropertyValues(request);
 // 解决文件上传申请
 MultipartRequest multipartRequest = WebUtils.getNativeRequest(request, MultipartRequest.class);
 if (multipartRequest != null) {bindMultipart(multipartRequest.getMultiFileMap(), mpvs);
 }
 // 把 url 中携带的参数也退出到 MutablePropertyValues
 addBindValues(mpvs, request);
 // 执行绑定（注入值）doBind(mpvs);
}
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因为调用档次过深，所以无奈一步步列出上面的步骤，doBind 办法的原理还是通过调用 POJO 对象里的 setter 办法设置值，能够查看最终的调试信息

依据调试信息能够看到，最终执行的还是 POJO 对象的 setter 办法，具体执行的类是 BeanWrapperImpl。

理解了参数的绑定，再来看返回值的解决。

返回值解决

invokeAndHandle

回到源码 invokeAndHandle 办法处（ServletInvocableHandlerMethod 类中），该办法定义如下

/**
 * 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 {
  // 真正解决不同类型返回值的办法
  this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
 }
 catch (Exception ex) {if (logger.isTraceEnabled()) {logger.trace(formatErrorForReturnValue(returnValue), ex);
  }
  throw ex;
 }
}
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真正解决不同类型的返回值的办法是 handleReturnValue 办法

handleReturnValue

/**
 * 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 {

 // 依据返回值个返回值类型选取适合的 HandlerMethodReturnValueHandler
 HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
 if (handler == null) {throw new IllegalArgumentException("Unknown return value type:" + returnType.getParameterType().getName());
 }
 // 真正的解决返回值
 handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
}
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selectHandler

@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;
}
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依据调试信息能够看到，SpringMVC 为返回值提供了 15 个 HandlerMethodReturnValueHandler 的实现了来解决不同类型的返回值。

事实上，用来解决 @ResponseBody 类型的是 RequestResponseBodyMethodProcessor。

如果对前文参数绑定还有印象的话，会发现 @RequestBody 类型参数绑定也是用的这个类。

持续跟进 RequestResponseBodyMethodProcessor 类的 handleReturnValue 办法

handleReturnValue

RequestResponseBodyMethodProcessor 类的 handleReturnValue 办法定义如下

这里设置了一个十分重要的属性 requestHandled，这个属性关系到是否须要返回 ModelAndView 对象

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

 // 设置该申请是否已在处理程序中齐全解决，例如 @ResponseBody 办法不须要视图解析器，此处就能够设置为 true。// 当控制器办法申明类型为 ServletResponse 或 OutputStream 的参数时，也能够设置此标记为 true。// 这个属性设置成 true 之后，下层 getModelAndView 获取 ModelAndView 时会返回 Null，因为不须要视图。// 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);
}
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持续深刻 writeWithMessageConverters 办法，一步步调试到最初，底层就是利用 java.io.OutputStreamWriter 类把返回值写到网络 IO

因为 handleReturnValue 把 requestHandled 设置成了 true，下层在调用 getModelAndView 办法时会返回 null，示意该申请不须要视图。感兴趣的同学本人调试一下便知。

总结

本文次要从源码的浏览和调试的角度，整体的解说了 SpringMVC 解决申请的整个流程，并且解说了参数的绑定以及返回值的解决。置信大家看完后，联合本人的调试信息，会对 SpringMVC 的申请处理过程有一个更深刻的了解。

链接：https://juejin.cn/post/690348…