SpringMVC中对返回值的数据基本分为两类：

1、响应数据

2、响应页面

一、响应数据

响应数据大多数都是将返回值的格式转换为JSON格式然后展示在页面或者保存i起来。

第一步：在SpringBoot中需要引入json场景

<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-json</artifactId><version>2.3.4.RELEASE</version><scope>compile</scope></dependency>

第二步：开启@ResponseBody注解

之后再写学习记录。

二、响应页面

第一个疑问：再创建Springboot项目时，spring Initializr说连接不到 URL，所以用Maven创建WebApp。但是之后写Demo的时候发现再SpringBoot项目下，webapp/WEB-INF/templates是查询不到的，报500错误，只能再resource下写templates才能访问页面，通过源码学习看看是为什么。

Demo：

控制器：

@Controller
public class helloController {@GetMapping("/index")public String helloTest(Model model){String msg = "thymeleaf渲染了";model.addAttribute("msg",msg);return "test";}
}

HTML页面：

<body><p th:text="${msg}">thymeleaf没渲染！！！！</p>
</body>

结果：

1、源码学习

这一段中有上面是学习过的参数解析器，下面是返回值解析器

这个返回值解析器中共有15种，之前响应数据并转换为JSON格式返回给页面的就是序号为11的解析器。

然后到执行这一步

这里获取到了返回值

所以还是看这个方法：

    @Nullablepublic Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {Object[] args = this.getMethodArgumentValues(request, mavContainer, providedArgs);if (this.logger.isTraceEnabled()) {this.logger.trace("Arguments: " + Arrays.toString(args));}return this.doInvoke(args);}

这个方法内部为：第一行获取控制器方法中的参数

最后通过反射调用控制器的方法。（再方法体内打了个断点）

public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {Object returnValue = this.invokeForRequest(webRequest, mavContainer, providedArgs);
......try {this.returnValueHandlers.handleReturnValue(returnValue, this.getReturnValueType(returnValue), mavContainer, webRequest);} catch (Exception var6) {if (this.logger.isTraceEnabled()) {this.logger.trace(this.formatErrorForReturnValue(returnValue), var6);}......}}

接着就是再try中去处理返回值，看看handleReturnValue方法

public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType, ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {HandlerMethodReturnValueHandler handler = this.selectHandler(returnValue, returnType);if (handler == null) {throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());} else {handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);}}

执行第一步之后我们得到了返回值解析器为ViewNameMethod...Handler，说明使用Thymeleaf响应页面的时候都是使用该解析器。

这个选择过程和之前选择对应的解析器的方法是一样的，都是一个接口，一个判断，一个执行。

接着找到了返回值解析器之后，会执行这个方法

这个方法内部为：

    public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType, ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {if (returnValue instanceof CharSequence) {String viewName = returnValue.toString();mavContainer.setViewName(viewName);if (this.isRedirectViewName(viewName)) {mavContainer.setRedirectModelScenario(true);}} else if (returnValue != null) {throw new UnsupportedOperationException("Unexpected return type: " + returnType.getParameterType().getName() + " in method: " + returnType.getMethod());}}

可以看到这里是判断你是不是重定向的场景的，如果是重定向就再这里会处理。目前的返回值只是一个test，通过thymeleaf渲染为 xx/test.html，所以不经过这一步。

执行完了之后mavContainer中包含了model和view

mavContainer中将给model中设置的值以及最后响应的view都保存再这里了。

并且，如果方法中的参数也是一个自定义类型的对象，也会再这了放到mavContainer中。

最后赋给变量var15也是这些数据

之后再DispatcherServlet中获取到了model 和view中的值

applyDefaultViewName是如果你返回值是空的，就给你设置一个默认的视图地址，这个默认的地址就是一开始访问的地址。比如说访问的url为 "/login" 但返回值为空，默认的视图地址就还是 “/login”。

之后就是再这里处理派发结果，也就是视图解析的原理：

方法内部如下：

    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) {this.logger.debug("ModelAndViewDefiningException encountered", exception);mv = ((ModelAndViewDefiningException)exception).getModelAndView();} else {Object handler = mappedHandler != null ? mappedHandler.getHandler() : null;mv = this.processHandlerException(request, response, handler, exception);errorView = mv != null;}}if (mv != null && !mv.wasCleared()) {this.render(mv, request, response);if (errorView) {WebUtils.clearErrorRequestAttributes(request);}} else if (this.logger.isTraceEnabled()) {this.logger.trace("No view rendering, null ModelAndView returned.");}if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {if (mappedHandler != null) {mappedHandler.triggerAfterCompletion(request, response, (Exception)null);}}}

其中先判断是否有异常或者有啥问题，没有的话就会执行这个render

render方法内部：

    protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {Locale locale = this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale();response.setLocale(locale);String viewName = mv.getViewName();View view;if (viewName != null) {view = this.resolveViewName(viewName, mv.getModelInternal(), locale, request);if (view == null) {throw new ServletException("Could not resolve view with name '" + mv.getViewName() + "' in servlet with name '" + this.getServletName() + "'");}} else {view = mv.getView();if (view == null) {throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a View object in servlet with name '" + this.getServletName() + "'");}}if (this.logger.isTraceEnabled()) {this.logger.trace("Rendering view [" + view + "] ");}try {if (mv.getStatus() != null) {response.setStatus(mv.getStatus().value());}view.render(mv.getModelInternal(), request, response);} catch (Exception var8) {if (this.logger.isDebugEnabled()) {this.logger.debug("Error rendering view [" + view + "]", var8);}throw var8;}}

可以看到如果 mv.getViewName()获取到的值不为空就会执行这个解析视图名称的过程。

接着再resolveViewName中，有一个视图解析器viewResolvers，也是springMVC准备好的。

    @Nullableprotected View resolveViewName(String viewName, @Nullable Map<String, Object> model, Locale locale, HttpServletRequest request) throws Exception {if (this.viewResolvers != null) {Iterator var5 = this.viewResolvers.iterator();while(var5.hasNext()) {ViewResolver viewResolver = (ViewResolver)var5.next();View view = viewResolver.resolveViewName(viewName, locale);if (view != null) {return view;}}}return null;}

通过这个视图解析器看哪个匹配获取到view对象。

最后选择的解析器是ContentNegotiatingViewResolver，在这个解析器的基础下选择了Thymeleaf视图对象。

现在具体看看是怎么获取到这个视图对象的。

调用了ContentNegotiatingViewResolver.resolveViewName这个方法

    @Nullablepublic View resolveViewName(String viewName, Locale locale) throws Exception {.......if (requestedMediaTypes != null) {List<View> candidateViews = this.getCandidateViews(viewName, locale, requestedMediaTypes);View bestView = this.getBestView(candidateViews, requestedMediaTypes, attrs);if (bestView != null) {return bestView;}}

这里有获取候选解析器和选择一个最合适的解析器。

    private List<View> getCandidateViews(String viewName, Locale locale, List<MediaType> requestedMediaTypes) throws Exception {List<View> candidateViews = new ArrayList();if (this.viewResolvers != null) {Assert.state(this.contentNegotiationManager != null, "No ContentNegotiationManager set");Iterator var5 = this.viewResolvers.iterator();while(var5.hasNext()) {ViewResolver viewResolver = (ViewResolver)var5.next();View view = viewResolver.resolveViewName(viewName, locale);if (view != null) {candidateViews.add(view);}......}

最后有两个满足：

所以先看看ThymeleafResolver.resolveViewName

    @Nullablepublic View resolveViewName(String viewName, Locale locale) throws Exception {if (!this.isCache()) {return this.createView(viewName, locale);} else {Object cacheKey = this.getCacheKey(viewName, locale);View view = (View)this.viewAccessCache.get(cacheKey);if (view == null) {synchronized(this.viewCreationCache) {view = (View)this.viewCreationCache.get(cacheKey);if (view == null) {view = this.createView(viewName, locale);if (view == null && this.cacheUnresolved) {view = UNRESOLVED_VIEW;}if (view != null && this.cacheFilter.filter(view, viewName, locale)) {this.viewAccessCache.put(cacheKey, view);this.viewCreationCache.put(cacheKey, view);}}}} else if (this.logger.isTraceEnabled()) {this.logger.trace(formatKey(cacheKey) + "served from cache");}return view != UNRESOLVED_VIEW ? view : null;}}

这里创建了一个View ，具体代码为：

   protected View createView(String viewName, Locale locale) throws Exception {if (!this.alwaysProcessRedirectAndForward && !this.canHandle(viewName, locale)) {vrlogger.trace("[THYMELEAF] View \"{}\" cannot be handled by ThymeleafViewResolver. Passing on to the next resolver in the chain.", viewName);return null;} else {String forwardUrl;if (viewName.startsWith("redirect:")) {vrlogger.trace("[THYMELEAF] View \"{}\" is a redirect, and will not be handled directly by ThymeleafViewResolver.", viewName);forwardUrl = viewName.substring("redirect:".length(), viewName.length());RedirectView view = new RedirectView(forwardUrl, this.isRedirectContextRelative(), this.isRedirectHttp10Compatible());return (View)this.getApplicationContext().getAutowireCapableBeanFactory().initializeBean(view, viewName);} else if (viewName.startsWith("forward:")) {vrlogger.trace("[THYMELEAF] View \"{}\" is a forward, and will not be handled directly by ThymeleafViewResolver.", viewName);forwardUrl = viewName.substring("forward:".length(), viewName.length());return new InternalResourceView(forwardUrl);} else if (this.alwaysProcessRedirectAndForward && !this.canHandle(viewName, locale)) {vrlogger.trace("[THYMELEAF] View \"{}\" cannot be handled by ThymeleafViewResolver. Passing on to the next resolver in the chain.", viewName);return null;} else {vrlogger.trace("[THYMELEAF] View {} will be handled by ThymeleafViewResolver and a {} instance will be created for it", viewName, this.getViewClass().getSimpleName());return this.loadView(viewName, locale);}}}

先判断能否处理，然后判断是不是以重定向开头或者是转发开头的，目前的请求是"/test"，所以都不是，所以直接进入else这里加载视图，最后返回了了一个ThymeleafView对象。

返回的第二个对象叫内部资源视图，这个暂时不清楚怎么用，先空下来。

然后再这两个view对象选最合适的最终获取到了ThymeleafView对象，最后响应页面也就是前面提到了使用 view对象的render方法。

也就是在