SpringMvc渲染视图

这篇博文讨论的问题是从ModelAndView如何渲染到页面。

首先要知道每个请求处理完之后都会返回一个ModelAndView对象。

这里我分6种情况来分析，代表6种返回类型：

1. ModelAndView
2. Map，ModelMap
3. Model
4. View
5. String
6. Void

我先贴出我的测试的后台代码：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xmlns:mvc="http://www.springframework.org/schema/mvc"
    xsi:schemaLocation="http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-4.0.xsd
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.0.xsd">
    <!-- 配置扫描的包 -->
    <context:component-scan base-package="com"></context:component-scan>
    
    
    <!-- 配置视图解析器，将方法的返回值映射到一个实际的物理视图 -->
    <bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="prefix" value="/pages/result/"></property>
        <property name="suffix" value=".jsp"></property>
    </bean>
    <mvc:default-servlet-handler/>
    <mvc:annotation-driven></mvc:annotation-driven>
 
  
</beans>

package com.mmc.modelandview;

import java.util.Date;
import java.util.Map;

import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.SessionAttributes;
import org.springframework.web.servlet.ModelAndView;
import org.springframework.web.servlet.View;
import org.springframework.web.servlet.view.InternalResourceView;
import org.springframework.web.servlet.view.jasperreports.JasperReportsPdfView;

import com.mmc.common.CommonParam;


@Controller
public class TestModelAndView {
    
    @RequestMapping("testModelAndView")
    public ModelAndView testModelAndView(){
        ModelAndView modelAndView=new ModelAndView(CommonParam.SUCCESS);
        modelAndView.addObject("time", new Date());
        return modelAndView;
    }
    
    
    
    @RequestMapping("testModel")
    public String testModel(Model model){
        model.addAttribute("time",new Date());
        return CommonParam.SUCCESS;
    }
    
    @RequestMapping("testMap")
    public String testMap(Map<String,Object> map){
        map.put("person", "lixiaolu");
        return  CommonParam.SUCCESS;
    }
    
    
    @RequestMapping("testView")
    public View testView(View view){
        view=new JasperReportsPdfView();
        return view;
    }
    
    
    @RequestMapping("testString")
    public String testString(){
        return CommonParam.SUCCESS;
    } 
    
    @RequestMapping("testForward")
    public String testForward(){
        return "forward:/hello";
    }
    
    @RequestMapping("testVoid")
    public void testVoid(){
        System.out.println("执行testVoid方法");
    }
      
}

第一种：返回值是ModelAndView类型：

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 {
                processedRequest = checkMultipart(request);
                multipartRequestParsed = processedRequest != request;

                // Determine handler for the current request.
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null || mappedHandler.getHandler() == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // Determine handler adapter for the current request.
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                // Process last-modified header, if supported by the handler.
                String method = request.getMethod();
                boolean isGet = "GET".equals(method);
                if (isGet || "HEAD".equals(method)) {
                    long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                    if (logger.isDebugEnabled()) {
                        String requestUri = urlPathHelper.getRequestUri(request);
                        logger.debug("Last-Modified value for [" + requestUri + "] is: " + lastModified);
                    }
                    if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                        return;
                    }
                }

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

                try {
                    // Actually invoke the handler.
                    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                }
                finally {
                    if (asyncManager.isConcurrentHandlingStarted()) {
                        return;
                    }
                }
　　　　　　　　   //检查view有没有设置，如果没有给他设置一个默认的
                applyDefaultViewName(request, mv);
　　　　　　　　   //传给我们的拦截器，也就是说在拦截器里我们可以操作视图的映射
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
　　　　　　　//就是去处理映射了
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) {
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Error err) {
            triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
        }
        finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                // Instead of postHandle and afterCompletion
                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                return;
            }
            // Clean up any resources used by a multipart request.
            if (multipartRequestParsed) {
                cleanupMultipart(processedRequest);
            }
        }
    }

这个方法进去之后，经过一些判断，会执行到一个render(mv, request, response);方法，这个方法就是渲染视图的方法。这个方法进去之后

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.resolveLocale(request);       
 response.setLocale(locale);

        View view;
        if (mv.isReference()) {
            // We need to resolve the view name.
　　　　　　　//返回一个View对象

            view = resolveViewName(mv.getViewName(), 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.isDebugEnabled()) {
            logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
        }
        try {
　　　　　　　　//渲染视图
            view.render(mv.getModelInternal(), request, response);
        }
        catch (Exception ex) {
            if (logger.isDebugEnabled()) {
                logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '"
                        + getServletName() + "'", ex);
            }
            throw ex;
        }
    }

@Override
    public void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) throws Exception {
        if (logger.isTraceEnabled()) {
            logger.trace("Rendering view with name '" + this.beanName + "' with model " + model +
                " and static attributes " + this.staticAttributes);
        }
　　　　　　//将我ModelAndView中modelAndView.addObject("time", new Date());的值放入mergeModel中
        Map<String, Object> mergedModel = createMergedOutputModel(model, request, response);
　　　　　　//判断是否读取本地缓存
        prepareResponse(request, response);
　　　　　　//进一步处理，下面贴出它的代码
        renderMergedOutputModel(mergedModel, request, response);
    }@Overriprotected void renderMergedOutputModel(

            Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {

        // Determine which request handle to expose to the RequestDispatcher.
　　　　　//确定哪些请求处理暴露给RequestDispatcher。

        HttpServletRequest requestToExpose = getRequestToExpose(request);

        // Expose the model object as request attributes.
　　　　　//把model里面的值放入request中

        exposeModelAsRequestAttributes(model, requestToExpose);

        // Expose helpers as request attributes, if any.
        exposeHelpers(requestToExpose);

        // Determine the path for the request dispatcher.
　　　　 //获取要返回的地址

        String dispatcherPath = prepareForRendering(requestToExpose, response);

        // Obtain a RequestDispatcher for the target resource (typically a JSP).
　　　　 //确定一个请求处理器为这些参数资源

        RequestDispatcher rd = getRequestDispatcher(requestToExpose, dispatcherPath);
        if (rd == null) {
            throw new ServletException("Could not get RequestDispatcher for [" + getUrl() +
                    "]: Check that the corresponding file exists within your web application archive!");
        }

        // If already included or response already committed, perform include, else forward.
        if (useInclude(requestToExpose, response)) {
            response.setContentType(getContentType());
            if (logger.isDebugEnabled()) {
                logger.debug("Including resource [" + getUrl() + "] in InternalResourceView '" + getBeanName() + "'");
            }
            rd.include(requestToExpose, response);
        }

        else {
            // Note: The forwarded resource is supposed to determine the content type itself.
            if (logger.isDebugEnabled()) {
                logger.debug("Forwarding to resource [" + getUrl() + "] in InternalResourceView '" + getBeanName() + "'");
            }
            rd.forward(requestToExpose, response);
        }
    }

当要转发的地址，和要携带的信息都放入到requestToExpose里之后，就调用rd.forward(requestToExpose, response);转发请求到页面。

第二种：返回类型是Map

InvocableHandlerMethod类中有下面的方法：

public final Object invokeForRequest(NativeWebRequest request, ModelAndViewContainer mavContainer,
            Object... providedArgs) throws Exception {
　　　　　//mavContainer对象初始化时，会初始化一个默认的BindingAwareModelMap属性。将默认的这个BindingAwareModelMap属性放入arg

Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs); if (logger.isTraceEnabled()) { StringBuilder sb = new StringBuilder("Invoking ["); sb.append(this.getBeanType().getSimpleName()).append("."); sb.append(getMethod().getName()).append("] method with arguments "); sb.append(Arrays.asList(args)); logger.trace(sb.toString()); }

　　　　 //执行我的业务方法，把arg数组传入，把我里面的mavContainer默认的modelMap赋上值
        Object returnValue = invoke(args);
        if (logger.isTraceEnabled()) {
            logger.trace("Method [" + getMethod().getName() + "] returned [" + returnValue + "]");
        }
        return returnValue;
    }

 然后将model取出来，和我返回的String一起构建ModelAndView对象。

private ModelAndView getModelAndView(ModelAndViewContainer mavContainer,
            ModelFactory modelFactory, NativeWebRequest webRequest) throws Exception {

        modelFactory.updateModel(webRequest, mavContainer);

        if (mavContainer.isRequestHandled()) {
            return null;
        }
　　　　　//取出model
        ModelMap model = mavContainer.getModel();
　　     //构建对象
        ModelAndView mav = new ModelAndView(mavContainer.getViewName(), model);
        if (!mavContainer.isViewReference()) {
            mav.setView((View) mavContainer.getView());
        }
        if (model instanceof RedirectAttributes) {
            Map<String, ?> flashAttributes = ((RedirectAttributes) model).getFlashAttributes();
            HttpServletRequest request = webRequest.getNativeRequest(HttpServletRequest.class);
            RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);
        }
        return mav;
    }

 第三种：返回类型是Model

需要分析的就是我们加入model中的参数是怎么加入到modelAndView对象的。

其实是和Map类型也是一样的，将

mavContainer对象初始化时，会初始化一个默认的BindingAwareModelMap属性。将默认的这个BindingAwareModelMap属性放入arg，然后把arg传入我们的业务方法，然后业务方法接收到这个参数，往这个参数放值。就像我的例子里那样

 public String testModel(Model model){
        model.addAttribute("time",new Date());
        return CommonParam.SUCCESS;
    }

放好之后，model就有值了，然后根据这个model和我返回的字符串一起构建ModelAndView对象。

第四种：返回类型是View

这种我还没搞懂怎么用

第五种：返回类型是String

这个类ServletInvocableHandlerMethod的方法里：

public final void invokeAndHandle(ServletWebRequest webRequest,
            ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {
　　　　  //这里返回我的String类型的值
        Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);

        setResponseStatus(webRequest);

        if (returnValue == null) {
            if (isRequestNotModified(webRequest) || hasResponseStatus() || mavContainer.isRequestHandled()) {
                mavContainer.setRequestHandled(true);
                return;
            }
        }
        else if (StringUtils.hasText(this.responseReason)) {
            mavContainer.setRequestHandled(true);
            return;
        }

        mavContainer.setRequestHandled(false);

        try {
　　　　　　　　//进一步处理返回值
            this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
        }
        catch (Exception ex) {
            if (logger.isTraceEnabled()) {
                logger.trace(getReturnValueHandlingErrorMessage("Error handling return value", returnValue), ex);
            }
            throw ex;
        }
    }

然后到ViewNameMethodReturnValueHandler类的这个方法里：

@Override
    public void handleReturnValue(
            Object returnValue, MethodParameter returnType,
            ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
            throws Exception {

        if (returnValue == null) {
            return;
        }
        else if (returnValue instanceof String) {
　　　　　　　//把返回值赋给mavContainer的ViewName
            String viewName = (String) returnValue;
            mavContainer.setViewName(viewName);
            if (isRedirectViewName(viewName)) {
                mavContainer.setRedirectModelScenario(true);
            }
        }
        else {
            // should not happen
            throw new UnsupportedOperationException("Unexpected return type: " +
                    returnType.getParameterType().getName() + " in method: " + returnType.getMethod());
        }
    }

然后通过mavContainer对象来构造一个ModelAndView对象，然后渲染视图。

第五种特别篇：返回值中带有redirect的字符串

在ViewNameMethodReturnValueHandler类中的handleReturnValue方法中：

@Override
    public void handleReturnValue(
            Object returnValue, MethodParameter returnType,
            ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
            throws Exception {

        if (returnValue == null) {
            return;
        }
        else if (returnValue instanceof String) {
　　　　　　　//同样是把带有redirect的字符串赋给mavContainer对象。
            String viewName = (String) returnValue;
            mavContainer.setViewName(viewName);
            if (isRedirectViewName(viewName)) {
                mavContainer.setRedirectModelScenario(true);
            }
        }
        else {
            // should not happen
            throw new UnsupportedOperationException("Unexpected return type: " +
                    returnType.getParameterType().getName() + " in method: " + returnType.getMethod());
        }
    }

然后在DispatcherServlet类中有这个方法：

protected View resolveViewName(String viewName, Map<String, Object> model, Locale locale,
            HttpServletRequest request) throws Exception {

        for (ViewResolver viewResolver : this.viewResolvers) {
　　　　　　　//获取view，View是一个接口，这里的view实际类型是RedirectView
            View view = viewResolver.resolveViewName(viewName, locale);
            if (view != null) {
                return view;
            }
        }
        return null;
    }

然后根据View类型的不同，调用的也是不同的renderMergedOutputModel方法，这是RedirectView类的方法：

@Override
    protected void renderMergedOutputModel(Map<String, Object> model, HttpServletRequest request,
            HttpServletResponse response) throws IOException {

        String targetUrl = createTargetUrl(model, request);
        targetUrl = updateTargetUrl(targetUrl, model, request, response);

        FlashMap flashMap = RequestContextUtils.getOutputFlashMap(request);
        if (!CollectionUtils.isEmpty(flashMap)) {
            UriComponents uriComponents = UriComponentsBuilder.fromUriString(targetUrl).build();
            flashMap.setTargetRequestPath(uriComponents.getPath());
            flashMap.addTargetRequestParams(uriComponents.getQueryParams());
            FlashMapManager flashMapManager = RequestContextUtils.getFlashMapManager(request);
            if (flashMapManager == null) {
                throw new IllegalStateException("FlashMapManager not found despite output FlashMap having been set");
            }
            flashMapManager.saveOutputFlashMap(flashMap, request, response);
        }

        sendRedirect(request, response, targetUrl, this.http10Compatible);
    }

然后就是response.sendRedirect(encodedRedirectURL);

第五种特别篇之带forward的字符串：

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.resolveLocale(request);
        response.setLocale(locale);

        View view;
        if (mv.isReference()) {
            // We need to resolve the view name.
　　　　　　　//选择不同的类型的视图处理器构建视图对象

            view = resolveViewName(mv.getViewName(), 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.isDebugEnabled()) {
            logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
        }
        try {
            view.render(mv.getModelInternal(), request, response);
        }
        catch (Exception ex) {
            if (logger.isDebugEnabled()) {
                logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '"
                        + getServletName() + "'", ex);
            }
            throw ex;
        }
    }

这个是DispatcherServlet中的resolveViewName方法。

然后是选择了AbstractCachingViewResolver这个视图处理器的resolveViewName方法，去创建视图。然后调用了他的子类UrlBasedViewResolver的创建视图方法：

@Override
    protected View createView(String viewName, Locale locale) throws Exception {
        // If this resolver is not supposed to handle the given view,
        // return null to pass on to the next resolver in the chain.
        if (!canHandle(viewName, locale)) {
            return null;
        }
        // Check for special "redirect:" prefix.
        if (viewName.startsWith(REDIRECT_URL_PREFIX)) {
            String redirectUrl = viewName.substring(REDIRECT_URL_PREFIX.length());
            RedirectView view = new RedirectView(redirectUrl, isRedirectContextRelative(), isRedirectHttp10Compatible());
            return applyLifecycleMethods(viewName, view);
        }
        // Check for special "forward:" prefix.
　　　　 //获得一个forwardUrl

        if (viewName.startsWith(FORWARD_URL_PREFIX)) {
            String forwardUrl = viewName.substring(FORWARD_URL_PREFIX.length());
            return new InternalResourceView(forwardUrl);
        }
        // Else fall back to superclass implementation: calling loadView.
        return super.createView(viewName, locale);
    }

然后调用InternalResourceView类的renderMergedOutputModel方法：

@Override
    protected void renderMergedOutputModel(
            Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {

        // Determine which request handle to expose to the RequestDispatcher.
        HttpServletRequest requestToExpose = getRequestToExpose(request);

        // Expose the model object as request attributes.
        exposeModelAsRequestAttributes(model, requestToExpose);

        // Expose helpers as request attributes, if any.
        exposeHelpers(requestToExpose);

        // Determine the path for the request dispatcher.
        String dispatcherPath = prepareForRendering(requestToExpose, response);

        // Obtain a RequestDispatcher for the target resource (typically a JSP).
        RequestDispatcher rd = getRequestDispatcher(requestToExpose, dispatcherPath);
        if (rd == null) {
            throw new ServletException("Could not get RequestDispatcher for [" + getUrl() +
                    "]: Check that the corresponding file exists within your web application archive!");
        }

        // If already included or response already committed, perform include, else forward.
        if (useInclude(requestToExpose, response)) {
            response.setContentType(getContentType());
            if (logger.isDebugEnabled()) {
                logger.debug("Including resource [" + getUrl() + "] in InternalResourceView '" + getBeanName() + "'");
            }
            rd.include(requestToExpose, response);
        }

        else {
            // Note: The forwarded resource is supposed to determine the content type itself.
            if (logger.isDebugEnabled()) {
                logger.debug("Forwarding to resource [" + getUrl() + "] in InternalResourceView '" + getBeanName() + "'");
            }
　　　　　　　//调用转发方法
            rd.forward(requestToExpose, response);
        }
    }

 第六种：返回类型是Void

这时我们需要考虑两个问题，第一：返回值是Void也就是没有返回值，他的ModalAndView对象是什么样的？第二：他将转到哪个页面，还是会直接报错？

带着这两个问题，我们看源码：

以DispatcherServlet中的这句代码往下看，

mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

一直到ServletInvocableHandlerMethod类的这个方法

public final void invokeAndHandle(ServletWebRequest webRequest,
            ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {

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

        setResponseStatus(webRequest);

        if (returnValue == null) {
            if (isRequestNotModified(webRequest) || hasResponseStatus() || mavContainer.isRequestHandled()) {
                mavContainer.setRequestHandled(true);
                return;
            }
        }
        else if (StringUtils.hasText(this.responseReason)) {
            mavContainer.setRequestHandled(true);
            return;
        }

        mavContainer.setRequestHandled(false);

        try {
　　　　　　  //主要看这里，handleReturnValue，处理返回值，那我们点进去看看他怎么处理null值
            this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
        }
        catch (Exception ex) {
            if (logger.isTraceEnabled()) {
                logger.trace(getReturnValueHandlingErrorMessage("Error handling return value", returnValue), ex);
            }
            throw ex;
        }
    }

然而结果就是他看到return是null，什么都没处理就返回了，只好再往下看。

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 {
                processedRequest = checkMultipart(request);
                multipartRequestParsed = processedRequest != request;

                // Determine handler for the current request.
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null || mappedHandler.getHandler() == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // Determine handler adapter for the current request.
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                // Process last-modified header, if supported by the handler.
                String method = request.getMethod();
                boolean isGet = "GET".equals(method);
                if (isGet || "HEAD".equals(method)) {
                    long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                    if (logger.isDebugEnabled()) {
                        String requestUri = urlPathHelper.getRequestUri(request);
                        logger.debug("Last-Modified value for [" + requestUri + "] is: " + lastModified);
                    }
                    if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                        return;
                    }
                }

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

                try {
                    // Actually invoke the handler.
                    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                }
                finally {
                    if (asyncManager.isConcurrentHandlingStarted()) {
                        return;
                    }
                }
　　　　　　　　　　//最后发现view为null时，这里处理了
                applyDefaultViewName(request, mv);
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) {
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Error err) {
            triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
        }
        finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                // Instead of postHandle and afterCompletion
                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                return;
            }
            // Clean up any resources used by a multipart request.
            if (multipartRequestParsed) {
                cleanupMultipart(processedRequest);
            }
        }
    }

接下来是组装地址的方法：这个是处理地址组装的类UrlBasedViewResolver

@Override
    protected View loadView(String viewName, Locale locale) throws Exception {
　　　　 //进行组装
        AbstractUrlBasedView view = buildView(viewName);
        View result = applyLifecycleMethods(viewName, view);
        return (view.checkResource(locale) ? result : null);
    }

protected AbstractUrlBasedView buildView(String viewName) throws Exception {
        AbstractUrlBasedView view = (AbstractUrlBasedView) BeanUtils.instantiateClass(getViewClass());
　　　　 //对应的我最上方spring.mvc里面的prefix和suffix的配置的地址，把它们连接起来
        view.setUrl(getPrefix() + viewName + getSuffix());
        String contentType = getContentType();
        if (contentType != null) {
            view.setContentType(contentType);
        }
        view.setRequestContextAttribute(getRequestContextAttribute());
        view.setAttributesMap(getAttributesMap());
        if (this.exposePathVariables != null) {
            view.setExposePathVariables(exposePathVariables);
        }
        return view;
    }

到此，要转向哪一个地址，ModelAndView对象是什么样都已经大致清楚了。然后就是调用rd.forward(requestToExpose, response);方法，渲染到页面

里面代码很多，思路也不是很清楚。有意义的地方只是给小伙伴们提供了几个方向，要想看springMvc是如何渲染视图的，可以从他的返回类型入手，然后debug一步步的看。源码我是在这里：

https://mvnrepository.com/search?q=spring-ui&p=2     下载的。我这里用的是4.0.0的版本。最后在总结一下吧。

首先，要构造一个ModelAndView对象，这个对象的构建思路是，我创建一个数组，把一个对象放在这个数组里，然后把数组给你，你往这个数组里放值，然后我去取这个数组里的值。然后获取的值我拿来

创建一个ModelAndView对象。

第二步，去渲染视图，有很多的渲染视图的处理器，系统会去判断选择一个处理器，来处理你的请求，大概的不同就是，我最终是调doFord还是doRedirect，我的返回值的header应该设置什么等等。