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：
- ModelAndView
- Map，ModelMap
- Model
- View
- String
- 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.xsdhttp://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsdhttp://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");}}
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;}}
@Overridepublic 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。
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;}
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。
这种我还没搞懂怎么用
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：
@Overridepublic 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ViewNameString viewName = (String) returnValue;mavContainer.setViewName(viewName);if (isRedirectViewName(viewName)) {mavContainer.setRedirectModelScenario(true);}}else {// should not happenthrow 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：
@Overridepublic 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 happenthrow 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：
@Overrideprotected 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：
@Overrideprotected 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：
@Overrideprotected 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);}}
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
@Overrideprotected 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。
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