前面讲解到DispatcherServlet的doDispatch过程,对视图的呈现处理是在processDispatchResult方法中完成的
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()) { render(mv, request, response); if (errorView) { WebUtils.clearErrorRequestAttributes(request); } } else { if (logger.isDebugEnabled()) { logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() + "': assuming HandlerAdapter completed request handling"); } } if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) { // Concurrent handling started during a forward return; } if (mappedHandler != null) { mappedHandler.triggerAfterCompletion(request, response, null); } }
/** * 视图呈现 * <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); //得到视图对象 View view; String viewName = mv.getViewName(); if (viewName != null) { //需要对视图名进行解析 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 { // 直接从mv中取得实际的视图对象 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 { if (mv.getStatus() != null) { response.setStatus(mv.getStatus().value()); }//通过视图的render完成视图呈现工作 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; } }
@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; }
ViewResolvers的解析过程可以参考常见的BeanNameViewResolver实现
@Override @Nullable public View resolveViewName(String viewName, Locale locale) throws BeansException { ApplicationContext context = obtainApplicationContext(); if (!context.containsBean(viewName)) { if (logger.isDebugEnabled()) { logger.debug("No matching bean found for view name '" + viewName + "'"); } // Allow for ViewResolver chaining... return null; } if (!context.isTypeMatch(viewName, View.class)) { if (logger.isDebugEnabled()) { logger.debug("Found matching bean for view name '" + viewName + "' - to be ignored since it does not implement View"); } // Since we're looking into the general ApplicationContext here, // let's accept this as a non-match and allow for chaining as well... return null; } return context.getBean(viewName, View.class);//从上下文中通过名称的对应关系把作为View对象的bean取到 }
下面分析一下JSP视图的实现
@Override public void render(@Nullable 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); } Map<String, Object> mergedModel = createMergedOutputModel(model, request, response); prepareResponse(request, response); renderMergedOutputModel(mergedModel, getRequestToExpose(request), response); }
/** * 这里把所有的相关信息都收集到一个Map中 */ protected Map<String, Object> createMergedOutputModel(@Nullable Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) { @SuppressWarnings("unchecked") Map<String, Object> pathVars = (this.exposePathVariables ? (Map<String, Object>) request.getAttribute(View.PATH_VARIABLES) : null); // Consolidate static and dynamic model attributes. int size = this.staticAttributes.size(); size += (model != null ? model.size() : 0); size += (pathVars != null ? pathVars.size() : 0); Map<String, Object> mergedModel = new LinkedHashMap<>(size); mergedModel.putAll(this.staticAttributes); if (pathVars != null) { mergedModel.putAll(pathVars); } if (model != null) { mergedModel.putAll(model); } // Expose RequestContext? if (this.requestContextAttribute != null) { mergedModel.put(this.requestContextAttribute, createRequestContext(request, response, mergedModel)); } return mergedModel; }
把所有的数据模型进行整合,放到一个mergedModel对象里面,然后调用renderMergedOutputModel方法,这个方法是一个模板方法,实现在InternalResourceView中完成。
@Override protected void renderMergedOutputModel( Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception { // 对数据进行处理,把模型对象存放到ServletContext中 exposeModelAsRequestAttributes(model, request); // Expose helpers as request attributes, if any. exposeHelpers(request); // 获取内部资源路径 String dispatcherPath = prepareForRendering(request, response); // 把请求转发到前面获取的内部资源路径中 RequestDispatcher rd = getRequestDispatcher(request, 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(request, response)) { response.setContentType(getContentType()); if (logger.isDebugEnabled()) { logger.debug("Including resource [" + getUrl() + "] in InternalResourceView '" + getBeanName() + "'"); } rd.include(request, 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(request, response); } }
首先对模型数据进行处理,这个处理是在exposeModelAsRequestAttributes方法中的,把ModelAndView中的模型数据和其他请求数据都放到HttpServletRequest的属性中。
protected void exposeModelAsRequestAttributes(Map<String, Object> model, HttpServletRequest request) throws Exception { model.forEach((modelName, modelValue) -> { if (modelValue != null) { request.setAttribute(modelName, modelValue); if (logger.isDebugEnabled()) { logger.debug("Added model object '" + modelName + "' of type [" + modelValue.getClass().getName() + "] to request in view with name '" + getBeanName() + "'"); } } else { request.removeAttribute(modelName); if (logger.isDebugEnabled()) { logger.debug("Removed model object '" + modelName + "' from request in view with name '" + getBeanName() + "'"); } } }); }
exposeHelpers进行数据处理
@Override protected void exposeHelpers(HttpServletRequest request) throws Exception { if (this.messageSource != null) { JstlUtils.exposeLocalizationContext(request, this.messageSource); } else { JstlUtils.exposeLocalizationContext(new RequestContext(request, getServletContext())); } }
实际的数据到页面的输出是由InternalResourceView完成的,render完成资源的重定向处理。
protected String prepareForRendering(HttpServletRequest request, HttpServletResponse response) throws Exception { String path = getUrl(); Assert.state(path != null, "'url' not set"); if (this.preventDispatchLoop) { String uri = request.getRequestURI(); if (path.startsWith("/") ? uri.equals(path) : uri.equals(StringUtils.applyRelativePath(uri, path))) { throw new ServletException("Circular view path [" + path + "]: would dispatch back " + "to the current handler URL [" + uri + "] again. Check your ViewResolver setup! " + "(Hint: This may be the result of an unspecified view, due to default view name generation.)"); } } return path; }从请求中得到URL的路径,并对路径进行处理,使用RequestDispatcher把请求转发到这个资源上,完成JSP页面的展现。