SpringMVC之源码分析--ViewResolver(一)
概述
本章开始进入另一重要的组件,即视图组件,Spring MVC处理视图组件使用两个主要的接口是ViewResolver和View。根据名称可知,ViewResolver即视图解析器,其作用是把逻辑视图名称解析为真正的视图,然后通过View对象进行渲染。View接口的作用是用于处理视图进行渲染。
延用之前的介绍流程,本章分两部分进行阐述:启动初始化和请求处理。
本系列文章是基于Spring5.0.5RELEASE。
ViewResolver初始化
Spring MVC初始化视图解析器策略与初始化其他策略一样,其入口是DispatcherSerlvet的initStrategies(context)方法,代码如下:
/** *初始化策略对象 */ protected void initStrategies(ApplicationContext context) { initMultipartResolver(context); initLocaleResolver(context); initThemeResolver(context); initHandlerMappings(context); initHandlerAdapters(context); initHandlerExceptionResolvers(context); initRequestToViewNameTranslator(context); // 初始化视图解析器ViewResolver initViewResolvers(context); initFlashMapManager(context); }
看过之前文章的可以了解,此方法是Spring MVC初始化策略组件的入口。针对视图解析器组件,调用initViewResolver(context)方法,加载视图处理策略,该方法源码如下:
private void initViewResolvers(ApplicationContext context) { // viewResolvers是视图解析器集合,接收到用户请求时,从该属性变量中获取到Spring MVC使用的视图解析器 this.viewResolvers = null; // 是否从Spring上下文中加载ViewResolver,detectAllViewResolvers属性变量默认为true,可在web部署描述文件中修改 if (this.detectAllViewResolvers) { // Find all ViewResolvers in the ApplicationContext, including ancestor contexts. // 按接口类型ViewResolver查找全部 // key为bean的id(name),value为bean的class对象 Map<String, ViewResolver> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, ViewResolver.class, true, false); if (!matchingBeans.isEmpty()) { this.viewResolvers = new ArrayList<>(matchingBeans.values()); // We keep ViewResolvers in sorted order. // 对ViewResolver进行排序,通过Ordered接口实现 AnnotationAwareOrderComparator.sort(this.viewResolvers); } } else { try { // 从Spring上下文中加载指定名字为"viewResolver"的bean,作为视图解析器 ViewResolver vr = context.getBean(VIEW_RESOLVER_BEAN_NAME, ViewResolver.class); this.viewResolvers = Collections.singletonList(vr); } catch (NoSuchBeanDefinitionException ex) { // Ignore, we'll add a default ViewResolver later. } } // Ensure we have at least one ViewResolver, by registering // a default ViewResolver if no other resolvers are found. // 为了确保至少有一个ViewResolver视图解析器,Spring MVC配置了默认的ViewResolver // 在DispatcherServlet.properties文件中定义,默认为InternalResourceViewResolver if (this.viewResolvers == null) { this.viewResolvers = getDefaultStrategies(context, ViewResolver.class); if (logger.isDebugEnabled()) { logger.debug("No ViewResolvers found in servlet '" + getServletName() + "': using default"); } } }
至此,Spring MVC即完成加载初始化ViewResolver视图解析器,即Spring MVC以具备解析处理逻辑视图名称的能力。
ViewResolver使用
ViewResolver的使用是指用户发起请求到Spring,Spring MVC经过HandlerMapping查找处理的handler,然后通过HandlerAdapter进行适配后处理用户请求,返回ModelAndView,最后使用ViewResolver对ModelAndView进行解析,即把逻辑视图名解析为真正的视图对象,由视图对象进行渲染的过程。
用户的请求处理流程由DispatcherServlet的doDispatcher(request,response)方法进行控制,主要代码如下:
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception { ... ... // 找到请求对应的handler mappedHandler = getHandler(processedRequest); ... ... // 找到对应handler的适配器 HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler()); ... ... // 执行用户请求的拦截器前置处理方法 if (!mappedHandler.applyPreHandle(processedRequest, response)) { return; } // 调用实际的handler处理方法 mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); ... ... // 设置默认的逻辑视图名称 // 如果handler返回null或者ViewName属性为null时,Spring进行取视图规则为:前缀+url+后缀 // 比如访问url为:http://localhost:8086/test,前缀配置/WEB-INF/jsp,后缀配置为.jsp,那么最终查找的是:/WEB-INF/jsp/test.jsp applyDefaultViewName(processedRequest, mv); // 执行用户请求的拦截器后置处理方法 mappedHandler.applyPostHandle(processedRequest, response, mv); ... ... // processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException); } /* *处理结果 */ private void processDispatchResult(HttpServletRequest request, HttpServletResponse response, @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv, @Nullable Exception exception) throws Exception { boolean errorView = false; // 异常处理 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? // 调用render方法 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); } } /* *渲染ModelAndView */ 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; // 获取逻辑视图名称 String viewName = mv.getViewName(); if (viewName != null) { // We need to resolve the view name. // 根据逻辑视图名解析视图,返回View对象 // 此处是视图解析器使用入口,见下面的方法 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.isDebugEnabled()) { logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'"); } try { if (mv.getStatus() != null) { response.setStatus(mv.getStatus().value()); } // 调用View的render方法进行视图渲染 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) { // 调用ViewResovler接口入口方法 View view = viewResolver.resolveViewName(viewName, locale); if (view != null) { return view; } } } return null; }
总结
本章概述了Spring MVC加载视图解析器策略,并且找到了ViewResolver的入口方法,下面的章节继续学习ViewResolver接口的实现类。
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