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ispatcherServlet执行流程及相关源码分析
在前一篇文章SpringMVC 启动流程及相关源码分析中，详细探讨了Spring MVC在Web容器中部署后的启动过程，以及相关源码分析，同时也讨论了DispatcherServlet类的初始化创建过程，相关内容在此不再赘述，如有需求可查阅。
本文主要讲解DispatcherServlet类获取用户请求到响应的全过程，并针对相关源码进行分析。对于基本的MVC架构本文不再进行讲解，有需要的读者可自行查阅。
首先，让我们站在Spring MVC的四大组件:DispatcherServlet、HandlerMapping、HandlerAdapter以及ViewResolver的角度来看一下Spring MVC对用户请求的处理过程，有如下时序图:
DispatcherServlet执行序列图
具体处理过程如下:
- 1、用户请求发送至
DispatcherServlet类进行处理。 - 2、
DispatcherServlet类遍历所有配置的HandlerMapping类请求查找Handler。 - 3、
HandlerMapping类根据request请求的URL等信息查找能够进行处理的Handler，以及相关拦截器interceptor并构造HandlerExecutionChain。 - 4、
HandlerMapping类将构造的HandlerExecutionChain类的对象返回给前端控制器DispatcherServlet类。 - 5、前端控制器拿着上一步的
Handler遍历所有配置的HandlerAdapter类请求执行Handler。 - 6、
HandlerAdapter类执行相关Handler并获取ModelAndView类的对象。 - 7、
HandlerAdapter类将上一步Handler执行结果的ModelAndView 类的对象返回给前端控制器。 - 8、
DispatcherServlet类遍历所有配置的ViewResolver类请求进行视图解析。 - 9、
ViewResolver类进行视图解析并获取View对象。 - 10、
ViewResolver类向前端控制器返回上一步骤的View对象。 - 11、
DispatcherServlet类进行视图View的渲染，填充Model。 - 12、
DispatcherServlet类向用户返回响应。
通过时序图和上面的讲解不难发现，整个Spring MVC对于用户请求的响应和处理都是以DispatcherServlet类为核心，其他三大组件均与前端控制器进行交互，三大组件之间没有交互并且互相解耦，因此，三大组件可以替换不同的实现而互相没有任何影响，提高了整个架构的稳定性并且降低了耦合度。接下来会按照上述的响应过程逐一进行讲解。
DispatcherServlet类本质上依旧是一个Servlet并且其父类实现了Servlet接口，我们知道，Servlet执行Service()方法对用户请求进行响应，根据前一篇文章的分析方法可以得到人如下的调用逻辑图:
service方法调用逻辑
从上图的源码调用逻辑可以看出，HttpServlet抽象类实现了Servlet接口的service(ServletRequest, ServletResponse)的方法，因此，用户请求的第一执行方法为该方法，该方法紧接着直接调用了service(HttpServletRequest, HttpServletResponse)方法，其子类FrameworkServlet抽象类重写了该方法，因为多态的特性最终是调用了FrameworkServlet抽象类的service(HttpServletRequest, HttpServletResponse)方法，FrameworkServlet抽象类同样也重写了doHead()、doPost()、doPut()、doDelete()、doOptions()、doTrace()方法，service(ServletRequest, ServletResponse)方法根据请求类型的不同分别调用上述方法，上述六个方法都调用了processRequest()方法，而该方法最终调用了DispatcherServlet类的doService()方法。通过层层分析，我们找到了最终要调用的处理用户请求的方法，doService()之前的方法调用都比较简单，这里不再逐一来查看源码，有兴趣的读者可以自行查阅。
查看doService()的源码如下:
/*** Exposes the DispatcherServlet-specific request attributes and delegates to {@link #doDispatch}* for the actual dispatching.*/@Overrideprotected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {if (logger.isDebugEnabled()) {String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +" processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");}// Keep a snapshot of the request attributes in case of an include,// to be able to restore the original attributes after the include.Map<String, Object> attributesSnapshot = null;if (WebUtils.isIncludeRequest(request)) {attributesSnapshot = new HashMap<String, Object>();Enumeration<?> attrNames = request.getAttributeNames();while (attrNames.hasMoreElements()) {String attrName = (String) attrNames.nextElement();if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {attributesSnapshot.put(attrName, request.getAttribute(attrName));}}}// Make framework objects available to handlers and view objects./*将当前Servlet的子IoC容器放入request请求中由此，我们可以访问到当前IoC子容器以及根IoC容器中的Bean*/request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);if (inputFlashMap != null) {request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));}request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);try {//真正进行用户请求的处理doDispatch(request, response);}finally {if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {// Restore the original attribute snapshot, in case of an include.if (attributesSnapshot != null) {restoreAttributesAfterInclude(request, attributesSnapshot);}}}}
doService()方法主要进行一些参数的设置，并将部分参数放入request请求中，真正执行用户请求并作出响应的方法则为doDispatch()方法，查看doDispatch()方法的源码如下:
/*** Process the actual dispatching to the handler.* <p>The handler will be obtained by applying the servlet's HandlerMappings in order.* The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters* to find the first that supports the handler class.* <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers* themselves to decide which methods are acceptable.* @param request current HTTP request* @param response current HTTP response* @throws Exception in case of any kind of processing failure*/protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {//用户的request请求HttpServletRequest processedRequest = request;//HandlerExecutionChain局部变量HandlerExecutionChain mappedHandler = null;//判断是否解析了文件类型的数据，如果有最终需要清理boolean multipartRequestParsed = false;WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);try {//ModelAndView局部变量ModelAndView mv = null;//处理异常局部变量Exception dispatchException = null;try {//检查是否包含文件等类型的数据processedRequest = checkMultipart(request);multipartRequestParsed = (processedRequest != request);// Determine handler for the current request.//向HandlerMapping请求查找HandlerExecutionChainmappedHandler = getHandler(processedRequest);//如果HandlerExecutionChain为null，则没有能够进行处理的Handler，抛出异常if (mappedHandler == null || mappedHandler.getHandler() == null) {noHandlerFound(processedRequest, response);return;}// Determine handler adapter for the current request.//根据查找到的Handler请求查找能够进行处理的HandlerAdapterHandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());// Process last-modified header, if supported by the handler.//判断自上次请求后是否有修改，没有修改直接返回响应String method = request.getMethod();boolean isGet = "GET".equals(method);if (isGet || "HEAD".equals(method)) {long lastModified = ha.getLastModified(request, mappedHandler.getHandler());if (logger.isDebugEnabled()) {logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);}if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {return;}}/*按顺序依次执行HandlerInterceptor的preHandle方法如果任一HandlerInterceptor的preHandle方法没有通过则不继续进行处理*/if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}// Actually invoke the handler.//通过HandlerAdapter执行查找到的handlermv = ha.handle(processedRequest, response, mappedHandler.getHandler());if (asyncManager.isConcurrentHandlingStarted()) {return;}applyDefaultViewName(processedRequest, mv);//逆序执行HandlerInterceptor的postHandle方法mappedHandler.applyPostHandle(processedRequest, response, mv);}catch (Exception ex) {dispatchException = ex;}catch (Throwable err) {// As of 4.3, we're processing Errors thrown from handler methods as well,// making them available for @ExceptionHandler methods and other scenarios.dispatchException = new NestedServletException("Handler dispatch failed", err);}//渲染视图填充Model，如果有异常渲染异常页面processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);}catch (Exception ex) {//如果有异常按倒序执行所有HandlerInterceptor的afterCompletion方法triggerAfterCompletion(processedRequest, response, mappedHandler, ex);}catch (Throwable err) {//如果有异常按倒序执行所有HandlerInterceptor的afterCompletion方法triggerAfterCompletion(processedRequest, response, mappedHandler,new NestedServletException("Handler processing failed", err));}finally {if (asyncManager.isConcurrentHandlingStarted()) {// Instead of postHandle and afterCompletionif (mappedHandler != null) {//倒序执行所有HandlerInterceptor的afterCompletion方法mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);}}else {// Clean up any resources used by a multipart request.//如果请求包含文件类型的数据则进行相关清理工作if (multipartRequestParsed) {cleanupMultipart(processedRequest);}}}}
根据上述源码并结合文章开始讲解的DispatcherServlet类结合三大组件对用户请求的处理过程不难理解相关处理流程。
doDispatch()方法通过调用getHandler()方法并传入reuqest通过HandlerMapping查找HandlerExecutionChain，查看其源码如下:
/*** Return the HandlerExecutionChain for this request.* <p>Tries all handler mappings in order.* @param request current HTTP request* @return the HandlerExecutionChain, or {@code null} if no handler could be found*/protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {for (HandlerMapping hm : this.handlerMappings) {if (logger.isTraceEnabled()) {logger.trace("Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");}HandlerExecutionChain handler = hm.getHandler(request);if (handler != null) {return handler;}}return null;}
getHandler()方法遍历了开发者配置的所有HandlerMapping类根据request请求来查找HandlerExecutionChain，从这里可以看出，Spring MVC是支持用户配置多个HandlerMapping类的，在处理用户请求时会逐一查找，找到后立即返回，因此，如果多个HandlerMapping类都能够处理同一request请求，只会返回第一个能够处理的HandlerMapping类构造的HandlerExecutionChain，所以在配置HandlerMapping类时需要注意不要对同一请求多次进行处理，由于篇幅问题HandlerMapping类如何具体查找Handler并构造HandlerExecutionChain的细节不在此进行讲解，如有兴趣可以查阅本系列文章的第三篇SpringMVC HandlerMapping源码分析。
如果没有找到对应的HandlerExecutionChain对象，则会执行noHandlerFound()方法，继续查看其源码如下:
/*** No handler found -> set appropriate HTTP response status.* @param request current HTTP request* @param response current HTTP response* @throws Exception if preparing the response failed*/protected void noHandlerFound(HttpServletRequest request, HttpServletResponse response) throws Exception {if (pageNotFoundLogger.isWarnEnabled()) {pageNotFoundLogger.warn("No mapping found for HTTP request with URI [" + getRequestUri(request) +"] in DispatcherServlet with name '" + getServletName() + "'");}if (this.throwExceptionIfNoHandlerFound) {throw new NoHandlerFoundException(request.getMethod(), getRequestUri(request),new ServletServerHttpRequest(request).getHeaders());}else {response.sendError(HttpServletResponse.SC_NOT_FOUND);}}
如果没有找到对应的HandlerExecutionChain则会抛出异常NoHandlerFoundException，在开发的过程中，如果我们将具体的URL写错了则会遇到这个404错误。
继续查看doDispatch()方法的源码，如果找到了HandlerExecutionChain接下来会调用getHandlerAdapter()方法来查找能够对Handler进行处理的HandlerAdapter，查看其源码如下:
/*** Return the HandlerAdapter for this handler object.* @param handler the handler object to find an adapter for* @throws ServletException if no HandlerAdapter can be found for the handler. This is a fatal error.*/protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {for (HandlerAdapter ha : this.handlerAdapters) {if (logger.isTraceEnabled()) {logger.trace("Testing handler adapter [" + ha + "]");}if (ha.supports(handler)) {return ha;}}throw new ServletException("No adapter for handler [" + handler +"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");}
与HandlerMapping类似，查找能够处理具体Handler的HandlerAdapter时同样会遍历所有配置了的HandlerAdapter，HandlerAdapter是一个接口包含一个support()方法，该方法根据Handler是否实现某个特定的接口来判断该HandlerAdapter是否能够处理这个具体的Handler，这里使用适配器模式，通过这样的方式就可以支持不同类型的HandlerAdapter。如果没有查找到能够处理Handler的HandlerAdapter则会抛出异常，如果在开发的过程中Handler在实现接口时出现了问题就可能会遇到上述异常。
查找到了对应的HandlerAdapter后就会调用HandlerExecutionChain的applyPreHandle()方法来执行配置的所有HandlerInteceptor的preHandle()方法，查看其源码如下:
/*** Apply preHandle methods of registered interceptors.* @return {@code true} if the execution chain should proceed with the* next interceptor or the handler itself. Else, DispatcherServlet assumes* that this interceptor has already dealt with the response itself.*/boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {HandlerInterceptor[] interceptors = getInterceptors();if (!ObjectUtils.isEmpty(interceptors)) {for (int i = 0; i < interceptors.length; i++) {HandlerInterceptor interceptor = interceptors[i];if (!interceptor.preHandle(request, response, this.handler)) {triggerAfterCompletion(request, response, null);return false;}this.interceptorIndex = i;}}return true;}
HandlerExecutionChain的applyPreHandle()方法会按照顺序依次调用HandlerInterceptor的preHandle()方法，但当任一HandlerInterceptor的preHandle()方法返回了false就不再继续执行其他HandlerInterceptor的preHandle()方法，而是直接跳转执行triggerAfterCompletion()方法，查看该方法源码如下:
/*** Trigger afterCompletion callbacks on the mapped HandlerInterceptors.* Will just invoke afterCompletion for all interceptors whose preHandle invocation* has successfully completed and returned true.*/void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, Exception ex)throws Exception {HandlerInterceptor[] interceptors = getInterceptors();if (!ObjectUtils.isEmpty(interceptors)) {for (int i = this.interceptorIndex; i >= 0; i--) {HandlerInterceptor interceptor = interceptors[i];try {interceptor.afterCompletion(request, response, this.handler, ex);}catch (Throwable ex2) {logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);}}}}
这里遍历的下标为interceptorIndex，该变量在前一个方法applyPreHandle()方法中赋值，如果preHandle()方法返回true该变量加一，因此该方法会逆序执行所有preHandle()方法返回了true的HandlerInterceptor的afterCompletion()方法。到这里读者已经掌握了HandlerInterceptor的preHandle()方法以及afterCompletion()方法的执行顺序，这些内容并不需要我们死记，需要知道其执行顺序查看源码是最好的方法。
继续阅读doDispatch()方法的源码，如果所有拦截器的preHandle()方法都返回了true没有进行拦截，接下来前端控制器会请求执行上文获取的Handler，这个Handler就是开发的时候编写的Controller，根据实现接口的不同执行相关方法，并获取到ModelAndView类的对象。
接下来会执行HandlerInterceptor的postHandle()方法，具体源码如下:
/*** Apply postHandle methods of registered interceptors.*/void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception {HandlerInterceptor[] interceptors = getInterceptors();if (!ObjectUtils.isEmpty(interceptors)) {for (int i = interceptors.length - 1; i >= 0; i--) {HandlerInterceptor interceptor = interceptors[i];interceptor.postHandle(request, response, this.handler, mv);}}}
可以发现，postHandle()方法是按照逆序执行。
执行完postHandle()方法后，doDispatch()方法调用了processDispatchResult()方法，其源码如下:
/*** Handle the result of handler selection and handler invocation, which is* either a ModelAndView or an Exception to be resolved to a ModelAndView.*/private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {boolean errorView = false;//判断HandlerMapping、HandlerAdapter处理时的异常是否为空if (exception != null) {//上述两个组件处理时的异常不为空//如果为ModelAndViewDefiningException异常，则获取一个异常视图if (exception instanceof ModelAndViewDefiningException) {logger.debug("ModelAndViewDefiningException encountered", exception);mv = ((ModelAndViewDefiningException) exception).getModelAndView();}//如果不为ModelAndViewDefiningException异常，进行异常视图的获取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?//判断mv是否为空，不管是正常的ModelAndView还是异常的ModelAndView，只要存在mv就进行视图渲染if (mv != null && !mv.wasCleared()) {render(mv, request, response);if (errorView) {WebUtils.clearErrorRequestAttributes(request);}}//否则记录无视图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 forwardreturn;}//执行相关HandlerInterceptor的afterCompletion()方法if (mappedHandler != null) {mappedHandler.triggerAfterCompletion(request, response, null);}}
该方法传入了一个异常类的对象dispatchException，阅读doDispatch()方法的源码可以看出，Spring MVC对整个doDispatch()方法用了嵌套的try-catch语句，内层的try-catch用于捕获HandlerMapping进行映射查找HandlerExecutionChain以及HandlerAdapter执行具体Handler时的处理异常，并将异常传入到上述processDispatchResult()方法中。
processDispatchResult()方法主要用于针对产生的异常来构造异常视图，接着不管视图是正常视图还是异常视图均调用render()方法来渲染，查看render()方法的具体源码如下:
/*** Render the given ModelAndView.* <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.resolveLocale(request);response.setLocale(locale);View view;if (mv.isReference()) {// We need to resolve the view name.// 解析视图名称获取对应视图Viewview = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);//如果视图View为空抛出异常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 {//设置Http响应状态字if (mv.getStatus() != null) {response.setStatus(mv.getStatus().value());}//调用视图View的render方法通过Model来渲染视图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;}}
render()方法通过调用resolveViewName()方法根据视图名称解析对应的视图View，该方法源码如下:
/*** Resolve the given view name into a View object (to be rendered).* <p>The default implementations asks all ViewResolvers of this dispatcher.* Can be overridden for custom resolution strategies, potentially based on* specific model attributes or request parameters.* @param viewName the name of the view to resolve* @param model the model to be passed to the view* @param locale the current locale* @param request current HTTP servlet request* @return the View object, or {@code null} if none found* @throws Exception if the view cannot be resolved* (typically in case of problems creating an actual View object)* @see ViewResolver#resolveViewName*/protected View resolveViewName(String viewName, Map<String, Object> model, Locale locale,HttpServletRequest request) throws Exception {for (ViewResolver viewResolver : this.viewResolvers) {View view = viewResolver.resolveViewName(viewName, locale);if (view != null) {return view;}}return null;}
resolveViewName()方法通过遍历配置的所有ViewResolver类根据视图名称来解析对应的视图View，如果找到则返回对应视图View，没有找到则返回null。
回到前一个render()方法，如果上述方法返回的视图为null则抛出异常，这个异常相信大多数人也见过，当开发时写错了返回的View视图名称时就会抛出该异常。接下来调用具体视图的render()方法来进行Model数据的渲染填充，最终构造成完整的视图。
到这里，doDispatch()的外层try-catch异常的作用我们就知道了，为了捕获渲染视图时的异常，通过两层嵌套的try-catch，Spring MVC就能够捕获到三大组件在处理用户请求时的异常，通过这样的方法能够很方便的实现统一的异常处理。
总结
通过前文的源码分析，我们能够清楚的认识到Spring MVC对用户请求的处理过程，进一步加深对Spring MVC的理解。
转载自：https://www.jianshu.com/p/0f981efdfbbd
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