Java从源码角度解析SpringMVC执行流程

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SpringMVC执行流程在面试中经常会被问到，本篇文章通过源码的方式简单的了解一下SpringMVC执行流程。

先看流程

先看一下SpringMVC执行流程再看源码，有助理解：

⽤户发送请求⾄前端控制器DispatcherServlet。
DispatcherServlet 收到请求调⽤ HandlerMapping 处理器映射器。
处理器映射器找到具体的处理器(可以根据xml配置、注解进⾏查找)，⽣成处理器及处理器拦截器(如果有则⽣成)⼀并返回给DispatcherServlet。
DispatcherServlet调⽤HandlerAdapter处理器适配器。
HandlerAdapter经过适配调⽤具体的处理器(Controller，也叫后端控制器)
Controller执⾏完成返回ModelAndView。
HandlerAdapter 将 Controller 执⾏结果 ModelAndView 返回给DispatcherServlet。
DispatcherServlet将ModelAndView传给ViewReslover视图解析器。
ViewReslover解析后返回具体View。
DispatcherServlet根据View进⾏渲染视图（即将模型数据填充⾄视图中）。
DispatcherServlet 响应⽤户。

SpringMVC执行流程

再看源码

我们都知道当从用户发起请求到后端是，首先走的就是DispatcherServlet，接着就会调用doService()方法执行业务逻辑，doService()方法也只是一个中转站，实际执行逻辑的是doDispatch()方法，且看源码：

@Override protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception { logRequest(request); // 省略部分源码 try { // 执行实际逻辑 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); } } ServletRequestPathUtils.setParsedRequestPath(previousRequestPath, request); } }

doDispatch 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); // 为当前请求获取映射处理器 mappedHandler = getHandler(processedRequest); if (mappedHandler == null) { noHandlerFound(processedRequest, response); return; } // 获取映射处理器适配器 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 (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) { return; } } if (!mappedHandler.applyPreHandle(processedRequest, response)) { return; } // 实际调用的Handler mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); if (asyncManager.isConcurrentHandlingStarted()) { return; } applyDefaultViewName(processedRequest, mv); 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); } //处理转发结果 processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException); } catch (Exception ex) { triggerAfterCompletion(processedRequest, response, mappedHandler, ex); } catch (Throwable err) { triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", err)); } finally { if (asyncManager.isConcurrentHandlingStarted()) { // Instead of postHandle and afterCompletion if (mappedHandler != null) { mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response); } } else { // Clean up any resources used by a multipart request. if (multipartRequestParsed) { cleanupMultipart(processedRequest); } } } }

下面来看一下其中几个重要的方法：

1.getHandler(HttpServletRequest request)方法：该方法是处理当前请求找到合适的HandlerMapping，并返回一个HandlerExecutionChain，HandlerExecutionChain，HandlerExecutionChain包含了具体的处理器(handler)和拦截器列表。

HandlerMapping 默认的实现有org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping 和 org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping。

@Nullable protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception { if (this.handlerMappings != null) { for (HandlerMapping mapping : this.handlerMappings) { HandlerExecutionChain handler = mapping.getHandler(request); if (handler != null) { return handler; } } } return null; }

2.getHandlerAdapter(Object handler) 根据HandlerExecutionChain中的handler来获取处理器适配器(HandlerAdapter)，

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException { if (this.handlerAdapters != null) { for (HandlerAdapter adapter : this.handlerAdapters) { if (adapter.supports(handler)) { return adapter; } } } throw new ServletException("No adapter for handler [" + handler + "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler"); }

HandlerAdapter有两个默认实现类，分别是 org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter和 org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter，前者用于没有使用模板引擎的请求，后者用于使用了模板引擎的接口。

实际处理请求的是HandlerAdapter的handle方法，如果是没有使用例如JSP等的模板引擎，handle方法就会返回null，如果使用了模板引擎就会返回一个ModelAndView对象。

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

handle方法最终调用的是Controller接口的 handleRequest(HttpServletRequest request, HttpServletResponse response) 方法来处理请求。

以SimpleControllerHandlerAdapter#handle方法源码为例：

@Override @Nullable public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception { return ((Controller) handler).handleRequest(request, response); }

3.processDispatchResult方法用于处理转发结果，该结果要么是一个ModelAndView，要么抛异常。

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); //处理异常 mv = processHandlerException(request, response, handler, exception); errorView = (mv != null); } } // Did the handler return a view to render? if (mv != null && !mv.wasCleared()) { //加载视图 render(mv, request, response); if (errorView) { WebUtils.clearErrorRequestAttributes(request); } } else { if (logger.isTraceEnabled()) { logger.trace("No view rendering, null ModelAndView returned."); } } if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) { // Concurrent handling started during a forward return; } if (mappedHandler != null) { // Exception (if any) is already handled.. mappedHandler.triggerAfterCompletion(request, response, null); } }

processDispatchResult方法中在正常情况下会调用render方法。

4.render方法用来通过名称呈现视图，它也是请求处理的最后一步。

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; String viewName = mv.getViewName(); if (viewName != null) { // 通过视图名称获取视图 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.isTraceEnabled()) { logger.trace("Rendering view [" + view + "] "); } try { if (mv.getStatus() != null) { response.setStatus(mv.getStatus().value()); } //渲染视图 view.render(mv.getModelInternal(), request, response); } catch (Exception ex) { if (logger.isDebugEnabled()) { logger.debug("Error rendering view [" + view + "]", ex); } throw ex; } }

DispatcherServlet的render方法是对视图View的封装，最后调用的还是View的render方法。

resolveViewName方法用于解析视图名称，它会通过视图解析器ViewResolver的resolveViewName方法解析视图并返回一个视图View，然后再通过View的render方法渲染视图，至于是怎么渲染视图的这里就不介绍了，感兴趣的可以自行查看源码。

protected View resolveViewName(String viewName, @Nullable Map 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; }

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