关于SpringMVC的执行流程

先贴一张SpringMVC的执行流程图

然后一步一步Debug去观察一下源码是怎么做到的

首先一个请求发送到后端时会把请求打到DispatcherServlet上，而主要处理请求的方法是doService方法

doService方法主要做的事

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;
        //处理Include标签的请求
		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("org.springframework.web.servlet")) {
					attributesSnapshot.put(attrName, request.getAttribute(attrName));
				}
			}
		}

		// Make framework objects available to handlers and view objects.
         // 将上下文存储到Attribute
		request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
         // 将国际化解析器存储到Attribute中
		request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
         // 将主题解析器存储到Attribute中
		request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
         // 将主题存储到Attribute中
		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);

概括一下doService方法的作用就是

1、保存请求快照
2、共享上下文信息，本地解析，主题解析等对象给 handler和view对象

然后会把请求交给doDispatch方法进行处理

try {
		doDispatch(request, response);
	}

然后进入doDispatch方法来看一下具体的处理(主要列出我认为比较重要的代码)

try {
    //判断是否为文件上传请求
   processedRequest = checkMultipart(request);
   multipartRequestParsed = (processedRequest != request);

   // Determine handler for the current request.
    //根据请求找到对应的handler，下一步会进入该方法去看
   mappedHandler = getHandler(processedRequest);
   if (mappedHandler == null || mappedHandler.getHandler() == null) {
      noHandlerFound(processedRequest, response);
      return;
   }

   // Determine handler adapter for the current request.
   // 为handler找到对应的HandlerAdapter(处理器适配器)
   HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

   // Process last-modified header, if supported by the handler.
   String method = request.getMethod();
   // 判断一下是不是GET方法
   boolean isGet = "GET".equals(method);
   if (isGet || "HEAD".equals(method)) {
       //getLastModified方法的作用下面也会说
      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;
      }
   }

   if (!mappedHandler.applyPreHandle(processedRequest, response)) {
      return;
   }

   // Actually invoke the handler.
   // 调用适配器，返回modelandreview
   mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

   if (asyncManager.isConcurrentHandlingStarted()) {
      return;
   }

   applyDefaultViewName(processedRequest, mv);
   mappedHandler.applyPostHandle(processedRequest, response, mv);

**来看一下mappedHandler = getHandler(processedRequest);**是怎么做的

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
         // 遍历handlerMappings
		for (HandlerMapping hm : this.handlerMappings) {
			if (logger.isTraceEnabled()) {
				logger.trace(
						"Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
			}
             //根据请求找到对应的处理器链
			HandlerExecutionChain handler = hm.getHandler(request);
			if (handler != null) {
				return handler;
			}
		}
		return null;
	}

在深入看一下hm.getHandler是怎么做的

	public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        // 从当前请求解析出要查找的url，然后继续寻找 handler
		Object handler = getHandlerInternal(request);
        // 如果handler为null，会去找默认的Handler
		if (handler == null) {
			handler = getDefaultHandler();
		}
		if (handler == null) {
			return null;
		}
		// Bean name or resolved handler?
        // 通过Bean去找handler
		if (handler instanceof String) {
			String handlerName = (String) handler;
			handler = getApplicationContext().getBean(handlerName);
		}
         // 返回处理器链
		HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);
		if (CorsUtils.isCorsRequest(request)) {
			CorsConfiguration globalConfig = this.corsConfigSource.getCorsConfiguration(request);
			CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);
			CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig);
			executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
		}
		return executionChain;
	}

getHandlerInternal中的两个方法

// 从请求头中获取lookPath，lookPath中存储着要查找的urlPath
String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
//找到真正需要执行的handlerMethod
HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);

来看一下比较重要的一行代码HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);

比较重要的代码

//将拦截器封装到chain中返回
for (HandlerInterceptor interceptor : this.adaptedInterceptors) {
			if (interceptor instanceof MappedInterceptor) {
				MappedInterceptor mappedInterceptor = (MappedInterceptor) interceptor;
				if (mappedInterceptor.matches(lookupPath, this.pathMatcher)) {
					chain.addInterceptor(mappedInterceptor.getInterceptor());
				}
			}

然后去找合适的处理器适配器HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

// 遍历handlerAdapters找到支持handler的处理器适配器
for (HandlerAdapter ha : this.handlerAdapters) {
			if (logger.isTraceEnabled()) {
				logger.trace("Testing handler adapter [" + ha + "]");
			}
			if (ha.supports(handler)) {
				return ha;
			}
		}

然后就是去调用处理器得到modelandview mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

	public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
			throws Exception {

		((HttpRequestHandler) handler).handleRequest(request, response);
		return null;
	}