SpringMVC详细执行流程

1. 当浏览器发起一个请求时，首先根据DNS查找域名对应的ip地址，先判断浏览器缓存中是否有映射关系，如果没有查询本机host文件，再没有就查询远端域名解析器

2. 找到ip后，根据ip逻辑与上子网掩码，得到目标机器属于哪个网络，然后用子网掩码取反与上ip地址，得到哪个具体主机

3. 找到主机后，根据端口号找到对应的服务器。

4. 请求首先被dispatchServlet拦截，从URL中提取URI，

5. 然后调用handlerMapping，根据URI查询对应的请求处理器，返回handler以及拦截器

6. 然后根据请求处理器查询handlerAdapter，进行数据校验和转换，然后调用handler，触发前置拦截

7. 调用handler后返回modelAndView，然后由视图解析器进行model和view的分离，进行页面渲染。

源码DispatcherServlet#doDispatch

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 {
          // 校验是否为上传请求 是上传请求执行解析 否则返回request
         processedRequest = checkMultipart(request);
         multipartRequestParsed = (processedRequest != request);

         // 根据访问的Handler 返回指定对应的HandlerExecutionChain对象 这里从HandlerMapping 集合中查找 HandlerExecutionChain 对象包含Handler与拦截器HandlerInterceptor列表
         mappedHandler = getHandler(processedRequest);
         if (mappedHandler == null) {
            noHandlerFound(processedRequest, response);
            return;
         }

         // 根据得到的Handler 获取对应的HandlerAdaptor对象
         HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

         // 处理GET、HEAD请求的Last-Modified
         String method = request.getMethod();
         boolean isGet = "GET".equals(method);
         if (isGet || "HEAD".equals(method)) {
            long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
            //当数据没有更改时，就直接返回上次的数据，提高效率
             if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
               return;
            }
         }

         //执行Interceptor的preHandle 
         if (!mappedHandler.applyPreHandle(processedRequest, response)) {
            return;
         }

         // 执行Handler 返回ModelAndView
         mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

          //如果需要异步处理，直接返回
         if (asyncManager.isConcurrentHandlingStarted()) {
            return;
         }

         //当view为空时，根据request设置默认view，如Handler返回值为void
         applyDefaultViewName(processedRequest, mv);
         //执行相应Interceptor的postHandle 
         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);
      }
       //处理返回结果，包括处理异常、渲染页面，发出完成通知触发Interceptor的afterCompletion
      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);
         }
      }
   }
}