从servlet到springboot（18）DispatcherServlet#doDispather详解与处理请求全过程

上一篇我们分析到doService的doDispatcher方法之前，这一篇我们将分析核心方法doDispatcher

 

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);

         // Determine handler for the current request.
         mappedHandler = getHandler(processedRequest);
         if (mappedHandler == null || mappedHandler.getHandler() == null) {
            noHandlerFound(processedRequest, response);
            return;
         }

         // Determine handler adapter for the current request.
         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 (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.
         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);
      }
      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);
         }
      }
   }
}

方法比较长，我们句句分析

这里需要优先了解下SpringMVC下的几大组件，关于这几个组件可以参考这篇文章https://www.cnblogs.com/leiqiannian/p/7807579.html

接下来我们详细说明

processedRequest = checkMultipart(request);这句话会检查请求是不是上传请求

点进去会发现有MultipartResolver这个组件，这个组件是专门用来处理上传请求的

mappedHandler = getHandler(processedRequest);根据request获取Handler

HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());根据Handler获取HandlerAdapter

这一段话会处理GET / HEAD请求的Last-Modified，关于Last-Modified，我们参考https://www.cnblogs.com/dcz2015/p/5751784.html这篇文章

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;
   }
}

mv = ha.handle(processedRequest, response, mappedHandler.getHandler());用HandlerAdapter处理Handler

processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException); 处理放回结果，包括返回view给用户

.

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

这句话会处理interceptor的前置方法

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

用handlerAdatper去处理request

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

如果是异步处理就立刻返回

mappedHandler.applyPostHandle(processedRequest, response, mv);这里是处理interceptor的后置方法

processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);

处理返回结果，这个方法里有一个比较重要的代码

mappedHandler.triggerAfterCompletion(request, response, null);

这个是触发interceptor的afterCompltetion方法

if (multipartRequestParsed) {
   cleanupMultipart(processedRequest);
}

这里如果是上传请求还会删除上传的资源

用一张图来归纳这个过程