SpringMVC处理请求流程

本文主要介绍在SpringMVC中，当一个URL请求发起后，SpringMVC如何根据URL将对应的请求分发到指定的Handler方法中(Controller中的指定方法)

前端请求发起时，会进入 HttpServlet 及继承了HttpServlet的类，进入对应的doGet,doPost方法;HttpServlet及其实现类中，只有FrameworkServlet类中实现了doGet及doPost,而在这两个方法中，实际调用的是doService这个方法，而FrameworkServlet中的doService是个抽象方法，其字子类DispatcherServlet 则实现了这个方法；在DispatcherServlet的doService方法中，调用了doDispatch(request, response) 真正处理请求,完成了SpringMVC处理的整个核心流程;

doDispatch()方法具体如下

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 {
         // 1 检查是否是文件上传的请求
         processedRequest = checkMultipart(request);
         multipartRequestParsed = (processedRequest != request);

         // Determine handler for the current request.
         // 2 取得处理当前请求的Handler即处理器  HandlerExecutionChain 请求处理链对象 该对象封装了Handler和Inteceptor ==》controller
         mappedHandler = getHandler(processedRequest);
         if (mappedHandler == null) {
            // 如果 handler 为空，则返回404
            noHandlerFound(processedRequest, response);
            return;
         }

         // Determine handler adapter for the current request.
         // 3 获取处理请求的处理器适配器 HandlerAdapter
         HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

         // Process last-modified header, if supported by the handler.
         // 处理 last-modified 请求头
         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;
         }

         // Actually invoke the handler.
         // 4 实际处理器处理请求，返回结果视图对象 
         mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

         if (asyncManager.isConcurrentHandlingStarted()) {
            return;
         }
         // 5. 结果视图对象的处理  如果没有配置返回页面，此方法将给赋一个默认页面
         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);
      }
        //5.  视图渲染及跳转  方法最后也会调用triggerAfterCompletion()
      processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
   }
   catch (Exception ex) {
      //最终会调用HandlerInterceptor的afterCompletion 方法
      triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
   }
   catch (Throwable err) {
      //最终会调用HandlerInterceptor的afterCompletion 方法
      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);
         }
      }
   }
}