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DispatcherServlet是怎么处理请求的?

时间:2022-12-12 14:56:18浏览次数:47  
标签:DispatcherServlet 请求 处理 mappedHandler request 处理器 null response

上篇文章总结了DispatcherServlet的核心功能,今天趁热打铁,系统梳理DispatcherServlet处理请求的流程。

DispatcherServlet处理请求的核心方法是doDispatch()。在处理过程中,会协同使用各组件的功能,共同完成对请求的处理。

以下是doDispatch()的执行流程图:

以下是doDispatch()的源码:

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、文件请求预处理
         processedRequest = checkMultipart(request);  
         multipartRequestParsed = (processedRequest != request);  
  
         // 2、获取请求处理器执行链  
         mappedHandler = getHandler(processedRequest);  
         if (mappedHandler == null) {  
            noHandlerFound(processedRequest, response);  
            return;  
         }  
  
         // 3、获取处理器适配器
         HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());  
  
         String method = request.getMethod();  
         boolean isGet = HttpMethod.GET.matches(method);  
         if (isGet || HttpMethod.HEAD.matches(method)) {  
            long lastModified = ha.getLastModified(request, mappedHandler.getHandler());  
            if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {  
               return;  
            }  
         }  
         
		 // 4、拦截器预处理
         if (!mappedHandler.applyPreHandle(processedRequest, response)) {  
            return;  
         }  
  
         // 5、处理器适配器处理请求
         mv = ha.handle(processedRequest, response, mappedHandler.getHandler());  
  
         if (asyncManager.isConcurrentHandlingStarted()) {  
            return;  
         }  
         applyDefaultViewName(processedRequest, mv);  
	     // 6、拦截器后处理
         mappedHandler.applyPostHandle(processedRequest, response, mv);  
      }  
      catch (Exception ex) {  
         dispatchException = ex;  
      }  
      catch (Throwable err) {  
      }  
      // 7、结果处理
      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()) {  
         if (mappedHandler != null) {  
            mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);  
         }  
      }  
      else {  
         // 8、文件请求后处理
         if (multipartRequestParsed) {  
            cleanupMultipart(processedRequest);  
         }  
      }  
   }  
}

1 文件请求预处理

DispatcherServlet在处理请求时,首先会使用MultipartResolver进行文件请求处理。
具体文件请求源码位于org.springframework.web.servlet.DispatcherServlet#checkMultipart

protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {  
   if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {  
      if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {  
         if (DispatcherType.REQUEST.equals(request.getDispatcherType())) {  
            logger.trace("Request already resolved to MultipartHttpServletRequest, e.g. by MultipartFilter");  
         }  
      }  
      else if (hasMultipartException(request)) {  
         logger.debug("Multipart resolution previously failed for current request - " +  
               "skipping re-resolution for undisturbed error rendering");  
      }  
      else {  
         try {  
            return this.multipartResolver.resolveMultipart(request);  
         }  
         catch (MultipartException ex) {  
            if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) != null) {  
               logger.debug("Multipart resolution failed for error dispatch", ex);  
               // Keep processing error dispatch with regular request handle below  
            }  
            else {  
               throw ex;  
            }  
         }  
      }  
   }  
   // If not returned before: return original request.  
   return request;  
}

首先,会判断当前请求是否是文件请求,主要通过请求头Content-Type是否以multipart/开头进行判断。
如果是文件请求,由于可能在Filter阶段进行文件处理,因此还需要判断是否已经进行过文件处理。
如果没有进行文件处理,则会使用MultipartResolver进行实际文件处理。
在Spring中,MultipartResolver有两种实现类:

  1. StandardServletMultipartResolver:底层根据Servlet 3.0+规范实现。
  2. CommonsMultipartResolver:底层根据Apache的commons-fileupload工具实现。
    虽然这两个实现类的底层原理有所不同,但它们对文件的处理方式基本一致:
  3. 将HTTP文件流保存到本地临时文件。
  4. 将本地临时文件封装成MultipartHttpServletRequest对象返回。

2. 获取请求处理器执行链

文件请求预处理完成后,会获取请求处理器执行链:

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;  
}

获取请求处理器执行链会经历以下两个步骤

  1. 根据请求地址/请求方法等信息,从HandlerMapping中获取对应的处理器(简单来说,就是开发人员定义在@Controller中定义的对应方法)。
  2. 根据配置信息,添加拦截器。

3 获取处理器适配器

获取完处理器执行链后,会根据处理器获取对应的处理器适配器。
HandlerMappingHandlerAdapter一一对应。
一般来说,不同HandlerMapping会返回不同的处理器,通过判断HandlerAdapter适配的处理器类型,即可获取到对应的处理器适配器:

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");  
}

4 拦截器预处理

获取处理器适配器后,接下来是正式对请求的处理。
首先会进行拦截器的预处理,会遍历拦截器依次进行处理,只有所有拦截器的预处理通过后,才能进行处理器的实际调用。

boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {  
   for (int i = 0; i < this.interceptorList.size(); i++) {  
      HandlerInterceptor interceptor = this.interceptorList.get(i);  
      if (!interceptor.preHandle(request, response, this.handler)) {  
         triggerAfterCompletion(request, response, null);  
         return false;  
      }  
      this.interceptorIndex = i;  
   }  
   return true;  
}

5 处理器适配器处理请求

当所有拦截器的预处理通过后,会使用处理器适配器处理请求。
该阶段是Spring MVC最核心也最复杂的步骤:

  1. 根据@RequestParam@RequestBody等注解将请求数据转换成形参对象。
  2. 根据@Validated等注解对请求数据进行校验。
  3. 通过反射执行处理器方法。
  4. 根据@ResponseBody等注解将返回值进行数据格式处理并输出。

6 拦截器后处理

处理器适配器处理请求完成后,会进行拦截器后处理。此时拦截器的调用顺序正好和预处理顺序相反:

void applyPostHandle(HttpServletRequest request, HttpServletResponse response, @Nullable ModelAndView mv)  
      throws Exception {  
  
   for (int i = this.interceptorList.size() - 1; i >= 0; i--) {  
      HandlerInterceptor interceptor = this.interceptorList.get(i);  
      interceptor.postHandle(request, response, this.handler, mv);  
   }  
}

7 结果处理

当拦截器后处理接收后,会对处理器适配器处理请求的结果进行处理:

  1. 对上述所有步骤中出现的异常进行统一处理。
  2. 视图渲染和输出(前后端分离项目可以不考虑这一步骤)
  3. 拦截器的最终处理
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);  
         mv = processHandlerException(request, response, handler, exception);  
         errorView = (mv != null);  
      }  
   }  
  
   // Did the handler return a view to render?  
   if (mv != null && !mv.wasCleared()) {  
      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);  
   }  
}

8 文件请求后处理

当上述请求都处理完成,会进行文件请求后处理,其本质上就是将文件预处理产生的本地临时文件删除,释放占用的系统资源。

标签:DispatcherServlet,请求,处理,mappedHandler,request,处理器,null,response
From: https://www.cnblogs.com/Xianhuii/p/16976018.html

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