HandlerInterceptor攔截Controller，實現(xiàn)pre和post方法。

HandlerExceptionResolver全局try-catch ?處理Controller里的異常。

這兩個類如何在DispatcherServlet中生效？

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.//判斷這個請求到底用哪個controller的哪個方法來處理。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;}}// 在執(zhí)行請求controller之前的操作，跟進可以看到里面是拿到所有注冊的 HandlerInterceptor， 執(zhí)行HandlerInterceptor.preHandle（HttpServletRequest request, HttpServletResponse response, Object handler）方法。if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}// Actually invoke the handler.//真正的去調用controller方法的地方，這里的ha是 AbstractHandlerMethodAdapter ，可以跟進看看其子類RequestMappingHandlerAdapter的 handleInternal（）方法。真正執(zhí)行controller方法的是 mav = invokeHandlerMethod(request, response, handlerMethod);這一步mv = ha.handle(processedRequest, response, mappedHandler.getHandler());if (asyncManager.isConcurrentHandlingStarted()) {return;}applyDefaultViewName(processedRequest, mv);//在執(zhí)行請求controller之后的操作，跟進可以看到里面是拿到所有注冊的 HandlerInterceptor， 執(zhí)行HandlerInterceptor.postHandle（request, response, this.handler, mv）方法。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);}//上面的兩個catch都可以看到用dispatchException 接收了異常，然后這一步是進行異常處理，可以看到的是用HandlerExceptionResolver 進行遍歷處理異常，只要處理到handlerExceptionResolver.resolveException(request, response, handler, ex)不為空的返回即break.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 afterCompletionif (mappedHandler != null) {mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);}}else {// Clean up any resources used by a multipart request.if (multipartRequestParsed) {cleanupMultipart(processedRequest);}}}}

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