Spring攔截器原理

前幾天學習了spring中異常處理器是怎麼加載進DispatcherServlet並且怎麼處理異常的，以及過濾器的原理。今天看下攔截器是怎麼工作的，都知道是在DispatcherServlet中執行handler的方法前後會執行攔截器的prehandle和posthandle方法，那麼是怎麼加載進DispatcherServlet以及怎麼執行的呢？

加載入DispatcherServlet

DispatcherServlet初始化時，調用onRefresh方法，裏面調用initStrategies 的this.initHandlerMappings(context);方法中實際加載攔截器。

protected void onRefresh(ApplicationContext context) {
        this.initStrategies(context);
    }
    protected void initStrategies(ApplicationContext context) {
        this.initMultipartResolver(context);
        this.initLocaleResolver(context);
        this.initThemeResolver(context);
        this.initHandlerMappings(context);
        this.initHandlerAdapters(context);
        this.initHandlerExceptionResolvers(context);
        this.initRequestToViewNameTranslator(context);
        this.initViewResolvers(context);
        this.initFlashMapManager(context);
    }

initHandlerMappings方法會加載所有實現HandlerMapping的類。

private void initHandlerMappings(ApplicationContext context) {
        this.handlerMappings = null;
        if (this.detectAllHandlerMappings) {
            Map<String, HandlerMapping> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.handlerMappings = new ArrayList(matchingBeans.values());
                AnnotationAwareOrderComparator.sort(this.handlerMappings);
            }
        } else {
            try {
                HandlerMapping hm = (HandlerMapping)context.getBean("handlerMapping", HandlerMapping.class);
                this.handlerMappings = Collections.singletonList(hm);
            } catch (NoSuchBeanDefinitionException var3) {
                ;
            }
        }

在doDispatch方法中，mappedHandler = this.getHandler(processedRequest); 實際是調用抽象類AbstractHandlerMapping 的public final HandlerExecutionChain getHandler(HttpServletRequest request)方法，獲取了一個HandlerExecutionChain對象,把匹配request的url的攔截器都加入HandlerExecutionChain。

protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
        HandlerExecutionChain chain = handler instanceof HandlerExecutionChain ? (HandlerExecutionChain)handler : new HandlerExecutionChain(handler);
        String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);
        Iterator var5 = this.adaptedInterceptors.iterator();

        while(var5.hasNext()) {
            HandlerInterceptor interceptor = (HandlerInterceptor)var5.next();
            if (interceptor instanceof MappedInterceptor) {
                MappedInterceptor mappedInterceptor = (MappedInterceptor)interceptor;
                if (mappedInterceptor.matches(lookupPath, this.pathMatcher)) {
                    chain.addInterceptor(mappedInterceptor.getInterceptor());
                }
            } else {
                chain.addInterceptor(interceptor);
            }
        }

執行

攔截器的preHandle 方法實際是調用HandlerExecutionChain對象的applyPreHandle方法,這個調用是在HandlerAdapter對象的handle方法也就是contrller本身的方法調用之前。postHandle 在之後，最後在processDispatchResult 處理返回結果和抓異常時執行攔截器的afterCompletion方法。

  boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HandlerInterceptor[] interceptors = this.getInterceptors();
        if (!ObjectUtils.isEmpty(interceptors)) {
            for(int i = 0; i < interceptors.length; this.interceptorIndex = i++) {
                HandlerInterceptor interceptor = interceptors[i];
                if (!interceptor.preHandle(request, response, this.handler)) {
                    this.triggerAfterCompletion(request, response, (Exception)null);
                    return false;
                }
            }
        }

        return true;
    }

附上doDispatch方法：

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HttpServletRequest processedRequest = request;
        HandlerExecutionChain mappedHandler = null;
        boolean multipartRequestParsed = false;
        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

        try {
            try {
                ModelAndView mv = null;
                Object dispatchException = null;

                try {
                    processedRequest = this.checkMultipart(request);
                    multipartRequestParsed = processedRequest != request;
                    //包含了一個匹配改url的攔截器list
                    mappedHandler = this.getHandler(processedRequest);
                    if (mappedHandler == null || mappedHandler.getHandler() == null) {
                        this.noHandlerFound(processedRequest, response);
                        return;
                    }
                            //這個是實際controller的代理類
                    HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
                    String method = request.getMethod();
                    boolean isGet = "GET".equals(method);
                    if (isGet || "HEAD".equals(method)) {
                        long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                        if (this.logger.isDebugEnabled()) {
                            this.logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
                        }

                        if ((new ServletWebRequest(request, response)).checkNotModified(lastModified) && isGet) {
                            return;
                        }
                    }
                                //執行preHandle方法
                    if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                        return;
                    }
                                //實際controller方法的執行
                    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                    if (asyncManager.isConcurrentHandlingStarted()) {
                        return;
                    }

                    this.applyDefaultViewName(processedRequest, mv);
                    //執行postHandle方法
                    mappedHandler.applyPostHandle(processedRequest, response, mv);
                } catch (Exception var20) {
                    dispatchException = var20;
                } catch (Throwable var21) {
                    dispatchException = new NestedServletException("Handler dispatch failed", var21);
                }
                        //處理返回結果並執行攔截器的afterCompletion方法,如果exception不爲null則執行HandlerExceptionResolver的邏輯
                this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);
            } catch (Exception var22) {
                //執行攔截器的afterCompletion方法
                this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22);
            } catch (Throwable var23) {
                //執行攔截器的afterCompletion方法
                this.triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", var23));
            }

        } finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                if (mappedHandler != null) {
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            } else if (multipartRequestParsed) {
                this.cleanupMultipart(processedRequest);
            }

        }
    }