SpringMVC 拦截器源码解析

前言：这两天学习了代理模式，自然想到了SpringMVC 的 AOP 使用的就是动态代理，拦截器使用的就是 jdk 的动态代理。今天看了看源码，记录一下。SpringMVC 的拦截器使用移步：Java Servlet 过滤器与 SpringMVC 拦截器的区别？

源码解析

springMVC 的所有连接入口都会进入 DispatcherServlet，然后在这里面去调用真正的 Controller。而拦截器要达到的作用则是在调用 Controller 前后去做一些事情。所以现在需要看看 DispatcherServlet 的源码。

DispatcherServlet

DispatcherServlet 的源码入口在 doService() 方法。

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
        if(this.logger.isDebugEnabled()) {
            String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult()?" resumed":"";
            this.logger.debug("DispatcherServlet with name '" + this.getServletName() + "'" + resumed + " processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");
        }

        Map<String, Object> attributesSnapshot = null;
        if(WebUtils.isIncludeRequest(request)) {
            attributesSnapshot = new HashMap();
            Enumeration attrNames = request.getAttributeNames();

            label108:
            while(true) {
                String attrName;
                do {
                    if(!attrNames.hasMoreElements()) {
                        break label108;
                    }

                    attrName = (String)attrNames.nextElement();
                } while(!this.cleanupAfterInclude && !attrName.startsWith("org.springframework.web.servlet"));

                attributesSnapshot.put(attrName, request.getAttribute(attrName));
            }
        }

        request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.getWebApplicationContext());
        request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
        request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
        request.setAttribute(THEME_SOURCE_ATTRIBUTE, this.getThemeSource());
        FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
        if(inputFlashMap != null) {
            request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
        }

        request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
        request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);

        try {
            this.doDispatch(request, response);
        } finally {
            if(!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted() && attributesSnapshot != null) {
                this.restoreAttributesAfterInclude(request, attributesSnapshot);
            }

        }

    }

doService() 方法里面处理了一些信息之后，调用了 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;
                    mappedHandler = this.getHandler(processedRequest);
                    if(mappedHandler == null || mappedHandler.getHandler() == null) {
                        this.noHandlerFound(processedRequest, response);
                        return;
                    }

                    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;
                        }
                    }

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

                    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                    if(asyncManager.isConcurrentHandlingStarted()) {
                        return;
                    }

                    this.applyDefaultViewName(processedRequest, mv);
                    mappedHandler.applyPostHandle(processedRequest, response, mv);
                } catch (Exception var20) {
                    dispatchException = var20;
                } catch (Throwable var21) {
                    dispatchException = new NestedServletException("Handler dispatch failed", var21);
                }

                this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);
            } catch (Exception var22) {
                this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22);
            } catch (Throwable var23) {
                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);
            }

        }
    }

doDispatch() 方法中通过 this.getHandler(processedRequest); 获取到了处理器链 HandlerExecutionChain mappedHandle 之后，去执行了  mappedHandler.applyPreHandle(processedRequest, response) 方法，从而执行了拦截器的 preHandle 方法，如果返回为 false，则会调用处理器链的 triggerAfterCompletion 方法，然后 DispatchServlet 类中会直接 retreturn；如果返回为 true，则继续。

然后调用了 HandlerAdapter 的 handler() 方法；

然后调用了 mappedHandler.applyPostHandle(processedRequest, response, mv); 方法，从而执行了拦截器的 postHandle 方法；

然后调用了 this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException); 方法；在 processDispatchResult() 方法里面渲染了对应的视图，然后调用了 mappedHandler.triggerAfterCompletion(request, response, (Exception)null); 方法，从而执行了拦截器的 afterCompletion 方法。

processDispatchResult() 源码如下：

    private void processDispatchResult(HttpServletRequest request, HttpServletResponse response, HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {
        boolean errorView = false;
        if(exception != null) {
            if(exception instanceof ModelAndViewDefiningException) {
                this.logger.debug("ModelAndViewDefiningException encountered", exception);
                mv = ((ModelAndViewDefiningException)exception).getModelAndView();
            } else {
                Object handler = mappedHandler != null?mappedHandler.getHandler():null;
                mv = this.processHandlerException(request, response, handler, exception);
                errorView = mv != null;
            }
        }

        if(mv != null && !mv.wasCleared()) {
            this.render(mv, request, response);
            if(errorView) {
                WebUtils.clearErrorRequestAttributes(request);
            }
        } else if(this.logger.isDebugEnabled()) {
            this.logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + this.getServletName() + "': assuming HandlerAdapter completed request handling");
        }

        if(!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
            if(mappedHandler != null) {
                mappedHandler.triggerAfterCompletion(request, response, (Exception)null);
            }

        }
    }

从 doDispatch() 方法这里主要需要知道 HandlerExecutionChain mappedHandler = this.getHandler(processedRequest); 这一句调用的 getHandler() 方法，源码如下。

getHandle() 方法里是通过 HandlerMapping 的 getHandler 方法返回 HandlerExecutionChain 的。

从代码中不难看出整个逻辑就是依次判断 servlet 中的每个 handlerMapping 是否能够匹配该请求，直到找到那个匹配的然后返回处理结果。

    protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        Iterator var2 = this.handlerMappings.iterator();

        HandlerExecutionChain handler;
        do {
            if(!var2.hasNext()) {
                return null;
            }

            HandlerMapping hm = (HandlerMapping)var2.next();
            if(this.logger.isTraceEnabled()) {
                this.logger.trace("Testing handler map [" + hm + "] in DispatcherServlet with name '" + this.getServletName() + "'");
            }

            handler = hm.getHandler(request);
        } while(handler == null);

        return handler;
    }

HandlerExecutionChain 类

applyPreHandle() 方法源码如下：可以看出是顺序调用拦截器的 preHandle() 方法。preHandle 返回 false 时调用了 triggerAfterCompletion 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;
    }

applyPostHandle() 方法源码如下：可以看出这里是逆序调用 postHandle 方法。

    void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception {
        HandlerInterceptor[] interceptors = this.getInterceptors();
        if(!ObjectUtils.isEmpty(interceptors)) {
            for(int i = interceptors.length - 1; i >= 0; --i) {
                HandlerInterceptor interceptor = interceptors[i];
                interceptor.postHandle(request, response, this.handler, mv);
            }
        }

    }

　triggerAfterCompletion() 方法源码如下：可以看出这里也是逆序调用 afterCompletion 方法。

    void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, Exception ex) throws Exception {
        HandlerInterceptor[] interceptors = this.getInterceptors();
        if(!ObjectUtils.isEmpty(interceptors)) {
            for(int i = this.interceptorIndex; i >= 0; --i) {
                HandlerInterceptor interceptor = interceptors[i];

                try {
                    interceptor.afterCompletion(request, response, this.handler, ex);
                } catch (Throwable var8) {
                    logger.error("HandlerInterceptor.afterCompletion threw exception", var8);
                }
            }
        }

    }