springMvc 拦截器源码解析

统一请求处理

dispatcherServlet 类图结构

前端请求过来先进入servlet 的 service 方法

// FrameWorkServlet
@Override
	protected void service(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
		if (httpMethod == HttpMethod.PATCH || httpMethod == null) { //<1>
			processRequest(request, response);
		}
		else {//<2>
			super.service(request, response);
		}
	}

无论是<1>还是<2> 都会进入FrameWorkServlet.processRequest方法

// FrameWorkServlet
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		long startTime = System.currentTimeMillis();
		Throwable failureCause = null;

		LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
		LocaleContext localeContext = buildLocaleContext(request);

		RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
		ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
		asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());

		initContextHolders(request, localeContext, requestAttributes);

		try {
			doService(request, response);//进入DispatcherServlet的doservice方法
		}
		catch (ServletException | IOException ex) {
			failureCause = ex;
			throw ex;
		}
		catch (Throwable ex) {
			failureCause = ex;
			throw new NestedServletException("Request processing failed", ex);
		}

		finally {
			resetContextHolders(request, previousLocaleContext, previousAttributes);
			if (requestAttributes != null) {
				requestAttributes.requestCompleted();
			}
			logResult(request, response, failureCause, asyncManager);
			publishRequestHandledEvent(request, response, startTime, failureCause);
		}
	}

再进入DispatcherServlet的doservice方法

再进入DispatcherServlet 的 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 {
			ModelAndView mv = null;
			Exception dispatchException = null;

			try {
				processedRequest = checkMultipart(request);
				multipartRequestParsed = (processedRequest != request);

				// Determine handler for the current request.
				// 获得处理器
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null) {
					noHandlerFound(processedRequest, response);
					return;
				}

				// Determine handler adapter for the current request.
				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 (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
						return;
					}
				}
				// 拦截器前置处理
				if (!mappedHandler.applyPreHandle(processedRequest, response)) {
					return;
				}

				// Actually invoke the handler.
				// 调用请求方法
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

				if (asyncManager.isConcurrentHandlingStarted()) {
					return;
				}

				applyDefaultViewName(processedRequest, mv);
				// 拦截器后置处理
				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);
			}
			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 afterCompletion
				if (mappedHandler != null) {
					mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
				}
			}
			else {
				// Clean up any resources used by a multipart request.
				if (multipartRequestParsed) {
					cleanupMultipart(processedRequest);
				}
			}
		}
	}

先通过getHandler 获得 HandlerExecutionChain，调用 HandlerExecutionChain 的 applyPreHandle执行前置处理，调用 applyPostHandle 执行后置处理，至此可以看到拦截器的处理请求，拦截器处理前置请求方法如下：

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

可以看到拦截器们实际上设置在HandlerExecutionChain .interceptors属性中，那么HandlerExecutionChain .interceptors是在哪里设置的呢？往下看

// AbstractHandlerMapping
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
		Object handler = getHandlerInternal(request);
		if (handler == null) {
			handler = getDefaultHandler();
		}
		if (handler == null) {
			return null;
		}
		// Bean name or resolved handler?
		if (handler instanceof String) {
			String handlerName = (String) handler;
			handler = obtainApplicationContext().getBean(handlerName);
		}

		HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);

		if (logger.isTraceEnabled()) {
			logger.trace("Mapped to " + handler);
		}
		else if (logger.isDebugEnabled() && !request.getDispatcherType().equals(DispatcherType.ASYNC)) {
			logger.debug("Mapped to " + executionChain.getHandler());
		}

		if (CorsUtils.isCorsRequest(request)) {
			CorsConfiguration globalConfig = this.corsConfigurationSource.getCorsConfiguration(request);
			CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);
			CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig);
			executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
		}

		return executionChain;
	}

// AbstractHandlerMapping
protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
		HandlerExecutionChain chain = (handler instanceof HandlerExecutionChain ?
				(HandlerExecutionChain) handler : new HandlerExecutionChain(handler));

		String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);
		for (HandlerInterceptor interceptor : this.adaptedInterceptors) {
			// 如果是 MappedInterceptor 类型的拦截器
			if (interceptor instanceof MappedInterceptor) {
				MappedInterceptor mappedInterceptor = (MappedInterceptor) interceptor;
				// 只有路径匹配的请求，才添加到拦截器链中
				if (mappedInterceptor.matches(lookupPath, this.pathMatcher)) {
					chain.addInterceptor(mappedInterceptor.getInterceptor());
				}
			}// 不是 MappedInterceptor 类型的拦截器，直接添加到拦截器链中
			else {
				chain.addInterceptor(interceptor);
			}
		}
		return chain;
	}

回到AbstractHandlerMapping 的 getHandler方法，通过getHandlerExecutionChain创建 HandlerExecutionChain对象，HandlerExecutionChain 中 的 拦截器就是通过AbstractHandlerMapping 的 adaptedInterceptors 过滤得到。而AbstractHandlerMapping.adaptedInterceptors是在哪里初始化的呢，看下面：

AbstractHandlerMapping 实现了 ApplicationContextAware 接口，而 spring 中实现 Aware 接口的类一般在它创建完成后的初始化方法中会进行设置属性，此过程大致如下：
AbstractBeanFactory.doGetBean
->AbstractAutowireCapableBeanFactory.doCreateBean
->AbstractAutowireCapableBeanFactory.initializeBean
->AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsBeforeInitialization
->ApplicationContextAwareProcessor.invokeAwareInterfaces
->ApplicationObjectSupport.setApplicationContext
->WebApplicationObjectSupport.initApplicationContext
->ApplicationObjectSupport.initApplicationContext
->AbstractHandlerMapping.initApplicationContext
->AbstractHandlerMapping.detectMappedInterceptors

protected void detectMappedInterceptors(List mappedInterceptors) {
		// 获取 spring 容器中 所有类型为 MappedInterceptor 的对象
		mappedInterceptors.addAll(
				BeanFactoryUtils.beansOfTypeIncludingAncestors(
						obtainApplicationContext(), MappedInterceptor.class, true, false).values());
	}

至此，也知道AbstractHandlerMapping 的 adaptedInterceptors是怎么初始化的了。
整个流程大概就是spring容器初始化的时候，把所有拦截器填充到了AbstractHandlerMapping.adaptedInterceptors属性中缓存起来，等请求来了的时候通过getHandlerExecutionChain将符合条件的拦截器存到HandlerExecutionChain对象里，然后调用拦截器的前置以及后置处理方法