SpringFramework之HandlerInterceptor

    最近在使用Spring时，总感受对HandlerInterceptor有点模糊，回头再来看看，记录下。

    SpringFramework的版本4.3.x.RELEASE.

    List-1

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.
			//一、获得handlerExecutionChain，里面含有对应的Controller,里面还有interceptor，
			mappedHandler = getHandler(processedRequest);
			if (mappedHandler == null || mappedHandler.getHandler() == null) {
				noHandlerFound(processedRequest, response);
				return;
			}

			// Determine handler adapter for the current request.
			//二、会根据Controller方法上的注解，将request中的请求转换为对象，转换为对应的pathVariable、body等
			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;
				}
			}
            //三、
			if (!mappedHandler.applyPreHandle(processedRequest, response)) {
				return;
			}

			// Actually invoke the handler.
			//四、真正进行反射操做，调用方法
			mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

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

			applyDefaultViewName(processedRequest, mv);
            //五、
			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);
		}
		//6
		processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
	}
	catch (Exception ex) {
	    //7
		triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
	}
	catch (Throwable err) {
	    //8
		triggerAfterCompletion(processedRequest, response, mappedHandler,
				new NestedServletException("Handler processing failed", err));
	}

    如List-1所示，

• 1位置，根据ServletRequest获得HandlerExecutionChain(具体怎么获得的有点复杂，后续再深刻分析)，它有点相似SpringSecurity中的FilterChainProxy。里面有HandlerInterceptor数组，和handler属性(经过debug能够看到是HandlerMethod)。
• 2位置，会根据Controller方法上的注解，将request中的请求转换为对象，转换为对应的pathVariable、body等。
• 3位置，调用HandlerExecutionChain的applyPreHandle，以下，逐个调用preHandle方法，只要preHandle返还false，就会你像调用afterCompletion，最后返还false。思考，经过源码可知在HandlerInterceptor中拦截后续preHandle中抛出的异常是拦截不到的。这个HandlerExceptionChain是线程安全的，每一个请求对应一个HandlerExceptionChain实例对象，因此源码中经过类属性interceptorIndex来控制下标。

    List-2

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;
}

• 位置4，到位置4，已经执行了全部handlerInterceptor的preHandle，而且已经将ServletRequest中的参数转换为了Controller方法上标有注解的参数。因此在位置经过反射调用Controller的方法。经过源码能够看出，HandlerInterceptor能够看出，只要Controller里面的业务代码抛出异常，那么位置5就执行不到，就不会执行HandlerInterceptor的postHandle方法。
• 位置5，以下List-3所示，会逆向执行HandlerInterceptor的postHandle方法。

    List-3

void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception {
	HandlerInterceptor[] interceptors = 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);
		}
	}
}

• 位置六、若是以前没有抛出异常，就调用processDispatchResult方法，方法processDispatchResult的最后，有以下List-4，调用HandlerExecutionChain的triggerAfterCompletion，会逆序调用HandlerInterceptor的afterCompletion，由List-5能够看出，afterCompletion中抛出异常，会被Spring框架吞噬，每一个HandlerInterceptor的afterCompletion方法中，拿到的异常都是null——看List-4中传入的参数。

    List-4

......
	if (mappedHandler != null) {
		mappedHandler.triggerAfterCompletion(request, response, null);
	}
}

    List-5

void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, Exception ex)
		throws Exception {

	HandlerInterceptor[] interceptors = 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 ex2) {
				logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
			}
		}
	}
}

• 位置7/8，若是以前的步骤抛出异常，就会到步骤7/8，调用triggerAfterCompletion，并将异常做为参数传入，由List-5能够看出，每一个HandlerInterceptor的afterCompletion都能接收到该异常。

    

            

                                                              图1 

    如图1所示，

1. 若是执行preHandle的链中抛出异常，那么逆向执行afterCompletion，不会执行postHandle；若是postHandle抛出异常，则逆向执行afterCompletion链。
2. 若是没有抛出异常，正常状况下，先执行完preHandle链，以后调用controller的方法，以后调用逆向postHandle链，以后逆向执行afterCompletion链，如图1中右边的竖行方向调用所示。

    经过源码可知，用HandlerInterceptor拦截全局异常，有点不靠谱，要深刻理解源码，否则有可能达不到预期的效果。

思考

1. 咱们自定义的，这些HandlerInterceptor是如何被接入到Spring中的？
2. Filter、HandlerInterceptor、@ControllerAdvice的调用顺序？
3. 定义多个HandlerIntercepto时，如何指定顺序?

Reference

1. SpringFramework源码