SpringBoot源码解析-ExceptionHandler处理异常的原理

在项目中，经常会使用ExceptionHandler来作为全局性的异常处理中心。那么ExceptionHandler处理异常的原理是什么呢，今天就来分析一下。

ExceptionHandler使用示例

@RestControllerAdvice
public class GlobalExceptionHandler {
    @ExceptionHandler(value = RuntimeException.class)
    public String handle(){
        return "error";
    }
}
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使用还是很简单的，在类上面添加ControllerAdvice注解，在方法上面添加ExceptionHandler注解，就可以在方法里处理相应的异常信息了。

原理剖析

ControllerAdvice和ExceptionHandler注解的作用

异常处理的核心类是ExceptionHandlerExceptionResolver，进入该类。查看afterPropertiesSet方法。

public void afterPropertiesSet() {
		// Do this first, it may add ResponseBodyAdvice beans
		initExceptionHandlerAdviceCache();
		...
	}

	private void initExceptionHandlerAdviceCache() {
		if (getApplicationContext() == null) {
			return;
		}
		//这行代码会找出所有标记了ControllerAdvice注解的类
		List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
		AnnotationAwareOrderComparator.sort(adviceBeans);

		for (ControllerAdviceBean adviceBean : adviceBeans) {
			Class<?> beanType = adviceBean.getBeanType();
			if (beanType == null) {
				throw new IllegalStateException("Unresolvable type for ControllerAdviceBean: " + adviceBean);
			}
			//遍历这些类，找出有ExceptionHandler注解标注的方法。
			ExceptionHandlerMethodResolver resolver = new ExceptionHandlerMethodResolver(beanType);
			if (resolver.hasExceptionMappings()) {
				this.exceptionHandlerAdviceCache.put(adviceBean, resolver);
			}
			if (ResponseBodyAdvice.class.isAssignableFrom(beanType)) {
				this.responseBodyAdvice.add(adviceBean);
			}
		}
		...
	}
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通过上述代码可以看出，在ExceptionHandlerExceptionResolver类中，该类扫描了所有标注有ExceptionHandler注解的方法，并将他们存入了exceptionHandlerAdviceCache中。

异常处理的原理

看过了ControllerAdvice和ExceptionHandler注解的作用后，我们来看一下异常处理的原理。进入DispatcherServlet的doDispatch方法

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
				...
				//处理controller方法
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

				...
			}
			catch (Exception ex) {
				dispatchException = ex;
			}
			catch (Throwable err) {
				dispatchException = new NestedServletException("Handler dispatch failed", err);
			}
			//异常处理中心
			processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
		}
		...
	}
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从doDispatch方法中可以看出，程序先处理了controller层的业务逻辑，对于业务逻辑抛出的异常，程序统一做了封装，然后进入了processDispatchResult方法中进行处理。所以我们进入该方法一探究竟。

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
			@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
			@Nullable Exception exception) throws Exception {

		boolean errorView = false;

		//如果程序发生了异常，就进行处理
		if (exception != null) {
			if (exception instanceof ModelAndViewDefiningException) {
				logger.debug("ModelAndViewDefiningException encountered", exception);
				mv = ((ModelAndViewDefiningException) exception).getModelAndView();
			}
			else {
				Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
				mv = processHandlerException(request, response, handler, exception);
				errorView = (mv != null);
			}
		}
		...
	}

	protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
			@Nullable Object handler, Exception ex) throws Exception {

		// Success and error responses may use different content types
		request.removeAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);

		// Check registered HandlerExceptionResolvers...
		ModelAndView exMv = null;
		if (this.handlerExceptionResolvers != null) {
		//遍历handlerExceptionResolvers处理异常信息
			for (HandlerExceptionResolver resolver : this.handlerExceptionResolvers) {
				exMv = resolver.resolveException(request, response, handler, ex);
				if (exMv != null) {
					break;
				}
			}
		}
		...
	}
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那这边的handlerExceptionResolvers是哪里来的呢？

private void initHandlerExceptionResolvers(ApplicationContext context) {
		this.handlerExceptionResolvers = null;

		if (this.detectAllHandlerExceptionResolvers) {
			// Find all HandlerExceptionResolvers in the ApplicationContext, including ancestor contexts.
			Map<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils
					.beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false);
			...
		}
		...
	}
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在DispatcherServlet初始化的时候，会去容器中找HandlerExceptionResolver类型的类。而刚刚的ExceptionHandlerExceptionResolver类就是继承了HandlerExceptionResolver接口，所以这个地方就将他放入了DispatcherServlet中。所以上面的遍历handlerExceptionResolvers处理异常信息的地方，就是调用了ExceptionHandlerExceptionResolver的resolveException方法。所以我们进入该方法。

public ModelAndView resolveException(
			HttpServletRequest request, HttpServletResponse response, @Nullable Object handler, Exception ex) {

		if (shouldApplyTo(request, handler)) {
			prepareResponse(ex, response);
			ModelAndView result = doResolveException(request, response, handler, ex);
			...
		}
	}

	protected final ModelAndView doResolveException(
			HttpServletRequest request, HttpServletResponse response, @Nullable Object handler, Exception ex) {

		return doResolveHandlerMethodException(request, response, (HandlerMethod) handler, ex);
	}

	protected ModelAndView doResolveHandlerMethodException(HttpServletRequest request,
			HttpServletResponse response, @Nullable HandlerMethod handlerMethod, Exception exception) {

		ServletInvocableHandlerMethod exceptionHandlerMethod = getExceptionHandlerMethod(handlerMethod, exception);
		...
			else {
				// Otherwise, just the given exception as-is
				exceptionHandlerMethod.invokeAndHandle(webRequest, mavContainer, exception, handlerMethod);
			}
		}
		...
	}

	public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
			Object... providedArgs) throws Exception {

		Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
		...
	}

	public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
			Object... providedArgs) throws Exception {
		//获取方法的参数
		Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
		if (logger.isTraceEnabled()) {
			logger.trace("Arguments: " + Arrays.toString(args));
		}
		//执行方法
		return doInvoke(args);
	}
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整个异常的执行逻辑如上面的代码，简单点说就是找到相应的异常处理方法，执行他。这个地方getMethodArgumentValues里面的逻辑和 SpringBoot源码解析-controller层参数的封装 是一样的，但是他们能处理的参数类型却不一样。

查看ExceptionHandlerExceptionResolver类的afterPropertiesSet方法:

public void afterPropertiesSet() {
		...
		if (this.argumentResolvers == null) {
			List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
			this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
		}
		...
	}

	protected List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() {
		List<HandlerMethodArgumentResolver> resolvers = new ArrayList<>();

		// Annotation-based argument resolution
		resolvers.add(new SessionAttributeMethodArgumentResolver());
		resolvers.add(new RequestAttributeMethodArgumentResolver());

		// Type-based argument resolution
		resolvers.add(new ServletRequestMethodArgumentResolver());
		resolvers.add(new ServletResponseMethodArgumentResolver());
		resolvers.add(new RedirectAttributesMethodArgumentResolver());
		resolvers.add(new ModelMethodProcessor());

		// Custom arguments
		if (getCustomArgumentResolvers() != null) {
			resolvers.addAll(getCustomArgumentResolvers());
		}

		return resolvers;
	}
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这边就是ExceptionHandler方法中可以接收的参数类型了。看一下，要比controller那边的类型少了许多，使用的时候注意一下即可。