开头

试想一下我们一般怎么统一处理异常呢，答：切面。但抛开切面不讲，如果对每一个controller方法抛出的异常做专门处理，那么着实太费劲了，有没有更好的方法呢？当然有，就是本篇文章接下来要介绍的springmvc的异常处理机制，用到了ControllerAdvice和ExceptionHandler注解，有点切面的感觉哈哈。

1.ExceptionHandlerExceptionResolver

首先从springmvc的异常处理解析器开始讲，当执行完controller方法后，不管有没有异常产生都会调用DispatcherServlet#doDispatch()方法中的processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException); 方法，接着会判断是否有异常，若无异常则走正常流程，若有异常则需要进行处理 mv = processHandlerException(request, response, handler, exception);  再接着就是遍历spring已经注册的异常处理解析器直到有处理器返回mav

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

		if (exception != null) {
			if (exception instanceof ModelAndViewDefiningException) {
				logger.debug("ModelAndViewDefiningException encountered", exception);
				mv = ((ModelAndViewDefiningException) exception).getModelAndView();
			}
			else {
				Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
				// 执行处理器产生的异常处理
				mv = processHandlerException(request, response, handler, exception);
				// 是否有异常视图返回
				errorView = (mv != null);
			}
		}

		// Did the handler return a view to render? 处理程序是否返回要渲染的视图
		if (mv != null && !mv.wasCleared()) {
			// 渲染视图
			render(mv, request, response);
			if (errorView) {
				WebUtils.clearErrorRequestAttributes(request);
			}
		}
		else {
			if (logger.isDebugEnabled()) {
				logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
						"': assuming HandlerAdapter completed request handling");
			}
		}
	}

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

		// Check registered HandlerExceptionResolvers...
		ModelAndView exMv = null;
		if (this.handlerExceptionResolvers != null) {
			for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {
				exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);
				if (exMv != null) {
					break;
				}
			}
		}
		if (exMv != null) {
			// 无视图view
			if (exMv.isEmpty()) {
				request.setAttribute(EXCEPTION_ATTRIBUTE, ex);
				return null;
			}
			// We might still need view name translation for a plain error model...
			if (!exMv.hasView()) {
				String defaultViewName = getDefaultViewName(request);
				if (defaultViewName != null) {
					exMv.setViewName(defaultViewName);
				}
			}
			WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
			return exMv;
		}

		throw ex;
	}

其中最重要也是最常使用的一个处理器就是ExceptionHandlerExceptionResolver，下面将着重介绍它，先来看看这个类的继承结构图，实现了InitializingBean接口，在这个bean创建完成之前会调用生命周期初始化方法afterPropertiesSet（），这里面包含了对@ControllerAdvice注解的解析，初始化完后的信息供后续解析异常使用。

实现HandlerExceptionResolver接口，实现解析方法resolveException()

public interface HandlerExceptionResolver {

	/**
	 * Try to resolve the given exception that got thrown during handler execution,
	 * returning a {@link ModelAndView} that represents a specific error page if appropriate.
	 * <p>The returned {@code ModelAndView} may be {@linkplain ModelAndView#isEmpty() empty}
	 * to indicate that the exception has been resolved successfully but that no view
	 * should be rendered, for instance by setting a status code.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @param handler the executed handler, or {@code null} if none chosen at the
	 * time of the exception (for example, if multipart resolution failed)
	 * @param ex the exception that got thrown during handler execution
	 * @return a corresponding {@code ModelAndView} to forward to,
	 * or {@code null} for default processing in the resolution chain
	 */
	@Nullable
	ModelAndView resolveException(
			HttpServletRequest request, HttpServletResponse response, @Nullable Object handler, Exception ex);

}

@Override
public void afterPropertiesSet() {
    // Do this first, it may add ResponseBodyAdvice beans
    // 初始化异常注解 @ControllerAdvice
    initExceptionHandlerAdviceCache();
}

private void initExceptionHandlerAdviceCache() {
    if (getApplicationContext() == null) {
        return;
    }
    if (logger.isDebugEnabled()) {
        logger.debug("Looking for exception mappings: " + getApplicationContext());
    }

    // 解析有@ControllerAdvice注解的bean，并将这个bean构建成ControllerAdviceBean对象
    List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
    // 将ControllerAdviceBean根据order排序
    AnnotationAwareOrderComparator.sort(adviceBeans);

    for (ControllerAdviceBean adviceBean : adviceBeans) {
        Class<?> beanType = adviceBean.getBeanType();
        if (beanType == null) {
            throw new IllegalStateException("Unresolvable type for ControllerAdviceBean: " + adviceBean);
        }
        ExceptionHandlerMethodResolver resolver = new ExceptionHandlerMethodResolver(beanType);
        // mappedMethods 映射不为空
        if (resolver.hasExceptionMappings()) {
            // 添加到缓存中
            this.exceptionHandlerAdviceCache.put(adviceBean, resolver);
            if (logger.isInfoEnabled()) {
                logger.info("Detected @ExceptionHandler methods in " + adviceBean);
            }
        }
        // 若实现了ResponseBodyAdvice接口（暂不介绍）
        if (ResponseBodyAdvice.class.isAssignableFrom(beanType)) {
            this.responseBodyAdvice.add(adviceBean);
            if (logger.isInfoEnabled()) {
                logger.info("Detected ResponseBodyAdvice implementation in " + adviceBean);
            }
        }
    }
}

 ExceptionHandlerMethodResolver resolver = new ExceptionHandlerMethodResolver(beanType); 这行代码会解析拥有@ControllerAdvice 注解的class，并且会遍历class中带有 @ExceptionHandler 注解的方法，获取方法注解带有的异常类型，将异常类型和方法放入到mappedMethods中供后面获取，获取的时候若对应处理此异常类型的method有多个，则需要进行排序，选取一个异常类型与method ExceptionHandler注解异常类型最近的一个（深度最小的那个也即是继承关系最少的那个）具体代码如下：

public class ExceptionHandlerMethodResolver {

	/**
	 * A filter for selecting {@code @ExceptionHandler} methods.
	 */
	public static final MethodFilter EXCEPTION_HANDLER_METHODS = method ->
			(AnnotationUtils.findAnnotation(method, ExceptionHandler.class) != null);


	/**
	 * 异常类型与方法的映射map
	 */
	private final Map<Class<? extends Throwable>, Method> mappedMethods = new HashMap<>(16);

	/**
	 * 缓存，用来存储先前碰到过的异常类型与处理方法的映射
	 */
	private final Map<Class<? extends Throwable>, Method> exceptionLookupCache = new ConcurrentReferenceHashMap<>(16);


	/**
	 * A constructor that finds {@link ExceptionHandler} methods in the given type.
	 * @param handlerType the type to introspect
	 */
	public ExceptionHandlerMethodResolver(Class<?> handlerType) {
		// 获取并遍历@ExceptionHandler注解的方法
		for (Method method : MethodIntrospector.selectMethods(handlerType, EXCEPTION_HANDLER_METHODS)) {
			for (Class<? extends Throwable> exceptionType : detectExceptionMappings(method)) {
				addExceptionMapping(exceptionType, method);
			}
		}
	}


	/**
	 * Extract exception mappings from the {@code @ExceptionHandler} annotation first,
	 * and then as a fallback from the method signature itself.
	 */
	@SuppressWarnings("unchecked")
	private List<Class<? extends Throwable>> detectExceptionMappings(Method method) {
		List<Class<? extends Throwable>> result = new ArrayList<>();
		// 将注解ExceptionHandler value值异常添加到result中
		detectAnnotationExceptionMappings(method, result);
		// 注解值为空的话再去获取参数的异常类型
		if (result.isEmpty()) {
			for (Class<?> paramType : method.getParameterTypes()) {
				if (Throwable.class.isAssignableFrom(paramType)) {
					result.add((Class<? extends Throwable>) paramType);
				}
			}
		}
		if (result.isEmpty()) {
			throw new IllegalStateException("No exception types mapped to " + method);
		}
		return result;
	}

	protected void detectAnnotationExceptionMappings(Method method, List<Class<? extends Throwable>> result) {
		ExceptionHandler ann = AnnotationUtils.findAnnotation(method, ExceptionHandler.class);
		Assert.state(ann != null, "No ExceptionHandler annotation");
		result.addAll(Arrays.asList(ann.value()));
	}

	private void addExceptionMapping(Class<? extends Throwable> exceptionType, Method method) {
		// 将异常类型以及对应的method添加到map中，且异常类型不能有重复否则会报错
		Method oldMethod = this.mappedMethods.put(exceptionType, method);
		if (oldMethod != null && !oldMethod.equals(method)) {
			throw new IllegalStateException("Ambiguous @ExceptionHandler method mapped for [" +
					exceptionType + "]: {" + oldMethod + ", " + method + "}");
		}
	}

	/**
	 * Whether the contained type has any exception mappings.
	 */
	public boolean hasExceptionMappings() {
		return !this.mappedMethods.isEmpty();
	}

	/**
	 * Find a {@link Method} to handle the given exception.
	 * Use {@link ExceptionDepthComparator} if more than one match is found.
	 * @param exception the exception
	 * @return a Method to handle the exception, or {@code null} if none found
	 */
	@Nullable
	public Method resolveMethod(Exception exception) {
		return resolveMethodByThrowable(exception);
	}

	/**
	 * Find a {@link Method} to handle the given Throwable.
	 * Use {@link ExceptionDepthComparator} if more than one match is found.
	 * @param exception the exception
	 * @return a Method to handle the exception, or {@code null} if none found
	 * @since 5.0
	 */
	@Nullable
	public Method resolveMethodByThrowable(Throwable exception) {
		Method method = resolveMethodByExceptionType(exception.getClass());
		if (method == null) {
			Throwable cause = exception.getCause();
			if (cause != null) {
				method = resolveMethodByExceptionType(cause.getClass());
			}
		}
		return method;
	}

	/**
	 * Find a {@link Method} to handle the given exception type. This can be
	 * useful if an {@link Exception} instance is not available (e.g. for tools).
	 * @param exceptionType the exception type
	 * @return a Method to handle the exception, or {@code null} if none found
	 */
	@Nullable
	public Method resolveMethodByExceptionType(Class<? extends Throwable> exceptionType) {
		Method method = this.exceptionLookupCache.get(exceptionType);
		if (method == null) {
			method = getMappedMethod(exceptionType);
			this.exceptionLookupCache.put(exceptionType, method);
		}
		return method;
	}

	/**
	 * Return the {@link Method} mapped to the given exception type, or {@code null} if none.
	 */
	@Nullable
	private Method getMappedMethod(Class<? extends Throwable> exceptionType) {
		List<Class<? extends Throwable>> matches = new ArrayList<>();
		for (Class<? extends Throwable> mappedException : this.mappedMethods.keySet()) {
			if (mappedException.isAssignableFrom(exceptionType)) {
				matches.add(mappedException);
			}
		}
		if (!matches.isEmpty()) {
			// exceptionType 到matchs父类异常类型的深度
			matches.sort(new ExceptionDepthComparator(exceptionType));
			return this.mappedMethods.get(matches.get(0));
		}
		else {
			return null;
		}
	}

}

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

    // exception为controller方法抛出的异常
    // 根据异常及其类型从上述的mappedMethods中获取对应的方法，再获取方法所在的对象 封装成ServletInvocableHandlerMethod
    ServletInvocableHandlerMethod exceptionHandlerMethod = getExceptionHandlerMethod(handlerMethod, exception);
    if (exceptionHandlerMethod == null) {
        return null;
    }

    // 设置参数解析器，主要用来获取方法的参数值的，供后续反射调用方法
    if (this.argumentResolvers != null) {
        exceptionHandlerMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
    }
    // 设置返回值解析器，当执行完方法后获取返回值，对返回值进行处理 或返回视图或将结果写入到response
    if (this.returnValueHandlers != null) {
        exceptionHandlerMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
    }

    ServletWebRequest webRequest = new ServletWebRequest(request, response);
    ModelAndViewContainer mavContainer = new ModelAndViewContainer();

    try {
        if (logger.isDebugEnabled()) {
            logger.debug("Invoking @ExceptionHandler method: " + exceptionHandlerMethod);
        }
        Throwable cause = exception.getCause();
        if (cause != null) {
            // Expose cause as provided argument as well
            // 执行异常处理方法，也就是我们的自定义的异常处理方法
            exceptionHandlerMethod.invokeAndHandle(webRequest, mavContainer, exception, cause, handlerMethod);
        }
        else {
            // Otherwise, just the given exception as-is
            exceptionHandlerMethod.invokeAndHandle(webRequest, mavContainer, exception, handlerMethod);
        }
    }
    catch (Throwable invocationEx) {
        // Any other than the original exception is unintended here,
        // probably an accident (e.g. failed assertion or the like).
        if (invocationEx != exception && logger.isWarnEnabled()) {
            logger.warn("Failed to invoke @ExceptionHandler method: " + exceptionHandlerMethod, invocationEx);
        }
        // Continue with default processing of the original exception...
        return null;
    }

    // 根据后续的返回值解析器设置的，将返回值写入到response中了直接返回空的mav
    if (mavContainer.isRequestHandled()) {
        return new ModelAndView();
    }
    else {
        ModelMap model = mavContainer.getModel();
        HttpStatus status = mavContainer.getStatus();
        ModelAndView mav = new ModelAndView(mavContainer.getViewName(), model, status);
        mav.setViewName(mavContainer.getViewName());
        // (this.view instanceof String)
        if (!mavContainer.isViewReference()) {
            mav.setView((View) mavContainer.getView());
        }
        if (model instanceof RedirectAttributes) {
            Map<String, ?> flashAttributes = ((RedirectAttributes) model).getFlashAttributes();
            RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);
        }
        return mav;
    }
}

exceptionHandlerMethod.invokeAndHandle(webRequest, mavContainer, exception, cause, handlerMethod); 此方法执行完成后已经完成了异常处理方法的调用，若方法返回值为视图ModelAndView或其他视图类型，则还需要借助视图解析器如InternalResourceViewResolver对视图进行解析渲染，若为其他类型的值则将值写入到response响应中。

2. demo

Controller类方法：

@Controller
@RequestMapping(value = "test")
public class HelloWorldController{

  @Data
  public static class User {
    private String username;

    private Integer age;

    private String address;
  }


  @RequestMapping(value = "user/get", method = RequestMethod.POST)
  @ResponseBody
  public Object testObject(@RequestBody @Valid User user, @RequestParam String address) {
    user.setAddress(address);
    // 这里特意抛出RuntimeException异常
    throw new RuntimeException("this is a exception");
  }

}

ExceptionHandlerController异常处理类

@ControllerAdvice
@ResponseBody
public class ExceptionHandlerController {

  @ExceptionHandler(value = Exception.class)
  public Object handleException(Exception e) {
    return CommonResult.fail("Exception:" + e.getMessage());
  }

  @ExceptionHandler(value = RuntimeException.class)
  public Object handlerRuntimeException(Exception e) {
    return CommonResult.fail("handlerRuntimeException:" + e.getMessage());
  }
}

ExceptionHandlerController类中定义了两个异常处理方法，一个处理Exception异常，一个处理RuntimeException异常，那个根据controller方法抛出的异常RuntimeException再结合上面的分析（RuntimeException到RuntimeException深度为0，RuntimeException到Exception中间继承了一次深度为1）可以得出抛出异常类型的处理方法为handlerRuntimeException 方法。 运行程序结果如下：

结语

初步解析ExceptionHandlerExceptionResolver源码，若写的有误或者有不理解的地方，欢迎指出讨论~