SpringMVC源码(五)-异常处理

时间 2019-11-15

原文原文链接

SpringMVC除了对请求URL的路由处理特别方便外，还支持对异常的统一处理机制，能够对业务操做时抛出的异常，unchecked异常以及状态码的异常进行统一处理。SpringMVC既提供简单的配置类，也提供了细粒度的异常控制机制。java

SpringMVC中全部的异常处理经过接口HandlerExceptionResolver来实现，接口中只定义了一个方法web

public interface HandlerExceptionResolver {

	ModelAndView resolveException(
		HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex);
}

方法中接受request和response信息，以及当前的处理Handler，和抛出的异常对象。而且提供抽象类AbstractHandlerExceptionResolver，实现resolveException方法，支持前置判断和处理，将实际处理抽象出doResolveException方法由子类来实现。ajax

1.SimpleMappingExceptionResolver

SimpleMappingExceptionResolver是SpringMVC提供的一个很是便捷的简易异常处理方式，在XML中进行配置便可使用。spring

<bean class="org.springframework.web.servlet.handler.SimpleMappingExceptionResolver">
	<!-- 默认异常视图 -->
	<property name="defaultErrorView" value="error"/>
	<!-- 视图中获取exception信息变量名 -->
	<property name="exceptionAttribute" value="ex"></property>
	<!-- 异常同视图映射关系 -->
	<property name="exceptionMappings">
		<props>
			<prop key="com.lcifn.springmvc.exception.BusinessException">businessEx</prop>
		</props>
	</property>
</bean>

这是极简的一种配置，exceptionMappings配置的是异常同视图之间的映射关系，它是一个Properties对象，key-value分别是异常的类路径和视图名称。defaultErrorView表示默认异常视图，若是抛出的异常没有匹配到任何视图，即会走默认异常视图。exceptionAttribute表示在视图中获取exception信息变量名，默认为exception。还有一些其余配置能够查看SimpleMappingExceptionResolver的源码来使用。mvc

2.@ExceptionHandler

SpringMVC提供了一种注解方式来灵活地配置异常处理，@ExceptionHandler中能够配置要处理的异常类型，而后定义在处理此种异常的方法上，方法只要写在Controller中，便可对Controller中全部请求方法有效。app

咱们定义一个BaseController，而且将须要处理的异常经过@ExceptionHandler定义好处理方法，这样业务Controller只须要继承这个基类就能够了。处理方法中支持Request/Response/Sessioin等相关的参数绑定。async

[@Controller](https://my.oschina.net/u/1774615)
public class BaseController {

	@ExceptionHandler(RuntimeException.class)
	public ModelAndView handleRuntimeException(HttpServletRequest req, HttpServletResponse resp, RuntimeException ex){
		return new ModelAndView("error");
	}
}

可是继承的方式仍是对业务代码形成侵入，Spring很是重要的特性就是非侵入性，于是SpringMVC提供了@ControllerAdvice，简单来讲就是Controller的切面，支持对可选择的Controller进行统一配置，用于异常处理简直再合适不过了，咱们只须要将BaseController稍稍改一下。ide

@ControllerAdvice
public class AdviceController {

	@ExceptionHandler(RuntimeException.class)
	public ModelAndView handleRuntimeException(HttpServletRequest req, HttpServletResponse resp, RuntimeException ex){
		return new ModelAndView("error");
	}
}

只须要在统一配置类上加上@ControllerAdvice注解，支持包路径，注解等过滤方式，便可完成对全部业务Controller进行控制，而业务Controller不用作anything。函数

若是请求为ajax方式，须要其余格式返回异常，在方法上加上@ResponseBody便可。this

3.异常处理原理

上面介绍了经常使用的两种异常处理的配置方式，所谓知其然要知其因此然，SpringMVC怎么在请求处理的过程当中完成对异常的统一处理的呢？咱们从源码来深度解读。

回到DispatcherServlet的doDispatcher方法

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

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

	// Determine handler adapter for the current request.
	HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

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

	// Actually invoke the handler.
	mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

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

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

能够看到对请求处理的核心处理使用一个大的try/catch，若是出现异常，统一封装成dispatchException交给processDispatchResult方法进行处理。咱们知道processDispatchResult方法用来对返回视图进行操做，而同时也对异常进行统一处理。

在processDispatchResult中，首先对异常进行判断。

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);
	}
}

若是不是特殊的ModelAndViewDefiningException，则由processHandlerException来操做。

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

	// Check registered HandlerExceptionResolvers...
	ModelAndView exMv = null;
	// 遍历全部注册的异常处理器，由异常处理器进行处理
	for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {
		exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);
		if (exMv != null) {
			break;
		}
	}
	// 若是异常视图存在，则转向异常视图
	if (exMv != null) {
		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()) {
			exMv.setViewName(getDefaultViewName(request));
		}
		if (logger.isDebugEnabled()) {
			logger.debug("Handler execution resulted in exception - forwarding to resolved error view: " + exMv, ex);
		}
		WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
		return exMv;
	}

	throw ex;
}

咱们主要关注异常处理器对异常的处理，SpringMVC经过HandlerExceptionResolver的resolveException调用实现类的实际实现方法doResolveException。

SimpleMappingExceptionResolver

来看SimpleMappingExceptionResolver的实现：

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

	// Expose ModelAndView for chosen error view.
	// 根据request和异常对象获取异常视图名称
	String viewName = determineViewName(ex, request);
	if (viewName != null) {
		// Apply HTTP status code for error views, if specified.
		// Only apply it if we're processing a top-level request.
		Integer statusCode = determineStatusCode(request, viewName);
		if (statusCode != null) {
			applyStatusCodeIfPossible(request, response, statusCode);
		}
		// 组装异常视图模型ModelAndView
		return getModelAndView(viewName, ex, request);
	}
	else {
		return null;
	}
}

determineViewName方法决定异常视图名称，getModelAndView方法返回ModelAndView对象

protected String determineViewName(Exception ex, HttpServletRequest request) {
	String viewName = null;
	if (this.excludedExceptions != null) {
		for (Class<?> excludedEx : this.excludedExceptions) {
			if (excludedEx.equals(ex.getClass())) {
				return null;
			}
		}
	}
	// Check for specific exception mappings.
	if (this.exceptionMappings != null) {
		viewName = findMatchingViewName(this.exceptionMappings, ex);
	}
	// Return default error view else, if defined.
	if (viewName == null && this.defaultErrorView != null) {
		if (logger.isDebugEnabled()) {
			logger.debug("Resolving to default view '" + this.defaultErrorView + "' for exception of type [" +
					ex.getClass().getName() + "]");
		}
		viewName = this.defaultErrorView;
	}
	return viewName;
}

在determineViewName方法中，咱们配置的defaultErrorView和exceptionMappings都起了做用。更细节的就不深刻了，有兴趣能够本身去看。

ExceptionHandlerExceptionResolver

ExceptionHandlerExceptionResolver支持了@ExceptionHandler注解的实现。它的抽象基类AbstractHandlerMethodExceptionResolver继承了AbstractHandlerExceptionResolver，doResolveException方法实际调用ExceptionHandlerExceptionResolver的doResolveHandlerMethodException方法。

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

	// 根据HandlerMethod和exception获取异常处理的Method
	ServletInvocableHandlerMethod exceptionHandlerMethod = getExceptionHandlerMethod(handlerMethod, exception);
	if (exceptionHandlerMethod == null) {
		return null;
	}

	// 设置异常处理方法的参数解析器和返回值解析器
	exceptionHandlerMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
	exceptionHandlerMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);

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

	// 执行异常处理方法
	try {
		if (logger.isDebugEnabled()) {
			logger.debug("Invoking @ExceptionHandler method: " + exceptionHandlerMethod);
		}
		Throwable cause = exception.getCause();
		if (cause != null) {
			// Expose cause as provided argument as well
			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;
	}

	// 对返回的视图模型进行处理
	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());
		if (!mavContainer.isViewReference()) {
			mav.setView((View) mavContainer.getView());
		}
		if (model instanceof RedirectAttributes) {
			Map<String, ?> flashAttributes = ((RedirectAttributes) model).getFlashAttributes();
			request = webRequest.getNativeRequest(HttpServletRequest.class);
			RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);
		}
		return mav;
	}
}

咱们主要关注的是如何匹配到异常处理方法的

protected ServletInvocableHandlerMethod getExceptionHandlerMethod(HandlerMethod handlerMethod, Exception exception) {
	Class<?> handlerType = (handlerMethod != null ? handlerMethod.getBeanType() : null);

	// 从当前Controller中匹配异常处理Method
	if (handlerMethod != null) {
		ExceptionHandlerMethodResolver resolver = this.exceptionHandlerCache.get(handlerType);
		if (resolver == null) {
			resolver = new ExceptionHandlerMethodResolver(handlerType);
			this.exceptionHandlerCache.put(handlerType, resolver);
		}
		Method method = resolver.resolveMethod(exception);
		if (method != null) {
			return new ServletInvocableHandlerMethod(handlerMethod.getBean(), method);
		}
	}

	// 从ControllerAdvice中匹配异常处理Method
	for (Entry<ControllerAdviceBean, ExceptionHandlerMethodResolver> entry : this.exceptionHandlerAdviceCache.entrySet()) {
		if (entry.getKey().isApplicableToBeanType(handlerType)) {
			ExceptionHandlerMethodResolver resolver = entry.getValue();
			Method method = resolver.resolveMethod(exception);
			if (method != null) {
				return new ServletInvocableHandlerMethod(entry.getKey().resolveBean(), method);
			}
		}
	}

	return null;
}

匹配异常处理方法的来源有两个，一个是当前Controller，一个是全部@ControllerAdvice类。能够看到两种方式都使用了cache的方式，那么ExceptionHandlerMethod的信息怎么初始化的呢？

当前Controller

对每一个请求HandlerMethod的Controller类型，都实例化一个ExceptionHandlerMethodResolver来处理异常。ExceptionHandlerMethodResolver的构造函数中初始化了当前Controller中的异常处理配置。

public ExceptionHandlerMethodResolver(Class<?> handlerType) {
	for (Method method : MethodIntrospector.selectMethods(handlerType, EXCEPTION_HANDLER_METHODS)) {
		// detectExceptionMappings方法执行探查
		for (Class<? extends Throwable> exceptionType : detectExceptionMappings(method)) {
			addExceptionMapping(exceptionType, method);
		}
	}
}

private List<Class<? extends Throwable>> detectExceptionMappings(Method method) {
	List<Class<? extends Throwable>> result = new ArrayList<Class<? extends Throwable>>();
	// 探查全部ExceptionHandler注解的方法
	detectAnnotationExceptionMappings(method, result);
	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);
	result.addAll(Arrays.asList(ann.value()));
}

@ControllerAdvice类

对@ControllerAdvice统一切面类的处理，则是在ExceptionHandlerExceptionResolver的初始化方法afterPropertiesSet中进行处理。

public void afterPropertiesSet() {
	// Do this first, it may add ResponseBodyAdvice beans
	// 初始化@ControllerAdvice中的@ExceptionHandler
	initExceptionHandlerAdviceCache();

	if (this.argumentResolvers == null) {
		List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
		this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
	}
	if (this.returnValueHandlers == null) {
		List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
		this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
	}
}

initExceptionHandlerAdviceCache方法遍历上下文中全部有@ControllerAdvice注解的Bean，而后实例化成ExceptionHandlerMethodResolver类，在构造函数中初始化全部@ExceptionHandler。

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

	// 查询全部@ControllerAdvice的Bean
	List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
	AnnotationAwareOrderComparator.sort(adviceBeans);

	// 遍历，实例化ExceptionHandlerMethodResolver
	for (ControllerAdviceBean adviceBean : adviceBeans) {
		ExceptionHandlerMethodResolver resolver = new ExceptionHandlerMethodResolver(adviceBean.getBeanType());
		if (resolver.hasExceptionMappings()) {
			this.exceptionHandlerAdviceCache.put(adviceBean, resolver);
			if (logger.isInfoEnabled()) {
				logger.info("Detected @ExceptionHandler methods in " + adviceBean);
			}
		}
		
		if (ResponseBodyAdvice.class.isAssignableFrom(adviceBean.getBeanType())) {
			this.responseBodyAdvice.add(adviceBean);
			if (logger.isInfoEnabled()) {
				logger.info("Detected ResponseBodyAdvice implementation in " + adviceBean);
			}
		}
	}
}

匹配到exceptionHandlerMethod后，设置一些方法执行的环境，而后调用ServletInvocableHandlerMethod中的invokeAndHandle去执行，这个调用过程和正常请求的调用就是一致了。这里也不向下扩展了，能够参看SpringMVC源码(四)-请求处理。

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

	// 执行请求方法
	Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
	setResponseStatus(webRequest);

	if (returnValue == null) {
		if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
			mavContainer.setRequestHandled(true);
			return;
		}
	}
	else if (StringUtils.hasText(getResponseStatusReason())) {
		mavContainer.setRequestHandled(true);
		return;
	}

	mavContainer.setRequestHandled(false);
	try {
		this.returnValueHandlers.handleReturnValue(
				returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
	}
	catch (Exception ex) {
		if (logger.isTraceEnabled()) {
			logger.trace(getReturnValueHandlingErrorMessage("Error handling return value", returnValue), ex);
		}
		throw ex;
	}
}

至此ExceptionHandlerExceptionResolver的异常处理已经基本完成。SpringMVC还内置了ResponseStatusExceptionResolver和DefaultHandlerExceptionResolver来对状态码异常和常见的请求响应异常进行统一处理。

4.web.xml的配置

某些状况下，SpringMVC的处理并无异常出现，但在最终的视图输出时找不到视图文件，就会显示404错误页面，很是影响用户体验。咱们能够在web.xml中对未捕获的异常以及此种4xx或5xx的异常经过<error-page>进行处理。

<error-page>
	<error-code>404</error-code>
	<location>/404</location>
</error-page>
<error-page>
	<exception-type>java.lang.Throwable</exception-type>
	<location>/500</location>
</error-page>

一般对于系统中的异常，业务相关的尽可能自定义异常处理方式，而一些系统异常经过统一错误页面进行处理。