JAVA高级面试进阶训练营视频教程

admin

Servlet传统异常处理

Servlet规范规定了当web应用发生异常时必须能够指明, 并确定了该如何处理, 规定了错误信息应该包含的内容和展示页面的方式.(详细可以参考servlet规范文档)

处理方式

• 处理状态码<error-code>
• 处理异常信息<exception-type>
• 处理服务地址<location>

Spring MVC 处理方式

所有的请求必然以某种方式转化为响应.

• Spring中特定的异常将自动映射为特定的HTTP状态码
• 使用@ResponseStatus注解可以映射某一异常到特定的HTTP状态码
• Controller方法上可以使用@ExceptionHandler注解使其用来处理异常
• 使用@ControllerAdvice 方式可以统一的方式处理全局异常

Spring boot 方式

• 实现ErrorPageRegistrar: 确定是页面处理的路径必须固定,优点是比较通用
• 注册ErrorPage
• 实现ErrorPage对应的服务

源码分析

一．接口HandlerExceptionResolver

该接口定义了Spring中该如何处理异常. 它只有一个方法resolveException(), 接口源码如下:

 // 由对象实现的接口，这些对象可以解决在处理程序映射或执行期间引发的异常，在典型的情况下是错误视图。在应用程序上下文中，实现器通常被注册为bean。
 // 错误视图类似于JSP错误页面，但是可以与任何类型的异常一起使用，包括任何已检查的异常，以及针对特定处理程序的潜在细粒度映射。
public interface HandlerExceptionResolver {
	@Nullable
	ModelAndView resolveException(HttpServletRequest request, HttpServletResponse response, @Nullable Object handler, Exception ex);
}

Spring 为该接口提供了若干实现类如下:

HandlerExceptionResolverComposite  				委托给其他HandlerExceptionResolver的实例列表
AbstractHandlerExceptionResolver				抽象基类
	AbstractHandlerMethodExceptionResolver		支持HandlerMethod处理器的抽象基类
		ExceptionHandlerExceptionResolver		通过 @ExceptionHandler 注解的方式实现的异常处理
	DefaultHandlerExceptionResolver				默认实现, 处理spring预定义的异常并将其对应到错误码
	ResponseStatusExceptionResolver				通过 @ResponseStatus 注解映射到错误码的异常
	SimpleMappingExceptionResolver				允许将异常类映射到视图名

二. DefaultHandlerExceptionResolver

这个类是Spring提供的默认实现, 用于将一些常见异常映射到特定的状态码. 这些状态码定义在接口HttpServletResponse中, 下面是几个状态码的代码片段

public interface HttpServletResponse extends ServletResponse {
	...
    public static final int SC_OK = 200;
    public static final int SC_MOVED_PERMANENTLY = 301;
    public static final int SC_MOVED_TEMPORARILY = 302;
    public static final int SC_FOUND = 302;
    public static final int SC_UNAUTHORIZED = 401;
    public static final int SC_INTERNAL_SERVER_ERROR = 500;
	...
}

实际上, DefaultHandlerExceptionResolver中并没有直接实现接口的resolveException方法, 而是实现了抽象类AbstractHandlerExceptionResolver的doResolveException()方法, 后者则在实现了接口的方法中委托给抽象方法doResolveException, 这个方法由子类去实现.

AbstractHandlerExceptionResolver的resolveException方法代码如下:

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

	// 判断是否当前解析器可用于handler
	if (shouldApplyTo(request, handler)) { 
		prepareResponse(ex, response);
		ModelAndView result = doResolveException(request, response, handler, ex);
		if (result != null) {
			// Print warn message when warn logger is not enabled...
			if (logger.isWarnEnabled() && (this.warnLogger == null || !this.warnLogger.isWarnEnabled())) {
				logger.warn("Resolved [" + ex + "]" + (result.isEmpty() ? "" : " to " + result));
			}
			// warnLogger with full stack trace (requires explicit config)
			logException(ex, request);
		}
		return result;
	}
	else {
		return null;
	}
}

接下来我们看DefaultHandlerExceptionResolver实现的doResolveException方法. 代码如下;

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

		try {
			if (ex instanceof HttpRequestMethodNotSupportedException) {
				return handleHttpRequestMethodNotSupported(
						(HttpRequestMethodNotSupportedException) ex, request, response, handler);
			}
			else if (ex instanceof HttpMediaTypeNotSupportedException) {
				return handleHttpMediaTypeNotSupported(
						(HttpMediaTypeNotSupportedException) ex, request, response, handler);
			}
			....
			else if (ex instanceof NoHandlerFoundException) {
				return handleNoHandlerFoundException(
						(NoHandlerFoundException) ex, request, response, handler);
			}
			.....
		}
		catch (Exception handlerEx) {
			if (logger.isWarnEnabled()) {
				logger.warn("Failure while trying to resolve exception [" + ex.getClass().getName() + "]", handlerEx);
			}
		}
		return null;
	}

可以看到代码中使用了大量的分支语句, 实际上是将方法传入的异常类型通过instanceof运算符测试, 通过测试的转化为特定的异常. 并调用处理该异常的特定方法. 我们挑一个比如处理NoHandlerFoundException这个异常类的方法, 这个方法将异常映射为404错误.

protected ModelAndView handleNoHandlerFoundException(NoHandlerFoundException ex,
		HttpServletRequest request, HttpServletResponse response, @Nullable Object handler) throws IOException {

	pageNotFoundLogger.warn(ex.getMessage());
	response.sendError(HttpServletResponse.SC_NOT_FOUND); //设置为404错误
	return new ModelAndView(); //返回个空视图
}

上面分析了Spring默认的异常处理实现类DefaultHandlerExceptionResolver.它处理的异常是Spring预定义的几种常见异常, 它将异常对应到HTTP的状态码. 而对于不属于这些类型的其他异常, 我们可以使用ResponseStatusExceptionResolver来处理, 将其对应到HTTP状态码.

三. ResponseStatusExceptionResolver

如何使用?

@GetMapping("/responseStatus")
@ResponseBody
public String responseStatus() throws MyException {
	throw new MyException();
}

@ResponseStatus(code = HttpStatus.BAD_GATEWAY)
public class MyException extends Exception{}

只需要在异常上使用@ResponseStatus注解即可将特定的自定义异常对应到Http的状态码.

四. ExceptionHandlerExceptionResolver

使用类似于普通的controller方法, 使用@ExceptionHandler注解的方法将作为处理该注解参数中异常的handler. 比如, 在一个controller中, 我们定义一个处理NPE的异常处理handler方法, 可以用来处理该controller中抛出的NPE. 代码如下:

 @GetMapping("/npe1")
@ResponseBody
public String npe1() throws NullPointerException {
	throw new NullPointerException();
}

@GetMapping("/npe2")
@ResponseBody
public String npe2() throws NullPointerException {
	throw new NullPointerException();
}

@ExceptionHandler(value = {NullPointerException.class})
@ResponseBody
public String npehandler(){
	return "test npe handler";
}

无论是请求/npe1还是请求/npe2, 系统都会抛出异常, 并交给对应的处理程序npehandler去处理. 使用@ExceptionHandler(value = {NullPointerException.class})注解的方法可以处理本controller范围内的所有方法排除的npe异常, 如果要将其作为应用中所有controller的异常处理器, 就要将其定义在@ControllerAdvice注解的类中.

@ControllerAdvice
public class ControllerAdvicer {

    @ExceptionHandler(value = {NullPointerException.class})
    @ResponseBody
    public String npehandler(){
        return "test npe handler in advice";
    }
}

要了解其原理, 需要查看ExceptionHandlerExceptionResolver中的方法doResolveHandlerMethodException

@Override
@Nullable
protected ModelAndView doResolveHandlerMethodException(HttpServletRequest request,
		HttpServletResponse response, @Nullable HandlerMethod handlerMethod, Exception exception) {
	
	// 获取异常对用的处理器, 就是@ExceptionHandler注解的方法包装, 注意参数handlerMethod, 在方法内部, 它将用来获取所在Controller的信息
	ServletInvocableHandlerMethod exceptionHandlerMethod = getExceptionHandlerMethod(handlerMethod, exception);
	if (exceptionHandlerMethod == null) {
		return null;
	}

	if (this.argumentResolvers != null) {
		exceptionHandlerMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
	}
	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();
		// 调用异常处理handler的方法.
		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;
	}

	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();
			RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);
		}
		return mav;
	}
}

可以看到在两个中文注释的地方, 其一是方法的开始部分获取到了异常的handler, 其二是调用这个handler的方法. 调用方法应该很好理解, 我们接下来查看方法getExceptionHandlerMethod.

// 找到给定异常对应的@ExceptionHandler注解方法, 默认先在controller类的继承结构中查找, 否则继续在@ControllerAdvice注解的 bean中查找.
@Nullable
protected ServletInvocableHandlerMethod getExceptionHandlerMethod(
		@Nullable HandlerMethod handlerMethod, Exception exception) {

	Class<?> handlerType = null;

	if (handlerMethod != null) {
		// Local exception handler methods on the controller class itself.
		// To be invoked through the proxy, even in case of an interface-based proxy.
		handlerType = handlerMethod.getBeanType();
		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);
		}
		// For advice applicability check below (involving base packages, assignable types
		// and annotation presence), use target class instead of interface-based proxy.
		if (Proxy.isProxyClass(handlerType)) {
			handlerType = AopUtils.getTargetClass(handlerMethod.getBean());
		}
	}

	// 在@ControllerAdvice注解的类中遍历查找
	for (Map.Entry<ControllerAdviceBean, ExceptionHandlerMethodResolver> entry : this.exceptionHandlerAdviceCache.entrySet()) {
		ControllerAdviceBean advice = entry.getKey();
		if (advice.isApplicableToBeanType(handlerType)) {
			ExceptionHandlerMethodResolver resolver = entry.getValue();
			Method method = resolver.resolveMethod(exception);
			if (method != null) {
				return new ServletInvocableHandlerMethod(advice.resolveBean(), method);
			}
		}
	}

	return null;
}

我们可以看到,它会首先查找controller中的方法, 如果找不到才去查找@ControllerAdvice注解的bean. 也就是说controller中的handler的优先级要高于advice.

上面我们了解了几个Exceptionresolver的使用, 并通过源代码简单看了他们各自处理的原理. 但这些Resolver如何加载我们还不知道, 接下来我们重点看下他们是如何加载进去的.

四. ExceptionResolver的加载

在本系列的上一篇Spring系列(六) Spring Web MVC 应用构建分析中, 我们大致提到了DispatcherServlet的启动调用关系如下:

整理下调用关系: DispatcherServlet initHandlerMappings <-- initStrategies <-- onRefresh <--
FrameworkServlet initWebApplicationContext <-- initServletBean <--
HttpServletBean init <--
GenericServlet init(ServletConfig config)
最后的GenericServlet是servlet Api的.

正是在initStrategies方法中, DispatcherServlet做了启动的一系列工作, 除了initHandlerMappings还可以看到一个initHandlerExceptionResolvers的方法, 其源码如下:

// 初始化HandlerExceptionResolver, 如果没有找到任何命名空间中定义的bean, 默认没有任何resolver
private void initHandlerExceptionResolvers(ApplicationContext context) {
	this.handlerExceptionResolvers = null;

	if (this.detectAllHandlerExceptionResolvers) {
		// 找到所有ApplicationContext中定义的 HandlerExceptionResolvers 包括在上级上下文中.
		Map<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils
				.beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false);
		if (!matchingBeans.isEmpty()) {
			this.handlerExceptionResolvers = new ArrayList<>(matchingBeans.values());
			// 保持有序.
			AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers);
		}
	}
	else {
		try {
			HandlerExceptionResolver her =
					context.getBean(HANDLER_EXCEPTION_RESOLVER_BEAN_NAME, HandlerExceptionResolver.class);
			this.handlerExceptionResolvers = Collections.singletonList(her);
		}
		catch (NoSuchBeanDefinitionException ex) {
			// Ignore, no HandlerExceptionResolver is fine too.
		}
	}

	// 确保有Resolver, 否则使用默认的
	if (this.handlerExceptionResolvers == null) {
		this.handlerExceptionResolvers = getDefaultStrategies(context, HandlerExceptionResolver.class);
		if (logger.isTraceEnabled()) {
			logger.trace("No HandlerExceptionResolvers declared in servlet '" + getServletName() +
					"': using default strategies from DispatcherServlet.properties");
		}
	}
}

好了, 现在我们加载了应用程序中所有定义的Resolver. 当有请求到达时, DispatcherServlet的doDispatch方法使用请求特定的handler处理, 当handler发生异常时, 变量dispatchException的值赋值为抛出的异常, 并委托给方法processDispatchResult

doDispatch的代码, 只摘录出与本议题有关的.

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
	....
	try {
		ModelAndView mv = null;
		mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
	}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);
	....
}

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

	boolean errorView = false;

	// 异常处理
	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);
		}
	}

	// handler是否返回了view
	if (mv != null && !mv.wasCleared()) {
		render(mv, request, response);
		if (errorView) {
			WebUtils.clearErrorRequestAttributes(request);
		}
	}
	else {
		if (logger.isTraceEnabled()) {
			logger.trace("No view rendering, null ModelAndView returned.");
		}
	}

	if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
		// Concurrent handling started during a forward
		return;
	}

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

从processDispatchResult方法中可以看到, 如果参数exception不为null, 则会处理异常, 对于ModelAndViewDefiningException类型的异常单独处理, 对于其他类型的异常, 转交给processHandlerException方法处理, 这个方法就是异常处理逻辑的核心.

@Nullable
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);

	// 使用注册的Resolver处理
	ModelAndView exMv = null;
	if (this.handlerExceptionResolvers != null) {
		for (HandlerExceptionResolver resolver : this.handlerExceptionResolvers) {
			exMv = resolver.resolveException(request, response, handler, ex);
			if (exMv != null) {
				break;
			}
		}
	}
	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()) {
			String defaultViewName = getDefaultViewName(request);
			if (defaultViewName != null) {
				exMv.setViewName(defaultViewName);
			}
		}
		if (logger.isTraceEnabled()) {
			logger.trace("Using resolved error view: " + exMv, ex);
		}
		if (logger.isDebugEnabled()) {
			logger.debug("Using resolved error view: " + exMv);
		}
		WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
		return exMv;
	}

	throw ex;
}

从上面代码可以看到, this.handlerExceptionResolvers就是在程序启动时初始化注册的, spring通过遍历Resolver列表的方式处理异常, 如果返回结果不为null, 说明处理成功, 就跳出循环.

总结

Spring的异常解析器实现全部继承自接口ResponseStatusExceptionResolver, 上面我们详细了解了该接口在Spring中的几种实现, 比如处理预定义异常的DefaultHandlerExceptionResolver, 可以映射异常到状态码的ResponseStatusExceptionResolver, 还有功能更为强大的ExceptionHandlerExceptionResolver. 同时也简单了解了其使用方式,使用@ExceptionHandler来将方法标记为异常处理器, 结合@ControllerAdvice处理全局异常.

最后我们探究了异常处理器的加载和处理方式, 我们知道了其通过 DispatcherServlet 的初始化方法initHandlerMappings完成加载器列表的注册初始化, 并且在具体处理请求的doDispatch中检测异常, 最终processDispatchResult方法委托给processHandlerException, 该方法循环注册的异常处理器列表完成处理过程.