Spring系列(七) Spring MVC 异常处理

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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)

• 处理状态码
• 处理异常信息
• 处理服务地址

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 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 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 FrameworkServlet initWebApplicationContext 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 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.

Spring系列(七) Spring MVC 异常处理

标签：ext.get   packages   scl   fresh   blog   out   简单   https   try   

原文地址：https://www.cnblogs.com/walkinhalo/p/9744656.html