概述

如果在请求映射期间发生异常或从请求处理程序(例如@Controller)抛出异常，DispatcherServlet将委托给HandlerExceptionResolver

(资料图片)

下表列出了可用的HandlerExceptionResolver实现：

HandlerExceptionResolver	描述
SimpleMappingExceptionResolver	异常类名和错误视图名之间的映射。用于在浏览器应用程序中渲染错误页面。
DefaultHandlerExceptionResolver	解析Spring MVC引发的异常，并将其映射为HTTP状态码。另见可选的ResponseEntityExceptionHandler和REST API异常。
ResponseStatusExceptionResolver	使用@ResponseStatus注解解析异常，并根据注解中的值将异常映射为HTTP状态码。
ExceptionHandlerExceptionResolver	通过在@Controller或@ControllerAdvice类中调用@ExceptionHandler方法来解决异常。

异常解析器链

你可以在Spring配置中声明多个HandlerExceptionResolver

HandlerExceptionResolver的约定规定它可以返回:

指向错误视图的ModelAndView如果异常是在解析器中处理的，则返回空的ModelAndView如果异常仍然未解决，则为null，以便后续的解析器尝试，如果异常在最后仍然存在，则允许它向上冒泡到Servlet容器。

MVC配置自动为默认的Spring MVC异常、@ResponseStatus注解的异常以及@ExceptionHandler方法声明了内置的解析器。

容器错误页配置

如果任何HandlerExceptionResolver都无法解决异常，因此需要传播，或者响应状态被设置为错误状态(即4xx、5xx)， Servlet容器可以在HTML中渲染一个默认的错误页面。要定制容器的默认错误页面，可以在web.xml中声明错误页面映射。如下面的例子所示:​

  /error

根据前面的示例，当出现异常冒泡或响应具有错误状态时，Servlet容器将在容器中向配置的URL(例如/error)发送错误。然后由DispatcherServlet处理，可能会将其映射到一个@Controller，这个@Controller可以实现为返回一个带有模型的错误视图名称，或者渲染一个JSON响应，如下面的例子所示:​

@RestControllerpublic class ErrorController {  @RequestMapping(path = "/error")  public Map handle(HttpServletRequest request) {    Map map = new HashMap();    map.put("status", request.getAttribute("javax.servlet.error.status_code"));    map.put("reason", request.getAttribute("javax.servlet.error.message"));    return map;  }}

错误处理原理

public class DispatcherServlet {  // 取得容器中所有的异常解析器  private List handlerExceptionResolvers;  protected void initStrategies(ApplicationContext context) {    // 初始化异常解析器    initHandlerExceptionResolvers(context);  }  private void initHandlerExceptionResolvers(ApplicationContext context) {    this.handlerExceptionResolvers = null;    if (this.detectAllHandlerExceptionResolvers) {      // 获取容器中所有的异常解析器      Map matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false);      if (!matchingBeans.isEmpty()) {        this.handlerExceptionResolvers = new ArrayList<>(matchingBeans.values());        AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers);      }    }  }}

发生异常后逻辑处理​

public class DispatcherServlet {  protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {    processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);  }  private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,     @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,     @Nullable Exception exception) throws Exception {    boolean errorView = false;    // 判断当前调用是否发生了异常    if (exception != null) {      if (exception instanceof ModelAndViewDefiningException) {        mv = ((ModelAndViewDefiningException) exception).getModelAndView();      } else {        // 获取当前处理句柄HandlerMethod        Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);        // 处理异常        mv = processHandlerException(request, response, handler, exception);        errorView = (mv != null);      }    }    if (mv != null && !mv.wasCleared()) {      render(mv, request, response);    }  }  protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,     @Nullable Object handler, Exception ex) throws Exception {    request.removeAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);    ModelAndView exMv = null;    if (this.handlerExceptionResolvers != null) {      // 遍历在上面初始化查找到的所有异常解析器      for (HandlerExceptionResolver resolver : this.handlerExceptionResolvers) {        // 执行异常解析        exMv = resolver.resolveException(request, response, handler, ex);        if (exMv != null) {          break;        }      }    }    if (exMv != null) {      // ...      return exMv ;    }    // 如果所有的异常解析器都返回null，则直接抛出异常，该异常会被Servlet进行处理    throw ex ;  }}

异常解析器默认配置

public class WebMvcAutoConfiguration {  @Configuration(proxyBeanMethods = false)  @EnableConfigurationProperties(WebProperties.class)  public static class EnableWebMvcConfiguration extends DelegatingWebMvcConfiguration {  }  }public class WebMvcConfigurationSupport {  @Bean  public HandlerExceptionResolver handlerExceptionResolver(@Qualifier("mvcContentNegotiationManager") ContentNegotiationManager contentNegotiationManager) {    List exceptionResolvers = new ArrayList<>();    // 处理自定义异常解析器的配置    configureHandlerExceptionResolvers(exceptionResolvers);    if (exceptionResolvers.isEmpty()) {      // 如果没有异常解析器，那么添加默认的异常解析器      addDefaultHandlerExceptionResolvers(exceptionResolvers, contentNegotiationManager);    }    extendHandlerExceptionResolvers(exceptionResolvers);    HandlerExceptionResolverComposite composite = new HandlerExceptionResolverComposite();    // 设置高优先级，Composite成为统一的入口    composite.setOrder(0);    composite.setExceptionResolvers(exceptionResolvers);    return composite;  }  // 默认配置  protected final void addDefaultHandlerExceptionResolvers(List exceptionResolvers, ContentNegotiationManager mvcContentNegotiationManager) {    ExceptionHandlerExceptionResolver exceptionHandlerResolver = createExceptionHandlerExceptionResolver();    exceptionHandlerResolver.setContentNegotiationManager(mvcContentNegotiationManager);    exceptionHandlerResolver.setMessageConverters(getMessageConverters());    exceptionHandlerResolver.setCustomArgumentResolvers(getArgumentResolvers());    exceptionHandlerResolver.setCustomReturnValueHandlers(getReturnValueHandlers());    if (jackson2Present) {      exceptionHandlerResolver.setResponseBodyAdvice(Collections.singletonList(new JsonViewResponseBodyAdvice()));    }    if (this.applicationContext != null) {      exceptionHandlerResolver.setApplicationContext(this.applicationContext);    }    exceptionHandlerResolver.afterPropertiesSet();    exceptionResolvers.add(exceptionHandlerResolver);    ResponseStatusExceptionResolver responseStatusResolver = new ResponseStatusExceptionResolver();    responseStatusResolver.setMessageSource(this.applicationContext);    exceptionResolvers.add(responseStatusResolver);    exceptionResolvers.add(new DefaultHandlerExceptionResolver());  }}

完毕！！！

关键词： 错误状态 抛出异常 应用程序 异常处理机制