Spring MVC核心功能异常处理机制原理详解

2023-02-28

this null 错误

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下表列出了可用的HandlerExceptionResolver实现：HandlerExceptionResolver描述S

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

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-page>
  <location>/error</location>
</error-page>1.
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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:

@RestController
public class ErrorController {


  @RequestMapping(path = "/error")
  public Map<String, Object> handle(HttpServletRequest request) {
    Map<String, Object> map = new HashMap<String, Object>();
    map.put("status", request.getAttribute("javax.servlet.error.status_code"));
    map.put("reason", request.getAttribute("javax.servlet.error.message"));
    return map;
  }
}1.
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错误处理原理

public class DispatcherServlet {
  // 取得容器中所有的异常解析器
  private List<HandlerExceptionResolver> 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<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false);
      if (!matchingBeans.isEmpty()) {
        this.handlerExceptionResolvers = new ArrayList<>(matchingBeans.values());
        AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers);
      }
    }
  }
}1.
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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 ;
  }
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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<HandlerExceptionResolver> 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<HandlerExceptionResolver> 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());
  }
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