SpringMVC的异常处理机制详细分析

public class CustomWebApplicationInitializer implements WebApplicationInitializer {

    @Override
    public void onStartup(ServletContext servletContext) {
        AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext();
        context.register(WebConfig.class);
        DispatcherServlet servlet = new DispatcherServlet(context);
        ServletRegistration.Dynamic registration = servletContext.addServlet("app", servlet);
        registration.setLoadOnStartup(1);
        registration.addMapping("/app/*");
    }
}
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protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
  try {
    Exception dispatchException = null;
    // 1.获取HandlerMapping（该对象就是当前请求与处理程序的一个映射关系）
    mappedHandler = getHandler(processedRequest);
    // 2.获取能够处理上一步得到的处理程序
    HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
    // 3.执行实际的调用（执行实际的处理程序）
    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
  } catch (Exception ex) {
    // 4.执行过程中发生异常记录到局部变量中
    dispatchException = ex;
  } catch (Throwable err) {
    dispatchException = new NestedServletException("Handler dispatch failed", err);
  }
  // 5.处理结果
  processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
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private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
      @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
      @Nullable Exception exception) throws Exception {

  boolean errorView = false;
  // 1.判断是否发生异常
  if (exception != null) {
    // 1.1.异常对象是否是该对象
    if (exception instanceof ModelAndViewDefiningException) {
      mv = ((ModelAndViewDefiningException) exception).getModelAndView();
    } else {
      // 如果不是上面的异常对象，则这里获取具体处理程序的Handler
      // 这里我们只考虑RequestMappingHandlerMapping情况，那么这里获取的将是HandlerMethod对象
      // 也就是Controller中的具体方法了
      Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
      // 处理异常，查看下面processhandlerException方法
      mv = processHandlerException(request, response, handler, exception);
      errorView = (mv != null);
    }
  }

  // Did the handler return a view to render?
  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) {
    // Exception (if any) is already handled..
    mappedHandler.triggerAfterCompletion(request, response, null);
  }
}
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protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
      @Nullable Object handler, Exception ex) throws Exception {
  ModelAndView exMv = null;
  // 判断当前的异常解析器是否存在；也就是从容器中获取所有HandlerExceptionResolver类型对象
  // 这里我们就不展开了，你可以在DispatcherServlet中查看初始化过程
  // 默认情况下，这里集合中有如下图1中所示
  if (this.handlerExceptionResolvers != null) {
    // 遍历每一个异常处理器，谁能返回ModelAndView就结束循环
    // 由于DefaultErrorAttributes内部方法直接返回了null，所以这里返回的是HandlerExceptionResolverComposite
    // 这是聚合类，聚合了其它3个具体的解析器，所以时间处理的还是其它类并非它
    for (HandlerExceptionResolver resolver : this.handlerExceptionResolvers) {
      // 结合上面说只考虑RequestMappingHandlerMapping处理Controller的情况
      // 那这里合理的解析器是ExceptionHandlerExceptionResolver
      exMv = resolver.resolveException(request, response, handler, ex);
      if (exMv != null) {
        break;
      }
    }
  }
  // 通过上面的执行如果获取到了ModelAndView对象，下面就是判断视图对象不同的情况如何进行处理了
  if (exMv != null) {
    if (exMv.isEmpty()) {
      request.setAttribute(EXCEPTION_ATTRIBUTE, ex);
      return null;
    }
    if (!exMv.hasView()) {
      String defaultViewName = getDefaultViewName(request);
      if (defaultViewName != null) {
        exMv.setViewName(defaultViewName);
      }
    }
    WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
    // 如果存在要想前端展示的视图，则返回。
    return exMv;
  }
  throw ex;
}
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