• SpringBoot中异常处理的流程图
  

在SpringBoot项目中，如果业务逻辑发生了异常，则首先看发生异常的类中有没有@ExceptionHandle能处理，如果能，则执行方法进行处理，如果没有，则去找全局的@ControllerAdvice注解，进行异常处理，如果还是不能处理，则继续看SpringMVC的异常处理机制能否处理，这里SpringMVC的异常处理机制就是可以处理默认的十几种异常，如请求类型错误Method Not Allowed,如请求体内容错误等，如果不能处理，则走SpringBoot提供的异常处理机制。SpringBoot的异常处理机制就是将当前异常请求重定向到/error请求，并且会自动根据内容协商是返回页面还是json数据。
所有有时当无法处理异常时，转发到/error请求后又会来带一些其他问题。

• ErrorMvcAutoConfiguration 自动配置类
  ErrorMvcAutoConfiguration 这个类是SpringBoot提供用于处理异常机制的。

首先这个类中给容器中注入了一个 Controller

	@Bean
	@ConditionalOnMissingBean(value = ErrorController.class, search = SearchStrategy.CURRENT)
	public BasicErrorController basicErrorController(ErrorAttributes errorAttributes,
			ObjectProvider<ErrorViewResolver> errorViewResolvers) {
		return new BasicErrorController(errorAttributes, this.serverProperties.getError(),
				errorViewResolvers.orderedStream().collect(Collectors.toList()));
	}

@Controller
@RequestMapping("${server.error.path:${error.path:/error}}")
public class BasicErrorController extends AbstractErrorController {

	private final ErrorProperties errorProperties;

	/**
	 * Create a new {@link BasicErrorController} instance.
	 * @param errorAttributes the error attributes
	 * @param errorProperties configuration properties
	 */
	public BasicErrorController(ErrorAttributes errorAttributes, ErrorProperties errorProperties) {
		this(errorAttributes, errorProperties, Collections.emptyList());
	}

	/**
	 * Create a new {@link BasicErrorController} instance.
	 * @param errorAttributes the error attributes
	 * @param errorProperties configuration properties
	 * @param errorViewResolvers error view resolvers
	 */
	public BasicErrorController(ErrorAttributes errorAttributes, ErrorProperties errorProperties,
			List<ErrorViewResolver> errorViewResolvers) {
		super(errorAttributes, errorViewResolvers);
		Assert.notNull(errorProperties, "ErrorProperties must not be null");
		this.errorProperties = errorProperties;
	}

	@RequestMapping(produces = MediaType.TEXT_HTML_VALUE)
	public ModelAndView errorHtml(HttpServletRequest request, HttpServletResponse response) {
		HttpStatus status = getStatus(request);
		Map<String, Object> model = Collections
			.unmodifiableMap(getErrorAttributes(request, getErrorAttributeOptions(request, MediaType.TEXT_HTML)));
		response.setStatus(status.value());
		ModelAndView modelAndView = resolveErrorView(request, response, status, model);
		return (modelAndView != null) ? modelAndView : new ModelAndView("error", model);
	}

	@RequestMapping
	public ResponseEntity<Map<String, Object>> error(HttpServletRequest request) {
		HttpStatus status = getStatus(request);
		if (status == HttpStatus.NO_CONTENT) {
			return new ResponseEntity<>(status);
		}
		Map<String, Object> body = getErrorAttributes(request, getErrorAttributeOptions(request, MediaType.ALL));
		return new ResponseEntity<>(body, status);
	}

	@ExceptionHandler(HttpMediaTypeNotAcceptableException.class)
	public ResponseEntity<String> mediaTypeNotAcceptable(HttpServletRequest request) {
		HttpStatus status = getStatus(request);
		return ResponseEntity.status(status).build();
	}

	protected ErrorAttributeOptions getErrorAttributeOptions(HttpServletRequest request, MediaType mediaType) {
		ErrorAttributeOptions options = ErrorAttributeOptions.defaults();
		if (this.errorProperties.isIncludeException()) {
			options = options.including(Include.EXCEPTION);
		}
		if (isIncludeStackTrace(request, mediaType)) {
			options = options.including(Include.STACK_TRACE);
		}
		if (isIncludeMessage(request, mediaType)) {
			options = options.including(Include.MESSAGE);
		}
		if (isIncludeBindingErrors(request, mediaType)) {
			options = options.including(Include.BINDING_ERRORS);
		}
		return options;
	}

	/**
	 * Determine if the stacktrace attribute should be included.
	 * @param request the source request
	 * @param produces the media type produced (or {@code MediaType.ALL})
	 * @return if the stacktrace attribute should be included
	 */
	protected boolean isIncludeStackTrace(HttpServletRequest request, MediaType produces) {
		switch (getErrorProperties().getIncludeStacktrace()) {
			case ALWAYS:
				return true;
			case ON_PARAM:
				return getTraceParameter(request);
			default:
				return false;
		}
	}

	/**
	 * Determine if the message attribute should be included.
	 * @param request the source request
	 * @param produces the media type produced (or {@code MediaType.ALL})
	 * @return if the message attribute should be included
	 */
	protected boolean isIncludeMessage(HttpServletRequest request, MediaType produces) {
		switch (getErrorProperties().getIncludeMessage()) {
			case ALWAYS:
				return true;
			case ON_PARAM:
				return getMessageParameter(request);
			default:
				return false;
		}
	}

	/**
	 * Determine if the errors attribute should be included.
	 * @param request the source request
	 * @param produces the media type produced (or {@code MediaType.ALL})
	 * @return if the errors attribute should be included
	 */
	protected boolean isIncludeBindingErrors(HttpServletRequest request, MediaType produces) {
		switch (getErrorProperties().getIncludeBindingErrors()) {
			case ALWAYS:
				return true;
			case ON_PARAM:
				return getErrorsParameter(request);
			default:
				return false;
		}
	}

	/**
	 * Provide access to the error properties.
	 * @return the error properties
	 */
	protected ErrorProperties getErrorProperties() {
		return this.errorProperties;
	}

}

通过分析这个类的源码得知，此类主要就是给SpringMVC映射了两个请求，来处理 /error请求。
而两个方法分别是返回html页面与json数据的。