SpringBoot的异常处理流程是什么样的?

public interface HandlerExceptionResolver {
    //解析处理异常
	ModelAndView resolveException(
        HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex);
    
}

public class DispatcherServlet extends FrameworkServlet {
    //异常解析器集合
	private List<HandlerExceptionResolver> handlerExceptionResolvers;
}

//处理分发结果
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
                                   HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {
    
    //判断HandlerMapping、HandlerAdapter处理时的异常是否为空
    if (exception != null) {
        Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
        //异常不为空，处理异常，进行异常视图的获取
        mv = processHandlerException(request, response, handler, exception);
    }

    
    //只要存在mv就进行视图渲染
    if (mv != null && !mv.wasCleared()) {
        //不管视图是正常视图还是异常视图，均进入视图渲染流程
        render(mv, request, response);
        if (errorView) {
            WebUtils.clearErrorRequestAttributes(request);
        }
    }
 	...略
    ...略
   	...略
}

//处理异常，进行异常视图的获取
protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
                                               Object handler, Exception ex) throws Exception {

    // Check registered HandlerExceptionResolvers...
    ModelAndView exMv = null;
    //遍历所有的处理器异常解析器handlerExceptionResolvers，看谁能够处理当前异常
    for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {
        //解析当前异常
        exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);
  
        if (exMv != null) {
            //ModelAndView 不为空时，异常视图获取成功，跳出方法，进行异常视图渲染。
            break;
        }
    }
   
    ...略
    ...略
    ...略
   //所有处理器异常解析器都不能处理该异常，抛出异常
   throw ex;
}

static class ErrorPageCustomizer implements ErrorPageRegistrar, Ordered {
    //关于服务器的一些配置，如端口号，编码方式等。在这里主要关注server.error.path 
    private final ServerProperties properties;

    private final DispatcherServletPath dispatcherServletPath;

    protected ErrorPageCustomizer(ServerProperties properties, DispatcherServletPath dispatcherServletPath) {
        this.properties = properties;
        this.dispatcherServletPath = dispatcherServletPath;
    }

    //注册错误页面
    @Override
    public void registerErrorPages(ErrorPageRegistry errorPageRegistry) {
        //ErrorPage里包含了状态码、异常、和错误控制器映射路径
        ErrorPage errorPage = new ErrorPage(
            //this.properties.getError().getPath()获取的就是server.error.path=/error(默认)
            this.dispatcherServletPath.getRelativePath(this.properties.getError().getPath()));
        //注册ErrorPage
        errorPageRegistry.addErrorPages(errorPage);
    }
}
====================DispatcherServletPath接口=================================
public interface DispatcherServletPath {   
    default String getRelativePath(String path) {
        String prefix = getPrefix();
        if (!path.startsWith("/")) {
            path = "/" + path;
        }
        return prefix + path;
    }
}

@Controller
//server.error.path 为空则用error.path, 再为空，再用/error
@RequestMapping("${server.error.path:${error.path:/error}}")
public class BasicErrorController extends AbstractErrorController {
    
     //1、产生html类型的数据；浏览器客户端发送的请求来到这个方法处理
    @RequestMapping(produces = MediaType.TEXT_HTML_VALUE)
	public ModelAndView errorHtml(HttpServletRequest request, HttpServletResponse response) {
        //状态码
		HttpStatus status = getStatus(request);
        //从DefaultErrorAttributes里获取可以在页面显示的数据
		Map<String, Object> model = Collections
				.unmodifiableMap(getErrorAttributes(request, getErrorAttributeOptions(request, MediaType.TEXT_HTML)));
		response.setStatus(status.value());
        //调用父类里的方法，寻找错误视图解析器，来解析错误视图
		ModelAndView modelAndView = resolveErrorView(request, response, status, model);
        //！！！如果没有配置具体的状态码错误页面或4xx,5xx这种视图，视图解析不成功，就会返回空的ModelAndView对象。此时就会构造一个默认的error错误视图，通过BeanNameViewResolver视图解析器，根据视图名(error)作为组件id去容器中找到View对象
		return (modelAndView != null) ? modelAndView : new ModelAndView("error", model);
	}
	
    //2、产生json数据，其他客户端来到这个方法处理
	@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);
	}
}

========================AbstractErrorController================================
protected ModelAndView resolveErrorView(HttpServletRequest request,HttpServletResponse response, HttpStatus status, Map<String, Object> model) {
    
    for (ErrorViewResolver resolver : this.errorViewResolvers) {
        //错误视图解析器获取错误视图，返回ModelAndView
        ModelAndView modelAndView = resolver.resolveErrorView(request, status, model);
        if (modelAndView != null) {
            return modelAndView;
        }
    }
    return null;
}

=========================ErrorMvcAutoConfiguration========================================
protected static class WhitelabelErrorViewConfiguration {

		private final StaticView defaultErrorView = new StaticView();
		
    	//向容器种注册默认的错误视图，id为error
		@Bean(name = "error")
		@ConditionalOnMissingBean(name = "error")
		public View defaultErrorView() {
			return this.defaultErrorView;
		}
}
//可以看到静态内部类StaticView，进行视图渲染的时候，构造了我们经常看到的默认错误页面Whitelabel Error Page。
private static class StaticView implements View {

		@Override
		public void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response)
				throws Exception {
			if (response.isCommitted()) {
				String message = getMessage(model);
				logger.error(message);
				return;
			}
			response.setContentType(TEXT_HTML_UTF8.toString());
			StringBuilder builder = new StringBuilder();
			Object timestamp = model.get("timestamp");
			Object message = model.get("message");
			Object trace = model.get("trace");
			if (response.getContentType() == null) {
				response.setContentType(getContentType());
			}
			builder.append("<html><body><h1>Whitelabel Error Page</h1>").append(
					"<p>This application has no explicit mapping for /error, so you are seeing this as a fallback.</p>")
					.append("<div id='created'>").append(timestamp).append("</div>")
					.append("<div>There was an unexpected error (type=").append(htmlEscape(model.get("error")))
					.append(", status=").append(htmlEscape(model.get("status"))).append(").</div>");
			if (message != null) {
				builder.append("<div>").append(htmlEscape(message)).append("</div>");
			}
			if (trace != null) {
				builder.append("<div style='white-space:pre-wrap;'>").append(htmlEscape(trace)).append("</div>");
			}
			builder.append("</body></html>");
			response.getWriter().append(builder.toString());
		}
}

public class DefaultErrorViewResolver implements ErrorViewResolver, Ordered {
    
    private static final Map<Series, String> SERIES_VIEWS;
    static {
        Map<Series, String> views = new EnumMap<>(Series.class);
        views.put(Series.CLIENT_ERROR, "4xx");
        views.put(Series.SERVER_ERROR, "5xx");
        SERIES_VIEWS = Collections.unmodifiableMap(views);
    }

    //解析错误视图，要去的错误页面
    @Override
    public ModelAndView resolveErrorView(HttpServletRequest request, HttpStatus status, Map<String, Object> model) {
        //先精确查找错误视图
        ModelAndView modelAndView = resolve(String.valueOf(status.value()), model);
        //精确查找不到，则查找4xx,5xx这种类型的错误页面
        if (modelAndView == null && SERIES_VIEWS.containsKey(status.series())) {
            modelAndView = resolve(SERIES_VIEWS.get(status.series()), model);
        }
        return modelAndView;
    }
    //真正解析 viewName是状态码
    private ModelAndView resolve(String viewName, Map<String, Object> model) {
     //！！！！默认SpringBoot可以去找到一个页面  error/404，注意定制错误页面要放在error文件夹下，如error/4xx.html
        String errorViewName = "error/" + viewName;

        //模板引擎可以解析这个页面地址就用模板引擎解析
        TemplateAvailabilityProvider provider = this.templateAvailabilityProviders
            .getProvider(errorViewName, this.applicationContext);
        if (provider != null) {
            //模板引擎可用的情况下返回到errorViewName指定的视图地址
            return new ModelAndView(errorViewName, model);
        }
        //模板引擎不可用，就在静态资源文件夹下找errorViewName对应的页面   error/404.html
        return resolveResource(errorViewName, model);
    }
    //模板引擎不可用，就去静态资源文件夹下寻找
    private ModelAndView resolveResource(String viewName, Map<String, Object> model) {
        //这里遍历的是"classpath:/META-INF/resources/","classpath:/resources/", "classpath:/static/", "classpath:/public/"
        for (String location : this.resourceProperties.getStaticLocations()) {
            try {
                Resource resource = this.applicationContext.getResource(location);
                resource = resource.createRelative(viewName + ".html");
                if (resource.exists()) {
                    return new ModelAndView(new HtmlResourceView(resource), model);
                }
            }
            catch (Exception ex) {
            }
        }
        return null;
    }
} 

public class DefaultErrorAttributes implements ErrorAttributes, HandlerExceptionResolver, Ordered {
    //帮我们在页面共享信息
    @Override
        public Map<String, Object> getErrorAttributes(RequestAttributes requestAttributes,
                boolean includeStackTrace) {
            Map<String, Object> errorAttributes = new LinkedHashMap<String, Object>();
            errorAttributes.put("timestamp", new Date());
            addStatus(errorAttributes, requestAttributes);
            addErrorDetails(errorAttributes, requestAttributes, includeStackTrace);
            addPath(errorAttributes, requestAttributes);
            return errorAttributes;
        }
}