SpringBoot的異常處理流程是什麼樣的?

public interface HandlerExceptionResolver {
    //解析處理異常
	ModelAndView resolveException(
        HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex);
    
}

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

//處理分發結果
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
                                   HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {
    
    //判斷HandlerMapping、HandlerAdapter處理時的異常是否為空
    if (exception != null) {
        Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
        //異常不為空，處理異常，進行異常視圖的獲取
        mv = processHandlerException(request, response, handler, exception);
    }

    
    //隻要存在mv就進行視圖渲染
    if (mv != null && !mv.wasCleared()) {
        //不管視圖是正常視圖還是異常視圖，均進入視圖渲染流程
        render(mv, request, response);
        if (errorView) {
            WebUtils.clearErrorRequestAttributes(request);
        }
    }
 	...略
    ...略
   	...略
}

//處理異常，進行異常視圖的獲取
protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
                                               Object handler, Exception ex) throws Exception {

    // Check registered HandlerExceptionResolvers...
    ModelAndView exMv = null;
    //遍歷所有的處理器異常解析器handlerExceptionResolvers，看誰能夠處理當前異常
    for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {
        //解析當前異常
        exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);
  
        if (exMv != null) {
            //ModelAndView 不為空時，異常視圖獲取成功，跳出方法，進行異常視圖渲染。
            break;
        }
    }
   
    ...略
    ...略
    ...略
   //所有處理器異常解析器都不能處理該異常，拋出異常
   throw ex;
}

static class ErrorPageCustomizer implements ErrorPageRegistrar, Ordered {
    //關於服務器的一些配置，如端口號，編碼方式等。在這裡主要關註server.error.path 
    private final ServerProperties properties;

    private final DispatcherServletPath dispatcherServletPath;

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

    //註冊錯誤頁面
    @Override
    public void registerErrorPages(ErrorPageRegistry errorPageRegistry) {
        //ErrorPage裡包含瞭狀態碼、異常、和錯誤控制器映射路徑
        ErrorPage errorPage = new ErrorPage(
            //this.properties.getError().getPath()獲取的就是server.error.path=/error(默認)
            this.dispatcherServletPath.getRelativePath(this.properties.getError().getPath()));
        //註冊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 為空則用error.path, 再為空，再用/error
@RequestMapping("${server.error.path:${error.path:/error}}")
public class BasicErrorController extends AbstractErrorController {
    
     //1、產生html類型的數據；瀏覽器客戶端發送的請求來到這個方法處理
    @RequestMapping(produces = MediaType.TEXT_HTML_VALUE)
	public ModelAndView errorHtml(HttpServletRequest request, HttpServletResponse response) {
        //狀態碼
		HttpStatus status = getStatus(request);
        //從DefaultErrorAttributes裡獲取可以在頁面顯示的數據
		Map<String, Object> model = Collections
				.unmodifiableMap(getErrorAttributes(request, getErrorAttributeOptions(request, MediaType.TEXT_HTML)));
		response.setStatus(status.value());
        //調用父類裡的方法，尋找錯誤視圖解析器，來解析錯誤視圖
		ModelAndView modelAndView = resolveErrorView(request, response, status, model);
        //！！！如果沒有配置具體的狀態碼錯誤頁面或4xx,5xx這種視圖，視圖解析不成功，就會返回空的ModelAndView對象。此時就會構造一個默認的error錯誤視圖，通過BeanNameViewResolver視圖解析器，根據視圖名(error)作為組件id去容器中找到View對象
		return (modelAndView != null) ? modelAndView : new ModelAndView("error", model);
	}
	
    //2、產生json數據，其他客戶端來到這個方法處理
	@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) {
        //錯誤視圖解析器獲取錯誤視圖，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();
		
    	//向容器種註冊默認的錯誤視圖，id為error
		@Bean(name = "error")
		@ConditionalOnMissingBean(name = "error")
		public View defaultErrorView() {
			return this.defaultErrorView;
		}
}
//可以看到靜態內部類StaticView，進行視圖渲染的時候，構造瞭我們經常看到的默認錯誤頁面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);
    }

    //解析錯誤視圖，要去的錯誤頁面
    @Override
    public ModelAndView resolveErrorView(HttpServletRequest request, HttpStatus status, Map<String, Object> model) {
        //先精確查找錯誤視圖
        ModelAndView modelAndView = resolve(String.valueOf(status.value()), model);
        //精確查找不到，則查找4xx,5xx這種類型的錯誤頁面
        if (modelAndView == null && SERIES_VIEWS.containsKey(status.series())) {
            modelAndView = resolve(SERIES_VIEWS.get(status.series()), model);
        }
        return modelAndView;
    }
    //真正解析 viewName是狀態碼
    private ModelAndView resolve(String viewName, Map<String, Object> model) {
     //！！！！默認SpringBoot可以去找到一個頁面  error/404，註意定制錯誤頁面要放在error文件夾下，如error/4xx.html
        String errorViewName = "error/" + viewName;

        //模板引擎可以解析這個頁面地址就用模板引擎解析
        TemplateAvailabilityProvider provider = this.templateAvailabilityProviders
            .getProvider(errorViewName, this.applicationContext);
        if (provider != null) {
            //模板引擎可用的情況下返回到errorViewName指定的視圖地址
            return new ModelAndView(errorViewName, model);
        }
        //模板引擎不可用，就在靜態資源文件夾下找errorViewName對應的頁面   error/404.html
        return resolveResource(errorViewName, model);
    }
    //模板引擎不可用，就去靜態資源文件夾下尋找
    private ModelAndView resolveResource(String viewName, Map<String, Object> model) {
        //這裡遍歷的是"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 {
    //幫我們在頁面共享信息
    @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;
        }
}