當我們使用瀏覽器訪問一個不存在的地址時，SpringBoot 默認提供如下類似的404頁面。  當如 Android、IOS 等非瀏覽器客戶端訪問時，比如使用 [Postman](https://www.postman.com/downloads/) 客戶端軟件模擬請求一個不存在的頁面時響應的是json數據。  SpringBoot 的錯誤處理機制由 ErrorMvcAutoConfiguration 類管理，它的源碼如下，它注冊了下面4個錯誤處理組件。 ```java -----ErrorMvcAutoConfiguration----- @Bean @ConditionalOnMissingBean( value = {ErrorAttributes.class}, search = SearchStrategy.CURRENT ) // 組件1：DefaultErrorAttributes public DefaultErrorAttributes errorAttributes() { return new DefaultErrorAttributes(); } @Bean @ConditionalOnMissingBean( value = {ErrorController.class}, search = SearchStrategy.CURRENT ) // 組件2：BasicErrorController public BasicErrorController basicErrorController(ErrorAttributes errorAttributes, ObjectProvider<ErrorViewResolver> errorViewResolvers) { return new BasicErrorController(errorAttributes, this.serverProperties.getError(), (List)errorViewResolvers.orderedStream().collect(Collectors.toList())); } @Bean // 組件3：ErrorPageCustomizer public ErrorMvcAutoConfiguration.ErrorPageCustomizer errorPageCustomizer(DispatcherServletPath dispatcherServletPath) { return new ErrorMvcAutoConfiguration.ErrorPageCustomizer(this.serverProperties, dispatcherServletPath); } @Bean @ConditionalOnBean({DispatcherServlet.class}) @ConditionalOnMissingBean({ErrorViewResolver.class}) 組件4：DefaultErrorViewResolver DefaultErrorViewResolver conventionErrorViewResolver() { return new DefaultErrorViewResolver(this.applicationContext, this.resourceProperties); } ``` 這4個組件的調用步驟如下： 1. 當系統出現4xx或者5xx之類的錯誤 ，調用ErrorPageCustomizer來定制錯誤的響應規則，它會獲取系統默認的/error頁面； ```java -----ErrorMvcAutoConfiguration----- public void registerErrorPages(ErrorPageRegistry errorPageRegistry) { ErrorPage errorPage = new ErrorPage(this.dispatcherServletPath.getRelativePath(this.properties.getError().▲▲▲▲getPath()▲▲▲▲)); errorPageRegistry.addErrorPages(new ErrorPage[]{errorPage}); } -> -----ErrorProperties----- public class ErrorProperties { @Value("${error.path:/error}") private String path = "/error"; private boolean includeException; ``` 2. 接著調用BasicErrorController來處理這個/error頁面，BasicErrorController 會根據請求的頭部信息來判斷是來自瀏覽器的請求，還是客戶端的請求，決定應該響應一個頁面，還是json數據。   ```java -----ErrorMvcAutoConfiguration----- @Bean @ConditionalOnMissingBean( value = {ErrorController.class}, search = SearchStrategy.CURRENT ) public ▲▲▲▲BasicErrorController▲▲▲▲ basicErrorController(ErrorAttributes errorAttributes, ObjectProvider<ErrorViewResolver> errorViewResolvers) { return new BasicErrorController(errorAttributes, this.serverProperties.getError(), (List)errorViewResolvers.orderedStream().collect(Collectors.toList())); } -> -----BasicErrorController----- @Controller // 如果沒有找到server.error.path，則采用errorp.path，如果還是沒有則最后采用/error錯誤配置 @RequestMapping({"${server.error.path:${error.path:/error}}"}) public class BasicErrorController extends AbstractErrorController { private final ErrorProperties errorProperties; @RequestMapping( produces = {"text/html"} ) // 當是瀏覽器訪問時，返回html public ModelAndView errorHtml(HttpServletRequest request, HttpServletResponse response) { HttpStatus status = this.getStatus(request); Map<String, Object> model = // 獲取模型數據 Collections.unmodifiableMap(▲▲▲▲this.getErrorAttributes(request, this.getErrorAttributeOptions(request, MediaType.TEXT_HTML))▲▲▲▲); response.setStatus(status.value()); // 去哪個頁面作為錯誤頁面 ModelAndView modelAndView = ▲▲▲▲this.resolveErrorView(request, response, status, model)▲▲▲▲; return modelAndView != null ? modelAndView : new ModelAndView("error", model); } // 當是非瀏覽器訪問時，返回json數據 @RequestMapping public ResponseEntity<Map<String, Object>> error(HttpServletRequest request) { HttpStatus status = this.getStatus(request); if (status == HttpStatus.NO_CONTENT) { return new ResponseEntity(status); } else { Map<String, Object> body = this.getErrorAttributes(request, this.getErrorAttributeOptions(request, MediaType.ALL)); return new ResponseEntity(body, status); } } ``` 3. 接著DefaultErrorViewResolver起作用； 當是瀏覽器訪問時，響應的是html，DefaultErrorViewResolver解析應該響應哪個頁面； ```java -----ErrorMvcAutoConfiguration----- @Bean @ConditionalOnBean( { DispatcherServlet.class } ) @ConditionalOnMissingBean( { ErrorViewResolver.class } ) ▲▲▲▲DefaultErrorViewResolver▲▲▲▲ conventionErrorViewResolver() { return new DefaultErrorViewResolver(this.applicationContext, this.resourceProperties); } ->-----DefaultErrorViewResolver----- // 解析出是客戶端錯誤，還是服務端錯誤，客戶端錯誤為4xx，服務端錯誤為5xx，將它們存入Map中 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); } // 返回對應的視圖 public ModelAndView resolveErrorView(HttpServletRequest request, HttpStatus status, Map<String, Object> model) { ModelAndView modelAndView=this.resolve(String.valueOf(status.value()), model); if (modelAndView==null && SERIES_VIEWS.containsKey(status.series())) { // 根據不同的狀態碼SERIES_VIEWS.get(status.series())解析出對應的視圖 modelAndView=this.resolve((String)SERIES_VIEWS.get(status.series()), model); } return modelAndView; } private ModelAndView resolve(String viewName, Map<String, Object> model) { // 將對應的視圖拼接為error/viewName，比如如果是404狀態碼，則為error/404.html String errorViewName="error/"+viewName; TemplateAvailabilityProvider provider=this.templateAvailabilityProviders.getProvider(errorViewName, this.applicationContext); // 如果模板引擎可用，而且解析的錯誤頁面可用，則用；如果不能用則用this.resolveResource(errorViewName, model)提供的錯誤頁面 return provider !=null ? new ModelAndView(errorViewName, model): this.resolveResource(errorViewName, model); } // 到靜態資源目錄下尋找可用的error頁面 private ModelAndView resolveResource(String viewName, Map<String, Object> model) { String[] var3=this.resourceProperties.getStaticLocations(); int var4=var3.length; for(int var5=0; var5 < var4; ++var5) { String location=var3[var5]; try { Resource resource=this.applicationContext.getResource(location); resource=resource.createRelative(viewName + ".html"); // 當靜態資源目錄下存在templates/error/404.html則用，不存在也是返回null if (resource.exists()) { return new ModelAndView(new DefaultErrorViewResolver.HtmlResourceView(resource), model); } } catch (Exception var8) {} } return null; } ``` 4. 最終由DefaultErrorAttributes來配置頁面的共享信息 ```java -----DefaultErrorAttributes----- errorAttributes.put("status", 999); // 狀態碼 errorAttributes.put("error", "None"); // 錯誤提示 errorAttributes.put("exception", error.getClass().getName()); // 異常對象 errorAttributes.put("message", message); // 消息 errorAttributes.put("errors", result.getAllErrors()); // jsr303數據校驗錯誤提示 errorAttributes.put("trace", stackTrace.toString()); errorAttributes.put("path", path); errorAttributes.put("timestamp", new Date()); ```