@RestController@RequestMapping("/demo")public class DemoController {
  @GetMapping("/index")  public Object index() {    System.out.println(1 / 0) ;    return "/demo/index" ;  }  }

public abstract class AbstractApplicationContext {  public void refresh() {    onRefresh();  }}

public class ServletWebServerApplicationContext {  protected void onRefresh() {    super.onRefresh();    try {      // 创建web服务      createWebServer();    } catch (Throwable ex) {      throw new ApplicationContextException("Unable to start web server", ex);    }  }  private void createWebServer() {    // 这里假设我们使用的是Tomcat容器，那么这里的factory = TomcatServletWebServerFactory    this.webServer = factory.getWebServer(getSelfInitializer());  }}

public class TomcatServletWebServerFactory {  public WebServer getWebServer(ServletContextInitializer... initializers) {    // 创建Tomcat实例    Tomcat tomcat = new Tomcat();    // ...    // 准备上下文    prepareContext(tomcat.getHost(), initializers);    return getTomcatWebServer(tomcat);  }  protected void prepareContext(Host host, ServletContextInitializer[] initializers) {    // 该类继承自StandardContext类（该类所属tomcat，每一个StandardContext代表了一个webapp）    TomcatEmbeddedContext context = new TomcatEmbeddedContext();    // ...    // 配置上下文    configureContext(context, initializersToUse);  }  // 配置上下文  protected void configureContext(Context context, ServletContextInitializer[] initializers) {    // 获取当前Spring容器中配置的ErrorPage，然后注册到Tomcat当前webapp的上下文中    // 这里其实对应的就是web.xml中配置的错误页    for (ErrorPage errorPage : getErrorPages()) {      org.apache.tomcat.util.descriptor.web.ErrorPage tomcatErrorPage = new org.apache.tomcat.util.descriptor.web.ErrorPage();      tomcatErrorPage.setLocation(errorPage.getPath());      tomcatErrorPage.setErrorCode(errorPage.getStatusCode());      tomcatErrorPage.setExceptionType(errorPage.getExceptionName());      context.addErrorPage(tomcatErrorPage);    }  }}

// TomcatServletWebServerFactory继承自AbstractConfigurableWebServerFactory// ConfigurableWebServerFactory接口继承了ErrorPageRegistry接口public abstract class AbstractConfigurableWebServerFactory implements ConfigurableWebServerFactory {  public void addErrorPages(ErrorPage... errorPages) {    this.errorPages.addAll(Arrays.asList(errorPages));  }}

@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class})public class ServletWebServerFactoryAutoConfiguration {}// BeanPostProcessorsRegistrar实现了ImportBeanDefinitionRegistrar，用来注册Beanpublic static class BeanPostProcessorsRegistrar implements ImportBeanDefinitionRegistrar, BeanFactoryAware {  public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {    // 注册了BeanPostProcessor类ErrorPageRegistrarBeanPostProcessor    registerSyntheticBeanIfMissing(registry, "errorPageRegistrarBeanPostProcessor", ErrorPageRegistrarBeanPostProcessor.class);  }}public class ErrorPageRegistrarBeanPostProcessor implements BeanPostProcessor, BeanFactoryAware {  public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {    // 判断当前的Bean是否实现了ErrorPageRegistry    if (bean instanceof ErrorPageRegistry errorPageRegistry) {      postProcessBeforeInitialization(errorPageRegistry);    }    return bean;  }  private void postProcessBeforeInitialization(ErrorPageRegistry registry) {    // 遍历所有的ErrorPageRegistrar，注册到当前实现了ErrorPageRegistry接口的Bean中    // 如上面的TomcatServletWebServerFactory    for (ErrorPageRegistrar registrar : getRegistrars()) {      registrar.registerErrorPages(registry);    }  }  private Collection<ErrorPageRegistrar> getRegistrars() {    if (this.registrars == null) {      // 获取容器中所有的ErrorPageRegistrar Bean对象      this.registrars = new ArrayList<>(this.beanFactory.getBeansOfType(ErrorPageRegistrar.class, false, false).values());      this.registrars.sort(AnnotationAwareOrderComparator.INSTANCE);      this.registrars = Collections.unmodifiableList(this.registrars);    }    return this.registrars;  }}

public class ErrorMvcAutoConfiguration {  @Bean  public ErrorPageCustomizer errorPageCustomizer(DispatcherServletPath dispatcherServletPath) {    return new ErrorPageCustomizer(this.serverProperties, dispatcherServletPath);  }  static class ErrorPageCustomizer implements ErrorPageRegistrar, Ordered {    // springboot配置文件中的server接口中获取error配置信息    private final ServerProperties properties;
    private final DispatcherServletPath dispatcherServletPath;
    protected ErrorPageCustomizer(ServerProperties properties, DispatcherServletPath dispatcherServletPath) {      this.properties = properties;      this.dispatcherServletPath = dispatcherServletPath;    }
    @Override    public void registerErrorPages(ErrorPageRegistry errorPageRegistry) {      // 获取server.error.path配置属性      ErrorPage errorPage = new ErrorPage(this.dispatcherServletPath.getRelativePath(this.properties.getError().getPath()));      errorPageRegistry.addErrorPages(errorPage);    }  }}@ConfigurationProperties(prefix = "server", ignoreUnknownFields = true)public class ServerProperties {  @NestedConfigurationProperty  private final ErrorProperties error = new ErrorProperties();}public class ErrorProperties {  // 读取error.path配置，如果没有配置，则默认是/error  @Value("${error.path:/error}")  private String path = "/error";}

public class ErrorMvcAutoConfiguration {  @Bean  @ConditionalOnMissingBean(value = ErrorAttributes.class, search = SearchStrategy.CURRENT)  public DefaultErrorAttributes errorAttributes() {    return new DefaultErrorAttributes();  }
  // 该Controller就是默认的/error错误跳转的类  @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().toList());  }}@Controller@RequestMapping("${server.error.path:${error.path:/error}}")public class BasicErrorController extends AbstractErrorController {  // 当请求的Accept=text/html时调用该方法  @RequestMapping(produces = MediaType.TEXT_HTML_VALUE)  public ModelAndView errorHtml(HttpServletRequest request, HttpServletResponse response) {    // ...  }  // 当请求的Accept=application/json时调用该方法  @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);  }}