SpringBoot错误页面的原理，你知道吗？

@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Bean
public 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);
  }
}