SpringBoot错误页面的原理，你知道吗？

环境：Springboot3.0.5

有如下接口：

@RestController
@RequestMapping("/demo")
public class DemoController {


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

当访问上面接口后，默认情况下Springboot会返回如下错误信息：

当请求的Accept是text/html返回的是HTML结果，当Accpet是application/json返回如下：

后台接口会根据不同的Accept返回不同的数据格式。

Springboot在启动过程中会执行如下处理：

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

ServletWebServerApplicationContext

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());
 }
}

TomcatServletWebServerFactory

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类实现了ErrorPageRegistry接口，有如下方法：

// 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));
 }
}

下面查看上面的addErrorPages方法是如何被调用的。

在自动配置类中导入了下面的类

@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;
 }
}

接下来就是从容器中获取ErrorPageRegistrar，然后注册到ErrorPageRegistry中。

在如下自动配置类中定义了一个默认的错误页对象。

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";
}

通过上面的分析，默认情况下springboot容器启动后会像tomcat容器中注册一个/error的错误页，这个/error又是谁呢？

在默认的错误页自动配置中：

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);
 }
}

以上就是当springboot默认情况下发生错误时的执行输出原理。