環境：Springboot2.4.11

環境配置

接下來的演示都是基于如下接口進行。

@RestController
@RequestMApping("/exceptions")
public class ExceptionsController {
    
  @GetMapping("/index")
  public Object index(int a) {
    if (a == 0) {
      throw new BusinessException() ;
    }
    return "exception" ;
  }
    
}

默認錯誤輸出

默認情況下，當請求一個接口發生異常時會有如下兩種情況的錯誤信息提示

• 基于html

• 基于JSON

上面兩個示例通過請求的Accept請求頭設置希望接受的數據類型，得到不同的響應數據類型。

標準web錯誤頁配置

在標準的JAVA web項目中我們一般是在web.xml文件中進行錯誤頁的配置，如下：

<error-page>
  <location>/error</location>
</error-page>

如上配置后，如發生了異常以后容器會自動地跳轉到錯誤頁面。

Spring實現原理

在Springboot中沒有web.xml，并且Servlet API也沒有提供相應的API進行錯誤頁的配置。那么在Springboot中又是如何實現錯誤頁的配置呢？

Springboot內置了應用服務，如Tomcat，Undertow，Jetty，默認是Tomcat。那接下來看下基于默認的Tomcat容器錯誤頁是如何進行配置的。

• Servlet Web服務自動配置

@EnableConfigurationProperties(ServerProperties.class)
@Import({ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class, 
         ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,...})
public class ServletWebServerFactoryAutoConfiguration {
  @Configuration(proxyBeanMethods = false)
  @ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
  @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
  static class EmbeddedTomcat {

    // 這里主要就是配置Web 容器服務，如這里使用的Tomcat
    // 注意該類實現了ErrorPageRegistry ，那么也就是說該類可以用來注冊錯誤頁的
    @Bean
    TomcatServletWebServerFactory tomcatServletWebServerFactory(
      ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,
      ObjectProvider<TomcatContextCustomizer> contextCustomizers,
      ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {
      TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();
      factory.getTomcatConnectorCustomizers().addAll(connectorCustomizers.orderedStream().collect(Collectors.toList()));
      factory.getTomcatContextCustomizers().addAll(contextCustomizers.orderedStream().collect(Collectors.toList()));
      factory.getTomcatProtocolHandlerCustomizers().addAll(protocolHandlerCustomizers.orderedStream().collect(Collectors.toList()));
      return factory;
    }

  }
}

在@Import中只列出了兩個比較重要的
BeanPostProcessorsRegistrar與EmbeddedTomcat

BeanPostProcessorsRegistrar注冊了兩個BeanPostProcessor處理器

public static class BeanPostProcessorsRegistrar implements ImportBeanDefinitionRegistrar, BeanFactoryAware {
  public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
    if (this.beanFactory == null) {
      return;
    }
    registerSyntheticBeanIfMissing(registry, "webServerFactoryCustomizerBeanPostProcessor", WebServerFactoryCustomizerBeanPostProcessor.class, WebServerFactoryCustomizerBeanPostProcessor::new);
    registerSyntheticBeanIfMissing(registry, "errorPageRegistrarBeanPostProcessor", ErrorPageRegistrarBeanPostProcessor.class, ErrorPageRegistrarBeanPostProcessor::new);
  }
}

通過名稱也能知道
WebServerFactoryCustomizerBeanPostProcessor用來處理Tomcat相關的自定義信息；
ErrorPageRegistrarBeanPostProcessor 這個就是重點了，這個就是用來配置我們的自定義錯誤頁面的。

public class ErrorPageRegistrarBeanPostProcessor implements BeanPostProcessor, BeanFactoryAware {
  @Override
  public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
    // 這里判斷了當前的bean對象是否是ErrorPageRegistry的實例
    // 當前類既然是BeanPostProcessor實例，同時上面注冊了一個TomcatServletWebServerFactory Bean實例
    // 那么在實例化TomcatServletWebServerFactory時一定是會調用該BeanPostProcessor處理器的
    if (bean instanceof ErrorPageRegistry) {
      postProcessBeforeInitialization((ErrorPageRegistry) bean);
    }
    return bean;
  }
  // 注冊錯誤頁面
  private void postProcessBeforeInitialization(ErrorPageRegistry registry) {
    for (ErrorPageRegistrar registrar : getRegistrars()) {
      registrar.registerErrorPages(registry);
    }
  }
  private Collection<ErrorPageRegistrar> getRegistrars() {
    if (this.registrars == null) {
      // Look up does not include the parent context
      // 從當前上下文中（比包括父上下文）查找ErrorPageRegistrar Bean對象
      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;
  }
}

注冊錯誤頁面

在上一步中知道了錯誤頁的注冊入口是在一個
ErrorPageRegistrarBeanPostProcessor Bean后處理器中進行注冊的，接下來繼續深入查看這個錯誤頁是如何被注冊的。

接著上一步在
ErrorPageRegistrarBeanPostProcessor中查找ErrorPageRegistrar類型的Bean對象。在另外一個自動配置中（ErrorMvcAutoConfiguration）有注冊ErrorPageRegistrar Bean對象

@AutoConfigureBefore(WebMvcAutoConfiguration.class)
@EnableConfigurationProperties({ ServerProperties.class, WebMvcProperties.class })
public class ErrorMvcAutoConfiguration {
  
  // 該類是ErrorPageRegistrar子類，那么在注冊錯誤頁的時候注冊的就是該類中生成的錯誤頁信息
  static class ErrorPageCustomizer implements ErrorPageRegistrar, Ordered {
    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) {
      // 錯誤頁的地址可以在配置文件中自定義server.error.path進行配置，默認：/error
      ErrorPage errorPage = new ErrorPage(this.dispatcherServletPath.getRelativePath(this.properties.getError().getPath()));
      errorPageRegistry.addErrorPages(errorPage);
    }
    @Override
    public int getOrder() {
      return 0;
    }
  }

}

關鍵代碼

//  errorPageRegistry對象的實例是TomcatServletWebServerFactory 
errorPageRegistry.addErrorPages(errorPage);

TomcatServletWebServerFactory中注冊錯誤頁信息，該類的父類（AbstractConfigurableWebServerFactory）方法中有添加錯誤也的方法

public abstract class AbstractConfigurableWebServerFactory {
  private Set<ErrorPage> errorPages = new LinkedHashSet<>();
  public void addErrorPages(ErrorPage... errorPages) {
    this.errorPages.addAll(Arrays.asList(errorPages));
  }
}

這個錯誤頁的注冊到Tomcat容器中又是如何實現的呢？

Tomcat中注冊錯誤頁

接下來看看這個錯誤頁是如何與Tomcat關聯在一起的。

Spring容器最核心的方法是refresh方法

public abstract class AbstractApplicationContext {
  public void refresh() {
    // ...
    // Initialize other special beans in specific context subclasses.
    onRefresh();
    // ...
  }
}

執行onRefresh方法

public class ServletWebServerApplicationContext extends GenericWebApplicationContext {
  protected void onRefresh() {
    super.onRefresh();
    try {
      // 創建Tomcat服務
      createWebServer();
    } catch (Throwable ex) {
      throw new ApplicationContextException("Unable to start web server", ex);
    }
  }
  private void createWebServer() {
    // ...
    // 返回應用于創建嵌入的Web服務器的ServletWebServerFactory。默認情況下，此方法在上下文本身中搜索合適的bean。
    // 在上面ServletWebServerFactoryAutoConfiguration自動配置中，已經自動的根據當前的環境創建了TomcatServletWebServerFactory對象
    ServletWebServerFactory factory = getWebServerFactory();
    // 獲取WebServer實例， factory = TomcatServletWebServerFactory
    this.webServer = factory.getWebServer(getSelfInitializer());
    // ...
  }
}

調用
TomcatServletWebServerFactory#getWebServer方法

public class TomcatServletWebServerFactory extends AbstractServletWebServerFactory {
  public WebServer getWebServer(ServletContextInitializer... initializers) {
    // ...
    Tomcat tomcat = new Tomcat();
    // ...
    // 預處理上下文
    prepareContext(tomcat.getHost(), initializers);
    return getTomcatWebServer(tomcat);
  }
  protected void prepareContext(Host host, ServletContextInitializer[] initializers) {
    // ...
    // 配置上下文
    configureContext(context, initializersToUse);
	}
  protected void configureContext(Context context, ServletContextInitializer[] initializers) {
    TomcatStarter starter = new TomcatStarter(initializers);
    // ...
    // 在這里就將錯誤的頁面注冊到了tomcat容器中
    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);
    }
    // ...
  }
}

到此你就知道了一個錯誤的頁是如何在Springboot中被注冊的。到目前為止我們看到的注冊到tomcat容器中的錯誤頁都是個地址，比如：默認是/error。那這個默認的/error又是怎么提供的接口呢？

默認錯誤頁

在Springboot中默認有個自動配置的錯誤頁，在上面有一個代碼片段你應該注意到了

@AutoConfigureBefore(WebMvcAutoConfiguration.class)
@EnableConfigurationProperties({ ServerProperties.class, WebMvcProperties.class })
public class ErrorMvcAutoConfiguration {
  @Bean
  @ConditionalOnMissingBean(value = ErrorAttributes.class, search = SearchStrategy.CURRENT)
  public DefaultErrorAttributes errorAttributes() {
    return new DefaultErrorAttributes();
  }
  @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().collect(Collectors.toList()));
  }
}

查看這個Controller

// 默認的錯誤頁地址是/error
@Controller
@RequestMapping("${server.error.path:${error.path:/error}}")
public class BasicErrorController extends AbstractErrorController {
  
  @RequestMapping(produces = MediaType.TEXT_HTML_VALUE)
  public ModelAndView errorHtml(HttpServletRequest request, HttpServletResponse response) {
    HttpStatus status = getStatus(request);
    Map<String, Object> model = Collections.unmodifiableMap(getErrorAttributes(request, getErrorAttributeOptions(request, MediaType.TEXT_HTML)));
    response.setStatus(status.value());
    ModelAndView modelAndView = resolveErrorView(request, response, status, model);
    return (modelAndView != null) ? modelAndView : new ModelAndView("error", model);
  }

  @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);
  }

}

這里有兩個方法，分別處理了不同的Accept請求頭。到此你是否真正地明白了Springboot中的錯誤處理的工作原理呢？

完畢！！！