1 、cloud nacos discovery 实现原理：

1、1注入自动配置类 ：
@EnableDiscoveryClient 扫描 nacos包中的spring.factories，加载NacosDiscoveryClientAutoConfiguration。

不贴代码了，springboot enable的标签都这么玩。

2、2 初始化配置和NacosAutoServiceRegistration对象。

@Configuration
@EnableConfigurationProperties
@ConditionalOnNacosDiscoveryEnabled
@ConditionalOnClass(name = "org.springframework.boot.web.servlet.context.ServletWebServerInitializedEvent")
@ConditionalOnProperty(value = "spring.cloud.service-registry.auto-registration.enabled", matchIfMissing = true)
@AutoConfigureBefore({ AutoServiceRegistrationAutoConfiguration.class,
        NacosDiscoveryClientAutoConfiguration.class })
public class NacosDiscoveryAutoConfiguration {
    //初始化核心类。
    @Bean
    @ConditionalOnBean(AutoServiceRegistrationProperties.class)
    public NacosAutoServiceRegistration nacosAutoServiceRegistration(
            NacosServiceRegistry registry,
            AutoServiceRegistrationProperties autoServiceRegistrationProperties,
            NacosRegistration registration) {
        return new NacosAutoServiceRegistration(registry,
                autoServiceRegistrationProperties, registration);
    }

NacosAutoServiceRegistration 继承AbstractAutoServiceRegistration：该类为spring cloud comoon中的基类，不管enruka 还是其他注册中心，都会继承该抽象类，AbstractAutoServiceRegistration有个bind 方法，监听了WebServerInitializedEvent事件，当webserver初始化完成后，实现注册中心注册的逻辑。

    @EventListener(WebServerInitializedEvent.class)
    public void bind(WebServerInitializedEvent event) {
        ApplicationContext context = event.getApplicationContext();
        if (context instanceof ConfigurableWebServerApplicationContext) {
            if ("management".equals(
                    ((ConfigurableWebServerApplicationContext) context).getServerNamespace())) {
                return;
            }
        }
        this.port.compareAndSet(0, event.getWebServer().getPort());
        this.start();
    }

bug 原因

现象：bind方法断点无法进入。事件没有监听到
分析：@EventListener 标签没有生效。
追踪：@EventListener 解析逻辑。通过注释@EventListener 标签通过EventListenerMethodProcessor来解析初始化的。
通过debug 发现spring 5.1 后新增了条件判断。导致NacosAutoServiceRegistration 无法作为事件注入到容器中。

    /**
     * Determine whether the given class is an {@code org.springframework}
     * bean class that is not annotated as a user or test {@link Component}...
     * which indicates that there is no {@link EventListener} to be found there.
     * @since 5.1
     */
    private static boolean isSpringContainerClass(Class<?> clazz) {
        return (clazz.getName().startsWith("org.springframework.") &&
                !AnnotatedElementUtils.isAnnotated(ClassUtils.getUserClass(clazz), Component.class));
    }