nacos 服务注册原理

springboot 的各种 starter 会根据 SPI 机制，读取 META-INFO/spring.factories 文件，自动注册一些 bean，spring-cloud-starter-alibaba-nacos-discovery 的 spring.factories 如下：

org.springframework.cloud.bootstrap.BootstrapConfiguration=\
  com.alibaba.cloud.nacos.discovery.configclient.NacosDiscoveryClientConfigServiceBootstrapConfiguration
org.springframework.boot.SpringApplicationRunListener=\
  com.alibaba.cloud.nacos.logging.NacosLoggingAppRunListener

发现并没有自动配置的自动装配的类？wtf？

从 spring boot2.7开始，慢慢不支持 META-INF/spring.factories 文件了，需要导入的自动配置类可以放在
/META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports，我这是 springcloud2023 版本，对应是 spring3.x 所以要看 META-INF/spring 下的文件

com.alibaba.cloud.nacos.discovery.NacosDiscoveryAutoConfiguration
com.alibaba.cloud.nacos.endpoint.NacosDiscoveryEndpointAutoConfiguration
com.alibaba.cloud.nacos.registry.NacosServiceRegistryAutoConfiguration
com.alibaba.cloud.nacos.discovery.NacosDiscoveryClientConfiguration
com.alibaba.cloud.nacos.discovery.NacosDiscoveryHeartBeatConfiguration
com.alibaba.cloud.nacos.discovery.reactive.NacosReactiveDiscoveryClientConfiguration
com.alibaba.cloud.nacos.discovery.configclient.NacosConfigServerAutoConfiguration
com.alibaba.cloud.nacos.loadbalancer.LoadBalancerNacosAutoConfiguration
com.alibaba.cloud.nacos.NacosServiceAutoConfiguration
com.alibaba.cloud.nacos.util.UtilIPv6AutoConfiguration

NacosServiceRegistryAutoConfiguration 主要看这个，这个类上面有一些注解，装配了 3 个 bean，这是 spring 的知识，可以翻翻前面的文章，这里就不赘述了，只要看 NacosAutoServiceRegistration 这个 bean（名字中带有 auto，意义也是一样的，完成自动注册）

@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties
@ConditionalOnNacosDiscoveryEnabled
@ConditionalOnProperty(value = "spring.cloud.service-registry.auto-registration.enabled",
		matchIfMissing = true)
@AutoConfigureAfter({ AutoServiceRegistrationConfiguration.class,
		AutoServiceRegistrationAutoConfiguration.class,
		NacosDiscoveryAutoConfiguration.class })
public class NacosServiceRegistryAutoConfiguration {

    // 注册服务
	@Bean
	public NacosServiceRegistry nacosServiceRegistry(
			NacosServiceManager nacosServiceManager,
			NacosDiscoveryProperties nacosDiscoveryProperties) {
		return new NacosServiceRegistry(nacosServiceManager, nacosDiscoveryProperties);
	}

	@Bean
	@ConditionalOnBean(AutoServiceRegistrationProperties.class)
	public NacosRegistration nacosRegistration(
			ObjectProvider<List<NacosRegistrationCustomizer>> registrationCustomizers,
			NacosDiscoveryProperties nacosDiscoveryProperties,
			ApplicationContext context) {
		return new NacosRegistration(registrationCustomizers.getIfAvailable(),
				nacosDiscoveryProperties, context);
	}

    // 触发注册
	@Bean
	@ConditionalOnBean(AutoServiceRegistrationProperties.class)
	public NacosAutoServiceRegistration nacosAutoServiceRegistration(
			NacosServiceRegistry registry,
			AutoServiceRegistrationProperties autoServiceRegistrationProperties,
			NacosRegistration registration) {
		return new NacosAutoServiceRegistration(registry,
				autoServiceRegistrationProperties, registration);
	}

}

这里只能知道注册了 3 个 bean，bean 的功能是啥现在也不知道，点进去看一下会发现：NacosAutoServiceRegistration 实现了一个接口 ApplicationListener<WebServerInitializedEvent>。事件监听也回去看看之前的文章，作用是 spring 容器启动会回调 AbstractAutoServiceRegistration#onApplicationEvent() 这个方法，这个方法源码调用链如下

看下注册到 nacos 的服务信息