服务注册入口

无论是将服务注册到哪种服务注册中心，服务注册的时机都是在AbstractApplicationContext#finishRefresh()方法中；即Spring容器加载完成、Web服务启动之后；

从SpringCloud规范来看，Spring-cloud-common包定义了一套服务注册、发现的规范；服务注册逻辑在ServiceRegistry类中；

package org.springframework.cloud.client.serviceregistry;

/**
 * Contract to register and deregister instances with a Service Registry.
 *
 * @param <R> registration meta data
 * @author Spencer Gibb
 * @since 1.2.0
 */
public interface ServiceRegistry<R extends Registration> {

	/**
	 * Registers the registration. A registration typically has information about an
	 * instance, such as its hostname and port.
	 * @param registration registration meta data
	 */
	void register(R registration);

	/**
	 * Deregisters the registration.
	 * @param registration registration meta data
	 */
	void deregister(R registration);

	/**
	 * Closes the ServiceRegistry. This is a lifecycle method.
	 */
	void close();

	/**
	 * Sets the status of the registration. The status values are determined by the
	 * individual implementations.
	 * @param registration The registration to update.
	 * @param status The status to set.
	 * @see org.springframework.cloud.client.serviceregistry.endpoint.ServiceRegistryEndpoint
	 */
	void setStatus(R registration, String status);

	/**
	 * Gets the status of a particular registration.
	 * @param registration The registration to query.
	 * @param <T> The type of the status.
	 * @return The status of the registration.
	 * @see org.springframework.cloud.client.serviceregistry.endpoint.ServiceRegistryEndpoint
	 */
	<T> T getStatus(R registration);

}

1、Eureka

从SpringBoot自动装配来看，spring-cloud-netflix-eureka-client包中有一个自动装配类EurekaClientAutoConfiguration；

其中使用@Bean注入一个类EurekaAutoServiceRegistration，它实现了SmartLifecycle接口

当Spring容器初始化完成之后会执行所有SmartLifecycle#start()方法，链路如下：

进而走到EurekaAutoServiceRegistration#start()方法中；

其中的serviceRegistry.register()负责服务注册到服务注册中⼼Eureka。Eureka中EurekaServiceRegistry类实现了ServiceRegistry接口，因此Eureka服务注册的入口为EurekaServiceRegistry#register()方法；

@Override
public void register(EurekaRegistration reg) {
	maybeInitializeClient(reg);

	if (log.isInfoEnabled()) {
		log.info("Registering application "
				+ reg.getApplicationInfoManager().getInfo().getAppName()
				+ " with eureka with status "
				+ reg.getInstanceConfig().getInitialStatus());
	}

	reg.getApplicationInfoManager()
			.setInstanceStatus(reg.getInstanceConfig().getInitialStatus());

	reg.getHealthCheckHandler().ifAvailable(healthCheckHandler -> reg
			.getEurekaClient().registerHealthCheck(healthCheckHandler));
}

2、Nacos

从SpringBoot自动装配来看，spring-cloud-alibaba-nacos-discovery 包中有一个自动装配类NacosServiceRegistryAutoConfiguration；NacosServiceRegistryAutoConfiguration中会使用@Bean注解装配NacosAutoServiceRegistration类；

public class NacosServiceRegistryAutoConfiguration {

	@Bean
	public NacosServiceRegistry nacosServiceRegistry(
			NacosDiscoveryProperties nacosDiscoveryProperties) {
		return new NacosServiceRegistry(nacosDiscoveryProperties);
	}

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

    ....
}

NacosAutoServiceRegistration间接实现了ApplicationListener接⼝；

其会监听Web服务初始化完成事件WebServerInitializedEvent；

在Spring生命周期中，上下文刷新完成阶段，即finishRefresh()方法中会执行所有SmartLifecycle#start()方法；

WebServerStartStopLifecycle实现了SmartLifecycle接口，进而走到WebServerStartStopLifecycle#start()方法，其中会发布Servlet服务初始化完成事件ServletWebServerInitializedEvent。

ServletWebServerInitializedEvent是WebServerInitializedEvent的子类。所以监听WebServerInitializedEvent的ApplicationListener可以监听到ServletWebServerInitializedEvent。

NacosAutoServiceRegistration的父类AbstractAutoServiceRegistration中基于ApplicationListener#onApplicationEvent()监听到事件；

然后走到NacosAutoServiceRegistration#register()方法开始做服务注册；

最终会走到ServiceRegistry接口的register(R registration)⽅法将服务注册到注册中⼼。Nacos中NacosServiceRegistry类实现了ServiceRegistry接口。

总结

从Eureka和Nacos做服务注册的入口来看，有两种方式让我们去做服务注册。

1. 基于扩展SmartLifecycle接口实现，在Spring上下文刷新完毕之后会执行SmartLifecycle#start()方法；
2. 基于监听Spring事件WebServerInitializedEvent，在ApplicationListener#onApplicationContext()方法中写逻辑；
   基于Spring事件机制的特性，可以做异步服务注册处理。
   本质上也是利用了SmartLifecycle接口，区别在于使用了SpringBoot web模块的WebServerStartStopLifecycle。

此外，还要遵循spring-cloud-common定义的规范，自定义服务注册的入口类一定要是ServiceRegistry接口的实现类。