我認爲SpringCloud中AbstractAutoServiceRegistration的「bug「

前話：zipkin-server與nacos結合時的坑

參考github解決方案

在解決Nacos社區認領的issue——對接zipkin時，遇到了一個奇怪的問題，當採用github的解決方案，即採用eureka-client時，zipkin-server可以自動註冊到Eureka Server中，可是當採用nacos-discovery時，卻怎麼也沒法實現zipkin-server服務自動註冊到nacos-server中，經過斷點調試以及參考Spring Cloud相關源碼以及文檔，終於發現了問題所在

Spring-Cloud認爲服務自動註冊的時機

SpringCloud自己認爲服務的註冊時機，應該是WebServerInitializedEvent事件發生後，進行服務的自動註冊，由於在接收到此事件時，會下發bind(Event)操做，由start()函數內部調用register()實現服務的自動註冊

@Override
@SuppressWarnings("deprecation")
public void onApplicationEvent(WebServerInitializedEvent event) {
	bind(event);
}

@Deprecated
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();
}

public void start() {
	if (!isEnabled()) {
		if (logger.isDebugEnabled()) {
			logger.debug("Discovery Lifecycle disabled. Not starting");
		}
		return;
	}

	// only initialize if nonSecurePort is greater than 0 and it isn't already running
	// because of containerPortInitializer below
	if (!this.running.get()) {
		this.context.publishEvent(
				new InstancePreRegisteredEvent(this, getRegistration()));
		register();
		if (shouldRegisterManagement()) {
			registerManagement();
		}
		this.context.publishEvent(
					new InstanceRegisteredEvent<>(this, getConfiguration()));
			this.running.compareAndSet(false, true);
	}
}
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存在的問題

由上面的代碼可知，SpringCloud自己對於服務註冊的時機，是在發生WebServerInitializedEvent事件以後，纔會去調用相應的register()方法實現服務註冊，可是這裏就可能存在一個小小的問題了。

在建立SpringBoot Web Application時，有一段代碼比較關鍵

protected ConfigurableApplicationContext createApplicationContext() {
	Class<?> contextClass = this.applicationContextClass;
	if (contextClass == null) {
		try {
			switch (this.webApplicationType) {
			case SERVLET:
				contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
				break;
			case REACTIVE:
				contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
				break;
			default:
				contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
			}
		}
		catch (ClassNotFoundException ex) {
			throw new IllegalStateException(
					"Unable create a default ApplicationContext, "
							+ "please specify an ApplicationContextClass",
					ex);
		}
	}
	return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}
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所以，這裏的contextClass對應着三種不一樣的類型，分別是AnnotationConfigServletWebServerApplicationContext、AnnotationConfigReactiveWebServerApplicationContext以及AnnotationConfigApplicationContext，而這三個中，只有前面兩個Context會觸發WebServerInitializedEvent事件的下發

AnnotationConfigServletWebServerApplicationContext

@Override
protected void finishRefresh() {
	super.finishRefresh();
	WebServer webServer = startWebServer();
	if (webServer != null) {
		publishEvent(new ServletWebServerInitializedEvent(webServer, this));
	}
}

public class ServletWebServerInitializedEvent extends WebServerInitializedEvent {
    ...
}
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AnnotationConfigReactiveWebServerApplicationContext

@Override
protected void finishRefresh() {
	super.finishRefresh();
	WebServer webServer = startReactiveWebServer();
	if (webServer != null) {
		publishEvent(new ReactiveWebServerInitializedEvent(webServer, this));
	}
}

public class ReactiveWebServerInitializedEvent extends WebServerInitializedEvent {
    ...
}
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AnnotationConfigApplicationContext

protected void finishRefresh() {
	// Clear context-level resource caches (such as ASM metadata from scanning).
	clearResourceCaches();

	// Initialize lifecycle processor for this context.
	initLifecycleProcessor();

	// Propagate refresh to lifecycle processor first.
	getLifecycleProcessor().onRefresh();

	// Publish the final event.
	publishEvent(new ContextRefreshedEvent(this));

	// Participate in LiveBeansView MBean, if active.
	LiveBeansView.registerApplicationContext(this);
}

public class ContextRefreshedEvent extends ApplicationContextEvent {
    ...
}
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所以，若是設置了spring.main.web-application-type=none時，createApplicationContext代碼中的switch分支就會執行contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);語句，此時獲取到的Context爲AnnotationConfigApplicationContext，而AnnotationConfigApplicationContext調用的finishRefresh方法所下發的事件ContextRefreshedEvent是沒有繼承WebServerInitializedEvent的，在這種狀況下，SpringCloud就沒法執行服務自動註冊

進行調整

調整的方法很簡單，因爲SpringBoot採用事件機制，所以當整個SpringBoot Application程序初始化完畢，執行listeners.running(context)時，會觸發ApplicationReadyEvent事件

SpringApplication

public ConfigurableApplicationContext run(String... args) {
	StopWatch stopWatch = new StopWatch();
	stopWatch.start();
	...

	try {
		listeners.running(context);
	}
	catch (Throwable ex) {
		handleRunFailure(context, ex, exceptionReporters, null);
		throw new IllegalStateException(ex);
	}
	return context;
}
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EventPublishingRunListener

@Override
public void running(ConfigurableApplicationContext context) {
	context.publishEvent(
				new ApplicationReadyEvent(this.application, this.args, context));
}
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所以，只須要改動以下代碼便可

org.springframework.cloud.client.serviceregistry.AbstractAutoServiceRegistration

public abstract class AbstractAutoServiceRegistration<R extends Registration> implements AutoServiceRegistration, ApplicationContextAware, ApplicationListener<ApplicationReadyEvent> {
    ...
}
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更改事件監聽類型爲ApplicationReadyEvent便可

原文