core：Java功能增強(qiáng) —— 事件機(jī)制(事件與監(jiān)聽(tīng)器)

Eureka提供了服務(wù)注冊(cè)與發(fā)現(xiàn)的功能，需要提供一個(gè)服務(wù)注冊(cè)中心(我這里是在spring boot項(xiàng)目啟動(dòng)類上使用@EnableEurekaServer泼疑，再配置相關(guān)屬性)切威，然后在我們的應(yīng)用服務(wù)(我這里是spring boot服務(wù))啟動(dòng)類上使用@EnableDiscoveryClient啟用服務(wù)注冊(cè)發(fā)現(xiàn)功能闯狱。那么問(wèn)題來(lái)了洽沟，我們的服務(wù)是怎么注冊(cè)到注冊(cè)中心的呢攘宙？

使用@EnableDiscoveryClient注解后啟動(dòng)服務(wù)丈莺，我們可以發(fā)現(xiàn)控制臺(tái)打出了如下log：

image.png

從圖中可以看出的信息有：1划煮、是在EurekaDiscoveryClientConfiguration中將服務(wù)器注冊(cè)到eureka的。2缔俄、DiscoveryClient類里面應(yīng)該有許多服務(wù)器與eureka之間的通信操作弛秋，如心跳續(xù)約等。3俐载、InstanceInfoReplicator應(yīng)該是一個(gè)線程類蟹略。

既然EurekaDiscoveryClientConfiguration是最開(kāi)始的地方，那就從它看起遏佣。

1挖炬、EurekaDiscoveryClientConfiguration

源碼如下：

@Configuration     //使用該注解來(lái)讓Spring容器發(fā)現(xiàn)并注冊(cè)里面的Bean
@EnableConfigurationProperties
@ConditionalOnClass({EurekaClientConfig.class})
@ConditionalOnProperty(
value = {"eureka.client.enabled"},
matchIfMissing = true
)
public class EurekaDiscoveryClientConfiguration implements SmartLifecycle, Ordered {

public void start() {
    if (this.port.get() != 0 && this.instanceConfig.getNonSecurePort() == 0) {
        this.instanceConfig.setNonSecurePort(this.port.get());
    }

    if (!this.running.get() && this.instanceConfig.getNonSecurePort() > 0) {
        this.maybeInitializeClient();
        if (log.isInfoEnabled()) {
            log.info("Registering application " + this.instanceConfig.getAppname() + " with eureka with status " + this.instanceConfig.getInitialStatus());
        }

        this.applicationInfoManager.setInstanceStatus(this.instanceConfig.getInitialStatus());
        if (this.healthCheckHandler != null) {
            this.eurekaClient.registerHealthCheck(this.healthCheckHandler);
        }
        //發(fā)布了一個(gè)注冊(cè)事件，在哪里監(jiān)聽(tīng)的状婶？還是說(shuō)是留給我們開(kāi)發(fā)者用的意敛？
        this.context.publishEvent(new InstanceRegisteredEvent(this, this.instanceConfig));
        this.running.set(true);
    }

}

private void maybeInitializeClient() {
    this.applicationInfoManager.getInfo();
    this.eurekaClient.getApplications();
}

public void stop() {
    if (this.applicationInfoManager.getInfo() != null) {
        if (log.isInfoEnabled()) {
            log.info("Unregistering application " + this.instanceConfig.getAppname() + " with eureka with status DOWN");
        }

        this.applicationInfoManager.setInstanceStatus(InstanceStatus.DOWN);
    }

    this.running.set(false);
}


//監(jiān)聽(tīng)，內(nèi)置容器啟動(dòng)時(shí)開(kāi)始注冊(cè)    
@EventListener({EmbeddedServletContainerInitializedEvent.class})
public void onApplicationEvent(EmbeddedServletContainerInitializedEvent event) {
    int localPort = event.getEmbeddedServletContainer().getPort();
    if (this.port.get() == 0) {
        log.info("Updating port to " + localPort);
        this.port.compareAndSet(0, localPort);
        this.start();
    }

}

//監(jiān)聽(tīng)膛虫，應(yīng)用關(guān)閉時(shí)關(guān)閉eureka客戶端
@EventListener({ContextClosedEvent.class})
public void onApplicationEvent(ContextClosedEvent event) {
    this.stop();
    this.eurekaClient.shutdown();
}
}

按猜想的來(lái)說(shuō)應(yīng)該有一個(gè)Rest請(qǐng)求什么的將服務(wù)器信息發(fā)送到eureka服務(wù)器才對(duì)空闲，好像上面的start()方法沒(méi)有做什么事情。

我們關(guān)閉了eureka注冊(cè)中心走敌，重新啟動(dòng)服務(wù)碴倾，發(fā)現(xiàn)報(bào)了如下錯(cuò)：

image.png

嗯，意料之中，肯定連不上啊跌榔，從這里可以看出异雁，發(fā)請(qǐng)求是RedirectingEurekaHttpClient請(qǐng)求客戶端做的，接著往下：

image.png

DiscoveryClient使用@Singleton修飾担平，里面有這么兩個(gè)函數(shù)示绊，可以看出是注冊(cè)與續(xù)約：

boolean register() throws Throwable {
    logger.info("DiscoveryClient_" + this.appPathIdentifier + ": registering service...");

    EurekaHttpResponse httpResponse;
    try {
        httpResponse = this.eurekaTransport.registrationClient.register(this.instanceInfo);
    } catch (Exception var3) {
        logger.warn("{} - registration failed {}", new Object[]{"DiscoveryClient_" + this.appPathIdentifier, var3.getMessage(), var3});
        throw var3;
    }

    if (logger.isInfoEnabled()) {
        logger.info("{} - registration status: {}", "DiscoveryClient_" + this.appPathIdentifier, httpResponse.getStatusCode());
    }

    return httpResponse.getStatusCode() == 204;
}

boolean renew() {
    try {
        EurekaHttpResponse<InstanceInfo> httpResponse = this.eurekaTransport.registrationClient.sendHeartBeat(this.instanceInfo.getAppName(), this.instanceInfo.getId(), this.instanceInfo, (InstanceStatus)null);
        logger.debug("{} - Heartbeat status: {}", "DiscoveryClient_" + this.appPathIdentifier, httpResponse.getStatusCode());
        if (httpResponse.getStatusCode() == 404) {
            this.REREGISTER_COUNTER.increment();
            logger.info("{} - Re-registering apps/{}", "DiscoveryClient_" + this.appPathIdentifier, this.instanceInfo.getAppName());
            return this.register();
        } else {
            return httpResponse.getStatusCode() == 200;
        }
    } catch (Throwable var3) {
        logger.error("{} - was unable to send heartbeat!", "DiscoveryClient_" + this.appPathIdentifier, var3);
        return false;
    }
}

這個(gè)register()就是InstanceInfoReplicator的run方法里調(diào)用的，而InstanceInfoReplicator是在DiscoveryClient的構(gòu)造方法中實(shí)例化的暂论。

在eureka服務(wù)器端則是將服務(wù)信息存放在一個(gè)雙層Map里面褐，第一層的Key是服務(wù)名，第二層的Key是實(shí)例名：

    private final ConcurrentHashMap<String, Map<String, Lease<InstanceInfo>>> registry = new ConcurrentHashMap();