概述
Spring Cloud中默認的服務發現採用的netflix的eureka，本篇文章就是閱讀Spring cloud中通過eureka做服務發現時的筆記。
順藤摸瓜
讀取Spring各種框架的時候，很多時候不知道從什麼地方開始，因為Spring中很多模塊的開啟就是通過一行註解，例如@EnableXXX。而Spring Cloud中的配置服務則是通過@EnableDiscoveryClient，(其實@EnableEurekaClient就是@EnableDiscoveryClient)
/*** Annotation to enable a DiscoveryClient implementation.* @author Spencer Gibb*/@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@Import(EnableDiscoveryClientImportSelector.class)public @interface EnableDiscoveryClient {/*** If true, the ServiceRegistry will automatically register the local server.*/boolean autoRegister() default true;}上面有一個@Import(EnableDiscoveryClientImportSelector.class),然後看一下EnableDiscoveryClientImportSelector這個類
public class EnableDiscoveryClientImportSelectorextends SpringFactoryImportSelector{@Overridepublic String[] selectImports(AnnotationMetadata metadata) {...boolean autoRegister = attributes.getBoolean("autoRegister");if (autoRegister) {List importsList = new ArrayList<>(Arrays.asList(imports));importsList.add("org.springframework.cloud.client.serviceregistry.AutoServiceRegistrationConfiguration");imports = importsList.toArray(new String[0]);}return imports;}}
一看代碼第一反應就是引入了org.springframework.cloud.client.serviceregistry.AutoServiceRegistrationConfiguration這個類，再看一下這個類
@Configuration@EnableConfigurationProperties(AutoServiceRegistrationProperties.class)public class AutoServiceRegistrationConfiguration {}什麼都沒有，怎麼回事。其實機關就是在SpringFactoryImportSelector
org.springframework.cloud.client.discovery.EnableDiscoveryClient=\org.springframework.cloud.netflix.eureka.EurekaDiscoveryClientConfiguration
這樣，Spring Cloud啟動的時候初始化Eureka就是在EurekaDiscoveryClientConfiguration這個類中。仔細一看，也不對啊，這個類只註冊了一個Marker類，而且這個類是個空類。再全局搜索一下哪裡引用了這個Marker類，終於發現真正初始化Eureka的類：EurekaClientAutoConfiguration
@Configuration@EnableConfigurationProperties@ConditionalOnClass(EurekaClientConfig.class)@ConditionalOnBean(EurekaDiscoveryClientConfiguration.Marker.class)@ConditionalOnProperty(value = "eureka.client.enabled", matchIfMissing = true)@AutoConfigureBefore({ NoopDiscoveryClientAutoConfiguration.class,CommonsClientAutoConfiguration.class, ServiceRegistryAutoConfiguration.class })@AutoConfigureAfter(name = "org.springframework.cloud.autoconfigure.RefreshAutoConfiguration")public class EurekaClientAutoConfiguration追根溯源
既然找到了EurekaClientAutoConfiguration這個配置類，那麼肯定有初始化和NetFlix相關類，仔細一看，就是
@Bean(destroyMethod = "shutdown")@ConditionalOnMissingBean(value = EurekaClient.class, search = SearchStrategy.CURRENT)@org.springframework.cloud.context.config.annotation.RefreshScope@Lazypublic EurekaClient eurekaClient(ApplicationInfoManager manager, EurekaClientConfig config, EurekaInstanceConfig instance) {manager.getInfo(); // force initializationreturn new CloudEurekaClient(manager, config, this.optionalArgs,this.context);}
終於發現和NetFlix的鏈接的地方，CloudEurekaClient繼承的DiscoveryClient其實就是NetFlix的服務發現類，這樣，就可以好好分析是怎麼初始化的。
直擊要害
CloudEurekaClient在初始化的時候主要是調用父類DiscoveryClient的構造函數，會做很多事，其中註冊服務和發現服務是通過調度任務來完成，調度任務的初始化是在initScheduledTasks這個而方法中，其中服務發現的代碼
ate void initScheduledTasks() { if (clientConfig.shouldFetchRegistry()) { // registry cache refresh timer int registryFetchIntervalSeconds = clientConfig.getRegistryFetchIntervalSeconds(); int expBackOffBound = clientConfig.getCacheRefreshExecutorExponentialBackOffBound(); scheduler.schedule( new TimedSupervisorTask( "cacheRefresh", scheduler, cacheRefreshExecutor, registryFetchIntervalSeconds, TimeUnit.SECONDS, expBackOffBound, new CacheRefreshThread() ), registryFetchIntervalSeconds, TimeUnit.SECONDS); }首先判斷是否需要進行服務發現，然後通過一個定時任務去刷新緩存信息。TimedSupervisorTask是支持timeout的調度任務，刷新緩存邏輯實際是在CacheRefreshThread()中，往後看就會看到通過Http請求和Erureka服務器交互。服務註冊的代碼緊接著在服務發現後面
if (clientConfig.shouldRegisterWithEureka()) { ... // Heartbeat timer scheduler.schedule( new TimedSupervisorTask( "heartbeat", scheduler, heartbeatExecutor, renewalIntervalInSecs, TimeUnit.SECONDS, expBackOffBound, new HeartbeatThread() ), renewalIntervalInSecs, TimeUnit.SECONDS); // InstanceInfo replicator instanceInfoReplicator = new InstanceInfoReplicator( this, instanceInfo, clientConfig.getInstanceInfoReplicationIntervalSeconds(), 2); // burstSize ... if (clientConfig.shouldOnDemandUpdateStatusChange()) { applicationInfoManager.registerStatusChangeListener(statusChangeListener); } instanceInfoReplicator.start(clientConfig.getInitialInstanceInfoReplicationIntervalSeconds()); }instanceInfoReplicator對當前服務信息（ instanceInfo）進行註冊。
public void run() { ... discoveryClient.refreshInstanceInfo(); Long dirtyTimestamp = instanceInfo.isDirtyWithTime(); if (dirtyTimestamp != null) { discoveryClient.register(); instanceInfo.unsetIsDirty(dirtyTimestamp); } ...}另外一個心跳檢測的定時任務則是對服務進行續約
boolean renew() { EurekaHttpResponse httpResponse; try { httpResponse = eurekaTransport.registrationClient.sendHeartBeat(instanceInfo.getAppName(), instanceInfo.getId(), instanceInfo, null); if (httpResponse.getStatusCode() == 404) { ... return register(); } return httpResponse.getStatusCode() == 200; } catch (Throwable e) { logger.error("{} - was unable to send heartbeat!", PREFIX + appPathIdentifier, e); return false; } } 可以看出如果續約失敗則重新發起一次註冊。那麼服務註冊的具體實現就在register()中
boolean register() throws Throwable { EurekaHttpResponse httpResponse; try { httpResponse = eurekaTransport.registrationClient.register(instanceInfo); } catch (Exception e) { logger.warn("{} - registration failed {}", PREFIX + appPathIdentifier, e.getMessage(), e); throw e; } if (logger.isInfoEnabled()) { logger.info("{} - registration status: {}", PREFIX + appPathIdentifier, httpResponse.getStatusCode()); } return httpResponse.getStatusCode() == 204;} 實現十分簡單，就是發起一次http調用，把當前instanceInfo傳遞過去。，那麼InstanceInfo都有什麼呢，其實就是一些appName ,url,port等等，通過這些信息我們就能發起一次基於http的rpc調用。
小結
可以看出，Spring cloud中的服務發現和註冊其實就是和通過http方式和eureka進行通信，實現手段則是通過定時任務進行定時操作，包括定時查詢，服務續約。
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