【SpringCloud Eureka源码】从Eureka Client发起注册请求到Eureka Server处理的整个服务注册过程（上）

本文使用Spring Cloud Eureka分析

Spring Cloud版本： Dalston.SR5

spring-cloud-starter-eureka版本： 1.3.6.RELEASE

netflix eureka版本： 1.6.2

Eureka Client启动并调用Eureka Server的注册接口

Eureka Client启动并调用Eureka Server的注册接口

Spring Cloud Eureka的自动配置

@EnableDiscoveryClient

首先从使用Eureka Client必须引入的@EnableDiscoveryClient注解说起

@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;
}

@EnableDiscoveryClient注解的作用：

autoRegister默认值为true，即服务发现客户端默认会自动注册到服务端
Import导入EnableDiscoveryClientImportSelector.class，其作用是
- 导入了 spring-cloud-eureka-client.jar!META-INFspring.factories 中的 EurekaDiscoveryClientConfiguration
```
org.springframework.cloud.client.discovery.EnableDiscoveryClient=
org.springframework.cloud.netflix.eureka.EurekaDiscoveryClientConfiguration
```
- 由于autoRegister默认为true，故还会导入AutoServiceRegistrationConfiguration，即启用自动服务注册的配置，等同于在配置文件中spring.cloud.service-registry.auto-registration.enabled = true

EurekaDiscoveryClientConfiguration

向Spring容器注册EurekaDiscoveryClientConfiguration.Marker.class，使得真正导入DiscoveryClient的EurekaClientAutoConfiguration配置类满足启用条件
创建监听RefreshScopeRefreshedEvent事件的监听器，满足在使用RefreshScope刷新时可以重建EurekaClient（不是本文重点）
在配置eureka.client.healthcheck.enabled=true的前提下，向Spring容器注册EurekaHealthCheckHandler用于健康检查（不是本文重点）

所以，EurekaDiscoveryClientConfiguration的主要作用是向Spring容器注册EurekaDiscoveryClientConfiguration.Marker.class，使得EurekaClientAutoConfiguration配置类满足启用条件

EurekaClientAutoConfiguration

EurekaClientAutoConfiguration配置类中涉及的内容比较多，主要内容：

1、注册了spring cloud包下的EurekaClientConfigBean，这是个对netflix的EurekaClientConfig客户端配置接口的实现
2、注册了spring cloud包下的EurekaInstanceConfigBean，这是个对netflix的EurekaInstanceConfig实例信息配置接口的实现
3、注册了一些AutoServiceRegistration，即客户端自动注册的组件，如
- EurekaRegistration： Eureka实例的服务注册信息（在开启客户端自动注册时才会注册）
- EurekaServiceRegistry： Eureka服务注册器
- EurekaAutoServiceRegistration： Eureka服务自动注册器，实现了SmartLifecycle，会在Spring容器的refresh的最后阶段被调用，通过EurekaServiceRegistry注册器注册EurekaRegistration信息
4、注册netflix的EurekaClient 和ApplicationInfoManager，注册时分为两种情况，即是否满足RefreshScope，如果满足，注入的Bean是带有 @Lazy + @RefreshScope 注解的
- ApplicationInfoManager：管理并初始化当前Instance实例的注册信息，并提供了实例状态监听机制
- EurekaClient ： netflix的接口类，用于和Eureka Server交互的客户端，而netflix的默认实现是DiscoveryClient，也是本文分析的重点
5、注册EurekaHealthIndicator，为/health端点提供Eureka相关信息，主要有Status当前实例状态和applications服务列表，在从Eureka Server获取服务列表正常的情况下，Status使用Eureka Server上的InstanceRemoteStatus，不正常情况下，代码中有一些判断逻辑

public class EurekaClientAutoConfiguration {
    ...省略

    /**
     * 1、注册EurekaClientConfigBean
     */
	@Bean
	@ConditionalOnMissingBean(value = EurekaClientConfig.class, search = SearchStrategy.CURRENT)
	public EurekaClientConfigBean eurekaClientConfigBean() {
		EurekaClientConfigBean client = new EurekaClientConfigBean();
		if ("bootstrap".equals(propertyResolver.getProperty("spring.config.name"))) {
			// We don't register during bootstrap by default, but there will be another
			// chance later.
			client.setRegisterWithEureka(false);
		}
		return client;
	}

    /**
     * 2、注册EurekaInstanceConfigBean
     */
	@Bean
	@ConditionalOnMissingBean(value = EurekaInstanceConfig.class, search = SearchStrategy.CURRENT)
	public EurekaInstanceConfigBean eurekaInstanceConfigBean(InetUtils inetUtils) throws MalformedURLException {
		PropertyResolver eurekaPropertyResolver = new RelaxedPropertyResolver(this.env, "eureka.instance.");
		String hostname = eurekaPropertyResolver.getProperty("hostname");

		boolean preferIpAddress = Boolean.parseBoolean(eurekaPropertyResolver.getProperty("preferIpAddress"));
		int nonSecurePort = Integer.valueOf(propertyResolver.getProperty("server.port", propertyResolver.getProperty("port", "8080")));
		int managementPort = Integer.valueOf(propertyResolver.getProperty("management.port", String.valueOf(nonSecurePort)));
		String managementContextPath = propertyResolver.getProperty("management.contextPath", propertyResolver.getProperty("server.contextPath", "/"));
		EurekaInstanceConfigBean instance = new EurekaInstanceConfigBean(inetUtils);
		instance.setNonSecurePort(nonSecurePort);
		instance.setInstanceId(getDefaultInstanceId(propertyResolver));
		instance.setPreferIpAddress(preferIpAddress);
		if (managementPort != nonSecurePort && managementPort != 0) {
			if (StringUtils.hasText(hostname)) {
				instance.setHostname(hostname);
			}
			String statusPageUrlPath = eurekaPropertyResolver.getProperty("statusPageUrlPath");
			String healthCheckUrlPath = eurekaPropertyResolver.getProperty("healthCheckUrlPath");
			if (!managementContextPath.endsWith("/")) {
				managementContextPath = managementContextPath + "/";
			}
			if (StringUtils.hasText(statusPageUrlPath)) {
				instance.setStatusPageUrlPath(statusPageUrlPath);
			}
			if (StringUtils.hasText(healthCheckUrlPath)) {
				instance.setHealthCheckUrlPath(healthCheckUrlPath);
			}
			String scheme = instance.getSecurePortEnabled() ? "https" : "http";
			URL base = new URL(scheme, instance.getHostname(), managementPort, managementContextPath);
			instance.setStatusPageUrl(new URL(base, StringUtils.trimLeadingCharacter(instance.getStatusPageUrlPath(), '/')).toString());
			instance.setHealthCheckUrl(new URL(base, StringUtils.trimLeadingCharacter(instance.getHealthCheckUrlPath(), '/')).toString());
		}
		return instance;
	}

    /**
     * 3、注册客户端自动注册相关组件
     *     EurekaRegistration： Eureka实例的服务注册信息（在开启客户端自动注册时才会注册）
     *     EurekaServiceRegistry： Eureka服务注册器
     *     EurekaAutoServiceRegistration： Eureka服务自动注册器，
     *                                     通过EurekaServiceRegistry注册器注册EurekaRegistration信息
     */
	@Bean
	public EurekaServiceRegistry eurekaServiceRegistry() {
		return new EurekaServiceRegistry();
	}

	@Bean
	@ConditionalOnBean(AutoServiceRegistrationProperties.class)
	@ConditionalOnProperty(value = "spring.cloud.service-registry.auto-registration.enabled", matchIfMissing = true)
	public EurekaRegistration eurekaRegistration(EurekaClient eurekaClient, CloudEurekaInstanceConfig instanceConfig, ApplicationInfoManager applicationInfoManager) {
		return EurekaRegistration.builder(instanceConfig)
				.with(applicationInfoManager)
				.with(eurekaClient)
				.with(healthCheckHandler)
				.build();
	}

	@Bean
	@ConditionalOnBean(AutoServiceRegistrationProperties.class)
	@ConditionalOnProperty(value = "spring.cloud.service-registry.auto-registration.enabled", matchIfMissing = true)
	public EurekaAutoServiceRegistration eurekaAutoServiceRegistration(ApplicationContext context, EurekaServiceRegistry registry, EurekaRegistration registration) {
		return new EurekaAutoServiceRegistration(context, registry, registration);
	}


	/**
     * 4、注册netflix的 EurekaClient 和 ApplicationInfoManager
     */
    // 如果禁用客户端自动注册，在此方法debug打断点会触发服务注册，状态为STARTING
    @Bean
	public DiscoveryClient discoveryClient(EurekaInstanceConfig config, EurekaClient client) {
		return new EurekaDiscoveryClient(config, client);
	}
    
    // 普通的EurekaClient配置（不可刷新）
	@Configuration
	@ConditionalOnMissingRefreshScope
	protected static class EurekaClientConfiguration {

		@Autowired
		private ApplicationContext context;

		@Autowired(required = false)
		private DiscoveryClientOptionalArgs optionalArgs;

		@Bean(destroyMethod = "shutdown")
		@ConditionalOnMissingBean(value = EurekaClient.class, search = SearchStrategy.CURRENT)
		public EurekaClient eurekaClient(ApplicationInfoManager manager, EurekaClientConfig config) {
			return new CloudEurekaClient(manager, config, this.optionalArgs,
					this.context);
		}

		@Bean
		@ConditionalOnMissingBean(value = ApplicationInfoManager.class, search = SearchStrategy.CURRENT)
		public ApplicationInfoManager eurekaApplicationInfoManager(
				EurekaInstanceConfig config) {
			InstanceInfo instanceInfo = new InstanceInfoFactory().create(config);
			return new ApplicationInfoManager(config, instanceInfo);
		}
	}

     // 可刷新的EurekaClient配置类
	@Configuration
	@ConditionalOnRefreshScope  //满足@ConditionalOnClass(RefreshScope.class)         
                                //    @ConditionalOnBean(RefreshAutoConfiguration.class)
	protected static class RefreshableEurekaClientConfiguration {

		@Autowired
		private ApplicationContext context;

		@Autowired(required = false)
		private DiscoveryClientOptionalArgs optionalArgs;

        // 注册CloudEurekaClient，是com.netflix.discovery.EurekaClient接口的实现类
		@Bean(destroyMethod = "shutdown")
		@ConditionalOnMissingBean(value = EurekaClient.class, search = SearchStrategy.CURRENT)
		@org.springframework.cloud.context.config.annotation.RefreshScope
		@Lazy
		public EurekaClient eurekaClient(ApplicationInfoManager manager, EurekaClientConfig config, EurekaInstanceConfig instance) {
			manager.getInfo(); // force initialization
			return new CloudEurekaClient(manager, config, this.optionalArgs,
					this.context);
		}

        // 注册ApplicationInfoManager
		@Bean
		@ConditionalOnMissingBean(value = ApplicationInfoManager.class, search = SearchStrategy.CURRENT)
		@org.springframework.cloud.context.config.annotation.RefreshScope
		@Lazy
		public ApplicationInfoManager eurekaApplicationInfoManager(EurekaInstanceConfig config) {
			InstanceInfo instanceInfo = new InstanceInfoFactory().create(config);
			return new ApplicationInfoManager(config, instanceInfo);
		}
	}

	
	/**
     * 5、注册 EurekaHealthIndicator
     */
	@Configuration
	@ConditionalOnClass(Endpoint.class)
	protected static class EurekaHealthIndicatorConfiguration {
		@Bean
		@ConditionalOnMissingBean
		public EurekaHealthIndicator eurekaHealthIndicator(EurekaClient eurekaClient,
														   EurekaInstanceConfig instanceConfig, EurekaClientConfig clientConfig) {
			return new EurekaHealthIndicator(eurekaClient, instanceConfig, clientConfig);
		}
	}
}

如上，在满足一系列Conditional条件后，会向Spring容器中注册CloudEurekaClient，它是com.netflix.discovery.EurekaClient接口的实现类，具体继承实现关系如下

DiscoveryClient继承实现关系

如上图所示，刚刚创建的CloudEurekaClient是 com.netflix.discovery.DiscoveryClient的子类，它们都实现了com.netflix.discovery.EurekaClient接口

EurekaClient是Netflix对服务发现客户端抽象的接口，包含很多方法，而DiscoveryClient是其默认实现，也是本文分析的重点，CloudEurekaClient是spring cloud的实现，根据类上注释，其主要重写了onCacheRefreshed()方法，这个方法主要是从Eureka Server fetchRegistry()获取服务列表之后用于以广播方式通知缓存刷新事件的，其实DiscoveryClient也有onCacheRefreshed()方法的实现，但由于DiscoveryClient是Netflix的类，只发送了com.netflix.discovery.EurekaEvent，而CloudEurekaClient使用Spring的ApplicationEventPublisher，发送了HeartbeatEvent

注意：

上面说的都是netflix的DiscoveryClient

还有另一个DiscoveryClient，是 org.springframework.cloud.client.discovery.DiscoveryClient

是Spring对服务发现客户端的抽象

创建DiscoveryClient的过程

DiscoveryClient构造方法

@Inject
DiscoveryClient(ApplicationInfoManager applicationInfoManager, 
                EurekaClientConfig config, 
                AbstractDiscoveryClientOptionalArgs args,
                Provider<BackupRegistry> backupRegistryProvider) {
    /**
     * AbstractDiscoveryClientOptionalArgs 是DiscoveryClient的可选参数，可理解为扩展点
     * 包含healthCheckHandlerProvider、healthCheckCallbackProvider、eventListeners等
     * spring cloud默认实现为MutableDiscoveryClientOptionalArgs，但此处相关成员变量赋值后认为空
     */
    if (args != null) {
        this.healthCheckHandlerProvider = args.healthCheckHandlerProvider;
        this.healthCheckCallbackProvider = args.healthCheckCallbackProvider;
        this.eventListeners.addAll(args.getEventListeners());
    } else {
        this.healthCheckCallbackProvider = null;
        this.healthCheckHandlerProvider = null;
    }
    
    this.applicationInfoManager = applicationInfoManager;
    InstanceInfo myInfo = applicationInfoManager.getInfo();

    clientConfig = config;
    staticClientConfig = clientConfig;
    transportConfig = config.getTransportConfig();
    instanceInfo = myInfo;
    if (myInfo != null) {
        appPathIdentifier = instanceInfo.getAppName() + "/" + instanceInfo.getId();
    } else {
        logger.warn("Setting instanceInfo to a passed in null value");
    }

    this.backupRegistryProvider = backupRegistryProvider;

    this.urlRandomizer = new EndpointUtils.InstanceInfoBasedUrlRandomizer(instanceInfo);
    localRegionApps.set(new Applications());

    fetchRegistryGeneration = new AtomicLong(0);

    remoteRegionsToFetch = new AtomicReference<String>(clientConfig.fetchRegistryForRemoteRegions());
    remoteRegionsRef = new AtomicReference<>(remoteRegionsToFetch.get() == null ? null : remoteRegionsToFetch.get().split(","));

    // 如果 shouldFetchRegistry=true，注册netflix servo监控
    if (config.shouldFetchRegistry()) {
        this.registryStalenessMonitor = new ThresholdLevelsMetric(this, METRIC_REGISTRY_PREFIX + "lastUpdateSec_", new long[]{15L, 30L, 60L, 120L, 240L, 480L});
    } else {
        this.registryStalenessMonitor = ThresholdLevelsMetric.NO_OP_METRIC;
    }

    // 如果 shouldRegisterWithEureka=true，注册netflix servo监控
    if (config.shouldRegisterWithEureka()) {
        this.heartbeatStalenessMonitor = new ThresholdLevelsMetric(this, METRIC_REGISTRATION_PREFIX + "lastHeartbeatSec_", new long[]{15L, 30L, 60L, 120L, 240L, 480L});
    } else {
        this.heartbeatStalenessMonitor = ThresholdLevelsMetric.NO_OP_METRIC;
    }

    logger.info("Initializing Eureka in region {}", clientConfig.getRegion());

    // 如果既不要向eureka server注册，又不要获取服务列表，就什么都不用初始化
    if (!config.shouldRegisterWithEureka() && !config.shouldFetchRegistry()) {
        logger.info("Client configured to neither register nor query for data.");
        scheduler = null;
        heartbeatExecutor = null;
        cacheRefreshExecutor = null;
        eurekaTransport = null;
        instanceRegionChecker = new InstanceRegionChecker(new PropertyBasedAzToRegionMapper(config), clientConfig.getRegion());

        // This is a bit of hack to allow for existing code using DiscoveryManager.getInstance()
        // to work with DI'd DiscoveryClient
        DiscoveryManager.getInstance().setDiscoveryClient(this);
        DiscoveryManager.getInstance().setEurekaClientConfig(config);

        initTimestampMs = System.currentTimeMillis();

        logger.info("Discovery Client initialized at timestamp {} with initial instances count: {}",
                initTimestampMs, this.getApplications().size());
        return;  // no need to setup up an network tasks and we are done
    }

    // 【重点】创建各种Executor 和 eurekaTransport、instanceRegionChecker
    try {
        // 执行定时任务的定时器，定时线程名为 DiscoveryClient-%d
        // 在定时器中用于定时执行TimedSupervisorTask监督任务，监督任务会强制超时 和 记录监控数据
        scheduler = Executors.newScheduledThreadPool(3,
                new ThreadFactoryBuilder()
                        .setNameFormat("DiscoveryClient-%d")
                        .setDaemon(true)
                        .build());

        // 执行heartbeat心跳任务的执行器，默认最大线程数=2，线程名为：DiscoveryClient-HeartbeatExecutor-%d
        heartbeatExecutor = new ThreadPoolExecutor(
                1, clientConfig.getHeartbeatExecutorThreadPoolSize(), 0, TimeUnit.SECONDS,
                new SynchronousQueue<Runnable>(),
                new ThreadFactoryBuilder()
                        .setNameFormat("DiscoveryClient-HeartbeatExecutor-%d")
                        .setDaemon(true)
                        .build()
        );  // use direct handoff

        // 执行服务列表缓存刷新的执行器，默认最大线程数=2，线程名为：DiscoveryClient-CacheRefreshExecutor-%d
        cacheRefreshExecutor = new ThreadPoolExecutor(
                1, clientConfig.getCacheRefreshExecutorThreadPoolSize(), 0, TimeUnit.SECONDS,
                new SynchronousQueue<Runnable>(),
                new ThreadFactoryBuilder()
                        .setNameFormat("DiscoveryClient-CacheRefreshExecutor-%d")
                        .setDaemon(true)
                        .build()
        );  // use direct handoff

        eurekaTransport = new EurekaTransport();
        // 初始化eurekaTransport在服务注册，获取服务列表时的client
        scheduleServerEndpointTask(eurekaTransport, args);

        AzToRegionMapper azToRegionMapper;
        if (clientConfig.shouldUseDnsForFetchingServiceUrls()) {
            azToRegionMapper = new DNSBasedAzToRegionMapper(clientConfig);
        } else {
            azToRegionMapper = new PropertyBasedAzToRegionMapper(clientConfig);
        }
        if (null != remoteRegionsToFetch.get()) {
            azToRegionMapper.setRegionsToFetch(remoteRegionsToFetch.get().split(","));
        }
        instanceRegionChecker = new InstanceRegionChecker(azToRegionMapper, clientConfig.getRegion());
    } catch (Throwable e) {
        throw new RuntimeException("Failed to initialize DiscoveryClient!", e);
    }

    // 如果需要从eureka server获取服务列表，并且尝试fetchRegistry(false)失败，调用BackupRegistry
    if (clientConfig.shouldFetchRegistry() && !fetchRegistry(false)) {
        fetchRegistryFromBackup();
    }

    // 【重点】初始化所有定时任务
    initScheduledTasks();
    
    // 添加servo监控
    try {
        Monitors.registerObject(this);
    } catch (Throwable e) {
        logger.warn("Cannot register timers", e);
    }

    // This is a bit of hack to allow for existing code using DiscoveryManager.getInstance()
    // to work with DI'd DiscoveryClient
    DiscoveryManager.getInstance().setDiscoveryClient(this);
    DiscoveryManager.getInstance().setEurekaClientConfig(config);

    initTimestampMs = System.currentTimeMillis();
    logger.info("Discovery Client initialized at timestamp {} with initial instances count: {}",
            initTimestampMs, this.getApplications().size());
}

上面的DiscoveryClient构造方法代码比较多，但多数都是一些赋值，本次分析的重点在注释中已经标出，创建了各种Executor 和 eurekaTransport、instanceRegionChecker，之后又调用initScheduledTasks()方法初始化所有这些定时任务

【重点】initScheduledTasks() 初始化定时任务

/**
 * Initializes all scheduled tasks.
 */
private void initScheduledTasks() {
    // 1、如果要从Eureka Server获取服务列表
    if (clientConfig.shouldFetchRegistry()) {
        // registry cache refresh timer
        
        // 从eureka服务器获取注册表信息的频率（默认30s）
        // 同时也是单次获取服务列表的超时时间
        int registryFetchIntervalSeconds = clientConfig.getRegistryFetchIntervalSeconds();
        // 如果缓存刷新超时，下一次执行的delay最大是registryFetchIntervalSeconds的几倍（默认10），默认每次执行是上一次的2倍
        int expBackOffBound = clientConfig.getCacheRefreshExecutorExponentialBackOffBound();
        
        /**
         * 【#### 执行CacheRefreshThread，服务列表缓存刷新任务 ####】
         * 执行TimedSupervisorTask监督任务的定时器，具体执行器为cacheRefreshExecutor，任务为CacheRefreshThread
         */
        scheduler.schedule(
                new TimedSupervisorTask(
                        "cacheRefresh",               //监控名
                        scheduler,
                        cacheRefreshExecutor,
                        registryFetchIntervalSeconds, //指定具体任务的超时时间
                        TimeUnit.SECONDS,
                        expBackOffBound,
                        new CacheRefreshThread()
                ),
                registryFetchIntervalSeconds, TimeUnit.SECONDS);
    }

    
    // 2、如果要注册到Eureka Server
    if (clientConfig.shouldRegisterWithEureka()) {
        // 续租的时间间隔（默认30s）
        int renewalIntervalInSecs = instanceInfo.getLeaseInfo().getRenewalIntervalInSecs();
        // 如果心跳任务超时，下一次执行的delay最大是renewalIntervalInSecs的几倍（默认10），默认每次执行是上一次的2倍
        int expBackOffBound = clientConfig.getHeartbeatExecutorExponentialBackOffBound();
        logger.info("Starting heartbeat executor: " + "renew interval is: " + renewalIntervalInSecs);

        // Heartbeat timer
        /**
         * 【#### 执行HeartbeatThread，发送心跳数据 ####】
         * 执行TimedSupervisorTask监督任务的定时器，具体执行器为heartbeatExecutor，任务为HeartbeatThread
         */
        scheduler.schedule(
                new TimedSupervisorTask(
                        "heartbeat",
                        scheduler,
                        heartbeatExecutor,
                        renewalIntervalInSecs,
                        TimeUnit.SECONDS,
                        expBackOffBound,
                        new HeartbeatThread()
                ),
                renewalIntervalInSecs, TimeUnit.SECONDS);

        // InstanceInfo replicator
        /** 
         * 【#### InstanceInfo复制器 ####】
         * 启动后台定时任务scheduler，线程名为 DiscoveryClient-InstanceInfoReplicator-%d
         * 默认每30s执行一次定时任务，查看Instance信息（DataCenterInfo、LeaseInfo、InstanceStatus）是否有变化
         * 如果有变化，执行 discoveryClient.register()
         */
        instanceInfoReplicator = new InstanceInfoReplicator(
               this,            //当前DiscoveryClient
               instanceInfo,    //当前实例信息
               clientConfig.getInstanceInfoReplicationIntervalSeconds(),//InstanceInfo的复制间隔（默认30s）
               2); // burstSize

        /**
         * 【StatusChangeListener 状态改变监听器】
         */
        statusChangeListener = new ApplicationInfoManager.StatusChangeListener() {
            @Override
            public String getId() {
                return "statusChangeListener";
            }

            @Override
            public void notify(StatusChangeEvent statusChangeEvent) {
                if (InstanceStatus.DOWN == statusChangeEvent.getStatus() ||
                        InstanceStatus.DOWN == statusChangeEvent.getPreviousStatus()) {
                    // log at warn level if DOWN was involved
                    logger.warn("Saw local status change event {}", statusChangeEvent);
                } else {
                    logger.info("Saw local status change event {}", statusChangeEvent);
                }
                
                //使用InstanceInfo复制器 scheduler.submit()一个Runnable任务
                //后台马上执行 discoveryClient.register()
                instanceInfoReplicator.onDemandUpdate();
            }
        };

        /**
         * 是否关注Instance状态变化，使用后台线程将状态同步到eureka server（默认true）
         * 调用 ApplicationInfoManager#setInstanceStatus(status) 会触发
         * 将 StatusChangeListener 注册到 ApplicationInfoManager
         */
        if (clientConfig.shouldOnDemandUpdateStatusChange()) {
            applicationInfoManager.registerStatusChangeListener(statusChangeListener);
        }

        // 启动InstanceInfo复制器
        instanceInfoReplicator.start(clientConfig.getInitialInstanceInfoReplicationIntervalSeconds());
    } 
    // 当前服务实例不注册到Eureka Server
    else {
        logger.info("Not registering with Eureka server per configuration");
    }
}

总的来说initScheduledTasks()做了以下几件事：

如果shouldFetchRegistry=true，即要从Eureka Server获取服务列表
- 启动刷新服务列表定时线程（DiscoveryClient-CacheRefreshExecutor-%d），默认registryFetchIntervalSeconds=30s执行一次，任务为CacheRefreshThread，即从Eureka Server获取服务列表，也刷新客户端缓存
如果shouldRegisterWithEureka=true，即要注册到Eureka Server
- 启动heartbeat心跳定时线程（DiscoveryClient-HeartbeatExecutor-%d），默认renewalIntervalInSecs=30s续约一次，任务为HeartbeatThread，即客户端向Eureka Server发送心跳
- 启动InstanceInfo复制器定时线程（DiscoveryClient-InstanceInfoReplicator-%d），开启定时线程检查当前Instance的DataCenterInfo、LeaseInfo、InstanceStatus，如果发现变更就执行discoveryClient.register()，将实例信息同步到Server端

Eureka Client复制InstanceInfo，发起注册

由创建DiscoveryClient的过程可知，创建了很多定时执行线程，如定时从Server端刷新服务列表的CacheRefreshThread，定时报心跳续约的HeartbeatThread，还有用于更新并复制本地实例状态到Server端的InstanceInfo复制器定时线程，而正是InstanceInfoReplicator#run()中的discoveryClient.register()发起了注册

那么怎么可以触发注册行为呢？

// InstanceInfoReplicator#run()
public void run() {
    try {
        /**
         * 刷新 InstanceInfo
         * 1、刷新 DataCenterInfo
         * 2、刷新 LeaseInfo 租约信息
         * 3、根据HealthCheckHandler获取InstanceStatus，并更新，如果状态发生变化会触发所有StatusChangeListener
         */
        discoveryClient.refreshInstanceInfo();

        // 如果isInstanceInfoDirty=true，返回dirtyTimestamp，否则是null
        Long dirtyTimestamp = instanceInfo.isDirtyWithTime();
        if (dirtyTimestamp != null) {
            discoveryClient.register();  //发起注册
            instanceInfo.unsetIsDirty(dirtyTimestamp);  //isInstanceInfoDirty置为false
        }
    } 
    catch (Throwable t) {
        logger.warn("There was a problem with the instance info replicator", t);
    } 
    finally { // 继续下次任务
        Future next = scheduler.schedule(this, replicationIntervalSeconds, TimeUnit.SECONDS);
        scheduledPeriodicRef.set(next);
    }
}

如上，先刷新InstanceInfo，刷新后如果发现有脏数据，即实例发生了变更，还未同步给Server的数据，就发起注册

那么在Eureka Client启动的这种场景下，怎样会触发有脏数据下的注册？

由InstanceInfoReplicator复制器的自动定时任务在刷新InstanceInfo时发现有脏数据，并更新
InstanceInfoReplicator复制器提供onDemandUpdate()按需更新方法，一旦调用，马上会submit()任务，其中会cancel自动更新任务，马上执行InstanceInfoReplicator#run()

InstanceInfoReplicator复制器自动定时更新

InstanceInfoReplicator复制器在启动创建DiscoveryClient时被创建并start()启动

// InstanceInfoReplicator#start()
public void start(int initialDelayMs) { // 默认40s
    if (started.compareAndSet(false, true)) {
        instanceInfo.setIsDirty();  // for initial register 初始化时会将instanceInfo设置为dirty
        Future next = scheduler.schedule(this, initialDelayMs, TimeUnit.SECONDS);
        scheduledPeriodicRef.set(next);
    }
}

所以当自动更新启动时会设置InstanceInfo为脏数据，因为要触发第一次向Server同步，那么在40s后会调用InstanceInfoReplicator#run()，假设InstanceInfo并没有其它变更，那么也会发起discoveryClient.register()

注意：

正常情况下是不会由延迟40s的第一次执行定时任务发起注册，而是下面的onDemandUpdate() 主动按需更新发起注册

如果设置@EnableDiscoveryClient(autoRegister = false) 或者 spring.cloud.service-registry.auto-registration.enabled=false，即放弃自动注册，并在EurekaClientAutoConfiguration的如下方法打断点
@Bean
public DiscoveryClient discoveryClient(EurekaInstanceConfig config, EurekaClient client) {
	return new EurekaDiscoveryClient(config, client);
}
会在断点生效时触发EurekaClient的实例化，而此EurekaClient就是一个DiscoveryClient，会启动InstanceInfoReplicator自动定时更新线程，但由于new InstanceInfoFactory().create(config)时本地实例状态为STARTING，所以注册到Server端的状态也是STARTING

onDemandUpdate() 主动按需更新

目前只有在ApplicationInfoManager#setInstanceStatus()更新实例状态，且实例状态真的发生变更，触发StatusChangeListener状态变更监听器时，会调用onDemandUpdate马上submit任务执行InstanceInfoReplicator#run()，再发起注册

由于Spring Cloud默认是启用服务自动注册AutoServiceRegistration的，所以在EurekaClientAutoConfiguration自动配置时会注册服务自动注册相关组件（EurekaRegistration、EurekaServiceRegistry、EurekaAutoServiceRegistration），其中EurekaAutoServiceRegistration实现了Spring的SmartLifecycle接口，会在Spring容器refresh要完毕时触发生命周期方法start()，其中会使用EurekaServiceRegistry服务注册器注册EurekaRegistration这个本地实例信息

// EurekaServiceRegistry#register()
public void register(EurekaRegistration reg) {
	maybeInitializeClient(reg);

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

    // 设置初始化状态
	reg.getApplicationInfoManager()
			.setInstanceStatus(reg.getInstanceConfig().getInitialStatus());

	if (reg.getHealthCheckHandler() != null) {
		reg.getEurekaClient().registerHealthCheck(reg.getHealthCheckHandler());
	}
}

主要是设置初始化状态步骤，而EurekaInstanceConfigBean本地实例信息的initialStatus初始化状态为 InstanceStatus.UP，所以状态与new InstanceInfo()时的STARTING不同，发生了状态变更，触发在创建DiscoveryClient时设置的StatusChangeListener

...省略

statusChangeListener = new ApplicationInfoManager.StatusChangeListener() {
    @Override
    public String getId() {
        return "statusChangeListener";
    }

    @Override
    public void notify(StatusChangeEvent statusChangeEvent) {
        if (InstanceStatus.DOWN == statusChangeEvent.getStatus() ||
                InstanceStatus.DOWN == statusChangeEvent.getPreviousStatus()) {
            // log at warn level if DOWN was involved
            logger.warn("Saw local status change event {}", statusChangeEvent);
        } else {
            logger.info("Saw local status change event {}", statusChangeEvent);
        }
        instanceInfoReplicator.onDemandUpdate();
    }
};

if (clientConfig.shouldOnDemandUpdateStatusChange()) {
    applicationInfoManager.registerStatusChangeListener(statusChangeListener);
}

...省略

其中会调用 InstanceInfoReplicator.onDemandUpdate() 实例信息复制器做按需更新，马上将UP状态更新/注册到Server端

所以，以我判断，Eureka Client启动时的自动注册大多数应该是Spring Cloud的服务自动注册机制，在Spring容器基本启动完毕时，触发服务自动注册操作，其中会使用ApplicationInfoManager更新实例状态为初始状态UP，一旦实例状态变更会被马上监听到，执行复制器的InstanceInfoReplicator.onDemandUpdate()按需更新，马上执行一次discoveryClient.register()操作

所以，下面就是分析 discoveryClient.register() 是怎么注册服务的

DiscoveryClient#register() 注册

// DiscoveryClient#register()
boolean register() throws Throwable {
    logger.info(PREFIX + appPathIdentifier + ": registering service...");
    EurekaHttpResponse<Void> 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;
}

如上，注册方法使用eurekaTransport的注册客户端registrationClient调用了register(instanceinfo)方法

EurekaTransport是DiscoveryClient的内部类，其中包含

registrationClient 和 registrationClientFactory：负责注册、续约相关工作的EurekaHttpClient 和 EurekaHttpClientFactory的实现类
queryClient 和 queryClientFactory：负责获取Server端服务列表的EurekaHttpClient 和 EurekaHttpClientFactory的实现类
TransportClientFactory：负责传输消息的客户端工厂（底层用于和Server交互的http框架是 Jersey，此处的工厂就和Jersey相关）

那么EurekaTransport的相关组件，尤其是registrationClient注册客户端是如何初始化的呢？

registrationClient - 服务注册相关的EurekaHttpClient

初始化是在DiscoveryClient的构造方法中

eurekaTransport = new EurekaTransport();
scheduleServerEndpointTask(eurekaTransport, args);

主要是scheduleServerEndpointTask()方法

private void scheduleServerEndpointTask(EurekaTransport eurekaTransport,
                                        AbstractDiscoveryClientOptionalArgs args) {

    Collection<?> additionalFilters = args == null
            ? Collections.emptyList()
            : args.additionalFilters;

    EurekaJerseyClient providedJerseyClient = args == null
            ? null
            : args.eurekaJerseyClient;
    
    TransportClientFactories argsTransportClientFactories = null;
    if (args != null && args.getTransportClientFactories() != null) {
        argsTransportClientFactories = args.getTransportClientFactories();
    }
    
    // Ignore the raw types warnings since the client filter interface changed between jersey 1/2
    @SuppressWarnings("rawtypes")
    TransportClientFactories transportClientFactories = argsTransportClientFactories == null
            ? new Jersey1TransportClientFactories()
            : argsTransportClientFactories;

    // If the transport factory was not supplied with args, assume they are using jersey 1 for passivity
    // 1、参数中是否提供了transportClientFactory的实现，没有就使用Jersey1TransportClientFactories
    eurekaTransport.transportClientFactory = providedJerseyClient == null
            ? transportClientFactories.newTransportClientFactory(clientConfig, additionalFilters, applicationInfoManager.getInfo())
            : transportClientFactories.newTransportClientFactory(additionalFilters, providedJerseyClient);

    ApplicationsResolver.ApplicationsSource applicationsSource = new ApplicationsResolver.ApplicationsSource() {
        @Override
        public Applications getApplications(int stalenessThreshold, TimeUnit timeUnit) {
            long thresholdInMs = TimeUnit.MILLISECONDS.convert(stalenessThreshold, timeUnit);
            long delay = getLastSuccessfulRegistryFetchTimePeriod();
            if (delay > thresholdInMs) {
                logger.info("Local registry is too stale for local lookup. Threshold:{}, actual:{}",
                        thresholdInMs, delay);
                return null;
            } else {
                return localRegionApps.get();
            }
        }
    };

    eurekaTransport.bootstrapResolver = EurekaHttpClients.newBootstrapResolver(
            clientConfig,
            transportConfig,
            eurekaTransport.transportClientFactory,
            applicationInfoManager.getInfo(),
            applicationsSource
    );

    /**
     * 是否要想Eureka Server注册
     * 2、创建RegistrationClient用于注册的客户端及工厂，并设置到eurekaTransport
     */ 
    if (clientConfig.shouldRegisterWithEureka()) {
        EurekaHttpClientFactory newRegistrationClientFactory = null;
        EurekaHttpClient newRegistrationClient = null;
        try {
            newRegistrationClientFactory = EurekaHttpClients.registrationClientFactory(
                    eurekaTransport.bootstrapResolver,
                    eurekaTransport.transportClientFactory,
                    transportConfig
            );
            newRegistrationClient = newRegistrationClientFactory.newClient();
        } catch (Exception e) {
            logger.warn("Transport initialization failure", e);
        }
        eurekaTransport.registrationClientFactory = newRegistrationClientFactory;
        eurekaTransport.registrationClient = newRegistrationClient;
    }

    // 
    /**
     * 是否要从Server端获取服务列表
     * 3、创建QueryClient用于查询服务列表的客户端及工厂，并设置到eurekaTransport
     */ 
    // new method (resolve from primary servers for read)
    // Configure new transport layer (candidate for injecting in the future)
    if (clientConfig.shouldFetchRegistry()) {
        EurekaHttpClientFactory newQueryClientFactory = null;
        EurekaHttpClient newQueryClient = null;
        try {
            newQueryClientFactory = EurekaHttpClients.queryClientFactory(
                    eurekaTransport.bootstrapResolver,
                    eurekaTransport.transportClientFactory,
                    clientConfig,
                    transportConfig,
                    applicationInfoManager.getInfo(),
                    applicationsSource
            );
            newQueryClient = newQueryClientFactory.newClient();
        } catch (Exception e) {
            logger.warn("Transport initialization failure", e);
        }
        eurekaTransport.queryClientFactory = newQueryClientFactory;
        eurekaTransport.queryClient = newQueryClient;
    }
}

所以，下面就是逐层深入分析RegistrationClient用于注册的客户端及工厂是如何创建的？

由于RegistrationClient其实是一种EurekaHttpClient，而EurekaHttpClient是接口，其实现类很多

查看源码发现，Netflix采用的是 Factory工厂 + 代理 的模式，从最外层创建的EurekaHttpClient工厂包含一个成员变量是另一个代理的EurekaHttpClient工厂，每个工厂生成的EurekaHttpClient功能不一样，在从外层执行一个操作时，最外层的工厂执行其相关功能后，使用代理的工厂新建EurekaHttpClient实例，再调用其相同的方法，也实现这个EurekaHttpClient的相关功能，就这样逐层深入，各司其职后，最后使用Jersey发送POST请求到Eureka Server发起注册，而这些EurekaHttpClient都是在com.netflix.discovery.shared.transport.decoratorEurekaHttpClient的包装类的包下的，由外到内大致是：

SessionedEurekaHttpClient：强制在一定时间间隔后重连EurekaHttpClient，防止永远只连接特定Eureka Server，反过来保证了在Server端集群拓扑发生变化时的负载重分配
- RetryableEurekaHttpClient：带有重试功能，默认最多3次，在配置的所有候选Server地址中尝试请求，成功重用，失败会重试另一Server，并维护隔离清单，下次跳过，当隔离数量达到阈值，清空隔离清单，重新开始
  - RedirectingEurekaHttpClient： Server端返回302重定向时，客户端shutdown原EurekaHttpClient，根据response header中的Location新建EurekaHttpClient
    - MetricsCollectingEurekaHttpClient：统计收集Metrics信息
      - JerseyApplicationClient： AbstractJerseyEurekaHttpClient的子类
        
        AbstractJerseyEurekaHttpClient：底层实现通过Jersery注册、发心跳等的核心类
        
        jerseyClient： Jersery客户端

SessionedEurekaHttpClient - 定时重连

// SessionedEurekaHttpClient#execute()
@Override
protected <R> EurekaHttpResponse<R> execute(RequestExecutor<R> requestExecutor) {
    long now = System.currentTimeMillis();
    long delay = now - lastReconnectTimeStamp;
    // 如果上次重连时间到现在已经超过了currentSessionDurationMs，关闭当前EurekaHttpClient
    if (delay >= currentSessionDurationMs) {
        logger.debug("Ending a session and starting anew");
        lastReconnectTimeStamp = now;
        currentSessionDurationMs = randomizeSessionDuration(sessionDurationMs);
        TransportUtils.shutdown(eurekaHttpClientRef.getAndSet(null));
    }

    // 如果EurekaHttpClient为空，clientFactory.newClient()重建
    EurekaHttpClient eurekaHttpClient = eurekaHttpClientRef.get();
    if (eurekaHttpClient == null) {
        eurekaHttpClient = TransportUtils.getOrSetAnotherClient(eurekaHttpClientRef, clientFactory.newClient());
    }
    
    // 继续执行后续
    return requestExecutor.execute(eurekaHttpClient);
}

RetryableEurekaHttpClient - 候选范围内失败重试

// RetryableEurekaHttpClient#execute()
@Override
protected <R> EurekaHttpResponse<R> execute(RequestExecutor<R> requestExecutor) {
    List<EurekaEndpoint> candidateHosts = null;
    int endpointIdx = 0;
    
    // 最多重试numberOfRetries（默认：3）
    for (int retry = 0; retry < numberOfRetries; retry++) {
        EurekaHttpClient currentHttpClient = delegate.get();//从AtomicReference<EurekaHttpClient>获取当前EurekaHttpClient
        EurekaEndpoint currentEndpoint = null;
        if (currentHttpClient == null) {
            if (candidateHosts == null) {
                candidateHosts = getHostCandidates(); //返回候选集合 排除 已经失败隔离的Host集合
                if (candidateHosts.isEmpty()) {
                    throw new TransportException("There is no known eureka server; cluster server list is empty");
                }
            }
            if (endpointIdx >= candidateHosts.size()) {
                throw new TransportException("Cannot execute request on any known server");
            }

            // 根据当前的下标获取Endpoint，并新建 JerseyClient
            currentEndpoint = candidateHosts.get(endpointIdx++);
            currentHttpClient = clientFactory.newClient(currentEndpoint);
        }

        try {
            // 继续后续执行
            EurekaHttpResponse<R> response = requestExecutor.execute(currentHttpClient);
            // 如果根据当前操作类型 和 返回状态码，满足状态计算器，记录currentHttpClient可用，下次继续使用
            // 返回状态码是：200、300、302，或者Register、SendHeartBeat情况下是404
            if (serverStatusEvaluator.accept(response.getStatusCode(), requestExecutor.getRequestType())) {
                delegate.set(currentHttpClient);
                if (retry > 0) {
                    logger.info("Request execution succeeded on retry #{}", retry);
                }
                return response;
            }
            logger.warn("Request execution failure with status code {}; retrying on another server if available", response.getStatusCode());
        } 
        catch (Exception e) {
            logger.warn("Request execution failed with message: {}", e.getMessage());  // just log message as the underlying client should log the stacktrace
        }

        // Connection error or 5xx from the server that must be retried on another server
        // 请求失败 或 报5xx错误，将delegate清空，重试另一个Server，并将当前Endpoint放到隔离集合
        delegate.compareAndSet(currentHttpClient, null);
        if (currentEndpoint != null) {
            quarantineSet.add(currentEndpoint);
        }
    }
    
    // 多次重试后仍无法成功返回结果，上抛异常
    throw new TransportException("Retry limit reached; giving up on completing the request");
}


//########## RetryableEurekaHttpClient#getHostCandidates() 
// 返回 所有候选的Host节点数据 与 隔离集合 的数据差集
private List<EurekaEndpoint> getHostCandidates() {
    List<EurekaEndpoint> candidateHosts = clusterResolver.getClusterEndpoints(); //所有候选节点数据
    quarantineSet.retainAll(candidateHosts); //确保quarantineSet隔离集合中的数据都在candidateHosts中
                                             //当candidateHosts发生变化时也能及时清理quarantineSet隔离集合

    // If enough hosts are bad, we have no choice but start over again
    // 默认：0.66百分比
    int threshold = (int) (candidateHosts.size() * transportConfig.getRetryableClientQuarantineRefreshPercentage());
    
    // 隔离集合为空
    if (quarantineSet.isEmpty()) {
        // no-op
    } 
    // 隔离数据已经大于阀值，不得已要重新开始，清空隔离集合
    else if (quarantineSet.size() >= threshold) { 
        logger.debug("Clearing quarantined list of size {}", quarantineSet.size());
        quarantineSet.clear();
    } 
    // 隔离集合不为空，也不大于阀值，排除隔离集合中的Endpoint后返回
    else { 
        List<EurekaEndpoint> remainingHosts = new ArrayList<>(candidateHosts.size());
        for (EurekaEndpoint endpoint : candidateHosts) {
            if (!quarantineSet.contains(endpoint)) {
                remainingHosts.add(endpoint);
            }
        }
        candidateHosts = remainingHosts;
    }

    return candidateHosts;
}


//########## ServerStatusEvaluators#LEGACY_EVALUATOR
// Eureka Server返回状态的计算器，计算不同场景下的不同状态码是否代表成功
private static final ServerStatusEvaluator LEGACY_EVALUATOR = new ServerStatusEvaluator() {
    @Override
    public boolean accept(int statusCode, RequestType requestType) {
        if (statusCode >= 200 && statusCode < 300 || statusCode == 302) {
            return true;
        } else if (requestType == RequestType.Register && statusCode == 404) {
            return true;
        } else if (requestType == RequestType.SendHeartBeat && statusCode == 404) {
            return true;
        } else if (requestType == RequestType.Cancel) {  // cancel is best effort
            return true;
        } else if (requestType == RequestType.GetDelta && (statusCode == 403 || statusCode == 404)) {
            return true;
        }
        return false;
    }
};

RedirectingEurekaHttpClient - 按Server端要求重定向到新Server

//########## RedirectingEurekaHttpClient#executeOnNewServer
// Server端返回302重定向时，客户端shutdown原EurekaHttpClient，根据response header中的Location新建EurekaHttpClient
private <R> EurekaHttpResponse<R> executeOnNewServer(RequestExecutor<R> requestExecutor,
                                                AtomicReference<EurekaHttpClient> currentHttpClientRef) {
    URI targetUrl = null;
    // 最多重定向默认10次
    for (int followRedirectCount = 0; followRedirectCount < MAX_FOLLOWED_REDIRECTS; followRedirectCount++) {
        EurekaHttpResponse<R> httpResponse = requestExecutor.execute(currentHttpClientRef.get());
        
        // 如果返回的不是302重定向，返回response
        if (httpResponse.getStatusCode() != 302) {
            if (followRedirectCount == 0) {
                logger.debug("Pinning to endpoint {}", targetUrl);
            } else {
                logger.info("Pinning to endpoint {}, after {} redirect(s)", targetUrl, followRedirectCount);
            }
            return httpResponse;
        }

        // 从response中获取Location，用于重建EurekaHttpClient
        targetUrl = getRedirectBaseUri(httpResponse.getLocation());
        if (targetUrl == null) {
            throw new TransportException("Invalid redirect URL " + httpResponse.getLocation());
        }

        currentHttpClientRef.getAndSet(null).shutdown();
        currentHttpClientRef.set(factory.newClient(new DefaultEndpoint(targetUrl.toString())));
    }
    String message = "Follow redirect limit crossed for URI " + serviceEndpoint.getServiceUrl();
    logger.warn(message);
    throw new TransportException(message);
}

MetricsCollectingEurekaHttpClient - 统计收集执行情况

// MetricsCollectingEurekaHttpClient#execute()
protected <R> EurekaHttpResponse<R> execute(RequestExecutor<R> requestExecutor) {
    EurekaHttpClientRequestMetrics requestMetrics = metricsByRequestType.get(requestExecutor.getRequestType());
    Stopwatch stopwatch = requestMetrics.latencyTimer.start(); //统计执行延时
    try {
        EurekaHttpResponse<R> httpResponse = requestExecutor.execute(delegate);
        requestMetrics.countersByStatus.get(mappedStatus(httpResponse)).increment(); //按状态统计
        return httpResponse;
    } catch (Exception e) {
        requestMetrics.connectionErrors.increment(); //统计错误
        exceptionsMetric.count(e); //按异常名统计
        throw e;
    } finally {
        stopwatch.stop();
    }
}

AbstractJerseyEurekaHttpClient - 底层通过Jersery发送注册、心跳请求

public abstract class AbstractJerseyEurekaHttpClient implements EurekaHttpClient {
    protected final Client jerseyClient; //真正处理请求的Jersery客户端
    protected final String serviceUrl; //连接的Server端地址
    
    protected AbstractJerseyEurekaHttpClient(Client jerseyClient, String serviceUrl) {
        this.jerseyClient = jerseyClient;
        this.serviceUrl = serviceUrl;
        logger.debug("Created client for url: {}", serviceUrl);
    }
    
    /**
     * 注册方法
     */
    @Override
    public EurekaHttpResponse<Void> register(InstanceInfo info) {
        String urlPath = "apps/" + info.getAppName(); //请求Eureka Server的【/apps/应用名】接口地址
        ClientResponse response = null;
        try {
            Builder resourceBuilder = jerseyClient.resource(serviceUrl).path(urlPath).getRequestBuilder();
            addExtraHeaders(resourceBuilder);
            response = resourceBuilder
                    .header("Accept-Encoding", "gzip")
                    .type(MediaType.APPLICATION_JSON_TYPE)
                    .accept(MediaType.APPLICATION_JSON)
                    .post(ClientResponse.class, info);  //实例InstanceInfo数据，通过Post请求body发过去
            return anEurekaHttpResponse(response.getStatus()).headers(headersOf(response)).build();
        } 
        finally {
            if (logger.isDebugEnabled()) {
                logger.debug("Jersey HTTP POST {}/{} with instance {}; statusCode={}", serviceUrl, urlPath, info.getId(),
                        response == null ? "N/A" : response.getStatus());
            }
            if (response != null) {
                response.close();
            }
        }
    }
    
    ...省略

Eureka Server接收到的注册请求详情

经过上面的步骤，客户端已经可以通过Jersery发送Http请求给Eureka Server端注册，具体请求如下：

POST /eureka/apps/应用名 HTTP/1.1
Accept-Encoding: gzip
Content-Type: application/json
Accept: application/json
DiscoveryIdentity-Name: DefaultClient
DiscoveryIdentity-Version: 1.4
DiscoveryIdentity-Id: 192.168.70.132
Transfer-Encoding: chunked
Host: localhost:8001
Connection: Keep-Alive
User-Agent: Java-EurekaClient/v1.6.2

1a0
{"instance":{
"instanceId":"192.168.70.132:应用名:10001",
"hostName":"192.168.70.132",
"app":"应用名",
"ipAddr":"192.168.70.132",
"status":"UP",
"overriddenstatus":"UNKNOWN",
"port": { "(":10001, "@enabled" : "true" }, "securePort": { ")":443, "@enabled" : "false"},
"countryId":1,
"dataCenterInfo":{"@class":"com.netflix.appinfo.InstanceInfo$DefaultDataCenterInfo",
"name":"MyOwn"
}

Eureka Client注册流程总结

大体来说，Eureka Client的注册是由Spring Cloud的AutoServiceRegistration自动注册发起，在设置应用实例Instance初始状态为UP时，触发了InstanceInfoReplicator#onDemandUpdate()按需更新方法，将实例Instance信息通过DiscoveryClient注册到Eureka Server，期间经过了一些EurekaHttpClient的装饰类，实现了诸如定期重连、失败重试、注册重定向、统计收集Metrics信息等功能，最后由JerseryClient发送POST请求调用Eureka Server的【/eureka/apps/应用名】端点，请求体携带InstanceInfo实例信息，完成注册

EurekaAutoServiceRegistration#start()：实现Spring的SmartLifecycle，在Spring容器refresh()最后一步finishRefresh()会调用生命周期的start()方法
- EurekaServiceRegistry#register(EurekaRegistration)：使用服务注册器注册服务信息
  - ApplicationInfoManager#setInstanceStatus(初始状态)：应用实例信息管理器更新初始状态为 UP
    - StatusChangeListener：触发实例状态监听（此Listener是在DiscoveryClient#initScheduledTasks()方法中设置的）
      - InstanceInfoReplicator.onDemandUpdate()：实例状态复制器执行按需状态更新
        
        DiscoveryClient#register()： DiscoveryClient发起注册实例信息
        
        EurekaHttpClientDecorator#execute()：执行EurekaHttpClient的装饰类，实现其各自功能
        
        SessionedEurekaHttpClient：定时重连
        
        RetryableEurekaHttpClient：候选范围内失败重试
        
        RedirectingEurekaHttpClient：按Eureka Server端要求重定向到新Server
        
        MetricsCollectingEurekaHttpClient：统计收集执行情况
        
        JerseyApplicationClient#register()：封装注册请求数据
        
        JerseyClient发送Post注册请求

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原文地址：https://www.cnblogs.com/trust-freedom/p/10218145.html