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说起微服务框架，可能大家首先想到的就是Spring Cloud的大家族了。微服务简单理解就是很小、功能相对集中的服务。在微服务的思想下，原有功能都集中在一起实现的业务，现在拆分出来很多的服务模块，那服务之间的互相调用就不可避免，同时服务的高可用也要能够有所保证。要解决上述需要，少不了微服务中至关重要的内容，即服务治理。
服务治理包括三大组件：服务注册中心、服务的注册、服务的发现。举个例子：就好比我们租房，房东呢，想要把房子租出去；房客想要租房。房东把自己的房子信息挂在租房平台上，房客通过租房平台了解到出租房屋的信息，进而可以给房东打电话租房。
这里呢租房平台就相当于服务注册中心，房东在平台上登记出租房屋相当于房屋的注册，房客从平台上获取出租信息相当于服务的发现。进入到程序的世界，Spring Cloud Eureka组件就提供了服务治理的功能。Eureka的服务端就是注册中心；客户端既可以是消费者也可以是提供者，承担着服务的注册和发现。
简单总结各种解决要提供的主要功能：
服务端：
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接收注册。
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接收续期。
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提供服务下线。
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提供服务列表
客户端：
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注册服务。
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续期。
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获取服务列表。
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维持心跳。
大体上了解之后，我们就来探索一下在底层是如何实现的。
搭建Eureka环境（单实例环境）
基于Springboot我们可以轻松的搭建Eureka的客户端和服务端。
一、服务端搭建：
引入依赖：
<dependency><groupId>org.springframework.cloud</groupId><artifactId>spring-cloud-starter-netflix-eureka-server</artifactId></dependency>
主类上添加注解：
@SpringBootApplication@EnableEurekaServerpublic class EurekaServer_6001 {public static void main(String[] args) {SpringApplication.run(EurekaServer_6001.class, args);}}
在application.properties中相关Eureka的配置：
eureka:instance:hostname: eurekapeer1 #eureka的注册实例名称client:register-with-eureka: false # 服务注册，不让自己注册到eureka中fetch-registry: false # 服务的发现 ， 自己不从服务中发现信息service-url:defaultZone: http://localhost:6001/eureka/
启动服务，使用浏览器访问配置的端口，可以看到Eureka的服务端的页面：
二、客户端搭建：
引入依赖：
<dependency><groupId>org.springframework.cloud</groupId><artifactId>spring-cloud-starter-netflix-eureka-client</artifactId></dependency>
主类添加注解：
@EnableDiscoveryClient@SpringBootApplicationpublic class ManagerZuulFilter {public static void main(String[] args) {SpringApplication.run(ManagerZuulFilter.class,args);}}
在application.properties中相关Eureka的配置：
eureka:client:register-with-eureka: true #服务注册开关fetch-registry: true #服务发现开关serviceUrl:defaultZone: http://localhost:6001/eureka/
启动服务，刷新服务端Eureka的页面，看到客户端服务注册到Eureka的页面。
关闭客户端服务，服务端Eureka的管理页面，客户端的服务从列表中下线。
源码详解
一、客户端源码分析
客户端服务的治理包括：服务的注册、服务的续期、服务的下线、服务列表的发现等。
Eureka的注册和发现的内容在DiscoveryClient类中。
@InjectDiscoveryClient(ApplicationInfoManager applicationInfoManager, EurekaClientConfig config, AbstractDiscoveryClientOptionalArgs args, Provider<BackupRegistry> backupRegistryProvider) {//...省略一些初始化的内容//不注册到Eureka server并且不从Eureka server中获取服务列表，相当于单节点的Eureka服务端if (!config.shouldRegisterWithEureka() && !config.shouldFetchRegistry()) {logger.info(\"Client configured to neither register nor query for data.\");this.scheduler = null;this.heartbeatExecutor = null;this.cacheRefreshExecutor = null;this.eurekaTransport = null;this.instanceRegionChecker = new InstanceRegionChecker(new PropertyBasedAzToRegionMapper(config), this.clientConfig.getRegion());DiscoveryManager.getInstance().setDiscoveryClient(this);DiscoveryManager.getInstance().setEurekaClientConfig(config);this.initTimestampMs = System.currentTimeMillis();logger.info(\"Discovery Client initialized at timestamp {} with initial instances count: {}\", this.initTimestampMs, this.getApplications().size());} else {try {//注册到Eureka并且从Eureka中获取服务列表的逻辑处理this.scheduler = Executors.newScheduledThreadPool(2, (new ThreadFactoryBuilder()).setNameFormat(\"DiscoveryClient-%d\").setDaemon(true).build());//创建心跳定时执行任务this.heartbeatExecutor = new ThreadPoolExecutor(1, this.clientConfig.getHeartbeatExecutorThreadPoolSize(), 0L, TimeUnit.SECONDS, new SynchronousQueue(), (new ThreadFactoryBuilder()).setNameFormat(\"DiscoveryClient-HeartbeatExecutor-%d\").setDaemon(true).build());//创建刷新缓存定时执行任务this.cacheRefreshExecutor = new ThreadPoolExecutor(1, this.clientConfig.getCacheRefreshExecutorThreadPoolSize(), 0L, TimeUnit.SECONDS, new SynchronousQueue(), (new ThreadFactoryBuilder()).setNameFormat(\"DiscoveryClient-CacheRefreshExecutor-%d\").setDaemon(true).build());//注册前处理器，可以在服务注册到Eureka之前做些操作if (this.preRegistrationHandler != null) {this.preRegistrationHandler.beforeRegistration();}//是否强制初始阶段就注册到Eurekaif (this.clientConfig.shouldRegisterWithEureka() && this.clientConfig.shouldEnforceRegistrationAtInit()) {try {if (!this.register()) {throw new IllegalStateException(\"Registration error at startup. Invalid server response.\");}} catch (Throwable var8) {logger.error(\"Registration error at startup: {}\", var8.getMessage());throw new IllegalStateException(var8);}}//执行定时任务this.initScheduledTasks();try {Monitors.registerObject(this);} catch (Throwable var7) {logger.warn(\"Cannot register timers\", var7);}DiscoveryManager.getInstance().setDiscoveryClient(this);DiscoveryManager.getInstance().setEurekaClientConfig(config);this.initTimestampMs = System.currentTimeMillis();logger.info(\"Discovery Client initialized at timestamp {} with initial instances count: {}\", this.initTimestampMs, this.getApplications().size());}}//初始化定时任务private void initScheduledTasks() {int renewalIntervalInSecs;int expBackOffBound;//定时获取服务列表任务开启if (this.clientConfig.shouldFetchRegistry()) {renewalIntervalInSecs = this.clientConfig.getRegistryFetchIntervalSeconds();expBackOffBound = this.clientConfig.getCacheRefreshExecutorExponentialBackOffBound();this.scheduler.schedule(new TimedSupervisorTask(\"cacheRefresh\", this.scheduler, this.cacheRefreshExecutor, renewalIntervalInSecs, TimeUnit.SECONDS, expBackOffBound, new DiscoveryClient.CacheRefreshThread()), (long)renewalIntervalInSecs, TimeUnit.SECONDS);}//注册到Eureka，心跳定时任务开始。要让Eureka Server知道客户端还活着if (this.clientConfig.shouldRegisterWithEureka()) {renewalIntervalInSecs = this.instanceInfo.getLeaseInfo().getRenewalIntervalInSecs();expBackOffBound = this.clientConfig.getHeartbeatExecutorExponentialBackOffBound();logger.info(\"Starting heartbeat executor: renew interval is: {}\", renewalIntervalInSecs);this.scheduler.schedule(new TimedSupervisorTask(\"heartbeat\", this.scheduler, this.heartbeatExecutor, renewalIntervalInSecs, TimeUnit.SECONDS, expBackOffBound, new DiscoveryClient.HeartbeatThread()), (long)renewalIntervalInSecs, TimeUnit.SECONDS);this.instanceInfoReplicator = new InstanceInfoReplicator(this, this.instanceInfo, this.clientConfig.getInstanceInfoReplicationIntervalSeconds(), 2);this.statusChangeListener = new StatusChangeListener() {public String getId() {return \"statusChangeListener\";}public void notify(StatusChangeEvent statusChangeEvent) {if (InstanceStatus.DOWN != statusChangeEvent.getStatus() && InstanceStatus.DOWN != statusChangeEvent.getPreviousStatus()) {DiscoveryClient.logger.info(\"Saw local status change event {}\", statusChangeEvent);} else {DiscoveryClient.logger.warn(\"Saw local status change event {}\", statusChangeEvent);}DiscoveryClient.this.instanceInfoReplicator.onDemandUpdate();}};if (this.clientConfig.shouldOnDemandUpdateStatusChange()) {this.applicationInfoManager.registerStatusChangeListener(this.statusChangeListener);}this.instanceInfoReplicator.start(this.clientConfig.getInitialInstanceInfoReplicationIntervalSeconds());} else {logger.info(\"Not registering with Eureka server per configuration\");}}//注册发送，REST请求，注册服务到Eureka Server中。boolean register() throws Throwable {logger.info(\"DiscoveryClient_{}: registering service...\", this.appPathIdentifier);EurekaHttpResponse httpResponse;try {httpResponse = this.eurekaTransport.registrationClient.register(this.instanceInfo);} catch (Exception var3) {logger.warn(\"DiscoveryClient_{} - registration failed {}\", new Object[]{this.appPathIdentifier, var3.getMessage(), var3});throw var3;}if (logger.isInfoEnabled()) {logger.info(\"DiscoveryClient_{} - registration status: {}\", this.appPathIdentifier, httpResponse.getStatusCode());}return httpResponse.getStatusCode() == 204;}
查看线程，发现确实有上述几类线程。
二、服务端源码分析
服务端的主要逻辑就是：接收客户端的注册/续期/下线等请求、提供服务列表、维护注册信息等。
#eureka server端的功能代码主要是在eureka-core包中主要涉及类和接口有：
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EurekaServerBootstrap
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InstanceRegistry
下面主要为大家讲一下客户端注册在服务端如何处理：eureka-server包下org.springframework.cloud.netflix.eureka.server.EurekaServerBootstrap初始化eurekserver相关的内容：
public class EurekaServerBootstrap {public void contextInitialized(ServletContext context) {try {this.initEurekaEnvironment();//初始化eureka的部署环境，默认testthis.initEurekaServerContext();//初始化eureka server的上下文context.setAttribute(EurekaServerContext.class.getName(), this.serverContext);} catch (Throwable var3) {log.error(\"Cannot bootstrap eureka server :\", var3);throw new RuntimeException(\"Cannot bootstrap eureka server :\", var3);}}}
eureka-server包下的org.springframework.cloud.netflix.eureka.server.InstanceRegistry中方法为跟服务治理有关系的内容。
public class InstanceRegistry extends PeerAwareInstanceRegistryImpl implements ApplicationContextAware {public void register(InstanceInfo info, int leaseDuration, boolean isReplication) {this.handleRegistration(info, leaseDuration, isReplication);super.register(info, leaseDuration, isReplication);//带有租期的注册，默认租期90s}}public abstract class AbstractInstanceRegistry implements InstanceRegistry {//存放注册到eureka server上的服务的数据接口//第一层的String key是指服务名//第二层的String key是指每个服务的instanceId：默认hostmame:applicationName:port//最里面的InstanceInfo就是一个服务的详细信息private final ConcurrentHashMap<String, Map<String, Lease<InstanceInfo>>> registry = new ConcurrentHashMap();//客户端注册到服务端的具体处理过程public void register(InstanceInfo registrant, int leaseDuration, boolean isReplication) {try {this.read.lock();//该Map存储的就是eureka server上的服务信息//根据应用名获取该应用名下的列表Map<String, Lease<InstanceInfo>> gMap = (Map)this.registry.get(registrant.getAppName());EurekaMonitors.REGISTER.increment(isReplication);if (gMap == null) {//没有注册过，则新创建ConcurrentHashMap<String, Lease<InstanceInfo>> gNewMap = new ConcurrentHashMap();gMap = (Map)this.registry.putIfAbsent(registrant.getAppName(), gNewMap);if (gMap == null) {gMap = gNewMap;}}//通过instanceId获取具体的服务Lease<InstanceInfo> existingLease = (Lease)((Map)gMap).get(registrant.getId());if (existingLease != null && existingLease.getHolder() != null) {Long existingLastDirtyTimestamp = ((InstanceInfo)existingLease.getHolder()).getLastDirtyTimestamp();Long registrationLastDirtyTimestamp = registrant.getLastDirtyTimestamp();logger.debug(\"Existing lease found (existing={}, provided={}\", existingLastDirtyTimestamp, registrationLastDirtyTimestamp);if (existingLastDirtyTimestamp > registrationLastDirtyTimestamp) {logger.warn(\"There is an existing lease and the existing lease\'s dirty timestamp {} is greater than the one that is being registered {}\", existingLastDirtyTimestamp, registrationLastDirtyTimestamp);logger.warn(\"Using the existing instanceInfo instead of the new instanceInfo as the registrant\");registrant = (InstanceInfo)existingLease.getHolder();}} else {synchronized(this.lock) {if (this.expectedNumberOfRenewsPerMin > 0) {this.expectedNumberOfRenewsPerMin += 2;this.numberOfRenewsPerMinThreshold = (int)((double)this.expectedNumberOfRenewsPerMin * this.serverConfig.getRenewalPercentThreshold());}}logger.debug(\"No previous lease information found; it is new registration\");}//创建新的租期信息Lease<InstanceInfo> lease = new Lease(registrant, leaseDuration);if (existingLease != null) {lease.setServiceUpTimestamp(existingLease.getServiceUpTimestamp());}//注册服务或者更新服务((Map)gMap).put(registrant.getId(), lease);synchronized(this.recentRegisteredQueue) {this.recentRegisteredQueue.add(new Pair(System.currentTimeMillis(), registrant.getAppName() + \"(\" + registrant.getId() + \")\"));}if (!InstanceStatus.UNKNOWN.equals(registrant.getOverriddenStatus())) {logger.debug(\"Found overridden status {} for instance {}. Checking to see if needs to be add to the overrides\", registrant.getOverriddenStatus(), registrant.getId());if (!this.overriddenInstanceStatusMap.containsKey(registrant.getId())) {logger.info(\"Not found overridden id {} and hence adding it\", registrant.getId());this.overriddenInstanceStatusMap.put(registrant.getId(), registrant.getOverriddenStatus());}}InstanceStatus overriddenStatusFromMap = (InstanceStatus)this.overriddenInstanceStatusMap.get(registrant.getId());if (overriddenStatusFromMap != null) {logger.info(\"Storing overridden status {} from map\", overriddenStatusFromMap);registrant.setOverriddenStatus(overriddenStatusFromMap);}InstanceStatus overriddenInstanceStatus = this.getOverriddenInstanceStatus(registrant, existingLease, isReplication);registrant.setStatusWithoutDirty(overriddenInstanceStatus);if (InstanceStatus.UP.equals(registrant.getStatus())) {lease.serviceUp();}registrant.setActionType(ActionType.ADDED);this.recentlyChangedQueue.add(new AbstractInstanceRegistry.RecentlyChangedItem(lease));registrant.setLastUpdatedTimestamp();this.invalidateCache(registrant.getAppName(), registrant.getVIPAddress(), registrant.getSecureVipAddress());logger.info(\"Registered instance {}/{} with status {} (replication={})\", new Object[]{registrant.getAppName(), registrant.getId(), registrant.getStatus(), isReplication});} finally {this.read.unlock();}}}
配置类
常用配置说明：客户端、服务端、以及实例配置可以直接在配置类的属性中查到：
客户端配置：前缀为eureka.client
org.springframework.cloud.netflix.eureka.EurekaClientConfigBean服务端配置：前缀为eureka.server
org.springframework.cloud.netflix.eureka.server.EurekaServerConfigBean实例配置：前缀eureka.instance
org.springframework.cloud.netflix.eureka.EurekaInstanceConfigBean常用配置
注册中心配置：
#关闭注册中心的保护机制，Eureka 会统计15分钟之内心跳失败的比例低于85%将会触发保护机制，不剔除服务提供者eureka.server.enable-self-preservation = false
服务实例配置：
#不使用主机名来定义注册中心的地址，而使用IP地址的形式eureka.instance.prefer-ip-address = true
服务注册类配置：
#获取服务注册列表eureka.client.fetch-registery = true#注册服务到Eureka Servereureka.client.register-with-eureka = true#从Eureka服务器端获取注册信息的间隔时间，单位：秒eureka.client.registery-fetch-interval-seconds = 30#读取EurekaServer信息的超时时间，单位：秒eureka.client.eureka-server-read-timeout-seconds = 8#连接EurekaServer的超时时间，单位：秒eureka.client.eureka-server-connect-timeout-seconds = 5
通用配置：
eureka.client.service-url.defaultZone = http://localhost:8761/eurekaSpring cloud Eureka中还提供了其他的一些功能，比如认证和限流的内容，有兴趣的可以了解一下。除Eureka提供的服务治理外，还有其他的框架也可以实现服务治理，如Dubbo+Zookeeper，有兴趣可以对比看一下。
框架可以算是一种工具，了解工具的内部是为了更好的让工具为我们服务。优秀框架的设计思想是我们要学习的重点，抽丝剥茧之后，原理其实很基础。
往期回顾
1、CDN加速原理
2、mybatis框架的插件机制
3、玩转Selenium——Web页面自动化实战
4、自定义控制器开发（一）
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作者｜董月
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