在上篇文章中,我们简单介绍了EurekaServer自动装配及启动流程解析，本篇文章则继续研究EurekaClient的相关代码
老规矩，先看spring.factories文件，其中引入了一个配置类EurekaDiscoveryClientConfigServiceBootstrapConfiguration
@ConditionalOnClass(ConfigServicePropertySourceLocator.class) @ConditionalOnProperty(value = "spring.cloud.config.discovery.enabled", matchIfMissing = false) @Configuration @Import({ EurekaDiscoveryClientConfiguration.class, EurekaClientAutoConfiguration.class }) public class EurekaDiscoveryClientConfigServiceBootstrapConfiguration { } 上方两个注解则是这个配置类是否能够开启的条件，这里就不再展开，直接看它引入的配置类吧
EurekaDiscoveryClientConfiguration
- 细心的读者可能会发现这里又注册了一个
Marker类，可以猜测也是某个地方的开关 EurekaClientConfigurationRefresher这个类看名字就知道这是当配置被动态刷新时的一个处理器，这里也不再展开了EurekaHealthCheckHandlerConfiguration这里面注册了一个Eureka健康检查的处理类，这个健康检查相关的原理分析可以参考这篇文章：SpringBoot健康检查实现原理
EurekaClientAutoConfiguration
这个类里面全是重点，也是我们本文的核心
注解
@Configuration @EnableConfigurationProperties @ConditionalOnClass(EurekaClientConfig.class) @Import(DiscoveryClientOptionalArgsConfiguration.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", "org.springframework.cloud.netflix.eureka.EurekaDiscoveryClientConfiguration", "org.springframework.cloud.client.serviceregistry.AutoServiceRegistrationAutoConfiguration"}) 首先可以看到这个类一共包含这些注解，我们来一一解析比较重要的几个注解吧
@Import(DiscoveryClientOptionalArgsConfiguration.class)
引入了两个bean，RestTemplateDiscoveryClientOptionalArgs和MutableDiscoveryClientOptionalArgs ，这两个类的作用暂且不说
@ConditionalOnBean(EurekaDiscoveryClientConfiguration.Marker.class)
刚才说的Marker类的作用出来了
@AutoConfigureBefore
既然必须在这三个类完成自动装配之后才能进行装配，那就代表着这三个类肯定大有用途，研究一下吧
NoopDiscoveryClientAutoConfiguration
故名思意，负责服务发现的类，咱们重点关注一下其中的几个方法
init
@PostConstruct public void init() { String host = "localhost"; try { host = InetAddress.getLocalHost().getHostName(); } catch (UnknownHostException e) { log.warn("Cannot get host info: (" + e.getMessage() + ")"); } int port = findPort(); this.serviceInstance = new DefaultServiceInstance( this.environment.getProperty("spring.application.name", "application"), host, port, false); } 这里构造了一个DefaultServiceInstance对象，这个对象包含了当前项目的ip+端口+项目名称 2. 注入beanNoopDiscoveryClient
@Bean public DiscoveryClient discoveryClient() { return new NoopDiscoveryClient(this.serviceInstance); } 再深入看一下这个类
public class NoopDiscoveryClient implements DiscoveryClient { public NoopDiscoveryClient(ServiceInstance instance) { } @Override public String description() { return "Spring Cloud No-op DiscoveryClient"; } @Override public List<ServiceInstance> getInstances(String serviceId) { return Collections.emptyList(); } @Override public List<String> getServices() { return Collections.emptyList(); } } 这个类包含了获取当前实例以及当前服务的方法，但是返回的都是空，那么是不是会在后面的某个地方被覆盖呢？
CommonsClientAutoConfiguration
进去深入了解一下，哎哟，注册了几个bean：DiscoveryClientHealthIndicator、DiscoveryCompositeHealthIndicator。原来是健康检查相关的东西，那就忽略了
ServiceRegistryAutoConfiguration
这个配置类中主要注册了一个bean：ServiceRegistryEndpoint这个类主要是对外提供对与Eureka状态的检查
@ReadOperation public ResponseEntity getStatus() { if (this.registration == null) { return ResponseEntity.status(HttpStatus.NOT_FOUND).body("no registration found"); } return ResponseEntity.ok().body(this.serviceRegistry.getStatus(this.registration)); } 而Eureka的状态则是通过serviceRegistry对象获取的，这个对象会再下方详细分析
注册bean
接着来看这个类注入的几个bean
EurekaClientConfigBean
@Bean @ConditionalOnMissingBean(value = EurekaClientConfig.class, search = SearchStrategy.CURRENT) public EurekaClientConfigBean eurekaClientConfigBean(ConfigurableEnvironment env) { EurekaClientConfigBean client = new EurekaClientConfigBean(); if ("bootstrap".equals(this.env.getProperty("spring.config.name"))) { client.setRegisterWithEureka(false); } return client; } 这个bean中包含了eureka.client.xxx系列的一些配置，详细的配置信息可以参考这里：https://github.com/shiyujun/syj-study-demo/blob/master/src/main/java/cn/shiyujun/EurekaConfig.md
EurekaInstanceConfigBean
这个bean中主要是包含eureka实例（eureka.instance.xxx系列）的一些配置信息，详细的配置信息同上
RefreshableEurekaClientConfiguration.DiscoveryClient
@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); } 其中CloudEurekaClient是DiscoveryClient的子类，而DiscoveryClient则是EurekaClient的核心类
public CloudEurekaClient(ApplicationInfoManager applicationInfoManager, EurekaClientConfig config, AbstractDiscoveryClientOptionalArgs<?> args, ApplicationEventPublisher publisher) { //这里会调用父类DiscoveryClient的构造方法 super(applicationInfoManager, config, args); this.applicationInfoManager = applicationInfoManager; this.publisher = publisher; this.eurekaTransportField = ReflectionUtils.findField(DiscoveryClient.class, "eurekaTransport"); ReflectionUtils.makeAccessible(this.eurekaTransportField); } 父类的构造方法中执行的代码块比较长，一些赋值操作等就忽略了,这里只摘出比较重要的部分
- 初始化拉取监控和心跳监控
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; } 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; } - 当当前实例不需要注册到EurekaServer时，构造方法走到这里就结束了
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()); 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; } - 初始化心跳线程和刷新线程以及它们的调度器
try { scheduler = Executors.newScheduledThreadPool(2, new ThreadFactoryBuilder() .setNameFormat("DiscoveryClient-%d") .setDaemon(true) .build()); heartbeatExecutor = new ThreadPoolExecutor( 1, clientConfig.getHeartbeatExecutorThreadPoolSize(), 0, TimeUnit.SECONDS, new SynchronousQueue<Runnable>(), new ThreadFactoryBuilder() .setNameFormat("DiscoveryClient-HeartbeatExecutor-%d") .setDaemon(true) .build() ); cacheRefreshExecutor = new ThreadPoolExecutor( 1, clientConfig.getCacheRefreshExecutorThreadPoolSize(), 0, TimeUnit.SECONDS, new SynchronousQueue<Runnable>(), new ThreadFactoryBuilder() .setNameFormat("DiscoveryClient-CacheRefreshExecutor-%d") .setDaemon(true) .build() ); - 从EurekaServer拉取注册信息
if (clientConfig.shouldFetchRegistry() && !fetchRegistry(false)) { fetchRegistryFromBackup(); } 这里fetchRegistry是第一次拉取注册信息，如果拉取不成功的话则执行fetchRegistryFromBackup从备份注册中心获取，同样，拉取的信息会放在之后的文章中
- 注册之前的扩展点
if (this.preRegistrationHandler != null) { this.preRegistrationHandler.beforeRegistration(); } 这里是个空的实现，可以通过实现PreRegistrationHandler接口做些什么操作 6. 向EurekaServer发起注册
if (clientConfig.shouldRegisterWithEureka() && clientConfig.shouldEnforceRegistrationAtInit()) { try { if (!register() ) { throw new IllegalStateException("Registration error at startup. Invalid server response."); } } catch (Throwable th) { logger.error("Registration error at startup: {}", th.getMessage()); throw new IllegalStateException(th); } } 注册方法为register，同样这里先不展开 7. 初始化几个定时任务
initScheduledTasks(); private void initScheduledTasks() { // 从 EurekaServer 拉取注册信息 if (clientConfig.shouldFetchRegistry()) { int registryFetchIntervalSeconds = clientConfig.getRegistryFetchIntervalSeconds(); int expBackOffBound = clientConfig.getCacheRefreshExecutorExponentialBackOffBound(); scheduler.schedule( new TimedSupervisorTask( "cacheRefresh", scheduler, cacheRefreshExecutor, registryFetchIntervalSeconds, TimeUnit.SECONDS, expBackOffBound, new CacheRefreshThread() ), registryFetchIntervalSeconds, TimeUnit.SECONDS); } // 向 EurekaServer 发送续租心跳 if (clientConfig.shouldRegisterWithEureka()) { int renewalIntervalInSecs = instanceInfo.getLeaseInfo().getRenewalIntervalInSecs(); int expBackOffBound = clientConfig.getHeartbeatExecutorExponentialBackOffBound(); logger.info("Starting heartbeat executor: " + "renew interval is: " + renewalIntervalInSecs); scheduler.schedule( new TimedSupervisorTask( "heartbeat", scheduler, heartbeatExecutor, renewalIntervalInSecs, TimeUnit.SECONDS, expBackOffBound, new HeartbeatThread() ), renewalIntervalInSecs, TimeUnit.SECONDS); instanceInfoReplicator = new InstanceInfoReplicator( this, instanceInfo, clientConfig.getInstanceInfoReplicationIntervalSeconds(), 2); 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()) { logger.warn("Saw local status change event {}", statusChangeEvent); } else { logger.info("Saw local status change event {}", statusChangeEvent); } instanceInfoReplicator.onDemandUpdate(); } }; if (clientConfig.shouldOnDemandUpdateStatusChange()) { //注册状态监听器 applicationInfoManager.registerStatusChangeListener(statusChangeListener); } instanceInfoReplicator.start(clientConfig.getInitialInstanceInfoReplicationIntervalSeconds()); } else { logger.info("Not registering with Eureka server per configuration"); } } 至此，EurekaClient的自动装配与启动流程就解析完毕了
来源：oschina
链接：https://my.oschina.net/u/3459011/blog/3118515