spring cloud默认使用eureka来做服务治理。作为client的核心功能有两个：服务发现和服务注册。
通过查看spring-cloud-netflix-eureka-client下的类发现功能核心类是CloudEurekaClient，他继承了netflix的DiscoveryClient。
查看DiscoveryClient源码，注释中写明了DiscoveryClient的四个功能：
下面一个个来看这些功能是如何实现的。
1.d)服务发现getApplications()
@Override
public Applications getApplications() {
return localRegionApps.get();
}
localRegionApps是一个成员变量
private final AtomicReference localRegionApps = new AtomicReference();
所以一定是有其他地方先set了Applications，getApplications() 方法才能正确返回。通过搜索代码，发现为localRegionApps设值的地方代码基本相似：
EurekaHttpResponse httpResponse = clientConfig.getRegistryRefreshSingleVipAddress() == null
? eurekaTransport.queryClient.getApplications(remoteRegionsRef.get())
: eurekaTransport.queryClient.getVip(clientConfig.getRegistryRefreshSingleVipAddress(), remoteRegionsRef.get());
Applications serverApps = httpResponse.getEntity();
localRegionApps.set(this.filterAndShuffle(serverApps));
可以看出localRegionApps的来源，通过eurekaTransport.queryClient获得一个EurekaHttpResponse
那现在就有两个问题：1.这段代码是由谁执行的，2.eurekaTransport.queryClient是如何工作的。
通过call Hierarchy发现调用链的最开始：
class CacheRefreshThread implements Runnable {
public void run() {
refreshRegistry();
}
}
查询CacheRefreshThread被使用的地方：
private void initScheduledTasks(){...}
而initScheduledTasks()被调用的地方是DiscoveryClient的构造函数，所以这一切在DiscoveryClient被创建时就开始了。
查看initScheduledTasks()代码：
scheduler.schedule(
new TimedSupervisorTask(
"cacheRefresh",
scheduler,
cacheRefreshExecutor,
registryFetchIntervalSeconds,
TimeUnit.SECONDS,
expBackOffBound,
new CacheRefreshThread()
),
registryFetchIntervalSeconds, TimeUnit.SECONDS);
...
scheduler.schedule(
new TimedSupervisorTask(
"heartbeat",
scheduler,
heartbeatExecutor,
renewalIntervalInSecs,
TimeUnit.SECONDS,
expBackOffBound,
new HeartbeatThread()
),
renewalIntervalInSecs, TimeUnit.SECONDS);
可以看出这是由线程池周期执行的任务，那现在就有两个地方：1.执行的线程池scheduler、cacheRefreshExecutor、cacheRefreshExecutor 2.被执行的回调：HeartbeatThread、CacheRefreshThread。CacheRefreshThread上面知道了是用来设置localRegionApps的，HeartbeatThread从注释来看是用来b)服务续约的:
/**
* The heartbeat task that renews the lease in the given intervals.
*/
private class HeartbeatThread implements Runnable {
public void run() {
if (renew()) {
lastSuccessfulHeartbeatTimestamp = System.currentTimeMillis();
}
}
}
找到三个线程池的初始化代码：
scheduler = Executors.newScheduledThreadPool(2,
new ThreadFactoryBuilder()
.setNameFormat("DiscoveryClient-%d")
.setDaemon(true)
.build());
heartbeatExecutor = new ThreadPoolExecutor(
1, clientConfig.getHeartbeatExecutorThreadPoolSize(), 0, TimeUnit.SECONDS,
new SynchronousQueue(),
new ThreadFactoryBuilder()
.setNameFormat("DiscoveryClient-HeartbeatExecutor-%d")
.setDaemon(true)
.build()
); // use direct handoff
cacheRefreshExecutor = new ThreadPoolExecutor(
1, clientConfig.getCacheRefreshExecutorThreadPoolSize(), 0, TimeUnit.SECONDS,
new SynchronousQueue(),
new ThreadFactoryBuilder()
.setNameFormat("DiscoveryClient-CacheRefreshExecutor-%d")
.setDaemon(true)
.build()
);
可以看出eureka默认用了两个线程来做任务调度，两个任务各有一个专属线程独自负责任务的实际执行。这种依靠线程池的隔离策略，在netflix组件中用到的地方有很多。
找到了CacheRefreshThread是被谁调用的，接着分析eurekaTransport.queryClient是如何工作的。eurekaTransport.queryClient的服务发现方法：getApplications()
@Override
public EurekaHttpResponse getApplications(String... regions) {
return getApplicationsInternal("apps/", regions);
}
private EurekaHttpResponse getApplicationsInternal(String urlPath,
String[] regions) {
String url = serviceUrl + urlPath;
if (regions != null && regions.length > 0)
urlPath = (urlPath.contains("?") ? "&" : "?") + "regions="
+ StringUtil.join(regions);
ResponseEntity response = restTemplate.exchange(url,
HttpMethod.GET, null, EurekaApplications.class);
return anEurekaHttpResponse(response.getStatusCodeValue(),
response.getStatusCode().value() == HttpStatus.OK.value()
&& response.hasBody() ? (Applications) response.getBody() : null)
.headers(headersOf(response)).build();
}
可以看出是向serviceUrl进行http请求获得相关数据的。而serviceUrl是DiscoveryClient在初始化eurekaTransport时传入的。DiscoveryClient初始化时主要有三个参数：ApplicationInfoManager、EurekaClientConfig、ApplicationEventPublisher，通过查看配置类EurekaClientAutoConfiguration可以看到，在SpringCloud中创建的其实是DiscoveryClient的子类CloudEurekaClient:
@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);
}
ApplicationInfoManager、EurekaClientConfig是自动注入的ApplicationInfoManager manager, EurekaClientConfig config而ApplicationEventPublisher是容器上下文ApplicationContext。继续查找代码可以发现：
@Bean
@ConditionalOnMissingBean(value = ApplicationInfoManager.class, search = SearchStrategy.CURRENT)
public ApplicationInfoManager eurekaApplicationInfoManager(
EurekaInstanceConfig config) {
InstanceInfo instanceInfo = new InstanceInfoFactory().create(config);
return new ApplicationInfoManager(config, instanceInfo);
}
@ConfigurationProperties(EurekaClientConfigBean.PREFIX)
public class EurekaClientConfigBean implements EurekaClientConfig {...}
ApplicationInfoManager来源于EurekaInstanceConfig ，而EurekaInstanceConfig 其实是由spring创建的EurekaInstanceConfigBean，通过@ConfigurationProperties("eureka.instance")收集了配置文件中的eureka.instance前缀的配置。
EurekaClientConfig其实是由spring实现并创建的EurekaClientConfigBean，通过@ConfigurationProperties("eureka.client")收集了配置文件中的eureka.client前缀的配置。
到这可以看出来springcloud做的工作其实就是收集配置并用来初始化DiscoveryClient。其实spring本质上作为一个beanfactory，工作就是创建并管理bean的生命周期。
2.ab)注册和续约
注册方法：register()，查看调用发现在DiscoveryClient的构造函数和renew()函数中被调用了。查看代码：
/**
* Register with the eureka service by making the appropriate REST call.
*/
boolean register() throws Throwable {
logger.info(PREFIX + "{}: registering service...", appPathIdentifier);
EurekaHttpResponse httpResponse;
try {
httpResponse = eurekaTransport.registrationClient.register(instanceInfo);
} catch (Exception e) {
logger.warn(PREFIX + "{} - registration failed {}", appPathIdentifier, e.getMessage(), e);
throw e;
}
if (logger.isInfoEnabled()) {
logger.info(PREFIX + "{} - registration status: {}", appPathIdentifier, httpResponse.getStatusCode());
}
return httpResponse.getStatusCode() == 204;
}
其实是通过eurekaTransport.registrationClient.register(instanceInfo)和server进行了一次rest-http调用。instanceInfo是这个客户端本身。
续约方法：renew()，上面说到了续约方法被放入new HeartbeatThread()中，被线程池周期执行。
/**
* Renew with the eureka service by making the appropriate REST call
*/
boolean renew() {
EurekaHttpResponse httpResponse;
try {
httpResponse = eurekaTransport.registrationClient.sendHeartBeat(instanceInfo.getAppName(), instanceInfo.getId(), instanceInfo, null);
logger.debug(PREFIX + "{} - Heartbeat status: {}", appPathIdentifier, httpResponse.getStatusCode());
if (httpResponse.getStatusCode() == 404) {
REREGISTER_COUNTER.increment();
logger.info(PREFIX + "{} - Re-registering apps/{}", appPathIdentifier, instanceInfo.getAppName());
long timestamp = instanceInfo.setIsDirtyWithTime();
boolean success = register();
if (success) {
instanceInfo.unsetIsDirty(timestamp);
}
return success;
}
return httpResponse.getStatusCode() == 200;
} catch (Throwable e) {
logger.error(PREFIX + "{} - was unable to send heartbeat!", appPathIdentifier, e);
return false;
}
}
可以看出也是通过 eurekaTransport.registrationClient进行一次http请求，如果请求失败，则调用一次register()从新注册，如果都失败则返回false。
3.c)注销
@PreDestroy
@Override
public synchronized void shutdown() {
if (isShutdown.compareAndSet(false, true)) {
logger.info("Shutting down DiscoveryClient ...");
if (statusChangeListener != null && applicationInfoManager != null) {
applicationInfoManager.unregisterStatusChangeListener(statusChangeListener.getId());
}
cancelScheduledTasks(); //取消上面所说的两个周期任务，并关闭线程池
// If APPINFO was registered
if (applicationInfoManager != null
&& clientConfig.shouldRegisterWithEureka()
&& clientConfig.shouldUnregisterOnShutdown()) {
applicationInfoManager.setInstanceStatus(InstanceStatus.DOWN);
unregister();
}
if (eurekaTransport != null) {
eurekaTransport.shutdown();
}
heartbeatStalenessMonitor.shutdown();
registryStalenessMonitor.shutdown();
logger.info("Completed shut down of DiscoveryClient");
}
}
可以看出主要工作有两部分，1.回收资源 2.调用unregister()通知eureka-server注销。查看unregister()代码：
void unregister() {
// It can be null if shouldRegisterWithEureka == false
if(eurekaTransport != null && eurekaTransport.registrationClient != null) {
try {
logger.info("Unregistering ...");
EurekaHttpResponse httpResponse = eurekaTransport.registrationClient.cancel(instanceInfo.getAppName(), instanceInfo.getId());
logger.info(PREFIX + "{} - deregister status: {}", appPathIdentifier, httpResponse.getStatusCode());
} catch (Exception e) {
logger.error(PREFIX + "{} - de-registration failed{}", appPathIdentifier, e.getMessage(), e);
}
}
}
可以看出这里也是通过eurekaTransport.registrationClient进行了一次http请求。
综上所述，eureka的主要靠独立的线程池执行周期性任务来执行http请求来进行服务发现的更新和服务续约。而spring所扮演的角色只是DiscoveryClient的创建和管理者，并没有改变eureka的内部功能。我们也可以通过自己创建和管理DiscoveryClient在非springcloud项目中独立地使用eureka，eureka功能完备，自己集成相对简单。总之，就是从server通过http请求获得服务数据而已，可以自己通过浏览器访问：http://localhost:8761/eureka/apps看看数据