【SpringCloud技术专题】「Eureka源码分析」从源码层面让你认识Eureka工作流程和运作机制（下）

承接上文的对应的Eureka的上篇介绍，我们开始介绍，详见 [【SpringCloud技术专题】「Eureka源码分析」从源码层面让你认识Eureka工作流程和运作机制（上）]

原理回顾

Eureka Server 提供服务注册服务，各个节点启动后，会在Eureka Server中进行注册，这样Eureka Server中的服务注册表中将会存储所有可用服务节点的信息，服务节点的信息可以在界面中直观的看到。
Eureka Client 是一个Java 客户端，用于简化与Eureka Server的交互，客户端同时也具备一个内置的、使用轮询负载算法的负载均衡器。
在应用启动后，将会向Eureka Server发送心跳(默认周期为30秒)，如果Eureka Server在多个心跳周期（默认3个心跳周期=90秒）没有收到某个节点的心跳，Eureka Server将会从服务注册表中把这个服务节点移除。
高可用情况下的：Eureka Server之间将会通过复制的方式完成数据的同步；
Eureka Client具有缓存的机制，即使所有的Eureka Server 都挂掉的话，客户端依然可以利用缓存中的信息消费其它服务的API；

EurekaServer 启动流程分析

EurekaServer 处理服务注册、集群数据复制

EurekaClient 是如何注册到 EurekaServer 的？

刚才在org.springframework.cloud.netflix.eureka.server.InstanceRegistry 的每个方法都打了一个断点，而且现在EurekaServer已经处于Debug运行状态，那么我们就随便找一个被 @EnableEurekaClient 的微服务启动试试微服务来试试吧，直接Run。

当启动后，就一定会调用注册register方法，那么就接着往下看，拭目以待；

实例注册方法机制

InstanceRegistry.register(final InstanceInfo info, final boolean isReplication) 方法进断点了。

InstanceRegistry.register顺着堆栈信息往上看，是 ApplicationResource.addInstance 方法被调用了，分析addInstance；

ApplicationResource 类

主要是处理接收 Http 的服务请求。

@POST@Consumes({"application/json", "application/xml"})public Response addInstance(InstanceInfo info,@HeaderParam(PeerEurekaNode.HEADER_REPLICATION) String isReplication) {logger.debug("Registering instance {} (replication={})", info.getId(), isReplication);// validate that the instanceinfo contains all the necessary required fieldsif (isBlank(info.getId())) {return Response.status(400).entity("Missing instanceId").build();} else if (isBlank(info.getHostName())) {return Response.status(400).entity("Missing hostname").build();} else if (isBlank(info.getAppName())) {return Response.status(400).entity("Missing appName").build();} else if (!appName.equals(info.getAppName())) {return Response.status(400).entity("Mismatched appName, expecting " + appName + " but was " + info.getAppName()).build();} else if (info.getDataCenterInfo() == null) {return Response.status(400).entity("Missing dataCenterInfo").build();} else if (info.getDataCenterInfo().getName() == null) {return Response.status(400).entity("Missing dataCenterInfo Name").build();}// handle cases where clients may be registering with bad DataCenterInfo with missing dataDataCenterInfo dataCenterInfo = info.getDataCenterInfo();if (dataCenterInfo instanceof UniqueIdentifier) {String dataCenterInfoId = ((UniqueIdentifier) dataCenterInfo).getId();if (isBlank(dataCenterInfoId)) {boolean experimental = "true".equalsIgnoreCase(serverConfig.getExperimental("registration.validation.dataCenterInfoId"));if (experimental) {String entity = "DataCenterInfo of type " + dataCenterInfo.getClass() + " must contain a valid id";return Response.status(400).entity(entity).build();} else if (dataCenterInfo instanceof AmazonInfo) {AmazonInfo amazonInfo = (AmazonInfo) dataCenterInfo;String effectiveId = amazonInfo.get(AmazonInfo.MetaDataKey.instanceId);if (effectiveId == null) {amazonInfo.getMetadata().put(AmazonInfo.MetaDataKey.instanceId.getName(), info.getId());}} else {logger.warn("Registering DataCenterInfo of type {} without an appropriate id", dataCenterInfo.getClass());}}}registry.register(info, "true".equals(isReplication));return Response.status(204).build();  // 204 to be backwards compatible}

这里的写法貌似看起来和我们之前 Controller 的 RESTFUL 写法有点不一样，仔细一看，原来是Jersey RESTful 框架，是一个产品级的RESTful service 和 client 框架。与Struts类似，它同样可以和hibernate,spring框架整合。
看到 registry.register(info, "true".equals(isReplication)); 注册啊，原来EurekaClient客户端启动后会调用会通过Http(s)请求，直接调到ApplicationResource.addInstance 方法，只要是和注册有关的，都会调用这个方法。
接着我们深入 registry.register(info, "true".equals(isReplication)) 查看；

@Overridepublic void register(final InstanceInfo info, final boolean isReplication) {handleRegistration(info, resolveInstanceLeaseDuration(info), isReplication);super.register(info, isReplication);}

handleRegistration(info, resolveInstanceLeaseDuration(info), isReplication) 方法；

private void handleRegistration(InstanceInfo info, int leaseDuration,boolean isReplication) {log("register " + info.getAppName() + ", vip " + info.getVIPAddress()+ ", leaseDuration " + leaseDuration + ", isReplication "+ isReplication);publishEvent(new EurekaInstanceRegisteredEvent(this, info, leaseDuration,isReplication));}

然后通过 ApplicationContext 发布了一个事件 EurekaInstanceRegisteredEvent 服务注册事件，可以给 EurekaInstanceRegisteredEvent 添加监听事件，那么用户就可以在此刻实现自己想要的一些业务逻辑。
然后我们再来看看 super.register(info, isReplication) 方法，该方法是 InstanceRegistry 的父类 PeerAwareInstanceRegistryImpl 的方法。

服务户厕机制

进入PeerAwareInstanceRegistryImpl 类的 register(final InstanceInfo info, final boolean isReplication) 方法；

@Overridepublic void register(final InstanceInfo info, final boolean isReplication) {// 注释：续约时间，默认时间是常量值 90 秒int leaseDuration = Lease.DEFAULT_DURATION_IN_SECS;// 注释：续约时间，当然也可以从配置文件中取出来，所以说续约时间值也是可以让我们自己自定义配置的if (info.getLeaseInfo() != null && info.getLeaseInfo().getDurationInSecs() > 0) {leaseDuration = info.getLeaseInfo().getDurationInSecs();}// 注释：将注册方的信息写入 EurekaServer 的注册表，父类为　AbstractInstanceRegistrysuper.register(info, leaseDuration, isReplication);// 注释：EurekaServer 节点之间的数据同步，复制到其他PeerreplicateToPeers(Action.Register, info.getAppName(), info.getId(), info, null, isReplication);}

进入super.register(info, leaseDuration, isReplication)，如何写入EurekaServer 的注册表的，进入AbstractInstanceRegistry.register(InstanceInfo registrant, int leaseDuration, boolean isReplication) 方法。

public void register(InstanceInfo registrant, int leaseDuration, boolean isReplication) {try {read.lock();// 注释：registry　这个变量，就是我们所谓的注册表，注册表是保存在内存中的；Map<String, Lease<InstanceInfo>> gMap = registry.get(registrant.getAppName());REGISTER.increment(isReplication);if (gMap == null) {final ConcurrentHashMap<String, Lease<InstanceInfo>> gNewMap = new ConcurrentHashMap<String, Lease<InstanceInfo>>();gMap = registry.putIfAbsent(registrant.getAppName(), gNewMap);if (gMap == null) {gMap = gNewMap;}}Lease<InstanceInfo> existingLease = gMap.get(registrant.getId());// Retain the last dirty timestamp without overwriting it, if there is already a leaseif (existingLease != null && (existingLease.getHolder() != null)) {Long existingLastDirtyTimestamp = 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 = existingLease.getHolder();}} else {// The lease does not exist and hence it is a new registrationsynchronized (lock) {if (this.expectedNumberOfRenewsPerMin > 0) {// Since the client wants to cancel it, reduce the threshold// (1// for 30 seconds, 2 for a minute)this.expectedNumberOfRenewsPerMin = this.expectedNumberOfRenewsPerMin + 2;this.numberOfRenewsPerMinThreshold =(int) (this.expectedNumberOfRenewsPerMin * serverConfig.getRenewalPercentThreshold());}}logger.debug("No previous lease information found; it is new registration");}Lease<InstanceInfo> lease = new Lease<InstanceInfo>(registrant, leaseDuration);if (existingLease != null) {lease.setServiceUpTimestamp(existingLease.getServiceUpTimestamp());}gMap.put(registrant.getId(), lease);synchronized (recentRegisteredQueue) {recentRegisteredQueue.add(new Pair<Long, String>(System.currentTimeMillis(),registrant.getAppName() + "(" + registrant.getId() + ")"));}// This is where the initial state transfer of overridden status happensif (!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 (!overriddenInstanceStatusMap.containsKey(registrant.getId())) {logger.info("Not found overridden id {} and hence adding it", registrant.getId());overriddenInstanceStatusMap.put(registrant.getId(), registrant.getOverriddenStatus());}}InstanceStatus overriddenStatusFromMap = overriddenInstanceStatusMap.get(registrant.getId());if (overriddenStatusFromMap != null) {logger.info("Storing overridden status {} from map", overriddenStatusFromMap);registrant.setOverriddenStatus(overriddenStatusFromMap);}// Set the status based on the overridden status rulesInstanceStatus overriddenInstanceStatus = getOverriddenInstanceStatus(registrant, existingLease, isReplication);registrant.setStatusWithoutDirty(overriddenInstanceStatus);// If the lease is registered with UP status, set lease service up timestampif (InstanceStatus.UP.equals(registrant.getStatus())) {lease.serviceUp();}registrant.setActionType(ActionType.ADDED);recentlyChangedQueue.add(new RecentlyChangedItem(lease));registrant.setLastUpdatedTimestamp();invalidateCache(registrant.getAppName(), registrant.getVIPAddress(), registrant.getSecureVipAddress());logger.info("Registered instance {}/{} with status {} (replication={})",registrant.getAppName(), registrant.getId(), registrant.getStatus(), isReplication);} finally {read.unlock();}}

发现这个方法有点长，大致阅读，主要更新了注册表的时间之外，还更新了缓存等其它东西，大家有兴趣的可以深究阅读该方法；

集群之间的复制

replicateToPeers(Action.Register, info.getAppName(), info.getId(), info, null, isReplication)　的这个方法。

private void replicateToPeers(Action action, String appName, String id,InstanceInfo info /* optional */,InstanceStatus newStatus /* optional */, boolean isReplication) {Stopwatch tracer = action.getTimer().start();try {if (isReplication) {numberOfReplicationsLastMin.increment();}// If it is a replication already, do not replicate again as this will create a poison replication// 注释：如果已经复制过，就不再复制if (peerEurekaNodes == Collections.EMPTY_LIST || isReplication) {return;}// 遍历Eureka Server集群中的所有节点，进行复制操作for (final PeerEurekaNode node : peerEurekaNodes.getPeerEurekaNodes()) {// If the url represents this host, do not replicate to yourself.if (peerEurekaNodes.isThisMyUrl(node.getServiceUrl())) {continue;}// 没有复制过，遍历Eureka Server集群中的node节点，依次操作，包括取消、注册、心跳、状态更新等。replicateInstanceActionsToPeers(action, appName, id, info, newStatus, node);}} finally {tracer.stop();}}

每当有注册请求，首先更新 EurekaServer 的注册表，然后再将信息同步到其它EurekaServer的节点上去；
接下来我们看看 node 节点是如何进行复制操作的，进入 replicateInstanceActionsToPeers 方法。

private void replicateInstanceActionsToPeers(Action action, String appName,String id, InstanceInfo info, InstanceStatus newStatus,PeerEurekaNode node) {try {InstanceInfo infoFromRegistry = null;CurrentRequestVersion.set(Version.V2);switch (action) {case Cancel:node.cancel(appName, id);break;case Heartbeat:InstanceStatus overriddenStatus = overriddenInstanceStatusMap.get(id);infoFromRegistry = getInstanceByAppAndId(appName, id, false);node.heartbeat(appName, id, infoFromRegistry, overriddenStatus, false);break;case Register:node.register(info);break;case StatusUpdate:infoFromRegistry = getInstanceByAppAndId(appName, id, false);node.statusUpdate(appName, id, newStatus, infoFromRegistry);break;case DeleteStatusOverride:infoFromRegistry = getInstanceByAppAndId(appName, id, false);node.deleteStatusOverride(appName, id, infoFromRegistry);break;}} catch (Throwable t) {logger.error("Cannot replicate information to {} for action {}", node.getServiceUrl(), action.name(), t);}}

节点之间的复制状态操作，都在这里体现的淋漓尽致，那么我们就拿 Register 类型 node.register(info) 来看，我们来看看 node 究竟是如何做到同步信息的，进入 node.register(info) 方法看看；

同级之间的复制机制

PeerEurekaNode.register(final InstanceInfo info) 方法，一窥究竟如何同步数据。

public void register(final InstanceInfo info) throws Exception {// 注释：任务过期时间给任务分发器处理，默认时间偏移当前时间 30秒long expiryTime = System.currentTimeMillis() + getLeaseRenewalOf(info);batchingDispatcher.process(taskId("register", info),new InstanceReplicationTask(targetHost, Action.Register, info, null, true) {public EurekaHttpResponse<Void> execute() {return replicationClient.register(info);}},expiryTime);}

这里涉及到了 Eureka 的任务批处理，通常情况下Peer之间的同步需要调用多次，如果EurekaServer一多的话，那么将会有很多http请求，所以自然而然的孕育出了任务批处理，但是也在一定程度上导致了注册和下线的一些延迟，突出优势的同时也势必会造成一些劣势，但是这些延迟情况还是能符合常理在容忍范围之内的。
在 expiryTime 超时时间之内，批次处理要做的事情就是合并任务为一个List，然后发送请求的时候，将这个批次List直接打包发送请求出去，这样的话，在这个批次的List里面，可能包含取消、注册、心跳、状态等一系列状态的集合List。
我们再接着看源码，batchingDispatcher.process 这么一调用，然后我们就直接看这个 TaskDispatchers.createBatchingTaskDispatcher 方法。

public static <ID, T> TaskDispatcher<ID, T> createBatchingTaskDispatcher(String id,int maxBufferSize,int workloadSize,int workerCount,long maxBatchingDelay,long congestionRetryDelayMs,long networkFailureRetryMs,TaskProcessor<T> taskProcessor) {final AcceptorExecutor<ID, T> acceptorExecutor = new AcceptorExecutor<>(id, maxBufferSize, workloadSize, maxBatchingDelay, congestionRetryDelayMs, networkFailureRetryMs);final TaskExecutors<ID, T> taskExecutor = TaskExecutors.batchExecutors(id, workerCount, taskProcessor, acceptorExecutor);return new TaskDispatcher<ID, T>() {@Overridepublic void process(ID id, T task, long expiryTime) {acceptorExecutor.process(id, task, expiryTime);}@Overridepublic void shutdown() {acceptorExecutor.shutdown();taskExecutor.shutdown();}};}

这里的 process 方法会将任务添加到队列中，有入队列自然有出队列，具体怎么取任务，我就不一一给大家讲解了，我就讲讲最后是怎么触发任务的。进入 final TaskExecutors<ID, T> taskExecutor = TaskExecutors.batchExecutors(id, workerCount, taskProcessor, acceptorExecutor) 这句代码的 TaskExecutors.batchExecutors 方法。

static <ID, T> TaskExecutors<ID, T> batchExecutors(final String name,int workerCount,final TaskProcessor<T> processor,final AcceptorExecutor<ID, T> acceptorExecutor) {final AtomicBoolean isShutdown = new AtomicBoolean();final TaskExecutorMetrics metrics = new TaskExecutorMetrics(name);return new TaskExecutors<>(new WorkerRunnableFactory<ID, T>() {@Overridepublic WorkerRunnable<ID, T> create(int idx) {return new BatchWorkerRunnable<>("TaskBatchingWorker-" +name + \'-\' + idx, isShutdown, metrics, processor, acceptorExecutor);}}, workerCount, isShutdown);}

我们发现 TaskExecutors 类中的 batchExecutors 这个静态方法，有个 BatchWorkerRunnable 返回的实现类，因此我们再次进入 BatchWorkerRunnable 类看看究竟,而且既然是 Runnable，那么势必会有 run 方法。

@Overridepublic void run() {try {while (!isShutdown.get()) {// 注释：获取信号量释放 batchWorkRequests.release()，返回任务集合列表List<TaskHolder<ID, T>> holders = getWork();metrics.registerExpiryTimes(holders);List<T> tasks = getTasksOf(holders);// 注释：将批量任务打包请求Peer节点ProcessingResult result = processor.process(tasks);switch (result) {case Success:break;case Congestion:case TransientError:taskDispatcher.reprocess(holders, result);break;case PermanentError:logger.warn("Discarding {} tasks of {} due to permanent error", holders.size(), workerName);}metrics.registerTaskResult(result, tasks.size());}} catch (InterruptedException e) {// Ignore} catch (Throwable e) {// Safe-guard, so we never exit this loop in an uncontrolled way.logger.warn("Discovery WorkerThread error", e);}}

这就是我们 BatchWorkerRunnable 类的 run 方法，这里面首先要获取信号量释放，才能获得任务集合，一旦获取到了任务集合的话，那么就直接调用 processor.process(tasks) 方法请求 Peer 节点同步数据，接下来我们看看 ReplicationTaskProcessor.process 方法；

@Overridepublic ProcessingResult process(List<ReplicationTask> tasks) {ReplicationList list = createReplicationListOf(tasks);try {// 注释：这里通过 JerseyReplicationClient 客户端对象直接发送list请求数据EurekaHttpResponse<ReplicationListResponse> response = replicationClient.submitBatchUpdates(list);int statusCode = response.getStatusCode();if (!isSuccess(statusCode)) {if (statusCode == 503) {logger.warn("Server busy (503) HTTP status code received from the peer {}; rescheduling tasks after delay", peerId);return ProcessingResult.Congestion;} else {// Unexpected error returned from the server. This should ideally never happen.logger.error("Batch update failure with HTTP status code {}; discarding {} replication tasks", statusCode, tasks.size());return ProcessingResult.PermanentError;}} else {handleBatchResponse(tasks, response.getEntity().getResponseList());}} catch (Throwable e) {if (isNetworkConnectException(e)) {logNetworkErrorSample(null, e);return ProcessingResult.TransientError;} else {logger.error("Not re-trying this exception because it does not seem to be a network exception", e);return ProcessingResult.PermanentError;}}return ProcessingResult.Success;}

感觉快要见到真相了，所以我们迫不及待的进入 JerseyReplicationClient.submitBatchUpdates(ReplicationList replicationList) 方法一窥究竟。

@Overridepublic EurekaHttpResponse<ReplicationListResponse> submitBatchUpdates(ReplicationList replicationList) {ClientResponse response = null;try {response = jerseyApacheClient.resource(serviceUrl)// 注释：这才是重点，请求目的相对路径，peerreplication/batch/.path(PeerEurekaNode.BATCH_URL_PATH).accept(MediaType.APPLICATION_JSON_TYPE).type(MediaType.APPLICATION_JSON_TYPE).post(ClientResponse.class, replicationList);if (!isSuccess(response.getStatus())) {return anEurekaHttpResponse(response.getStatus(), ReplicationListResponse.class).build();}ReplicationListResponse batchResponse = response.getEntity(ReplicationListResponse.class);return anEurekaHttpResponse(response.getStatus(), batchResponse).type(MediaType.APPLICATION_JSON_TYPE).build();} finally {if (response != null) {response.close();}}}

看到了相对路径地址，我们搜索下"batch"这样的字符串看看有没有对应的接收方法或者被@Path注解进入的；在 eureka-core-1.4.12.jar 这个包下面，果然搜到到了 @Path("batch") 这样的字样，直接进入，发现这是 PeerReplicationResource 类的方法 batchReplication，我们进入这方法看看。

@Path("batch")@POSTpublic Response batchReplication(ReplicationList replicationList) {try {ReplicationListResponse batchResponse = new ReplicationListResponse();// 注释：这里将收到的任务列表，依次循环解析处理，主要核心方法在 dispatch 方法中。for (ReplicationInstance instanceInfo : replicationList.getReplicationList()) {try {batchResponse.addResponse(dispatch(instanceInfo));} catch (Exception e) {batchResponse.addResponse(new ReplicationInstanceResponse(Status.INTERNAL_SERVER_ERROR.getStatusCode(), null));logger.error(instanceInfo.getAction() + " request processing failed for batch item "+ instanceInfo.getAppName() + \'/\' + instanceInfo.getId(), e);}}return Response.ok(batchResponse).build();} catch (Throwable e) {logger.error("Cannot execute batch Request", e);return Response.status(Status.INTERNAL_SERVER_ERROR).build();}}

看到了循环一次遍历任务进行处理，不知不觉觉得心花怒放，胜利的重点马上就要到来了，我们进入 PeerReplicationResource.dispatch 方法看看。

private ReplicationInstanceResponse dispatch(ReplicationInstance instanceInfo) {ApplicationResource applicationResource = createApplicationResource(instanceInfo);InstanceResource resource = createInstanceResource(instanceInfo, applicationResource);String lastDirtyTimestamp = toString(instanceInfo.getLastDirtyTimestamp());String overriddenStatus = toString(instanceInfo.getOverriddenStatus());String instanceStatus = toString(instanceInfo.getStatus());Builder singleResponseBuilder = new Builder();switch (instanceInfo.getAction()) {case Register:singleResponseBuilder = handleRegister(instanceInfo, applicationResource);break;case Heartbeat:singleResponseBuilder = handleHeartbeat(resource, lastDirtyTimestamp, overriddenStatus, instanceStatus);break;case Cancel:singleResponseBuilder = handleCancel(resource);break;case StatusUpdate:singleResponseBuilder = handleStatusUpdate(instanceInfo, resource);break;case DeleteStatusOverride:singleResponseBuilder = handleDeleteStatusOverride(instanceInfo, resource);break;}return singleResponseBuilder.build();}

随便抓一个类型，那我们也拿 Register 类型来看，进入 PeerReplicationResource.handleRegister 看看。

private static Builder handleRegister(ReplicationInstance instanceInfo, ApplicationResource applicationResource) {// 注释：private static final String REPLICATION = "true"; 定义的一个常量值，而且还是回调 ApplicationResource.addInstance 方法applicationResource.addInstance(instanceInfo.getInstanceInfo(), REPLICATION);return new Builder().setStatusCode(Status.OK.getStatusCode());}

Peer节点的同步旅程终于结束了，最终又回调到了 ApplicationResource.addInstance 这个方法，这个方法在最终是EurekaClient启动后注册调用的方法，然而Peer节点的信息同步也调用了这个方法，仅仅只是通过一个变量 isReplication 为true还是false来判断是否是节点复制。剩下的ApplicationResource.addInstance流程前面已经提到过了，相信大家已经明白了注册的流程是如何扭转的，包括批量任务是如何处理EurekaServer节点之间的信息同步的了。

EurekaClient 启动流程分析

调换运行模式

Run运行discovery-eureka服务，Debug 运行 provider-user 服务，先观察日志先；

2017-10-23 19:43:07.688  INFO 1488 --- [           main] o.s.c.support.DefaultLifecycleProcessor  : Starting beans in phase 02017-10-23 19:43:07.694  INFO 1488 --- [           main] o.s.c.n.eureka.InstanceInfoFactory       : Setting initial instance status as: STARTING2017-10-23 19:43:07.874  INFO 1488 --- [           main] c.n.d.provider.DiscoveryJerseyProvider   : Using JSON encoding codec LegacyJacksonJson2017-10-23 19:43:07.874  INFO 1488 --- [           main] c.n.d.provider.DiscoveryJerseyProvider   : Using JSON decoding codec LegacyJacksonJson2017-10-23 19:43:07.971  INFO 1488 --- [           main] c.n.d.provider.DiscoveryJerseyProvider   : Using XML encoding codec XStreamXml2017-10-23 19:43:07.971  INFO 1488 --- [           main] c.n.d.provider.DiscoveryJerseyProvider   : Using XML decoding codec XStreamXml2017-10-23 19:43:08.134  INFO 1488 --- [           main] c.n.d.s.r.aws.ConfigClusterResolver      : Resolving eureka endpoints via configuration2017-10-23 19:43:08.344  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Disable delta property : false2017-10-23 19:43:08.344  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Single vip registry refresh property : null2017-10-23 19:43:08.344  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Force full registry fetch : false2017-10-23 19:43:08.344  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Application is null : false2017-10-23 19:43:08.344  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Registered Applications size is zero : true2017-10-23 19:43:08.344  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Application version is -1: true2017-10-23 19:43:08.345  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Getting all instance registry info from the eureka server2017-10-23 19:43:08.630  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : The response status is 2002017-10-23 19:43:08.631  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Starting heartbeat executor: renew interval is: 302017-10-23 19:43:08.634  INFO 1488 --- [           main] c.n.discovery.InstanceInfoReplicator     : InstanceInfoReplicator onDemand update allowed rate per min is 42017-10-23 19:43:08.637  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Discovery Client initialized at timestamp 1508758988637 with initial instances count: 02017-10-23 19:43:08.657  INFO 1488 --- [           main] c.n.e.EurekaDiscoveryClientConfiguration : Registering application springms-provider-user with eureka with status UP2017-10-23 19:43:08.658  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Saw local status change event StatusChangeEvent [timestamp=1508758988658, current=UP, previous=STARTING]2017-10-23 19:43:08.659  INFO 1488 --- [nfoReplicator-0] com.netflix.discovery.DiscoveryClient    : DiscoveryClient_SPRINGMS-PROVIDER-USER/springms-provider-user:192.168.3.101:7900: registering service...2017-10-23 19:43:08.768  INFO 1488 --- [           main] s.b.c.e.t.TomcatEmbeddedServletContainer : Tomcat started on port(s): 7900 (http)2017-10-23 19:43:08.768  INFO 1488 --- [           main] c.n.e.EurekaDiscoveryClientConfiguration : Updating port to 79002017-10-23 19:43:08.773  INFO 1488 --- [           main] c.s.cloud.MsProviderUserApplication      : Started ProviderApplication in 882.1 seconds (JVM running for 10.398)

服务提供方主体加载流程

【1】：仔细查看下日志，先是 DefaultLifecycleProcessor 类处理了一些 bean,然后接下来肯定会调用一些实现 SmartLifecycle 类的start 方法；
【2】：接着初始化设置了EurekaClient的状态为 STARTING，初始化编码使用的格式，哪些用JSON，哪些用XML；
【3】：紧接着打印了强制获取注册信息状态为false，已注册的应用大小为0，客户端发送心跳续约，心跳续约间隔为30秒，最后打印Client初始化完成；

EnableEurekaClient 组件。

@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@EnableDiscoveryClientpublic @interface EnableEurekaClient {}

@EnableEurekaClient

这个注解类竟然也使用了注解 @EnableDiscoveryClient，那么我们有必要去这个注解类看看。

@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@Import(EnableDiscoveryClientImportSelector.class)public @interface EnableDiscoveryClient {}

@EnableDiscoveryClient

这个注解类有个比较特殊的注解 @Import，由此我们猜想，这里的大多数逻辑是不是都写在这个 EnableDiscoveryClientImportSelector 类呢？

EnableDiscoveryClientImportSelector

@Order(Ordered.LOWEST_PRECEDENCE - 100)public class EnableDiscoveryClientImportSelectorextends SpringFactoryImportSelector<EnableDiscoveryClient> {@Overrideprotected boolean isEnabled() {return new RelaxedPropertyResolver(getEnvironment()).getProperty("spring.cloud.discovery.enabled", Boolean.class, Boolean.TRUE);}@Overrideprotected boolean hasDefaultFactory() {return true;}}

EnableDiscoveryClientImportSelector 类继承了 SpringFactoryImportSelector 类，但是重写了一个 isEnabled() 方法，默认值返回 true，为什么会返回true。

/*** Select and return the names of which class(es) should be imported based on* the {@link AnnotationMetadata} of the importing @{@link Configuration} class.*/@Overridepublic String[] selectImports(AnnotationMetadata metadata) {if (!isEnabled()) {return new String[0];}AnnotationAttributes attributes = AnnotationAttributes.fromMap(metadata.getAnnotationAttributes(this.annotationClass.getName(), true));Assert.notNull(attributes, "No " + getSimpleName() + " attributes found. Is "+ metadata.getClassName() + " annotated with @" + getSimpleName() + "?");// Find all possible auto configuration classes, filtering duplicatesList<String> factories = new ArrayList<>(new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(this.annotationClass, this.beanClassLoader)));if (factories.isEmpty() && !hasDefaultFactory()) {throw new IllegalStateException("Annotation @" + getSimpleName()+ " found, but there are no implementations. Did you forget to include a starter?");}if (factories.size() > 1) {// there should only ever be one DiscoveryClient, but there might be more than// one factorylog.warn("More than one implementation " + "of @" + getSimpleName()+ " (now relying on @Conditionals to pick one): " + factories);}return factories.toArray(new String[factories.size()]);}

EnableDiscoveryClientImportSelector.selectImports

首先通过注解获取了一些属性，然后加载了一些类名称，我们进入loadFactoryNames 方法看看。

public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {String factoryClassName = factoryClass.getName();try {// 注释：public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";// 注释：这个 jar 包下的一个配置文件Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));List<String> result = new ArrayList<String>();while (urls.hasMoreElements()) {URL url = urls.nextElement();Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));String factoryClassNames = properties.getProperty(factoryClassName);result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));}return result;}catch (IOException ex) {throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +"] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);}}

加载了一个配置文件，配置文件里面写了啥呢？打开SpringFactoryImportSelector该文件所在的jar包的spring.factories文件一看。

# AutoConfigurationorg.springframework.boot.autoconfigure.EnableAutoConfiguration=\\org.springframework.cloud.client.CommonsClientAutoConfiguration,\\org.springframework.cloud.client.discovery.noop.NoopDiscoveryClientAutoConfiguration,\\org.springframework.cloud.client.hypermedia.CloudHypermediaAutoConfiguration,\\org.springframework.cloud.client.loadbalancer.LoadBalancerAutoConfiguration,\\org.springframework.cloud.commons.util.UtilAutoConfiguration# Environment Post Processorsorg.springframework.boot.env.EnvironmentPostProcessor=\\org.springframework.cloud.client.HostInfoEnvironmentPostProcessor

都是一些 Configuration 后缀的类名，所以这些都是加载的一堆堆的配置文件类。factories对象里面只有一个类名路径为 org.springframework.cloud.netflix.eureka.EurekaDiscoveryClientConfiguration 。

EurekaDiscoveryClientConfiguration

@Configuration@EnableConfigurationProperties@ConditionalOnClass(EurekaClientConfig.class)@ConditionalOnProperty(value = "eureka.client.enabled", matchIfMissing = true)@CommonsLogpublic class EurekaDiscoveryClientConfiguration implements SmartLifecycle, Ordered {@Overridepublic void start() {// only set the port if the nonSecurePort is 0 and this.port != 0if (this.port.get() != 0 && this.instanceConfig.getNonSecurePort() == 0) {this.instanceConfig.setNonSecurePort(this.port.get());}// only initialize if nonSecurePort is greater than 0 and it isn\'t already running// because of containerPortInitializer belowif (!this.running.get() && this.instanceConfig.getNonSecurePort() > 0) {maybeInitializeClient();if (log.isInfoEnabled()) {log.info("Registering application " + this.instanceConfig.getAppname()+ " with eureka with status "+ this.instanceConfig.getInitialStatus());}this.applicationInfoManager.setInstanceStatus(this.instanceConfig.getInitialStatus());if (this.healthCheckHandler != null) {this.eurekaClient.registerHealthCheck(this.healthCheckHandler);}this.context.publishEvent(new InstanceRegisteredEvent<>(this, this.instanceConfig));this.running.set(true);}}｝

首先看到该类实现了SmartLifecycle 接口，那么就肯定会实现 start 方法，而且这个 start 方法感觉应在会被加载执行的。this.applicationInfoManager.setInstanceStatus(this.instanceConfig.getInitialStatus()) 这段代码有一个观察者模式的回调存在。

// ApplicationInfoManager.setInstanceStatus 的方法public synchronized void setInstanceStatus(InstanceStatus status) {// 打上断点InstanceStatus prev = instanceInfo.setStatus(status);if (prev != null) {for (StatusChangeListener listener : listeners.values()) {try {listener.notify(new StatusChangeEvent(prev, status));} catch (Exception e) {logger.warn("failed to notify listener: {}", listener.getId(), e);}}}}

这个方法会因为状态的改变而回调所有实现 StatusChangeListener 这个类的地方，前提得先注册到 listeners 中去才行。
于是乎，我们断定，若想要回调，那么就必须有地方先注册这个事件，而且这个注册还必须提前执行在 start 方法前执行，于是我们得先在ApplicationInfoManager 这个类中找到注册到 listeners 的这个方法。

public void registerStatusChangeListener(StatusChangeListener listener) {// 打上断点listeners.put(listener.getId(), listener);}

于是我们逆向找下 registerStatusChangeListener 被调用的地方。
很不巧的是，尽然只有1个地方被调用，这个地方就是 DiscoveryClient.initScheduledTasks 方法，而且 initScheduledTasks方法又是在 DiscoveryClient 的构造函数里面调用的，同时我们也对 initScheduledTasks 以及 initScheduledTasks 被调用的构造方法地方打上断点。

果不其然，EurekaDiscoveryClientConfiguration.start 方法被调用了，紧接着 this.applicationInfoManager.setInstanceStatus(this.instanceConfig.getInitialStatus()) 也进入断点，然后在往下走，又进入的DiscoveryClient.initScheduledTasks 方法中的 notify 回调处。

看着断点依次经过我们上述分析的地方，然后也符合日志打印的顺序，所以我们现在应该是有必要好好看看 DiscoveryClient.initScheduledTasks 这个方法究竟干了什么伟大的事情。然而又想了想，还不如看看 initScheduledTasks 被调用的构造方法。

DiscoveryClient 经过 @Inject 注解过的构造方法。

@InjectDiscoveryClient(ApplicationInfoManager applicationInfoManager, EurekaClientConfig config, DiscoveryClientOptionalArgs args, Provider<BackupRegistry> backupRegistryProvider) {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(","));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;}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 DiscoveryClientDiscoveryManager.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}try {// 注释：定时任务调度准备scheduler = Executors.newScheduledThreadPool(3,new ThreadFactoryBuilder().setNameFormat("DiscoveryClient-%d").setDaemon(true).build());// 注释：实例化心跳定时任务线程池heartbeatExecutor = new ThreadPoolExecutor(1, clientConfig.getHeartbeatExecutorThreadPoolSize(), 0, TimeUnit.SECONDS,new SynchronousQueue<Runnable>(),new ThreadFactoryBuilder().setNameFormat("DiscoveryClient-HeartbeatExecutor-%d").setDaemon(true).build());  // use direct handoff// 注释：实例化缓存刷新定时任务线程池cacheRefreshExecutor = new ThreadPoolExecutor(1, clientConfig.getCacheRefreshExecutorThreadPoolSize(), 0, TimeUnit.SECONDS,new SynchronousQueue<Runnable>(),new ThreadFactoryBuilder().setNameFormat("DiscoveryClient-CacheRefreshExecutor-%d").setDaemon(true).build());  // use direct handoffeurekaTransport = new EurekaTransport();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);}if (clientConfig.shouldFetchRegistry() && !fetchRegistry(false)) {fetchRegistryFromBackup();}// 注释：初始化调度任务initScheduledTasks();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 DiscoveryClientDiscoveryManager.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());}

从往下看，initScheduledTasks 这个方法顾名思义就是初始化调度任务，所以这里面的内容应该就是重头戏，进入看看。

private void initScheduledTasks() {if (clientConfig.shouldFetchRegistry()) {// registry cache refresh timer// 注释：间隔多久去拉取服务注册信息，默认时间 30秒int registryFetchIntervalSeconds = clientConfig.getRegistryFetchIntervalSeconds();int expBackOffBound = clientConfig.getCacheRefreshExecutorExponentialBackOffBound();// 注释：定时任务，每间隔 30秒 去拉取一次服务注册信息scheduler.schedule(new TimedSupervisorTask("cacheRefresh",scheduler,cacheRefreshExecutor,registryFetchIntervalSeconds,TimeUnit.SECONDS,expBackOffBound,new CacheRefreshThread()),registryFetchIntervalSeconds, TimeUnit.SECONDS);}if (clientConfig.shouldRegisterWithEureka()) {// 注释：间隔多久发送一次心跳续约，默认间隔时间 30 秒int renewalIntervalInSecs = instanceInfo.getLeaseInfo().getRenewalIntervalInSecs();int expBackOffBound = clientConfig.getHeartbeatExecutorExponentialBackOffBound();logger.info("Starting heartbeat executor: " + "renew interval is: " + renewalIntervalInSecs);// Heartbeat timer// 注释：定时任务，每间隔 30秒 去想 EurekaServer 发送一次心跳续约scheduler.schedule(new TimedSupervisorTask("heartbeat",scheduler,heartbeatExecutor,renewalIntervalInSecs,TimeUnit.SECONDS,expBackOffBound,new HeartbeatThread()),renewalIntervalInSecs, TimeUnit.SECONDS);// InstanceInfo replicator// 注释：实例信息复制器，定时刷新dataCenterInfo数据中心信息，默认30秒instanceInfoReplicator = new InstanceInfoReplicator(this,instanceInfo,clientConfig.getInstanceInfoReplicationIntervalSeconds(),2); // burstSize// 注释：实例化状态变化监听器statusChangeListener = new ApplicationInfoManager.StatusChangeListener() {@Overridepublic String getId() {return "statusChangeListener";}@Overridepublic void notify(StatusChangeEvent statusChangeEvent) {if (InstanceStatus.DOWN == statusChangeEvent.getStatus() ||InstanceStatus.DOWN == statusChangeEvent.getPreviousStatus()) {// log at warn level if DOWN was involvedlogger.warn("Saw local status change event {}", statusChangeEvent);} else {logger.info("Saw local status change event {}", statusChangeEvent);}// 注释：状态有变化的话，会回调这个方法instanceInfoReplicator.onDemandUpdate();}};// 注释：注册状态变化监听器if (clientConfig.shouldOnDemandUpdateStatusChange()) {applicationInfoManager.registerStatusChangeListener(statusChangeListener);}instanceInfoReplicator.start(clientConfig.getInitialInstanceInfoReplicationIntervalSeconds());} else {logger.info("Not registering with Eureka server per configuration");}}

在这个方法从上往下一路注释分析下来，干了EurekaClient我们最想知道的一些事情，定时任务获取注册信息，定时任务刷新缓存，定时任务心跳续约，定时任务同步数据中心数据，状态变化监听回调等。但是唯独没看到注册，这是怎么回事呢？
instanceInfoReplicator.onDemandUpdate() 就是在状态改变的时候。

public boolean onDemandUpdate() {if (rateLimiter.acquire(burstSize, allowedRatePerMinute)) {scheduler.submit(new Runnable() {@Overridepublic void run() {logger.debug("Executing on-demand update of local InstanceInfo");Future latestPeriodic = scheduledPeriodicRef.get();if (latestPeriodic != null && !latestPeriodic.isDone()) {logger.debug("Canceling the latest scheduled update, it will be rescheduled at the end of on demand update");latestPeriodic.cancel(false);}// 注释：这里进行了实例信息刷新和注册InstanceInfoReplicator.this.run();}});return true;} else {logger.warn("Ignoring onDemand update due to rate limiter");return false;}}

onDemandUpdate 这个方法，唯独 InstanceInfoReplicator.this.run() 这个方法还有点用，而且还是 run 方法呢，感情 InstanceInfoReplicator 这个类还是实现了 Runnable 接口？经过查看这个类，还真是实现了 Runnable 接口。
这个方法应该我们要找的注册所在的地方。

public void run() {try {discoveryClient.refreshInstanceInfo();Long dirtyTimestamp = instanceInfo.isDirtyWithTime();if (dirtyTimestamp != null) {discoveryClient.register();instanceInfo.unsetIsDirty(dirtyTimestamp);}} catch (Throwable t) {logger.warn("There was a problem with the instance info replicator", t);} finally {Future next = scheduler.schedule(this, replicationIntervalSeconds, TimeUnit.SECONDS);scheduledPeriodicRef.set(next);}}

discoveryClient.register() 这个 register 方法，原来注册方法就是这个。

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;}

原来调用了 EurekaHttpClient 封装的客户端请求对象来进行注册的，再继续深探 registrationClient.register 方法，于是我们来到了 AbstractJerseyEurekaHttpClient.register 方法。

@Overridepublic EurekaHttpResponse<Void> register(InstanceInfo info) {String urlPath = "apps/" + info.getAppName();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)// 注释：打包带上当前应用的所有信息 info.post(ClientResponse.class, info);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();}}}

调用的是 Jersey RESTful 框架来进行请求的，然后在 EurekaServer 那边就会在 ApplicationResource.addInstance 方法接收客户端的注册请求，因此我们的 EurekaClient 是如何注册的就到此为止了。