【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

1. 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。
2. 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。
3. 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。
4. 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；
5. 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 fields
    if (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 data
    DataCenterInfo 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；

@Override
public 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；

@Override
public 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　AbstractInstanceRegistry
    super.register(info, leaseDuration, isReplication);
    // 注释：EurekaServer 节点之间的数据同步，复制到其他Peer
    replicateToPeers(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> gMap = registry.get(registrant.getAppName());
        REGISTER.increment(isReplication);
        if (gMap == null) {
            final ConcurrentHashMap> gNewMap = new ConcurrentHashMap>();
            gMap = registry.putIfAbsent(registrant.getAppName(), gNewMap);
            if (gMap == null) {
                gMap = gNewMap;
            }
        }
        Lease existingLease = gMap.get(registrant.getId());
        // Retain the last dirty timestamp without overwriting it, if there is already a lease
        if (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 registration
            synchronized (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 lease = new Lease(registrant, leaseDuration);
        if (existingLease != null) {
            lease.setServiceUpTimestamp(existingLease.getServiceUpTimestamp());
        }
        gMap.put(registrant.getId(), lease);
        synchronized (recentRegisteredQueue) {
            recentRegisteredQueue.add(new Pair(
                    System.currentTimeMillis(),
                    registrant.getAppName() + "(" + registrant.getId() + ")"));
        }
        // This is where the initial state transfer of overridden status happens
        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 (!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 rules
        InstanceStatus overriddenInstanceStatus = getOverriddenInstanceStatus(registrant, existingLease, isReplication);
        registrant.setStatusWithoutDirty(overriddenInstanceStatus);

        // If the lease is registered with UP status, set lease service up timestamp
        if (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 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  TaskDispatcher createBatchingTaskDispatcher(String id,
                                                                             int maxBufferSize,
                                                                             int workloadSize,
                                                                             int workerCount,
                                                                             long maxBatchingDelay,
                                                                             long congestionRetryDelayMs,
                                                                             long networkFailureRetryMs,
                                                                             TaskProcessor taskProcessor) {
        final AcceptorExecutor acceptorExecutor = new AcceptorExecutor<>(
                id, maxBufferSize, workloadSize, maxBatchingDelay, congestionRetryDelayMs, networkFailureRetryMs
        );
        final TaskExecutors taskExecutor = TaskExecutors.batchExecutors(id, workerCount, taskProcessor, acceptorExecutor);
        return new TaskDispatcher() {
            @Override
            public void process(ID id, T task, long expiryTime) {
                acceptorExecutor.process(id, task, expiryTime);
            }

            @Override
            public 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 taskExecutor = TaskExecutors.batchExecutors(id, workerCount, taskProcessor, acceptorExecutor) 这句代码的 TaskExecutors.batchExecutors 方法。

static  TaskExecutors batchExecutors(final String name,
                                                   int workerCount,
                                                   final TaskProcessor processor,
                                                   final AcceptorExecutor acceptorExecutor) {
    final AtomicBoolean isShutdown = new AtomicBoolean();
    final TaskExecutorMetrics metrics = new TaskExecutorMetrics(name);
    return new TaskExecutors<>(new WorkerRunnableFactory() {
        @Override
        public WorkerRunnable 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。

@Override
public 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> holders = getWork();
            metrics.registerExpiryTimes(holders);

            List 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；

@Override
public ProcessingResult process(List 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 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。

@Override
public EurekaHttpResponse 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")
@POST
public 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 0
2017-10-23 19:43:07.694  INFO 1488 --- [           main] o.s.c.n.eureka.InstanceInfoFactory       : Setting initial instance status as: STARTING
2017-10-23 19:43:07.874  INFO 1488 --- [           main] c.n.d.provider.DiscoveryJerseyProvider   : Using JSON encoding codec LegacyJacksonJson
2017-10-23 19:43:07.874  INFO 1488 --- [           main] c.n.d.provider.DiscoveryJerseyProvider   : Using JSON decoding codec LegacyJacksonJson
2017-10-23 19:43:07.971  INFO 1488 --- [           main] c.n.d.provider.DiscoveryJerseyProvider   : Using XML encoding codec XStreamXml
2017-10-23 19:43:07.971  INFO 1488 --- [           main] c.n.d.provider.DiscoveryJerseyProvider   : Using XML decoding codec XStreamXml
2017-10-23 19:43:08.134  INFO 1488 --- [           main] c.n.d.s.r.aws.ConfigClusterResolver      : Resolving eureka endpoints via configuration
2017-10-23 19:43:08.344  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Disable delta property : false
2017-10-23 19:43:08.344  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Single vip registry refresh property : null
2017-10-23 19:43:08.344  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Force full registry fetch : false
2017-10-23 19:43:08.344  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Application is null : false
2017-10-23 19:43:08.344  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Registered Applications size is zero : true
2017-10-23 19:43:08.344  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Application version is -1: true
2017-10-23 19:43:08.345  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Getting all instance registry info from the eureka server
2017-10-23 19:43:08.630  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : The response status is 200
2017-10-23 19:43:08.631  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Starting heartbeat executor: renew interval is: 30
2017-10-23 19:43:08.634  INFO 1488 --- [           main] c.n.discovery.InstanceInfoReplicator     : InstanceInfoReplicator onDemand update allowed rate per min is 4
2017-10-23 19:43:08.637  INFO 1488 --- [           main] com.netflix.discovery.DiscoveryClient    : Discovery Client initialized at timestamp 1508758988637 with initial instances count: 0
2017-10-23 19:43:08.657  INFO 1488 --- [           main] c.n.e.EurekaDiscoveryClientConfiguration : Registering application springms-provider-user with eureka with status UP
2017-10-23 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 7900
2017-10-23 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
@EnableDiscoveryClient
public @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 EnableDiscoveryClientImportSelector
        extends SpringFactoryImportSelector {
    @Override
    protected boolean isEnabled() {
        return new RelaxedPropertyResolver(getEnvironment()).getProperty(
                "spring.cloud.discovery.enabled", Boolean.class, Boolean.TRUE);
    }
    @Override
    protected 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.
 */
@Override
public 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 duplicates
    List 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 factory
        log.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 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 urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
        List result = new ArrayList();
        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。

# AutoConfiguration
org.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 Processors
org.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)
@CommonsLog
public class EurekaDiscoveryClientConfiguration implements SmartLifecycle, Ordered {
    @Override
    public void start() {
        // only set the port if the nonSecurePort is 0 and this.port != 0
        if (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 below
        if (!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。

@Inject
DiscoveryClient(ApplicationInfoManager applicationInfoManager, EurekaClientConfig config, DiscoveryClientOptionalArgs args, Provider 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(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 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
    }

    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(),
                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(),
                new ThreadFactoryBuilder()
                        .setNameFormat("DiscoveryClient-CacheRefreshExecutor-%d")
                        .setDaemon(true)
                        .build()
        );  // use direct handoff

        eurekaTransport = 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 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());
}

• 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() {
            @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);
                }

                // 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() {
            @Override
            public 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 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。

@Override
public EurekaHttpResponse 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。