eureka客户端启动流程

1. 获取eureka服务端的地址

com.netflix.discovery.endpoint.EndpointUtils#getServiceUrlsFromDNS

2. 初始化延时任务

初始化任务位于这个方法中：com.netflix.discovery.DiscoveryClient#initScheduledTasks
把延迟任务封装到这个类里面com.netflix.discovery.TimedSupervisorTask#TimedSupervisorTask，
使得每个定时任务运行结束后根据运行时间动态调整下次启动任务的时间，既起到了定时任务的作用，又起到了动态调整启动任务的时间。

public void run() {
        Future future = null;
        try {
            future = executor.submit(task);
            threadPoolLevelGauge.set((long) executor.getActiveCount());
            future.get(timeoutMillis, TimeUnit.MILLISECONDS);  // block until done or timeout
            delay.set(timeoutMillis);
            threadPoolLevelGauge.set((long) executor.getActiveCount());
        } catch (TimeoutException e) {
            logger.error("task supervisor timed out", e);
            timeoutCounter.increment();
			//如果当前任务超时，获取超时时间*2或者最大延时时间中的最大值，作为下次的延时启动时间。
            long currentDelay = delay.get();
            long newDelay = Math.min(maxDelay, currentDelay * 2);
            delay.compareAndSet(currentDelay, newDelay);

        } catch (RejectedExecutionException e) {
            if (executor.isShutdown() || scheduler.isShutdown()) {
                logger.warn("task supervisor shutting down, reject the task", e);
            } else {
                logger.error("task supervisor rejected the task", e);
            }

            rejectedCounter.increment();
        } catch (Throwable e) {
            if (executor.isShutdown() || scheduler.isShutdown()) {
                logger.warn("task supervisor shutting down, can't accept the task");
            } else {
                logger.error("task supervisor threw an exception", e);
            }

            throwableCounter.increment();
        } finally {
            if (future != null) {
                future.cancel(true);
            }
           //启动下一次的延时任务
            if (!scheduler.isShutdown()) {
                scheduler.schedule(this, delay.get(), TimeUnit.MILLISECONDS);
            }
        }
    }

2.1. 定时从eureka服务端获取服务列表（服务获取）

if (clientConfig.shouldFetchRegistry()) {
            // registry cache refresh timer
            int registryFetchIntervalSeconds = clientConfig.getRegistryFetchIntervalSeconds();
            int expBackOffBound = clientConfig.getCacheRefreshExecutorExponentialBackOffBound();
            scheduler.schedule(
                    new TimedSupervisorTask(
                            "cacheRefresh",
                            scheduler,
                            cacheRefreshExecutor,
                            registryFetchIntervalSeconds,
                            TimeUnit.SECONDS,
                            expBackOffBound,
                            new CacheRefreshThread()
                    ),
                    registryFetchIntervalSeconds, TimeUnit.SECONDS);
        }

2.2. 定时发心跳信息到eureka服务端 （服务续约）

// Heartbeat timer
            scheduler.schedule(
                    new TimedSupervisorTask(
                            "heartbeat",
                            scheduler,
                            heartbeatExecutor,
                            renewalIntervalInSecs,
                            TimeUnit.SECONDS,
                            expBackOffBound,
                            new HeartbeatThread()
                    ),
                    renewalIntervalInSecs, TimeUnit.SECONDS);

2.3. 启动配置更新的监听器和把自己注册到eureka服务端的延时任务（服务注册）

// InstanceInfo replicator
            instanceInfoReplicator = new InstanceInfoReplicator(
                    this,
                    instanceInfo,
                    clientConfig.getInstanceInfoReplicationIntervalSeconds(),
                    2); // burstSize
			//启动监听器
            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);
                    }
                    //如果配置有更改时回调的方法
                    instanceInfoReplicator.onDemandUpdate();
                }
            };
			//注册监听器
            if (clientConfig.shouldOnDemandUpdateStatusChange()) {
                applicationInfoManager.registerStatusChangeListener(statusChangeListener);
            }
			//启动延时任务
            instanceInfoReplicator.start(clientConfig.getInitialInstanceInfoReplicationIntervalSeconds());

• 每次延时任务结束会在finally方法中启动下一个延时任务，并把任务放到原子引用中。

启动延时任务的start方法，最是会在线程池里面启动run方法

public void start(int initialDelayMs) {
        if (started.compareAndSet(false, true)) {
            instanceInfo.setIsDirty();  // for initial register
            Future next = scheduler.schedule(this, initialDelayMs, TimeUnit.SECONDS);
            scheduledPeriodicRef.set(next);
        }
    }

run方法的最后finally方法块里面会启动下一个延时任务并放到scheduledPeriodicRef变量中

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);
        }
    }

• 若配置有更新，把原子任务中的延时任务拿出并cancle，根据当前任务启动一个新的延时任务。

配置有更新时把旧的延时任务删除，因为旧的配置与更新后的不一致再想eureka注册没有意义，所以在原子引用中把任务拿出并cancle，最后重新调用run方法。

public boolean onDemandUpdate() {
        if (rateLimiter.acquire(burstSize, allowedRatePerMinute)) {
            if (!scheduler.isShutdown()) {
                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);
                        }
    
                        InstanceInfoReplicator.this.run();
                    }
                });
                return true;
            } else {
                logger.warn("Ignoring onDemand update due to stopped scheduler");
                return false;
            }
        } else {
            logger.warn("Ignoring onDemand update due to rate limiter");
            return false;
        }
    }

具体的注册功能其实就是把自己的元数据通过http的方式发送到eureka

/**
     * Register with the eureka service by making the appropriate REST call.
     */
    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;
    }