序

之前说过，要把一个SpringBoot应用注册到Eureka Server或者是从Eureka Server上获取服务列表，主要做了一下两件事：
1.在应用主类中配置@EnableDiscoveryClient注解。
2.在application.yml中配置eureka.client.service-url.defaultZone指定注册中心的位置。
关于@EnableDiscoveryClient注解在上篇文章中已经介绍过了，今天来看一看客户端是怎么获取到注册中心的位置的。

getDiscoveryServiceUrls方法

获取注册中心的方法是DiscoveryClient的getDiscoveryServiceUrls方法。先看一下这个方法的代码。

/**
     * @deprecated see replacement in {@link com.netflix.discovery.endpoint.EndpointUtils}
     *
     * Get the list of all eureka service urls from properties file for the eureka client to talk to.
     *
     * @param instanceZone The zone in which the client resides
     * @param preferSameZone true if we have to prefer the same zone as the client, false otherwise
     * @return The list of all eureka service urls for the eureka client to talk to
     */
    @Deprecated
    @Override
    public List<String> getServiceUrlsFromConfig(String instanceZone, boolean preferSameZone) {
        return EndpointUtils.getServiceUrlsFromConfig(clientConfig, instanceZone, preferSameZone);
    }

可以看到这个方法已经不再被使用的，而是link到了EndpointUtils的getServiceUrlsFromConfig方法。那么来看一下getServiceUrlsFromConfig的代码。

/**
     * Get the list of all eureka service urls from properties file for the eureka client to talk to.
     *
     * @param clientConfig the clientConfig to use
     * @param instanceZone The zone in which the client resides
     * @param preferSameZone true if we have to prefer the same zone as the client, false otherwise
     * @return The list of all eureka service urls for the eureka client to talk to
     */
    public static List<String> getServiceUrlsFromConfig(EurekaClientConfig clientConfig, String instanceZone, boolean preferSameZone) {
        List<String> orderedUrls = new ArrayList<String>();
        String region = getRegion(clientConfig);
        String[] availZones = clientConfig.getAvailabilityZones(clientConfig.getRegion());
        if (availZones == null || availZones.length == 0) {
            availZones = new String[1];
            availZones[0] = DEFAULT_ZONE;
        }
        logger.debug("The availability zone for the given region {} are {}", region, availZones);
        int myZoneOffset = getZoneOffset(instanceZone, preferSameZone, availZones);

        List<String> serviceUrls = clientConfig.getEurekaServerServiceUrls(availZones[myZoneOffset]);
        if (serviceUrls != null) {
            orderedUrls.addAll(serviceUrls);
        }
        int currentOffset = myZoneOffset == (availZones.length - 1) ? 0 : (myZoneOffset + 1);
        while (currentOffset != myZoneOffset) {
            serviceUrls = clientConfig.getEurekaServerServiceUrls(availZones[currentOffset]);
            if (serviceUrls != null) {
                orderedUrls.addAll(serviceUrls);
            }
            if (currentOffset == (availZones.length - 1)) {
                currentOffset = 0;
            } else {
                currentOffset++;
            }
        }

        if (orderedUrls.size() < 1) {
            throw new IllegalArgumentException("DiscoveryClient: invalid serviceUrl specified!");
        }
        return orderedUrls;
    }

先看一下这个方法的入参。clientConfig包含了一些默认配置以及我们在application.yml里面添加或修改的配置。如果我们在application.yml里配置了region，region下面配置了多个zone的话，那么instanceZone就默认取region下配置的第一个zone。配置文件如下：

server:
  port: 8882
spring:
  application:
    name: product-service
eureka:
  client:
    prefer-same-zone-eureka: true
    region: beijing
    availability-zones:
      beijing: zone-2,zone-1
    service-url:
      zone-1: http://peer1:8880/eureka/
      zone-2: http://peer2:8881/eureka/

如果我们没有自己配置region，那么instanceZone默认为“defaultZone”，这时我们在service-url后面需要配置为defaultZone，如果配置为其他名称的话会找不到注册中心的url。配置文件如下：

server:
  port: 8882
spring:
  application:
    name: product-service
eureka:
  client:
    prefer-same-zone-eureka: true
#    region: beijing
#    availability-zones:
#      beijing: zone-2,zone-1
    service-url:
      defaultZone: http://peer1:8880/eureka/,http://peer2:8881/eureka/
#      zone-1: http://peer1:8880/eureka/
#      zone-2: http://peer2:8881/eureka/

关于region和zone的说明可以参考https://www.cnblogs.com/junjiang3/p/9061867.html
最后来看preferSameZone这个入参。preferSameZone对应的是配置文件中的 prefer-same-zone-eureka，为true时优先选择client实例所在的zone，一般默认为true。
如果prefer-same-zone-eureka为false，按照service-url下的 list取第一个注册中心来注册，并和其维持心跳检测。不会再向list内的其它的注册中心注册和维持心跳。只有在第一个注册失败的情况下，才会依次向其它的注册中心注册，总共重试3次，如果3个service-url都没有注册成功，则注册失败。每隔一个心跳时间，会再次尝试。
如果prefer-same-zone-eureka为true，先通过region取availability-zones内的第一个zone，然后通过这个zone取service-url下的list，并向list内的第一个注册中心进行注册和维持心跳，不会再向list内的其它的注册中心注册和维持心跳。只有在第一个注册失败的情况下，才会依次向其它的注册中心注册，总共重试3次，如果3个service-url都没有注册成功，则注册失败。每隔一个心跳时间，会再次尝试。

继续看getServiceUrlsFromConfig方法。

List<String> orderedUrls = new ArrayList<String>();
        String region = getRegion(clientConfig);
        String[] availZones = clientConfig.getAvailabilityZones(clientConfig.getRegion());
        if (availZones == null || availZones.length == 0) {
            availZones = new String[1];
            availZones[0] = DEFAULT_ZONE;
        }
        logger.debug("The availability zone for the given region {} are {}", region, availZones);
        int myZoneOffset = getZoneOffset(instanceZone, preferSameZone, availZones);

        List<String> serviceUrls = clientConfig.getEurekaServerServiceUrls(availZones[myZoneOffset]);
        if (serviceUrls != null) {
            orderedUrls.addAll(serviceUrls);
        }

这部分的逻辑是先从clientConfig里获取region，然后或者这个region里面的zone，通过getAvailabilityZones方法返回一个数组availZones 。如果availZones 为空或null的话，就默认一个值“default”。但是availZones 其实并不会为空或者null。可以看一下getAvailabilityZones方法的代码（有两个实现类，这里看EurekaClientConfigBean里的实现方法）。

public String[] getAvailabilityZones(String region) {
        String value = (String)this.availabilityZones.get(region);
        if (value == null) {
            value = "defaultZone";
        }

        return value.split(",");
    }

可以看到，如果我们没有在region下面配置zone的话，那么返回的String数组里默认一个值“defaultZone”。
接下来看getZoneOffset方法。

private static int getZoneOffset(String myZone, boolean preferSameZone, String[] availZones) {
        for(int i = 0; i < availZones.length; ++i) {
            if (myZone != null && availZones[i].equalsIgnoreCase(myZone.trim()) == preferSameZone) {
                return i;
            }
        }

        logger.warn("DISCOVERY: Could not pick a zone based on preferred zone settings. My zone - {}, preferSameZone - {}. Defaulting to {}", new Object[]{myZone, preferSameZone, availZones[0]});
        return 0;
    }

当preferSameZone为true是，返回myZone，也就是region下面第一个zone在service-url里面的index。当preferSameZone为false时，返回region下面第一个非myZone元素在service-url里面的index。
然后就是根据这个index获取service-url了。
但是getServiceUrlsFromConfig方法返回的是所有service-url的集合，刚才我们只是获取了一个url，获取所有url的方法可以看以下代码。

int currentOffset = myZoneOffset == (availZones.length - 1) ? 0 : (myZoneOffset + 1);
while (currentOffset != myZoneOffset) {
    serviceUrls = clientConfig.getEurekaServerServiceUrls(availZones[currentOffset]);
    if (serviceUrls != null) {
        orderedUrls.addAll(serviceUrls);
    }
    if (currentOffset == (availZones.length - 1)) {
        currentOffset = 0;
    } else {
        currentOffset++;
    }
}

这段代码的意思是，判断我们前面得到的那个zone是否是region下面所有zone集合的最后一个元素，如果是的话，给currentOffset赋值为0；如果不是的话，给currentOffset赋值为zone数组中下一个元素的index。后面进入while循环，跳出的条件是currentOffset==myZoneOffset。如果currentOffset到了zone数组的最后一位，那么给currentOffset赋值0，从头开始遍历zone数组，直到currentOffset==myZoneOffset。这样我们就获得了所有的url。

关于断点调试

要理清getServiceUrlsFromConfig方法的逻辑，最好的办法还是打断点进去一步一步跟代码。但是直接在getServiceUrlsFromConfig里面打断点，启动项目的时候不会走进去。但是在getServiceUrlsFromConfig下面有一个getServiceUrlsMapFromConfig方法，这个方法的逻辑和getServiceUrlsFromConfig完全一样，唯一不同的是返回值是Map而不是List。在项目启动初始化的时候会进入这个方法。为什么会走到getServiceUrlsMapFromConfig方法里呢？回到DiscoveryClient这个类里面。当DiscoveryClient初始化时，它的构造方法如下：

@Inject
DiscoveryClient(ApplicationInfoManager applicationInfoManager, EurekaClientConfig config, AbstractDiscoveryClientOptionalArgs args,
                Provider<BackupRegistry> backupRegistryProvider, EndpointRandomizer endpointRandomizer) {
    if (args != null) {
        this.healthCheckHandlerProvider = args.healthCheckHandlerProvider;
        this.healthCheckCallbackProvider = args.healthCheckCallbackProvider;
        this.eventListeners.addAll(args.getEventListeners());
        this.preRegistrationHandler = args.preRegistrationHandler;
    } else {
        this.healthCheckCallbackProvider = null;
        this.healthCheckHandlerProvider = null;
        this.preRegistrationHandler = 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.endpointRandomizer = endpointRandomizer;
    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;
    }

    logger.info("Initializing Eureka in region {}", clientConfig.getRegion());

    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();
        initRegistrySize = this.getApplications().size();
        registrySize = initRegistrySize;
        logger.info("Discovery Client initialized at timestamp {} with initial instances count: {}",
                initTimestampMs, initRegistrySize);

        return;  // no need to setup up an network tasks and we are done
    }

    try {
        // default size of 2 - 1 each for heartbeat and cacheRefresh
        scheduler = Executors.newScheduledThreadPool(2,
                new ThreadFactoryBuilder()
                        .setNameFormat("DiscoveryClient-%d")
                        .setDaemon(true)
                        .build());

        heartbeatExecutor = new ThreadPoolExecutor(
                1, clientConfig.getHeartbeatExecutorThreadPoolSize(), 0, TimeUnit.SECONDS,
                new SynchronousQueue<Runnable>(),
                new ThreadFactoryBuilder()
                        .setNameFormat("DiscoveryClient-HeartbeatExecutor-%d")
                        .setDaemon(true)
                        .build()
        );  // use direct handoff

        cacheRefreshExecutor = new ThreadPoolExecutor(
                1, clientConfig.getCacheRefreshExecutorThreadPoolSize(), 0, TimeUnit.SECONDS,
                new SynchronousQueue<Runnable>(),
                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();
    }

    // call and execute the pre registration handler before all background tasks (inc registration) is started
    if (this.preRegistrationHandler != null) {
        this.preRegistrationHandler.beforeRegistration();
    }

    if (clientConfig.shouldRegisterWithEureka() && clientConfig.shouldEnforceRegistrationAtInit()) {
        try {
            if (!register() ) {
                throw new IllegalStateException("Registration error at startup. Invalid server response.");
            }
        } catch (Throwable th) {
            logger.error("Registration error at startup: {}", th.getMessage());
            throw new IllegalStateException(th);
        }
    }

    // finally, init the schedule tasks (e.g. cluster resolvers, heartbeat, instanceInfo replicator, fetch
    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();
    initRegistrySize = this.getApplications().size();
    registrySize = initRegistrySize;
    logger.info("Discovery Client initialized at timestamp {} with initial instances count: {}",
            initTimestampMs, initRegistrySize);
}

里面有这样一段

eurekaTransport.bootstrapResolver = EurekaHttpClients.newBootstrapResolver(
                clientConfig,
                transportConfig,
                eurekaTransport.transportClientFactory,
                applicationInfoManager.getInfo(),
                applicationsSource,
                endpointRandomizer
        );

再进入newBootstrapResolver方法里。

public static ClosableResolver<AwsEndpoint> newBootstrapResolver(
            final EurekaClientConfig clientConfig,
            final EurekaTransportConfig transportConfig,
            final TransportClientFactory transportClientFactory,
            final InstanceInfo myInstanceInfo,
            final ApplicationsResolver.ApplicationsSource applicationsSource,
            final EndpointRandomizer randomizer) {
        if (COMPOSITE_BOOTSTRAP_STRATEGY.equals(transportConfig.getBootstrapResolverStrategy())) {
            if (clientConfig.shouldFetchRegistry()) {
                return compositeBootstrapResolver(
                        clientConfig,
                        transportConfig,
                        transportClientFactory,
                        myInstanceInfo,
                        applicationsSource,
                        randomizer
                );
            } else {
                logger.warn("Cannot create a composite bootstrap resolver if registry fetch is disabled." +
                        " Falling back to using a default bootstrap resolver.");
            }
        }

        // if all else fails, return the default
        return defaultBootstrapResolver(clientConfig, myInstanceInfo, randomizer);
    }

最后一句的引用了defaultBootstrapResolver方法，继续进入defaultBootstrapResolver里看一看。

/**
 * @return a bootstrap resolver that resolves eureka server endpoints based on either DNS or static config,
 *         depending on configuration for one or the other. This resolver will warm up at the start.
 */
static ClosableResolver<AwsEndpoint> defaultBootstrapResolver(final EurekaClientConfig clientConfig,
                                                              final InstanceInfo myInstanceInfo,
                                                              final EndpointRandomizer randomizer) {
    String[] availZones = clientConfig.getAvailabilityZones(clientConfig.getRegion());
    String myZone = InstanceInfo.getZone(availZones, myInstanceInfo);

    ClusterResolver<AwsEndpoint> delegateResolver = new ZoneAffinityClusterResolver(
            new ConfigClusterResolver(clientConfig, myInstanceInfo),
            myZone,
            true,
            randomizer
    );

    List<AwsEndpoint> initialValue = delegateResolver.getClusterEndpoints();
    if (initialValue.isEmpty()) {
        String msg = "Initial resolution of Eureka server endpoints failed. Check ConfigClusterResolver logs for more info";
        logger.error(msg);
        failFastOnInitCheck(clientConfig, msg);
    }

    return new AsyncResolver<>(
            EurekaClientNames.BOOTSTRAP,
            delegateResolver,
            initialValue,
            1,
            clientConfig.getEurekaServiceUrlPollIntervalSeconds() * 1000
    );
}

这个方法是用来解析eureka的端点，两种途径DNS和配置文件。
里面有List<AwsEndpoint> initialValue = delegateResolver.getClusterEndpoints();这么一句。getClusterEndpoints这个方法有多个实现，我们选择ConfigClusterResolver类中的实现方法。

public List<AwsEndpoint> getClusterEndpoints() {
    if (clientConfig.shouldUseDnsForFetchingServiceUrls()) {
        if (logger.isInfoEnabled()) {
            logger.info("Resolving eureka endpoints via DNS: {}", getDNSName());
        }
        return getClusterEndpointsFromDns();
    } else {
        logger.info("Resolving eureka endpoints via configuration");
        return getClusterEndpointsFromConfig();
    }
}

可以看到这个方法中飞为了从dns中获取url或者从config中获取。直接进入getClusterEndpointsFromConfig方法。

private List<AwsEndpoint> getClusterEndpointsFromConfig() {
    String[] availZones = clientConfig.getAvailabilityZones(clientConfig.getRegion());
    String myZone = InstanceInfo.getZone(availZones, myInstanceInfo);

    Map<String, List<String>> serviceUrls = EndpointUtils
            .getServiceUrlsMapFromConfig(clientConfig, myZone, clientConfig.shouldPreferSameZoneEureka());

    List<AwsEndpoint> endpoints = new ArrayList<>();
    for (String zone : serviceUrls.keySet()) {
        for (String url : serviceUrls.get(zone)) {
            try {
                endpoints.add(new AwsEndpoint(url, getRegion(), zone));
            } catch (Exception ignore) {
                logger.warn("Invalid eureka server URI: {}; removing from the server pool", url);
            }
        }
    }

    logger.debug("Config resolved to {}", endpoints);

    if (endpoints.isEmpty()) {
        logger.error("Cannot resolve to any endpoints from provided configuration: {}", serviceUrls);
    }

    return endpoints;
}

不容易，终于看到getServiceUrlsMapFromConfig的调用了。所以为了理解client是如何获取eureka server的url的，我们可以在启动client的时候在getServiceUrlsMapFromConfig里面打上端点调试。