spring cloud源码学习_use-dns-for-fetching-service-urls-优快云博客

本文链接：https://blog.youkuaiyun.com/duqingqing5666/article/details/119395052

eureka服务治理

spring-boot-starter-parent：2.5.3
spring-cloud-starter-netflix-eureka-server：2020.0.3

一、spring cloud服务端自动配置

1,使用eureka需要在类中添加@EnableEurekaServer注解，那么这个注解主要是干什么的？

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(EurekaServerMarkerConfiguration.class)
public @interface EnableEurekaServer {

}

我们可以看到主要就是导入了EurekaServerMarkerConfiguration类，那么这个类又干了什么？

@Configuration(proxyBeanMethods = false)
public class EurekaServerMarkerConfiguration {

	@Bean
	public Marker eurekaServerMarkerBean() {
		return new Marker();
	}

	class Marker {

	}

}

可以看到这个类是把空的marker类变成了一个spring管理的bean。那么这个bean是用来干什么的？

不急，我们去看看EurekaServerAutoConfiguration类。

判断EurekaServerMarkerConfiguration.Marker这个Bean是否存在。如果存在才会解析这个自动配置类EurekaServerAutoConfiguration。

@Configuration(proxyBeanMethods = false)
@Import(EurekaServerInitializerConfiguration.class)
@ConditionalOnBean(EurekaServerMarkerConfiguration.Marker.class)
@EnableConfigurationProperties({ EurekaDashboardProperties.class, InstanceRegistryProperties.class })
@PropertySource("classpath:/eureka/server.properties")
public class EurekaServerAutoConfiguration implements WebMvcConfigurer {

	/**
	 * List of packages containing Jersey resources required by the Eureka server.
	 */
	private static final String[] EUREKA_PACKAGES = new String[] { "com.netflix.discovery", "com.netflix.eureka" };

	@Autowired
	private ApplicationInfoManager applicationInfoManager;

	@Autowired
	private EurekaServerConfig eurekaServerConfig;

	@Autowired
	private EurekaClientConfig eurekaClientConfig;

	@Autowired
	private EurekaClient eurekaClient;

	@Autowired
	private InstanceRegistryProperties instanceRegistryProperties;

	/**
	 * A {@link CloudJacksonJson} instance.
	 */
	public static final CloudJacksonJson JACKSON_JSON = new CloudJacksonJson();

	@Bean
	public HasFeatures eurekaServerFeature() {
		return HasFeatures.namedFeature("Eureka Server", EurekaServerAutoConfiguration.class);
	}

	@Bean
	@ConditionalOnProperty(prefix = "eureka.dashboard", name = "enabled", matchIfMissing = true)
	public EurekaController eurekaController() {
		return new EurekaController(this.applicationInfoManager);
	}

	static {
		CodecWrappers.registerWrapper(JACKSON_JSON);
		EurekaJacksonCodec.setInstance(JACKSON_JSON.getCodec());
	}

	@Bean
	public ServerCodecs serverCodecs() {
		return new CloudServerCodecs(this.eurekaServerConfig);
	}

	private static CodecWrapper getFullJson(EurekaServerConfig serverConfig) {
		CodecWrapper codec = CodecWrappers.getCodec(serverConfig.getJsonCodecName());
		return codec == null ? CodecWrappers.getCodec(JACKSON_JSON.codecName()) : codec;
	}

	private static CodecWrapper getFullXml(EurekaServerConfig serverConfig) {
		CodecWrapper codec = CodecWrappers.getCodec(serverConfig.getXmlCodecName());
		return codec == null ? CodecWrappers.getCodec(CodecWrappers.XStreamXml.class) : codec;
	}

	@Bean
	@ConditionalOnMissingBean
	public ReplicationClientAdditionalFilters replicationClientAdditionalFilters() {
		return new ReplicationClientAdditionalFilters(Collections.emptySet());
	}

	@Bean
	public PeerAwareInstanceRegistry peerAwareInstanceRegistry(ServerCodecs serverCodecs) {
		this.eurekaClient.getApplications(); // force initialization
		return new InstanceRegistry(this.eurekaServerConfig, this.eurekaClientConfig, serverCodecs, this.eurekaClient,
				this.instanceRegistryProperties.getExpectedNumberOfClientsSendingRenews(),
				this.instanceRegistryProperties.getDefaultOpenForTrafficCount());
	}

	@Bean
	@ConditionalOnMissingBean
	public PeerEurekaNodes peerEurekaNodes(PeerAwareInstanceRegistry registry, ServerCodecs serverCodecs,
			ReplicationClientAdditionalFilters replicationClientAdditionalFilters) {
		return new RefreshablePeerEurekaNodes(registry, this.eurekaServerConfig, this.eurekaClientConfig, serverCodecs,
				this.applicationInfoManager, replicationClientAdditionalFilters);
	}

	@Bean
	@ConditionalOnMissingBean
	public EurekaServerContext eurekaServerContext(ServerCodecs serverCodecs, PeerAwareInstanceRegistry registry,
			PeerEurekaNodes peerEurekaNodes) {
		return new DefaultEurekaServerContext(this.eurekaServerConfig, serverCodecs, registry, peerEurekaNodes,
				this.applicationInfoManager);
	}

	@Bean
	public EurekaServerBootstrap eurekaServerBootstrap(PeerAwareInstanceRegistry registry,
			EurekaServerContext serverContext) {
		return new EurekaServerBootstrap(this.applicationInfoManager, this.eurekaClientConfig, this.eurekaServerConfig,
				registry, serverContext);
	}

	/**
	 * Register the Jersey filter.
	 * @param eurekaJerseyApp an {@link Application} for the filter to be registered
	 * @return a jersey {@link FilterRegistrationBean}
	 */
	@Bean
	public FilterRegistrationBean<?> jerseyFilterRegistration(javax.ws.rs.core.Application eurekaJerseyApp) {
		FilterRegistrationBean<Filter> bean = new FilterRegistrationBean<Filter>();
		bean.setFilter(new ServletContainer(eurekaJerseyApp));
		bean.setOrder(Ordered.LOWEST_PRECEDENCE);
		bean.setUrlPatterns(Collections.singletonList(EurekaConstants.DEFAULT_PREFIX + "/*"));

		return bean;
	}

	/**
	 * Construct a Jersey {@link javax.ws.rs.core.Application} with all the resources
	 * required by the Eureka server.
	 * @param environment an {@link Environment} instance to retrieve classpath resources
	 * @param resourceLoader a {@link ResourceLoader} instance to get classloader from
	 * @return created {@link Application} object
	 */
	@Bean
	public javax.ws.rs.core.Application jerseyApplication(Environment environment, ResourceLoader resourceLoader) {

		ClassPathScanningCandidateComponentProvider provider = new ClassPathScanningCandidateComponentProvider(false,
				environment);

		// Filter to include only classes that have a particular annotation.
		//
		provider.addIncludeFilter(new AnnotationTypeFilter(Path.class));
		provider.addIncludeFilter(new AnnotationTypeFilter(Provider.class));

		// Find classes in Eureka packages (or subpackages)
		//
		Set<Class<?>> classes = new HashSet<>();
		for (String basePackage : EUREKA_PACKAGES) {
			Set<BeanDefinition> beans = provider.findCandidateComponents(basePackage);
			for (BeanDefinition bd : beans) {
				Class<?> cls = ClassUtils.resolveClassName(bd.getBeanClassName(), resourceLoader.getClassLoader());
				classes.add(cls);
			}
		}

		// Construct the Jersey ResourceConfig
		Map<String, Object> propsAndFeatures = new HashMap<>();
		propsAndFeatures.put(
				// Skip static content used by the webapp
				ServletContainer.PROPERTY_WEB_PAGE_CONTENT_REGEX,
				EurekaConstants.DEFAULT_PREFIX + "/(fonts|images|css|js)/.*");

		DefaultResourceConfig rc = new DefaultResourceConfig(classes);
		rc.setPropertiesAndFeatures(propsAndFeatures);

		return rc;
	}

	@Bean
	@ConditionalOnBean(name = "httpTraceFilter")
	public FilterRegistrationBean<?> traceFilterRegistration(@Qualifier("httpTraceFilter") Filter filter) {
		FilterRegistrationBean<Filter> bean = new FilterRegistrationBean<Filter>();
		bean.setFilter(filter);
		bean.setOrder(Ordered.LOWEST_PRECEDENCE - 10);
		return bean;
	}

	@Configuration(proxyBeanMethods = false)
	protected static class EurekaServerConfigBeanConfiguration {

		@Bean
		@ConditionalOnMissingBean
		public EurekaServerConfig eurekaServerConfig(EurekaClientConfig clientConfig) {
			EurekaServerConfigBean server = new EurekaServerConfigBean();
			if (clientConfig.shouldRegisterWithEureka()) {
				// Set a sensible default if we are supposed to replicate
				server.setRegistrySyncRetries(5);
			}
			return server;
		}

	}

	/**
	 * {@link PeerEurekaNodes} which updates peers when /refresh is invoked. Peers are
	 * updated only if <code>eureka.client.use-dns-for-fetching-service-urls</code> is
	 * <code>false</code> and one of following properties have changed.
	 * <p>
	 * </p>
	 * <ul>
	 * <li><code>eureka.client.availability-zones</code></li>
	 * <li><code>eureka.client.region</code></li>
	 * <li><code>eureka.client.service-url.&lt;zone&gt;</code></li>
	 * </ul>
	 */
	static class RefreshablePeerEurekaNodes extends PeerEurekaNodes
			implements ApplicationListener<EnvironmentChangeEvent> {

		private ReplicationClientAdditionalFilters replicationClientAdditionalFilters;

		RefreshablePeerEurekaNodes(final PeerAwareInstanceRegistry registry, final EurekaServerConfig serverConfig,
				final EurekaClientConfig clientConfig, final ServerCodecs serverCodecs,
				final ApplicationInfoManager applicationInfoManager,
				final ReplicationClientAdditionalFilters replicationClientAdditionalFilters) {
			super(registry, serverConfig, clientConfig, serverCodecs, applicationInfoManager);
			this.replicationClientAdditionalFilters = replicationClientAdditionalFilters;
		}

		@Override
		protected PeerEurekaNode createPeerEurekaNode(String peerEurekaNodeUrl) {
			JerseyReplicationClient replicationClient = JerseyReplicationClient.createReplicationClient(serverConfig,
					serverCodecs, peerEurekaNodeUrl);

			this.replicationClientAdditionalFilters.getFilters().forEach(replicationClient::addReplicationClientFilter);

			String targetHost = hostFromUrl(peerEurekaNodeUrl);
			if (targetHost == null) {
				targetHost = "host";
			}
			return new PeerEurekaNode(registry, targetHost, peerEurekaNodeUrl, replicationClient, serverConfig);
		}

		@Override
		public void onApplicationEvent(final EnvironmentChangeEvent event) {
			if (shouldUpdate(event.getKeys())) {
				updatePeerEurekaNodes(resolvePeerUrls());
			}
		}

		/*
		 * Check whether specific properties have changed.
		 */
		protected boolean shouldUpdate(final Set<String> changedKeys) {
			assert changedKeys != null;

			// if eureka.client.use-dns-for-fetching-service-urls is true, then
			// service-url will not be fetched from environment.
			if (this.clientConfig.shouldUseDnsForFetchingServiceUrls()) {
				return false;
			}

			if (changedKeys.contains("eureka.client.region")) {
				return true;
			}

			for (final String key : changedKeys) {
				// property keys are not expected to be null.
				if (key.startsWith("eureka.client.service-url.")
						|| key.startsWith("eureka.client.availability-zones.")) {
					return true;
				}
			}
			return false;
		}

	}

	class CloudServerCodecs extends DefaultServerCodecs {

		CloudServerCodecs(EurekaServerConfig serverConfig) {
			super(getFullJson(serverConfig), CodecWrappers.getCodec(CodecWrappers.JacksonJsonMini.class),
					getFullXml(serverConfig), CodecWrappers.getCodec(CodecWrappers.JacksonXmlMini.class));
		}

	}

}

上面这个类上面的注解，加载了三个键值对的属性EurekaDashboardProperties、

InstanceRegistryProperties、classpath:/eureka/server.properties，并且导入了

EurekaServerInitializerConfiguration这个类。另外，还注入了如下的类

a.DashBoard--看板

b.ReplicationClientAdditionalFilters--复制客户端附加过滤器

c.PeerAwareInstanceRegistry--发现实例注册表（重要）

d.PeerEurekaNodes--RefreshablePeerEurekaNodes

e.EurekaServerContext--DefaultEurekaServerContext

f.EurekaServerBootstrap--启动引导

g.FilterRegistrationBean

h.Application#jerseyApplication--Jersey提供rpc调用

i:FilterRegistrationBean#traceFilterRegistration

@Configuration(proxyBeanMethods = false)
public class EurekaServerInitializerConfiguration implements ServletContextAware, SmartLifecycle, Ordered {
@Override
	public void start() {
		new Thread(() -> {
			try {
				// 初始化了EurekaEnvironment和EurekaServerContext（从相邻的集群节点当中同步注册信息、注册一个统计器）
				eurekaServerBootstrap.contextInitialized(EurekaServerInitializerConfiguration.this.servletContext);
				log.info("Started Eureka Server");

				publish(new EurekaRegistryAvailableEvent(getEurekaServerConfig()));
				EurekaServerInitializerConfiguration.this.running = true;
				publish(new EurekaServerStartedEvent(getEurekaServerConfig()));
			}
			catch (Exception ex) {
				// Help!
				log.error("Could not initialize Eureka servlet context", ex);
			}
		}).start();
	}
}

2.服务注册的入口

@Produces({"application/xml", "application/json"})
public class ApplicationResource {
 @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.getIPAddr())) {
            return Response.status(400).entity("Missing ip address").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
    }

}

public class InstanceRegistry extends PeerAwareInstanceRegistryImpl implements ApplicationContextAware {

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

父类的方法

@Singleton
public class PeerAwareInstanceRegistryImpl extends AbstractInstanceRegistry implements PeerAwareInstanceRegistry {
 @Override
    public void register(final InstanceInfo info, final boolean isReplication) {
        int leaseDuration = Lease.DEFAULT_DURATION_IN_SECS;
        if (info.getLeaseInfo() != null && info.getLeaseInfo().getDurationInSecs() > 0) {
            leaseDuration = info.getLeaseInfo().getDurationInSecs();
        }
        super.register(info, leaseDuration, isReplication);
        replicateToPeers(Action.Register, info.getAppName(), info.getId(), info, null, isReplication);
    }
}

它的父类的方法

public abstract class AbstractInstanceRegistry implements InstanceRegistry {


public void register(InstanceInfo registrant, int leaseDuration, boolean isReplication) {
        read.lock();
        try {
            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 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);

                // this is a > instead of a >= because if the timestamps are equal, we still take the remote transmitted
                // InstanceInfo instead of the server local copy.
                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.expectedNumberOfClientsSendingRenews > 0) {
                        // Since the client wants to register it, increase the number of clients sending renews
                        this.expectedNumberOfClientsSendingRenews = this.expectedNumberOfClientsSendingRenews + 1;
                        updateRenewsPerMinThreshold();
                    }
                }
                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);
            recentRegisteredQueue.add(new Pair<Long, String>(
                    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();
        }
    }

}

存储结构如下：

{
    "app名字": {
        "实例的唯一ID": { // 实例标识符
            "lease": { // 持有实例信息
                "instanceInfo": { // 实例信息
                    "appName": "app名字",
                    "instanceId": "实例的唯一ID",
                    "ipAddr": "IP地址",
                    "port": "调用端口"
                }
            }
        }
    }
}

2）复制到其他节点PeerAwareInstanceRegistryImpl#register里的第二步调用

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
            if (peerEurekaNodes == Collections.EMPTY_LIST || isReplication) {
                return;
            }

            for (final PeerEurekaNode node : peerEurekaNodes.getPeerEurekaNodes()) {
                // If the url represents this host, do not replicate to yourself.
                if (peerEurekaNodes.isThisMyUrl(node.getServiceUrl())) {
                    continue;
                }
                replicateInstanceActionsToPeers(action, appName, id, info, newStatus, node);
            }
        } finally {
            tracer.stop();
        }
    }

private void replicateInstanceActionsToPeers(Action action, String appName,
                                                 String id, InstanceInfo info, InstanceStatus newStatus,
                                                 PeerEurekaNode node) {
        try {
            InstanceInfo infoFromRegistry;
            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);
        } finally {
            CurrentRequestVersion.remove();
        }
    }

 public void register(final InstanceInfo info) throws Exception {
        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
        );
    }

public abstract class AbstractJerseyEurekaHttpClient implements EurekaHttpClient {
public EurekaHttpResponse<Void> register(InstanceInfo info) {
        String urlPath = "apps/" + info.getAppName();
        ClientResponse response = null;

        EurekaHttpResponse var5;
        try {
            Builder resourceBuilder = this.jerseyClient.resource(this.serviceUrl).path(urlPath).getRequestBuilder();
            this.addExtraHeaders(resourceBuilder);
            response = (ClientResponse)((Builder)((Builder)((Builder)resourceBuilder.header("Accept-Encoding", "gzip")).type(MediaType.APPLICATION_JSON_TYPE)).accept(new String[]{"application/json"})).post(ClientResponse.class, info);
            var5 = EurekaHttpResponse.anEurekaHttpResponse(response.getStatus()).headers(headersOf(response)).build();
        } finally {
            if (logger.isDebugEnabled()) {
                logger.debug("Jersey HTTP POST {}/{} with instance {}; statusCode={}", new Object[]{this.serviceUrl, urlPath, info.getId(), response == null ? "N/A" : response.getStatus()});
            }

            if (response != null) {
                response.close();
            }

        }

        return var5;
    }


}

3.服务端获取服务列表

ApplicationsResource#getContainers，从responseCache中拿数据

 @GET
    public Response getContainers(@PathParam("version") String version,
                                  @HeaderParam(HEADER_ACCEPT) String acceptHeader,
                                  @HeaderParam(HEADER_ACCEPT_ENCODING) String acceptEncoding,
                                  @HeaderParam(EurekaAccept.HTTP_X_EUREKA_ACCEPT) String eurekaAccept,
                                  @Context UriInfo uriInfo,
                                  @Nullable @QueryParam("regions") String regionsStr) {

        boolean isRemoteRegionRequested = null != regionsStr && !regionsStr.isEmpty();
        String[] regions = null;
        if (!isRemoteRegionRequested) {
            EurekaMonitors.GET_ALL.increment();
        } else {
            regions = regionsStr.toLowerCase().split(",");
            Arrays.sort(regions); // So we don't have different caches for same regions queried in different order.
            EurekaMonitors.GET_ALL_WITH_REMOTE_REGIONS.increment();
        }

        // Check if the server allows the access to the registry. The server can
        // restrict access if it is not
        // ready to serve traffic depending on various reasons.
        if (!registry.shouldAllowAccess(isRemoteRegionRequested)) {
            return Response.status(Status.FORBIDDEN).build();
        }
        CurrentRequestVersion.set(Version.toEnum(version));
        KeyType keyType = Key.KeyType.JSON;
        String returnMediaType = MediaType.APPLICATION_JSON;
        if (acceptHeader == null || !acceptHeader.contains(HEADER_JSON_VALUE)) {
            keyType = Key.KeyType.XML;
            returnMediaType = MediaType.APPLICATION_XML;
        }

        Key cacheKey = new Key(Key.EntityType.Application,
                ResponseCacheImpl.ALL_APPS,
                keyType, CurrentRequestVersion.get(), EurekaAccept.fromString(eurekaAccept), regions
        );

        Response response;
        if (acceptEncoding != null && acceptEncoding.contains(HEADER_GZIP_VALUE)) {
            response = Response.ok(responseCache.getGZIP(cacheKey))
                    .header(HEADER_CONTENT_ENCODING, HEADER_GZIP_VALUE)
                    .header(HEADER_CONTENT_TYPE, returnMediaType)
                    .build();
        } else {
            response = Response.ok(responseCache.get(cacheKey))
                    .build();
        }
        CurrentRequestVersion.remove();
        return response;
    }

最终调用的是ResponseCacheImpl中的getValue

 @VisibleForTesting
    Value getValue(final Key key, boolean useReadOnlyCache) {
        Value payload = null;
        try {
            if (useReadOnlyCache) {
                final Value currentPayload = readOnlyCacheMap.get(key);
                if (currentPayload != null) {
                    payload = currentPayload;
                } else {
                    payload = readWriteCacheMap.get(key);
                    readOnlyCacheMap.put(key, payload);
                }
            } else {
                payload = readWriteCacheMap.get(key);
            }
        } catch (Throwable t) {
            logger.error("Cannot get value for key : {}", key, t);
        }
        return payload;
    }

可以看到，是从readWriteCacheMap中获取值的。

this.readWriteCacheMap =
                CacheBuilder.newBuilder().initialCapacity(serverConfig.getInitialCapacityOfResponseCache())
                        .expireAfterWrite(serverConfig.getResponseCacheAutoExpirationInSeconds(), TimeUnit.SECONDS)
                        .removalListener(new RemovalListener<Key, Value>() {
                            @Override
                            public void onRemoval(RemovalNotification<Key, Value> notification) {
                                Key removedKey = notification.getKey();
                                if (removedKey.hasRegions()) {
                                    Key cloneWithNoRegions = removedKey.cloneWithoutRegions();
                                    regionSpecificKeys.remove(cloneWithNoRegions, removedKey);
                                }
                            }
                        })
                        .build(new CacheLoader<Key, Value>() {
                            @Override
                            public Value load(Key key) throws Exception {
                                if (key.hasRegions()) {
                                    Key cloneWithNoRegions = key.cloneWithoutRegions();
                                    regionSpecificKeys.put(cloneWithNoRegions, key);
                                }
                                Value value = generatePayload(key);
                                return value;
                            }
                        });

看下这个是怎么获取应用数据的

  /*
     * Generate pay load for the given key.
     */
    private Value generatePayload(Key key) {
        Stopwatch tracer = null;
        try {
            String payload;
            switch (key.getEntityType()) {
                case Application:
                    boolean isRemoteRegionRequested = key.hasRegions();

                    if (ALL_APPS.equals(key.getName())) {
                        if (isRemoteRegionRequested) {
                            tracer = serializeAllAppsWithRemoteRegionTimer.start();
                            payload = getPayLoad(key, registry.getApplicationsFromMultipleRegions(key.getRegions()));
                        } else {
                            tracer = serializeAllAppsTimer.start();
                            payload = getPayLoad(key, registry.getApplications());
                        }
                    } else if (ALL_APPS_DELTA.equals(key.getName())) {
                        if (isRemoteRegionRequested) {
                            tracer = serializeDeltaAppsWithRemoteRegionTimer.start();
                            versionDeltaWithRegions.incrementAndGet();
                            versionDeltaWithRegionsLegacy.incrementAndGet();
                            payload = getPayLoad(key,
                                    registry.getApplicationDeltasFromMultipleRegions(key.getRegions()));
                        } else {
                            tracer = serializeDeltaAppsTimer.start();
                            versionDelta.incrementAndGet();
                            versionDeltaLegacy.incrementAndGet();
                            payload = getPayLoad(key, registry.getApplicationDeltas());
                        }
                    } else {
                        tracer = serializeOneApptimer.start();
                        payload = getPayLoad(key, registry.getApplication(key.getName()));
                    }
                    break;
                case VIP:
                case SVIP:
                    tracer = serializeViptimer.start();
                    payload = getPayLoad(key, getApplicationsForVip(key, registry));
                    break;
                default:
                    logger.error("Unidentified entity type: {} found in the cache key.", key.getEntityType());
                    payload = "";
                    break;
            }
            return new Value(payload);
        } finally {
            if (tracer != null) {
                tracer.stop();
            }
        }
    }

核心方法是getPayLoad

    private String getPayLoad(Key key, Application app) {
        if (app == null) {
            return EMPTY_PAYLOAD;
        }

        EncoderWrapper encoderWrapper = serverCodecs.getEncoder(key.getType(), key.getEurekaAccept());
        try {
            return encoderWrapper.encode(app);
        } catch (Exception e) {
            logger.error("Failed to encode the payload for application {}", app.getName(), e);
            return "";
        }
    }

二、spring cloud客户端自动配置

@EnableDiscoveryClient注解，autoRegister默认为注册，
导入了EnableDiscoveryClientImportSelector类

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

导入了AutoServiceRegistrationConfiguration类

/*
 * Copyright 2012-2020 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.cloud.client.discovery;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.LinkedHashMap;
import java.util.List;

import org.springframework.cloud.commons.util.SpringFactoryImportSelector;
import org.springframework.core.Ordered;
import org.springframework.core.annotation.AnnotationAttributes;
import org.springframework.core.annotation.Order;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.env.Environment;
import org.springframework.core.env.MapPropertySource;
import org.springframework.core.type.AnnotationMetadata;

/**
 * @author Spencer Gibb
 */
@Order(Ordered.LOWEST_PRECEDENCE - 100)
public class EnableDiscoveryClientImportSelector extends SpringFactoryImportSelector<EnableDiscoveryClient> {

	@Override
	public String[] selectImports(AnnotationMetadata metadata) {
		String[] imports = super.selectImports(metadata);

		AnnotationAttributes attributes = AnnotationAttributes
				.fromMap(metadata.getAnnotationAttributes(getAnnotationClass().getName(), true));

		boolean autoRegister = attributes.getBoolean("autoRegister");

		if (autoRegister) {
			List<String> importsList = new ArrayList<>(Arrays.asList(imports));
			importsList.add("org.springframework.cloud.client.serviceregistry.AutoServiceRegistrationConfiguration");
			imports = importsList.toArray(new String[0]);
		}
		else {
			Environment env = getEnvironment();
			if (ConfigurableEnvironment.class.isInstance(env)) {
				ConfigurableEnvironment configEnv = (ConfigurableEnvironment) env;
				LinkedHashMap<String, Object> map = new LinkedHashMap<>();
				map.put("spring.cloud.service-registry.auto-registration.enabled", false);
				MapPropertySource propertySource = new MapPropertySource("springCloudDiscoveryClient", map);
				configEnv.getPropertySources().addLast(propertySource);
			}

		}
//["org.springframework.cloud.client.serviceregistry.AutoServiceRegistrationConfiguration"]

		return imports;
	}

	@Override
	protected boolean isEnabled() {
		return getEnvironment().getProperty("spring.cloud.discovery.enabled", Boolean.class, Boolean.TRUE);
	}

	@Override
	protected boolean hasDefaultFactory() {
		return true;
	}

}

上述类是导入了 AutoServiceRegistrationConfiguration类，我们来看看这个类做了些什么？

@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties(AutoServiceRegistrationProperties.class)
@ConditionalOnProperty(value = "spring.cloud.service-registry.auto-registration.enabled", matchIfMissing = true)
public class AutoServiceRegistrationConfiguration {

}

@ConfigurationProperties("spring.cloud.service-registry.auto-registration")
public class AutoServiceRegistrationProperties {

	/** Whether service auto-registration is enabled. Defaults to true. */
	private boolean enabled = true;

	/** Whether to register the management as a service. Defaults to true. */
	private boolean registerManagement = true;

	/**
	 * Whether startup fails if there is no AutoServiceRegistration. Defaults to false.
	 */
	private boolean failFast = false;

	public boolean isEnabled() {
		return this.enabled;
	}

	public void setEnabled(boolean enabled) {
		this.enabled = enabled;
	}

	public boolean isRegisterManagement() {
		return this.registerManagement;
	}

	public void setRegisterManagement(boolean registerManagement) {
		this.registerManagement = registerManagement;
	}

	@Deprecated
	public boolean shouldRegisterManagement() {
		return this.registerManagement;
	}

	public boolean isFailFast() {
		return this.failFast;
	}

	public void setFailFast(boolean failFast) {
		this.failFast = failFast;
	}

}

就是加载了一些属性。

下面来看spring.factories里默认加载的类EnableAutoConfiguration，该类中其实是加载了一些配置

/*
 * Copyright 2013-2020 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.cloud.netflix.eureka;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.util.Map;

import com.netflix.appinfo.ApplicationInfoManager;
import com.netflix.appinfo.EurekaInstanceConfig;
import com.netflix.appinfo.HealthCheckHandler;
import com.netflix.appinfo.InstanceInfo;
import com.netflix.discovery.AbstractDiscoveryClientOptionalArgs;
import com.netflix.discovery.EurekaClient;
import com.netflix.discovery.EurekaClientConfig;

import org.springframework.aop.support.AopUtils;
import org.springframework.beans.factory.ObjectProvider;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.actuate.autoconfigure.health.ConditionalOnEnabledHealthIndicator;
import org.springframework.boot.actuate.health.Health;
import org.springframework.boot.autoconfigure.AutoConfigureAfter;
import org.springframework.boot.autoconfigure.AutoConfigureBefore;
import org.springframework.boot.autoconfigure.condition.AnyNestedCondition;
import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.boot.autoconfigure.condition.SearchStrategy;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.cloud.autoconfigure.RefreshAutoConfiguration;
import org.springframework.cloud.client.CommonsClientAutoConfiguration;
import org.springframework.cloud.client.ConditionalOnDiscoveryEnabled;
import org.springframework.cloud.client.actuator.HasFeatures;
import org.springframework.cloud.client.serviceregistry.AutoServiceRegistrationProperties;
import org.springframework.cloud.client.serviceregistry.ServiceRegistryAutoConfiguration;
import org.springframework.cloud.commons.util.InetUtils;
import org.springframework.cloud.context.scope.refresh.RefreshScope;
import org.springframework.cloud.netflix.eureka.metadata.DefaultManagementMetadataProvider;
import org.springframework.cloud.netflix.eureka.metadata.ManagementMetadata;
import org.springframework.cloud.netflix.eureka.metadata.ManagementMetadataProvider;
import org.springframework.cloud.netflix.eureka.serviceregistry.EurekaAutoServiceRegistration;
import org.springframework.cloud.netflix.eureka.serviceregistry.EurekaRegistration;
import org.springframework.cloud.netflix.eureka.serviceregistry.EurekaServiceRegistry;
import org.springframework.cloud.util.ProxyUtils;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Lazy;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.util.StringUtils;

import static org.springframework.cloud.commons.util.IdUtils.getDefaultInstanceId;

/**
 * @author Dave Syer
 * @author Spencer Gibb
 * @author Jon Schneider
 * @author Matt Jenkins
 * @author Ryan Baxter
 * @author Daniel Lavoie
 * @author Olga Maciaszek-Sharma
 * @author Tim Ysewyn
 */
@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties
@ConditionalOnClass(EurekaClientConfig.class)
@ConditionalOnProperty(value = "eureka.client.enabled", matchIfMissing = true)
@ConditionalOnDiscoveryEnabled
@AutoConfigureBefore({ CommonsClientAutoConfiguration.class, ServiceRegistryAutoConfiguration.class })
@AutoConfigureAfter(name = { "org.springframework.cloud.netflix.eureka.config.DiscoveryClientOptionalArgsConfiguration",
		"org.springframework.cloud.autoconfigure.RefreshAutoConfiguration",
		"org.springframework.cloud.netflix.eureka.EurekaDiscoveryClientConfiguration",
		"org.springframework.cloud.client.serviceregistry.AutoServiceRegistrationAutoConfiguration" })
public class EurekaClientAutoConfiguration {

	private ConfigurableEnvironment env;

	public EurekaClientAutoConfiguration(ConfigurableEnvironment env) {
		this.env = env;
	}

	@Bean
	public HasFeatures eurekaFeature() {
		return HasFeatures.namedFeature("Eureka Client", EurekaClient.class);
	}

	@Bean
	@ConditionalOnMissingBean(value = EurekaClientConfig.class, search = SearchStrategy.CURRENT)
	public EurekaClientConfigBean eurekaClientConfigBean(ConfigurableEnvironment env) {
		return new EurekaClientConfigBean();
	}

	@Bean
	@ConditionalOnMissingBean
	public ManagementMetadataProvider serviceManagementMetadataProvider() {
		return new DefaultManagementMetadataProvider();
	}

	private String getProperty(String property) {
		return this.env.containsProperty(property) ? this.env.getProperty(property) : "";
	}
        //配置当前实例信息
	@Bean
	@ConditionalOnMissingBean(value = EurekaInstanceConfig.class, search = SearchStrategy.CURRENT)
	public EurekaInstanceConfigBean eurekaInstanceConfigBean(InetUtils inetUtils,
			ManagementMetadataProvider managementMetadataProvider) {
		String hostname = getProperty("eureka.instance.hostname");
		boolean preferIpAddress = Boolean.parseBoolean(getProperty("eureka.instance.prefer-ip-address"));
		String ipAddress = getProperty("eureka.instance.ip-address");
		boolean isSecurePortEnabled = Boolean.parseBoolean(getProperty("eureka.instance.secure-port-enabled"));

		String serverContextPath = env.getProperty("server.servlet.context-path", "/");
		int serverPort = Integer.parseInt(env.getProperty("server.port", env.getProperty("port", "8080")));

		Integer managementPort = env.getProperty("management.server.port", Integer.class);

		String managementContextPath = env.getProperty("management.server.servlet.context-path");
		if (!StringUtils.hasText(managementContextPath)) {
			managementContextPath = env.getProperty("management.server.base-path");
		}

		Integer jmxPort = env.getProperty("com.sun.management.jmxremote.port", Integer.class);
		EurekaInstanceConfigBean instance = new EurekaInstanceConfigBean(inetUtils);

		instance.setNonSecurePort(serverPort);
		instance.setInstanceId(getDefaultInstanceId(env));
		instance.setPreferIpAddress(preferIpAddress);
		instance.setSecurePortEnabled(isSecurePortEnabled);
		if (StringUtils.hasText(ipAddress)) {
			instance.setIpAddress(ipAddress);
		}

		if (isSecurePortEnabled) {
			instance.setSecurePort(serverPort);
		}

		if (StringUtils.hasText(hostname)) {
			instance.setHostname(hostname);
		}
		String statusPageUrlPath = getProperty("eureka.instance.status-page-url-path");
		String healthCheckUrlPath = getProperty("eureka.instance.health-check-url-path");

		if (StringUtils.hasText(statusPageUrlPath)) {
			instance.setStatusPageUrlPath(statusPageUrlPath);
		}
		if (StringUtils.hasText(healthCheckUrlPath)) {
			instance.setHealthCheckUrlPath(healthCheckUrlPath);
		}

		ManagementMetadata metadata = managementMetadataProvider.get(instance, serverPort, serverContextPath,
				managementContextPath, managementPort);

		if (metadata != null) {
			instance.setStatusPageUrl(metadata.getStatusPageUrl());
			instance.setHealthCheckUrl(metadata.getHealthCheckUrl());
			if (instance.isSecurePortEnabled()) {
				instance.setSecureHealthCheckUrl(metadata.getSecureHealthCheckUrl());
			}
			Map<String, String> metadataMap = instance.getMetadataMap();
			metadataMap.computeIfAbsent("management.port", k -> String.valueOf(metadata.getManagementPort()));
		}
		else {
			// without the metadata the status and health check URLs will not be set
			// and the status page and health check url paths will not include the
			// context path so set them here
			if (StringUtils.hasText(managementContextPath)) {
				instance.setHealthCheckUrlPath(managementContextPath + instance.getHealthCheckUrlPath());
				instance.setStatusPageUrlPath(managementContextPath + instance.getStatusPageUrlPath());
			}
		}

		setupJmxPort(instance, jmxPort);
		return instance;
	}

	private void setupJmxPort(EurekaInstanceConfigBean instance, Integer jmxPort) {
		Map<String, String> metadataMap = instance.getMetadataMap();
		if (metadataMap.get("jmx.port") == null && jmxPort != null) {
			metadataMap.put("jmx.port", String.valueOf(jmxPort));
		}
	}
        /**负责注册的bean
        **/
	@Bean
	public EurekaServiceRegistry eurekaServiceRegistry() {
		return new EurekaServiceRegistry();
	}
        //待注册的对象 
	// @Bean
	// @ConditionalOnBean(AutoServiceRegistrationProperties.class)
	// @ConditionalOnProperty(value =
	// "spring.cloud.service-registry.auto-registration.enabled", matchIfMissing = true)
	// public EurekaRegistration eurekaRegistration(EurekaClient eurekaClient,
	// CloudEurekaInstanceConfig instanceConfig, ApplicationInfoManager
	// applicationInfoManager, ObjectProvider<HealthCheckHandler> healthCheckHandler) {
	// return EurekaRegistration.builder(instanceConfig)
	// .with(applicationInfoManager)
	// .with(eurekaClient)
	// .with(healthCheckHandler)
	// .build();
	// }
      
       //自动注册的bean
	@Bean
	@ConditionalOnBean(AutoServiceRegistrationProperties.class)
	@ConditionalOnProperty(value = "spring.cloud.service-registry.auto-registration.enabled", matchIfMissing = true)
	public EurekaAutoServiceRegistration eurekaAutoServiceRegistration(ApplicationContext context,
			EurekaServiceRegistry registry, EurekaRegistration registration) {
		return new EurekaAutoServiceRegistration(context, registry, registration);
	}

	@Configuration(proxyBeanMethods = false)
	@ConditionalOnMissingRefreshScope
	protected static class EurekaClientConfiguration {

		@Autowired
		private ApplicationContext context;

		@Autowired
		private AbstractDiscoveryClientOptionalArgs<?> optionalArgs;
        
//这个类里会加载服务端的注册中心列表
		@Bean(destroyMethod = "shutdown")
		@ConditionalOnMissingBean(value = EurekaClient.class, search = SearchStrategy.CURRENT)
		public EurekaClient eurekaClient(ApplicationInfoManager manager, EurekaClientConfig config) {
			return new CloudEurekaClient(manager, config, this.optionalArgs, this.context);
		}

		@Bean
		@ConditionalOnMissingBean(value = ApplicationInfoManager.class, search = SearchStrategy.CURRENT)
		public ApplicationInfoManager eurekaApplicationInfoManager(EurekaInstanceConfig config) {
			InstanceInfo instanceInfo = new InstanceInfoFactory().create(config);
			return new ApplicationInfoManager(config, instanceInfo);
		}
                /**
                Eureka待注册的对象
                  **/
		@Bean
		@ConditionalOnBean(AutoServiceRegistrationProperties.class)
		@ConditionalOnProperty(value = "spring.cloud.service-registry.auto-registration.enabled", matchIfMissing = true)
		public EurekaRegistration eurekaRegistration(EurekaClient eurekaClient,
				CloudEurekaInstanceConfig instanceConfig, ApplicationInfoManager applicationInfoManager,
				@Autowired(required = false) ObjectProvider<HealthCheckHandler> healthCheckHandler) {
			return EurekaRegistration.builder(instanceConfig).with(applicationInfoManager).with(eurekaClient)
					.with(healthCheckHandler).build();
		}

	}

	@Configuration(proxyBeanMethods = false)
	@ConditionalOnRefreshScope
	protected static class RefreshableEurekaClientConfiguration {

		@Autowired
		private ApplicationContext context;

		@Autowired
		private AbstractDiscoveryClientOptionalArgs<?> optionalArgs;

		@Bean(destroyMethod = "shutdown")
		@ConditionalOnMissingBean(value = EurekaClient.class, search = SearchStrategy.CURRENT)
		@org.springframework.cloud.context.config.annotation.RefreshScope
		@Lazy
		public EurekaClient eurekaClient(ApplicationInfoManager manager, EurekaClientConfig config,
				EurekaInstanceConfig instance, @Autowired(required = false) HealthCheckHandler healthCheckHandler) {
			// If we use the proxy of the ApplicationInfoManager we could run into a
			// problem
			// when shutdown is called on the CloudEurekaClient where the
			// ApplicationInfoManager bean is
			// requested but wont be allowed because we are shutting down. To avoid this
			// we use the
			// object directly.
			ApplicationInfoManager appManager;
			if (AopUtils.isAopProxy(manager)) {
				appManager = ProxyUtils.getTargetObject(manager);
			}
			else {
				appManager = manager;
			}
			CloudEurekaClient cloudEurekaClient = new CloudEurekaClient(appManager, config, this.optionalArgs,
					this.context);
			cloudEurekaClient.registerHealthCheck(healthCheckHandler);
			return cloudEurekaClient;
		}

		@Bean
		@ConditionalOnMissingBean(value = ApplicationInfoManager.class, search = SearchStrategy.CURRENT)
		@org.springframework.cloud.context.config.annotation.RefreshScope
		@Lazy
		public ApplicationInfoManager eurekaApplicationInfoManager(EurekaInstanceConfig config) {
			InstanceInfo instanceInfo = new InstanceInfoFactory().create(config);
			return new ApplicationInfoManager(config, instanceInfo);
		}

		@Bean
		@org.springframework.cloud.context.config.annotation.RefreshScope
		@ConditionalOnBean(AutoServiceRegistrationProperties.class)
		@ConditionalOnProperty(value = "spring.cloud.service-registry.auto-registration.enabled", matchIfMissing = true)
		public EurekaRegistration eurekaRegistration(EurekaClient eurekaClient,
				CloudEurekaInstanceConfig instanceConfig, ApplicationInfoManager applicationInfoManager,
				@Autowired(required = false) ObjectProvider<HealthCheckHandler> healthCheckHandler) {
			return EurekaRegistration.builder(instanceConfig).with(applicationInfoManager).with(eurekaClient)
					.with(healthCheckHandler).build();
		}

	}

	@Target({ ElementType.TYPE, ElementType.METHOD })
	@Retention(RetentionPolicy.RUNTIME)
	@Documented
	@Conditional(OnMissingRefreshScopeCondition.class)
	@interface ConditionalOnMissingRefreshScope {

	}

	@Target({ ElementType.TYPE, ElementType.METHOD })
	@Retention(RetentionPolicy.RUNTIME)
	@Documented
	@ConditionalOnClass(RefreshScope.class)
	@ConditionalOnBean(RefreshAutoConfiguration.class)
	@ConditionalOnProperty(value = "eureka.client.refresh.enable", havingValue = "true", matchIfMissing = true)
	@interface ConditionalOnRefreshScope {

	}

	private static class OnMissingRefreshScopeCondition extends AnyNestedCondition {

		OnMissingRefreshScopeCondition() {
			super(ConfigurationPhase.REGISTER_BEAN);
		}

		@ConditionalOnMissingClass("org.springframework.cloud.context.scope.refresh.RefreshScope")
		static class MissingClass {

		}

		@ConditionalOnMissingBean(RefreshAutoConfiguration.class)
		static class MissingScope {

		}

		@ConditionalOnProperty(value = "eureka.client.refresh.enable", havingValue = "false")
		static class OnPropertyDisabled {

		}

	}

	@Configuration(proxyBeanMethods = false)
	@ConditionalOnClass(Health.class)
	protected static class EurekaHealthIndicatorConfiguration {

		@Bean
		@ConditionalOnMissingBean
		@ConditionalOnEnabledHealthIndicator("eureka")
		public EurekaHealthIndicator eurekaHealthIndicator(EurekaClient eurekaClient,
				EurekaInstanceConfig instanceConfig, EurekaClientConfig clientConfig) {
			return new EurekaHealthIndicator(eurekaClient, instanceConfig, clientConfig);
		}

	}

}

EurekaAutoServiceRegistration类是负责自动注册的，下面是它的源码

/*
 * Copyright 2013-2020 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.cloud.netflix.eureka.serviceregistry;

import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.boot.web.context.WebServerInitializedEvent;
import org.springframework.cloud.client.discovery.event.InstanceRegisteredEvent;
import org.springframework.cloud.client.serviceregistry.AutoServiceRegistration;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationEvent;
import org.springframework.context.SmartLifecycle;
import org.springframework.context.event.ContextClosedEvent;
import org.springframework.context.event.SmartApplicationListener;
import org.springframework.core.Ordered;

/**
 * @author Dave Syer
 * @author Spencer Gibb
 * @author Jon Schneider
 * @author Jakub Narloch
 * @author Raiyan Raiyan
 */
public class EurekaAutoServiceRegistration
		implements AutoServiceRegistration, SmartLifecycle, Ordered, SmartApplicationListener {

	private static final Log log = LogFactory.getLog(EurekaAutoServiceRegistration.class);

	private AtomicBoolean running = new AtomicBoolean(false);

	private int order = 0;

	private AtomicInteger port = new AtomicInteger(0);

	private ApplicationContext context;

	private EurekaServiceRegistry serviceRegistry;

	private EurekaRegistration registration;

	public EurekaAutoServiceRegistration(ApplicationContext context, EurekaServiceRegistry serviceRegistry,
			EurekaRegistration registration) {
		this.context = context;
		this.serviceRegistry = serviceRegistry;
		this.registration = registration;
	}

	@Override
	public void start() {
		// only set the port if the nonSecurePort or securePort is 0 and this.port != 0
		if (this.port.get() != 0) {
			if (this.registration.getNonSecurePort() == 0) {
				this.registration.setNonSecurePort(this.port.get());
			}

			if (this.registration.getSecurePort() == 0 && this.registration.isSecure()) {
				this.registration.setSecurePort(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.registration.getNonSecurePort() > 0) {

			this.serviceRegistry.register(this.registration);

			this.context.publishEvent(new InstanceRegisteredEvent<>(this, this.registration.getInstanceConfig()));
			this.running.set(true);
		}
	}

	@Override
	public void stop() {
		this.serviceRegistry.deregister(this.registration);
		this.running.set(false);
	}

	@Override
	public boolean isRunning() {
		return this.running.get();
	}

	@Override
	public int getPhase() {
		return 0;
	}

	@Override
	public boolean isAutoStartup() {
		return true;
	}

	@Override
	public void stop(Runnable callback) {
		stop();
		callback.run();
	}

	@Override
	public int getOrder() {
		return this.order;
	}

	@Override
	public boolean supportsEventType(Class<? extends ApplicationEvent> eventType) {
		return WebServerInitializedEvent.class.isAssignableFrom(eventType)
				|| ContextClosedEvent.class.isAssignableFrom(eventType);
	}

	@Override
	public void onApplicationEvent(ApplicationEvent event) {
		if (event instanceof WebServerInitializedEvent) {
			onApplicationEvent((WebServerInitializedEvent) event);
		}
		else if (event instanceof ContextClosedEvent) {
			onApplicationEvent((ContextClosedEvent) event);
		}
	}

	public void onApplicationEvent(WebServerInitializedEvent event) {
		// TODO: take SSL into account
		String contextName = event.getApplicationContext().getServerNamespace();
		if (contextName == null || !contextName.equals("management")) {
			int localPort = event.getWebServer().getPort();
			if (this.port.get() == 0) {
				log.info("Updating port to " + localPort);
				this.port.compareAndSet(0, localPort);
				start();
			}
		}
	}

	public void onApplicationEvent(ContextClosedEvent event) {
		if (event.getApplicationContext() == context) {
			stop();
		}
	}

}

其中这句this.serviceRegistry.register(this.registration);实际调用的是下面这个类中的方法

/*
 * Copyright 2013-2020 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.cloud.netflix.eureka.serviceregistry;

import java.util.HashMap;

import com.netflix.appinfo.InstanceInfo;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.cloud.client.serviceregistry.ServiceRegistry;

import static com.netflix.appinfo.InstanceInfo.InstanceStatus.UNKNOWN;

/**
 * @author Spencer Gibb
 */
public class EurekaServiceRegistry implements ServiceRegistry<EurekaRegistration> {

	private static final Log log = LogFactory.getLog(EurekaServiceRegistry.class);

	@Override
	public void register(EurekaRegistration reg) {
		maybeInitializeClient(reg);

		if (log.isInfoEnabled()) {
			log.info("Registering application " + reg.getApplicationInfoManager().getInfo().getAppName()
					+ " with eureka with status " + reg.getInstanceConfig().getInitialStatus());
		}

		reg.getApplicationInfoManager().setInstanceStatus(reg.getInstanceConfig().getInitialStatus());

		reg.getHealthCheckHandler()
				.ifAvailable(healthCheckHandler -> reg.getEurekaClient().registerHealthCheck(healthCheckHandler));
	}

	private void maybeInitializeClient(EurekaRegistration reg) {
		// force initialization of possibly scoped proxies
		reg.getApplicationInfoManager().getInfo();
		reg.getEurekaClient().getApplications();
	}

	@Override
	public void deregister(EurekaRegistration reg) {
		if (reg.getApplicationInfoManager().getInfo() != null) {

			if (log.isInfoEnabled()) {
				log.info("Unregistering application " + reg.getApplicationInfoManager().getInfo().getAppName()
						+ " with eureka with status DOWN");
			}

			reg.getApplicationInfoManager().setInstanceStatus(InstanceInfo.InstanceStatus.DOWN);

			// shutdown of eureka client should happen with EurekaRegistration.close()
			// auto registration will create a bean which will be properly disposed
			// manual registrations will need to call close()
		}
	}

	@Override
	public void setStatus(EurekaRegistration registration, String status) {
		InstanceInfo info = registration.getApplicationInfoManager().getInfo();

		// TODO: howto deal with delete properly?
		if ("CANCEL_OVERRIDE".equalsIgnoreCase(status)) {
			registration.getEurekaClient().cancelOverrideStatus(info);
			return;
		}

		// TODO: howto deal with status types across discovery systems?
		InstanceInfo.InstanceStatus newStatus = InstanceInfo.InstanceStatus.toEnum(status);
		registration.getEurekaClient().setStatus(newStatus, info);
	}

	@Override
	public Object getStatus(EurekaRegistration registration) {
		String appname = registration.getApplicationInfoManager().getInfo().getAppName();
		String instanceId = registration.getApplicationInfoManager().getInfo().getId();
		InstanceInfo info = registration.getEurekaClient().getInstanceInfo(appname, instanceId);

		HashMap<String, Object> status = new HashMap<>();
		if (info != null) {
			status.put("status", info.getStatus().toString());
			status.put("overriddenStatus", info.getOverriddenStatus().toString());
		}
		else {
			status.put("status", UNKNOWN.toString());
		}

		return status;
	}

	public void close() {
	}

}

DiscoveryClient里的registerHealthCheck方法

@Override
    public void registerHealthCheck(HealthCheckHandler healthCheckHandler) {
        if (instanceInfo == null) {
            logger.error("Cannot register a healthcheck handler when instance info is null!");
        }
        if (healthCheckHandler != null) {
            this.healthCheckHandlerRef.set(healthCheckHandler);
            // schedule an onDemand update of the instanceInfo when a new healthcheck handler is registered
            if (instanceInfoReplicator != null) {
                instanceInfoReplicator.onDemandUpdate();
            }
        }
    }

InstanceInfoReplicator#onDemandUpdate

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);
                        }
                        //单线程异步执行run方法    
                        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;
        }
    }

    public void run() {
        try {
            //刷新实例
            discoveryClient.refreshInstanceInfo();

            Long dirtyTimestamp = instanceInfo.isDirtyWithTime();
            if (dirtyTimestamp != null) {
//注册
                discoveryClient.register();
                instanceInfo.unsetIsDirty(dirtyTimestamp);
            }
        } catch (Throwable t) {
            logger.warn("There was a problem with the instance info replicator", t);
        } finally {
            //下一次延迟执行，默认每隔30s执行一次
            Future next = scheduler.schedule(this, replicationIntervalSeconds, TimeUnit.SECONDS);
            scheduledPeriodicRef.set(next);
        }
    }

DiscoveryClient#register方法

    boolean register() throws Throwable {
        logger.info(PREFIX + "{}: registering service...", appPathIdentifier);
        EurekaHttpResponse<Void> httpResponse;
        try {
            httpResponse = eurekaTransport.registrationClient.register(instanceInfo);
        } catch (Exception e) {
            logger.warn(PREFIX + "{} - registration failed {}", appPathIdentifier, e.getMessage(), e);
            throw e;
        }
        if (logger.isInfoEnabled()) {
            logger.info(PREFIX + "{} - registration status: {}", appPathIdentifier, httpResponse.getStatusCode());
        }
        return httpResponse.getStatusCode() == Status.NO_CONTENT.getStatusCode();
    }

调用注册客户端的register方法，发送http请求

 @Override
    public 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)
                    .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();
            }
        }
    }

三、openFeign

@EnableFeignClients注解，导入FeignClientsRegistrar类

/*
 * Copyright 2013-2020 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.cloud.openfeign;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import org.springframework.context.annotation.Import;

/**
 * Scans for interfaces that declare they are feign clients (via
 * {@link org.springframework.cloud.openfeign.FeignClient} <code>@FeignClient</code>).
 * Configures component scanning directives for use with
 * {@link org.springframework.context.annotation.Configuration}
 * <code>@Configuration</code> classes.
 *
 * @author Spencer Gibb
 * @author Dave Syer
 * @since 1.0
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(FeignClientsRegistrar.class)
public @interface EnableFeignClients {

	/**
	 * Alias for the {@link #basePackages()} attribute. Allows for more concise annotation
	 * declarations e.g.: {@code @ComponentScan("org.my.pkg")} instead of
	 * {@code @ComponentScan(basePackages="org.my.pkg")}.
	 * @return the array of 'basePackages'.
	 */
	String[] value() default {};

	/**
	 * Base packages to scan for annotated components.
	 * <p>
	 * {@link #value()} is an alias for (and mutually exclusive with) this attribute.
	 * <p>
	 * Use {@link #basePackageClasses()} for a type-safe alternative to String-based
	 * package names.
	 * @return the array of 'basePackages'.
	 */
	String[] basePackages() default {};

	/**
	 * Type-safe alternative to {@link #basePackages()} for specifying the packages to
	 * scan for annotated components. The package of each class specified will be scanned.
	 * <p>
	 * Consider creating a special no-op marker class or interface in each package that
	 * serves no purpose other than being referenced by this attribute.
	 * @return the array of 'basePackageClasses'.
	 */
	Class<?>[] basePackageClasses() default {};

	/**
	 * A custom <code>@Configuration</code> for all feign clients. Can contain override
	 * <code>@Bean</code> definition for the pieces that make up the client, for instance
	 * {@link feign.codec.Decoder}, {@link feign.codec.Encoder}, {@link feign.Contract}.
	 *
	 * @see FeignClientsConfiguration for the defaults
	 * @return list of default configurations
	 */
	Class<?>[] defaultConfiguration() default {};

	/**
	 * List of classes annotated with @FeignClient. If not empty, disables classpath
	 * scanning.
	 * @return list of FeignClient classes
	 */
	Class<?>[] clients() default {};

}

向Bean容器种注册添加BeanDefinition

class FeignClientsRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware, EnvironmentAware {
@Override
	public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
         // 注册默认配置
		registerDefaultConfiguration(metadata, registry);
    // 注册FeignClient接口的Bean
		registerFeignClients(metadata, registry);
	}

private void registerDefaultConfiguration(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
		Map<String, Object> defaultAttrs = metadata.getAnnotationAttributes(EnableFeignClients.class.getName(), true);

		if (defaultAttrs != null && defaultAttrs.containsKey("defaultConfiguration")) {
			String name;
			if (metadata.hasEnclosingClass()) {
				name = "default." + metadata.getEnclosingClassName();
			}
			else {
				name = "default." + metadata.getClassName();
			}
			registerClientConfiguration(registry, name, defaultAttrs.get("defaultConfiguration"));
		}
	}

public void registerFeignClients(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {

		LinkedHashSet<BeanDefinition> candidateComponents = new LinkedHashSet<>();
		Map<String, Object> attrs = metadata.getAnnotationAttributes(EnableFeignClients.class.getName());
		final Class<?>[] clients = attrs == null ? null : (Class<?>[]) attrs.get("clients");
		if (clients == null || clients.length == 0) {
			ClassPathScanningCandidateComponentProvider scanner = getScanner();
			scanner.setResourceLoader(this.resourceLoader);
			scanner.addIncludeFilter(new AnnotationTypeFilter(FeignClient.class));
			Set<String> basePackages = getBasePackages(metadata);
			for (String basePackage : basePackages) {
				candidateComponents.addAll(scanner.findCandidateComponents(basePackage));
			}
		}
		else {
			for (Class<?> clazz : clients) {
				candidateComponents.add(new AnnotatedGenericBeanDefinition(clazz));
			}
		}

		for (BeanDefinition candidateComponent : candidateComponents) {
			if (candidateComponent instanceof AnnotatedBeanDefinition) {
				// verify annotated class is an interface
				AnnotatedBeanDefinition beanDefinition = (AnnotatedBeanDefinition) candidateComponent;
				AnnotationMetadata annotationMetadata = beanDefinition.getMetadata();
				Assert.isTrue(annotationMetadata.isInterface(), "@FeignClient can only be specified on an interface");

				Map<String, Object> attributes = annotationMetadata
						.getAnnotationAttributes(FeignClient.class.getCanonicalName());

				String name = getClientName(attributes);
				registerClientConfiguration(registry, name, attributes.get("configuration"));

				registerFeignClient(registry, annotationMetadata, attributes);
			}
		}
	}
}

扫描了@FeignClient注解的接口，生成FeignClientFactoryBean的BeanDefinition注册到容器当中。而具体的Bean对象，将会通过调用FeignClientFactoryBean的getObject->getTarget方法来获取

<T> T getTarget() {
		FeignContext context = beanFactory != null ? beanFactory.getBean(FeignContext.class)
				: applicationContext.getBean(FeignContext.class);
		Feign.Builder builder = feign(context);

		if (!StringUtils.hasText(url)) {
			if (url != null && LOG.isWarnEnabled()) {
				LOG.warn("The provided URL is empty. Will try picking an instance via load-balancing.");
			}
			else if (LOG.isDebugEnabled()) {
				LOG.debug("URL not provided. Will use LoadBalancer.");
			}
			if (!name.startsWith("http")) {
				url = "http://" + name;
			}
			else {
				url = name;
			}
			url += cleanPath();
			return (T) loadBalance(builder, context, new HardCodedTarget<>(type, name, url));
		}
		if (StringUtils.hasText(url) && !url.startsWith("http")) {
			url = "http://" + url;
		}
		String url = this.url + cleanPath();
		Client client = getOptional(context, Client.class);
		if (client != null) {
			if (client instanceof FeignBlockingLoadBalancerClient) {
				// not load balancing because we have a url,
				// but Spring Cloud LoadBalancer is on the classpath, so unwrap
				client = ((FeignBlockingLoadBalancerClient) client).getDelegate();
			}
			if (client instanceof RetryableFeignBlockingLoadBalancerClient) {
				// not load balancing because we have a url,
				// but Spring Cloud LoadBalancer is on the classpath, so unwrap
				client = ((RetryableFeignBlockingLoadBalancerClient) client).getDelegate();
			}
			builder.client(client);
		}
		Targeter targeter = get(context, Targeter.class);
		return (T) targeter.target(this, builder, context, new HardCodedTarget<>(type, name, url));
	}


protected <T> T loadBalance(Feign.Builder builder, FeignContext context, HardCodedTarget<T> target) {
		Client client = getOptional(context, Client.class);
		if (client != null) {
			builder.client(client);
			Targeter targeter = get(context, Targeter.class);
			return targeter.target(this, builder, context, target);
		}

		throw new IllegalStateException(
				"No Feign Client for loadBalancing defined. Did you forget to include spring-cloud-starter-loadbalancer?");
	}

	@Override
	public <T> T target(FeignClientFactoryBean factory, Feign.Builder feign, FeignContext context,
			Target.HardCodedTarget<T> target) {
		return feign.target(target);
	}

public abstract class Feign {
public <T> T target(Target<T> target) {
      return build().newInstance(target);
    }

    public Feign build() {
      Client client = Capability.enrich(this.client, capabilities);
      Retryer retryer = Capability.enrich(this.retryer, capabilities);
      List<RequestInterceptor> requestInterceptors = this.requestInterceptors.stream()
          .map(ri -> Capability.enrich(ri, capabilities))
          .collect(Collectors.toList());
      Logger logger = Capability.enrich(this.logger, capabilities);
      Contract contract = Capability.enrich(this.contract, capabilities);
      Options options = Capability.enrich(this.options, capabilities);
      Encoder encoder = Capability.enrich(this.encoder, capabilities);
      Decoder decoder = Capability.enrich(this.decoder, capabilities);
      InvocationHandlerFactory invocationHandlerFactory =
          Capability.enrich(this.invocationHandlerFactory, capabilities);
      QueryMapEncoder queryMapEncoder = Capability.enrich(this.queryMapEncoder, capabilities);

      SynchronousMethodHandler.Factory synchronousMethodHandlerFactory =
          new SynchronousMethodHandler.Factory(client, retryer, requestInterceptors, logger,
              logLevel, decode404, closeAfterDecode, propagationPolicy, forceDecoding);
      ParseHandlersByName handlersByName =
          new ParseHandlersByName(contract, options, encoder, decoder, queryMapEncoder,
              errorDecoder, synchronousMethodHandlerFactory);
      return new ReflectiveFeign(handlersByName, invocationHandlerFactory, queryMapEncoder);
    }
  }

}

jdk的动态代理获取的代理对象

public class ReflectiveFeign extends Feign {
@Override
  public <T> T newInstance(Target<T> target) {
    Map<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target);
    Map<Method, MethodHandler> methodToHandler = new LinkedHashMap<Method, MethodHandler>();
    List<DefaultMethodHandler> defaultMethodHandlers = new LinkedList<DefaultMethodHandler>();

    for (Method method : target.type().getMethods()) {
      if (method.getDeclaringClass() == Object.class) {
        continue;
      } else if (Util.isDefault(method)) {
        DefaultMethodHandler handler = new DefaultMethodHandler(method);
        defaultMethodHandlers.add(handler);
//MethodHandler将包含发起http请求的实现
        methodToHandler.put(method, handler);
      } else {
//MethodHandler将包含发起http请求的实现
        methodToHandler.put(method, nameToHandler.get(Feign.configKey(target.type(), method)));
      }
    }

    InvocationHandler handler = factory.create(target, methodToHandler);
    T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(),
        new Class<?>[] {target.type()}, handler);

    for (DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) {
      defaultMethodHandler.bindTo(proxy);
    }
    return proxy;
  }
}

public interface InvocationHandlerFactory {

  InvocationHandler create(Target target, Map<Method, MethodHandler> dispatch);

  /**
   * Like {@link InvocationHandler#invoke(Object, java.lang.reflect.Method, Object[])}, except for a
   * single method.
   */
  interface MethodHandler {

    Object invoke(Object[] argv) throws Throwable;
  }

  static final class Default implements InvocationHandlerFactory {

    @Override
    public InvocationHandler create(Target target, Map<Method, MethodHandler> dispatch) {
      return new ReflectiveFeign.FeignInvocationHandler(target, dispatch);
    }
  }
}

调用FeignInvocationHandler的invoke方法

  static class FeignInvocationHandler implements InvocationHandler {
@Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      if ("equals".equals(method.getName())) {
        try {
          Object otherHandler =
              args.length > 0 && args[0] != null ? Proxy.getInvocationHandler(args[0]) : null;
          return equals(otherHandler);
        } catch (IllegalArgumentException e) {
          return false;
        }
      } else if ("hashCode".equals(method.getName())) {
        return hashCode();
      } else if ("toString".equals(method.getName())) {
        return toString();
      }

      return dispatch.get(method).invoke(args);
    }
}

SynchronousMethodHandler#invoke发起http请求

final class SynchronousMethodHandler implements MethodHandler {
 @Override
  public Object invoke(Object[] argv) throws Throwable {
    RequestTemplate template = buildTemplateFromArgs.create(argv);
    Options options = findOptions(argv);
    Retryer retryer = this.retryer.clone();
    while (true) {
      try {
        return executeAndDecode(template, options);
      } catch (RetryableException e) {
        try {
          retryer.continueOrPropagate(e);
        } catch (RetryableException th) {
          Throwable cause = th.getCause();
          if (propagationPolicy == UNWRAP && cause != null) {
            throw cause;
          } else {
            throw th;
          }
        }
        if (logLevel != Logger.Level.NONE) {
          logger.logRetry(metadata.configKey(), logLevel);
        }
        continue;
      }
    }
  }

  Object executeAndDecode(RequestTemplate template, Options options) throws Throwable {
    Request request = targetRequest(template);

    if (logLevel != Logger.Level.NONE) {
      logger.logRequest(metadata.configKey(), logLevel, request);
    }

    Response response;
    long start = System.nanoTime();
    try {
      response = client.execute(request, options);
      // ensure the request is set. TODO: remove in Feign 12
      response = response.toBuilder()
          .request(request)
          .requestTemplate(template)
          .build();
    } catch (IOException e) {
      if (logLevel != Logger.Level.NONE) {
        logger.logIOException(metadata.configKey(), logLevel, e, elapsedTime(start));
      }
      throw errorExecuting(request, e);
    }
    long elapsedTime = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - start);


    if (decoder != null)
      return decoder.decode(response, metadata.returnType());

    CompletableFuture<Object> resultFuture = new CompletableFuture<>();
    asyncResponseHandler.handleResponse(resultFuture, metadata.configKey(), response,
        metadata.returnType(),
        elapsedTime);

    try {
      if (!resultFuture.isDone())
        throw new IllegalStateException("Response handling not done");

      return resultFuture.join();
    } catch (CompletionException e) {
      Throwable cause = e.getCause();
      if (cause != null)
        throw cause;
      throw e;
    }
  }
}

负载均衡实现

public class FeignBlockingLoadBalancerClient implements Client {
@Override
	public Response execute(Request request, Request.Options options) throws IOException {
		final URI originalUri = URI.create(request.url());
		String serviceId = originalUri.getHost();
		Assert.state(serviceId != null, "Request URI does not contain a valid hostname: " + originalUri);
		String hint = getHint(serviceId);
		DefaultRequest<RequestDataContext> lbRequest = new DefaultRequest<>(
				new RequestDataContext(buildRequestData(request), hint));
		Set<LoadBalancerLifecycle> supportedLifecycleProcessors = LoadBalancerLifecycleValidator
				.getSupportedLifecycleProcessors(
						loadBalancerClientFactory.getInstances(serviceId, LoadBalancerLifecycle.class),
						RequestDataContext.class, ResponseData.class, ServiceInstance.class);
		supportedLifecycleProcessors.forEach(lifecycle -> lifecycle.onStart(lbRequest));
		ServiceInstance instance = loadBalancerClient.choose(serviceId, lbRequest);
		org.springframework.cloud.client.loadbalancer.Response<ServiceInstance> lbResponse = new DefaultResponse(
				instance);
		if (instance == null) {
			String message = "Load balancer does not contain an instance for the service " + serviceId;
			if (LOG.isWarnEnabled()) {
				LOG.warn(message);
			}
			supportedLifecycleProcessors.forEach(lifecycle -> lifecycle
					.onComplete(new CompletionContext<ResponseData, ServiceInstance, RequestDataContext>(
							CompletionContext.Status.DISCARD, lbRequest, lbResponse)));
			return Response.builder().request(request).status(HttpStatus.SERVICE_UNAVAILABLE.value())
					.body(message, StandardCharsets.UTF_8).build();
		}
		String reconstructedUrl = loadBalancerClient.reconstructURI(instance, originalUri).toString();
		Request newRequest = buildRequest(request, reconstructedUrl);
		return executeWithLoadBalancerLifecycleProcessing(delegate, options, newRequest, lbRequest, lbResponse,
				supportedLifecycleProcessors);
	}
}

public class BlockingLoadBalancerClient implements LoadBalancerClient {
        @Override
	public <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException {
		String hint = getHint(serviceId);
		LoadBalancerRequestAdapter<T, DefaultRequestContext> lbRequest = new LoadBalancerRequestAdapter<>(request,
				new DefaultRequestContext(request, hint));
		Set<LoadBalancerLifecycle> supportedLifecycleProcessors = getSupportedLifecycleProcessors(serviceId);
		supportedLifecycleProcessors.forEach(lifecycle -> lifecycle.onStart(lbRequest));
		ServiceInstance serviceInstance = choose(serviceId, lbRequest);
		if (serviceInstance == null) {
			supportedLifecycleProcessors.forEach(lifecycle -> lifecycle.onComplete(
					new CompletionContext<>(CompletionContext.Status.DISCARD, lbRequest, new EmptyResponse())));
			throw new IllegalStateException("No instances available for " + serviceId);
		}
		return execute(serviceId, serviceInstance, lbRequest);
	}

@Override
	public <T> ServiceInstance choose(String serviceId, Request<T> request) {
		ReactiveLoadBalancer<ServiceInstance> loadBalancer = loadBalancerClientFactory.getInstance(serviceId);
		if (loadBalancer == null) {
			return null;
		}
		Response<ServiceInstance> loadBalancerResponse = Mono.from(loadBalancer.choose(request)).block();
		if (loadBalancerResponse == null) {
			return null;
		}
		return loadBalancerResponse.getServer();
	}

}

轮询算法获取其中的一个实例

public class RoundRobinLoadBalancer implements ReactorServiceInstanceLoadBalancer {

@Override
	// see original
	// https://github.com/Netflix/ocelli/blob/master/ocelli-core/
	// src/main/java/netflix/ocelli/loadbalancer/RoundRobinLoadBalancer.java
	public Mono<Response<ServiceInstance>> choose(Request request) {
		ServiceInstanceListSupplier supplier = serviceInstanceListSupplierProvider
				.getIfAvailable(NoopServiceInstanceListSupplier::new);
		return supplier.get(request).next()
				.map(serviceInstances -> processInstanceResponse(supplier, serviceInstances));
	}


private Response<ServiceInstance> processInstanceResponse(ServiceInstanceListSupplier supplier,
			List<ServiceInstance> serviceInstances) {
		Response<ServiceInstance> serviceInstanceResponse = getInstanceResponse(serviceInstances);
		if (supplier instanceof SelectedInstanceCallback && serviceInstanceResponse.hasServer()) {
			((SelectedInstanceCallback) supplier).selectedServiceInstance(serviceInstanceResponse.getServer());
		}
		return serviceInstanceResponse;
	}

private Response<ServiceInstance> getInstanceResponse(List<ServiceInstance> instances) {
		if (instances.isEmpty()) {
			if (log.isWarnEnabled()) {
				log.warn("No servers available for service: " + serviceId);
			}
			return new EmptyResponse();
		}
		// TODO: enforce order?
		int pos = Math.abs(this.position.incrementAndGet());

		ServiceInstance instance = instances.get(pos % instances.size());

		return new DefaultResponse(instance);
	}
}