前言：
前面我们分析了 Nacos 客户端和服务端的服务注册流程，服务注册的最终目的是为了服务间的调用的，服务间的调用，需要知道目标服务有哪些实例，也就是服务发现（订阅），本篇我们来分析一下 Nacos 的服务订阅源码。
Nacos 系列文章传送门：
Nacos 配置管理模型 – 命名空间（Namespace）、配置分组（Group）和配置集ID（Data ID）
服务发现
我们在分析 服务启动何时触发 Nacos 的注册流程时分析到 NacosServiceRegistryAutoConfiguration 源码，NacosServiceRegistryAutoConfiguration 由，Spring Boot 自动配置加载的，最终注入到 NacosAutoServiceRegistration 完成了服务注册，我们查看 spring-cloud-starter-alibaba-nacos-discovery.jar 包路径下的 spring.factories 文件，如下：
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.alibaba.cloud.nacos.discovery.NacosDiscoveryAutoConfiguration,\
com.alibaba.cloud.nacos.endpoint.NacosDiscoveryEndpointAutoConfiguration,\
com.alibaba.cloud.nacos.registry.NacosServiceRegistryAutoConfiguration,\
com.alibaba.cloud.nacos.discovery.NacosDiscoveryClientConfiguration,\
com.alibaba.cloud.nacos.discovery.reactive.NacosReactiveDiscoveryClientConfiguration,\
com.alibaba.cloud.nacos.discovery.configclient.NacosConfigServerAutoConfiguration,\
com.alibaba.cloud.nacos.NacosServiceAutoConfiguration
org.springframework.cloud.bootstrap.BootstrapConfiguration=\
com.alibaba.cloud.nacos.discovery.configclient.NacosDiscoveryClientConfigServiceBootstrapConfiguration
我们注意更客户端服务发现有关的有两个自动配置类 NacosDiscoveryAutoConfiguration 和 NacosDiscoveryClientConfiguration，源码如下：
package com.alibaba.cloud.nacos.discovery;
import com.alibaba.cloud.nacos.ConditionalOnNacosDiscoveryEnabled;
import com.alibaba.cloud.nacos.NacosDiscoveryProperties;
import com.alibaba.cloud.nacos.NacosServiceManager;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.cloud.client.ConditionalOnDiscoveryEnabled;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
@Configuration(
proxyBeanMethods = false
)
@ConditionalOnDiscoveryEnabled
@ConditionalOnNacosDiscoveryEnabled
public class NacosDiscoveryAutoConfiguration {
public NacosDiscoveryAutoConfiguration() {
}
@Bean
@ConditionalOnMissingBean
public NacosDiscoveryProperties nacosProperties() {
return new NacosDiscoveryProperties();
}
//创建 NacosServiceDiscovery 对象
@Bean
@ConditionalOnMissingBean
public NacosServiceDiscovery nacosServiceDiscovery(NacosDiscoveryProperties discoveryProperties, NacosServiceManager nacosServiceManager) {
return new NacosServiceDiscovery(discoveryProperties, nacosServiceManager);
}
}
package com.alibaba.cloud.nacos.discovery;
import com.alibaba.cloud.nacos.ConditionalOnNacosDiscoveryEnabled;
import com.alibaba.cloud.nacos.NacosDiscoveryProperties;
import com.alibaba.cloud.nacos.NacosServiceManager;
import org.springframework.beans.factory.ObjectProvider;
import org.springframework.boot.autoconfigure.AutoConfigureAfter;
import org.springframework.boot.autoconfigure.AutoConfigureBefore;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
import org.springframework.cloud.client.CommonsClientAutoConfiguration;
import org.springframework.cloud.client.ConditionalOnBlockingDiscoveryEnabled;
import org.springframework.cloud.client.ConditionalOnDiscoveryEnabled;
import org.springframework.cloud.client.discovery.DiscoveryClient;
import org.springframework.cloud.client.discovery.simple.SimpleDiscoveryClientAutoConfiguration;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.scheduling.concurrent.ThreadPoolTaskScheduler;
@Configuration(
proxyBeanMethods = false
)
@ConditionalOnDiscoveryEnabled
@ConditionalOnBlockingDiscoveryEnabled
@ConditionalOnNacosDiscoveryEnabled
@AutoConfigureBefore({SimpleDiscoveryClientAutoConfiguration.class, CommonsClientAutoConfiguration.class})
//在 NacosDiscoveryAutoConfiguration 自动装配完成后才执行
@AutoConfigureAfter({NacosDiscoveryAutoConfiguration.class})
public class NacosDiscoveryClientConfiguration {
public NacosDiscoveryClientConfiguration() {
}
//创建 DiscoveryClient 对象
@Bean
public DiscoveryClient nacosDiscoveryClient(NacosServiceDiscovery nacosServiceDiscovery) {
return new NacosDiscoveryClient(nacosServiceDiscovery);
}
@Bean
@ConditionalOnMissingBean
@ConditionalOnProperty(
value = {"spring.cloud.nacos.discovery.watch.enabled"},
matchIfMissing = true
)
public NacosWatch nacosWatch(NacosServiceManager nacosServiceManager, NacosDiscoveryProperties nacosDiscoveryProperties, ObjectProvider<ThreadPoolTaskScheduler> taskExecutorObjectProvider) {
return new NacosWatch(nacosServiceManager, nacosDiscoveryProperties, taskExecutorObjectProvider);
}
}
NacosDiscoveryAutoConfiguration 类自动配置类中，创建了一个 NacosServiceDiscovery 的对象，NacosDiscoveryClientConfiguration 类自动装配时，会创建一个 DiscoveryClient 对象，需要用到前面创建的 NacosServiceDiscovery 对象，因此 NacosDiscoveryClientConfiguration 类的自动装配必须要在 NacosDiscoveryAutoConfiguration 类之后。
NacosDiscoveryClient 源码分析
NacosDiscoveryClient 实现了 Spring Cloud 的 DiscoveryClient 接口，我们需要重点关注的是 getInstances() 和 getServices()方法，这两个方法都是由 NacosServiceDiscovery 类实现的。
package com.alibaba.cloud.nacos.discovery;
import java.util.Collections;
import java.util.List;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.cloud.client.ServiceInstance;
import org.springframework.cloud.client.discovery.DiscoveryClient;
public class NacosDiscoveryClient implements DiscoveryClient {
private static final Logger log = LoggerFactory.getLogger(NacosDiscoveryClient.class);
public static final String DESCRIPTION = "Spring Cloud Nacos Discovery Client";
private NacosServiceDiscovery serviceDiscovery;
public NacosDiscoveryClient(NacosServiceDiscovery nacosServiceDiscovery) {
this.serviceDiscovery = nacosServiceDiscovery;
}
public String description() {
return "Spring Cloud Nacos Discovery Client";
}
//获取服务实例列表
public List<ServiceInstance> getInstances(String serviceId) {
try {
return this.serviceDiscovery.getInstances(serviceId);
} catch (Exception var3) {
throw new RuntimeException("Can not get hosts from nacos server. serviceId: " + serviceId, var3);
}
}
//获取服务列表
public List<String> getServices() {
try {
return this.serviceDiscovery.getServices();
} catch (Exception var2) {
log.error("get service name from nacos server fail,", var2);
return Collections.emptyList();
}
}
}
NacosServiceDiscovery 源码分析
NacosServiceDiscovery 类的代码也非常简单，就提供了 getInstances() 和 getServices() 方法，分别去获取服务实例信息和服务信息，具体请看如下源码分析：
package com.alibaba.cloud.nacos.discovery;
import com.alibaba.cloud.nacos.NacosDiscoveryProperties;
import com.alibaba.cloud.nacos.NacosServiceInstance;
import com.alibaba.nacos.api.exception.NacosException;
import com.alibaba.nacos.api.naming.pojo.Instance;
import com.alibaba.nacos.api.naming.pojo.ListView;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import org.springframework.cloud.client.ServiceInstance;
public class NacosServiceDiscovery {
//Nacos 配置 属性注入的时候已经赋值了
private NacosDiscoveryProperties discoveryProperties;
public NacosServiceDiscovery(NacosDiscoveryProperties discoveryProperties) {
this.discoveryProperties = discoveryProperties;
}
//获取服务实例列表
public List<ServiceInstance> getInstances(String serviceId) throws NacosException {
//获取组 group
String group = this.discoveryProperties.getGroup();
//根据 serviceId 和 group 获取服务实例列表
//通过 NacosFactory 反射创建 NacosNamingService
List<Instance> instances = this.discoveryProperties.namingServiceInstance().selectInstances(serviceId, group, true);
//封装服务实例数据
return hostToServiceInstanceList(instances, serviceId);
}
//获取服务列表
public List<String> getServices() throws NacosException {
//获取组 group
String group = this.discoveryProperties.getGroup();
//获取服务列表
ListView<String> services = this.discoveryProperties.namingServiceInstance().getServicesOfServer(1, 2147483647, group);
return services.getData();
}
//封装服务实例
public static List<ServiceInstance> hostToServiceInstanceList(List<Instance> instances, String serviceId) {
//迭代遍历获取到的服务实例列表
List<ServiceInstance> result = new ArrayList(instances.size());
Iterator var3 = instances.iterator();
while(var3.hasNext()) {
//实例对象
Instance instance = (Instance)var3.next();
//封装实例 会对实例状态进行判断
ServiceInstance serviceInstance = hostToServiceInstance(instance, serviceId);
if (serviceInstance != null) {
result.add(serviceInstance);
}
}
return result;
}
//封装服务实例对象
public static ServiceInstance hostToServiceInstance(Instance instance, String serviceId) {
//实例为空判断
//实例是否开启
//实例是否健康
if (instance != null && instance.isEnabled() && instance.isHealthy()) {
//创建 Nacos 服务实例对象
NacosServiceInstance nacosServiceInstance = new NacosServiceInstance();
//设置 ip
nacosServiceInstance.setHost(instance.getIp());
//设置端口
nacosServiceInstance.setPort(instance.getPort());
//设置服务id
nacosServiceInstance.setServiceId(serviceId);
//元数据
Map<String, String> metadata = new HashMap();
//实例id
metadata.put("nacos.instanceId", instance.getInstanceId());
//实例权重
metadata.put("nacos.weight", instance.getWeight() + "");
//实例健康状态
metadata.put("nacos.healthy", instance.isHealthy() + "");
//集群名称
metadata.put("nacos.cluster", instance.getClusterName() + "");
//实例所有元数据
metadata.putAll(instance.getMetadata());
//设置 Nacos 服务实例对象 元数据
nacosServiceInstance.setMetadata(metadata);
//安全 设置
if (metadata.containsKey("secure")) {
boolean secure = Boolean.parseBoolean((String)metadata.get("secure"));
nacosServiceInstance.setSecure(secure);
}
return nacosServiceInstance;
} else {
return null;
}
}
}
NacosNamingService#selectInstances 方法源码分析
NacosNamingService#selectInstances 方法主要通过重载方法 HostReactor#getServiceInfo 方法，获取服务实例列表返回。
//com.alibaba.nacos.client.naming.NacosNamingService#selectInstances(java.lang.String, java.lang.String, boolean)
public List<Instance> selectInstances(String serviceName, String groupName, boolean healthy) throws NacosException {
return this.selectInstances(serviceName, groupName, healthy, true);
}
//重载方法
//com.alibaba.nacos.client.naming.NacosNamingService#selectInstances(java.lang.String, java.lang.String, boolean, boolean)
public List<Instance> selectInstances(String serviceName, String groupName, boolean healthy, boolean subscribe) throws NacosException {
return this.selectInstances(serviceName, groupName, new ArrayList(), healthy, subscribe);
}
//com.alibaba.nacos.client.naming.NacosNamingService#selectInstances(java.lang.String, java.lang.String, java.util.List<java.lang.String>, boolean, boolean)
public List<Instance> selectInstances(String serviceName, String groupName, List<String> clusters, boolean healthy, boolean subscribe) throws NacosException {
//服务信息
ServiceInfo serviceInfo;
//是否订阅服务 subscribe 默认 true
if (subscribe) {
//订阅服务信息
serviceInfo = this.hostReactor.getServiceInfo(NamingUtils.getGroupedName(serviceName, groupName), StringUtils.join(clusters, ","));
} else {
//直接从 Nacos 拉取服务信息
serviceInfo = this.hostReactor.getServiceInfoDirectlyFromServer(NamingUtils.getGroupedName(serviceName, groupName), StringUtils.join(clusters, ","));
}
//从服务信息中获取服务实例列表返回
return this.selectInstances(serviceInfo, healthy);
}
HostReactor#getServiceInfo 方法源码分析
HostReactor#getServiceInfo 先从本地 serviceInfoMap 缓存获取服务实例列表，如果本地 serviceInfoMap 中获取不到，则创建一个空的服务信息对象，存入到本地缓存中，然后立刻去更新服务信息（实际是发送请求到 Nacos 服务端，获取服务列表），如果本地缓存有，则开启定时更新功能，并返回缓存结果， 需要注意的是不管是立即更新服务列表，还是定时更新服务列表，最终都会执行 HostReactor 中的updateService()方法。
//com.alibaba.nacos.client.naming.core.HostReactor#getServiceInfo
public ServiceInfo getServiceInfo(final String serviceName, final String clusters) {
NAMING_LOGGER.debug("failover-mode: " + failoverReactor.isFailoverSwitch());
//拼接 key 服务名@@集群名
String key = ServiceInfo.getKey(serviceName, clusters);
//是否打开故障转移开关
if (failoverReactor.isFailoverSwitch()) {
return failoverReactor.getService(key);
}
//读取本地服务列表的缓存
//private final Map<String, ServiceInfo> serviceInfoMap;
ServiceInfo serviceObj = getServiceInfo0(serviceName, clusters);
//服务信息为空判断
if (null == serviceObj) {
//为空 创建一个 ServiceInfo
serviceObj = new ServiceInfo(serviceName, clusters);
//存入缓存
serviceInfoMap.put(serviceObj.getKey(), serviceObj);
//存入待更新的服务列表
//private final Map<String, Object> updatingMap;
updatingMap.put(serviceName, new Object());
//立刻更新服务列表
updateServiceNow(serviceName, clusters);
//从待更新的服务列表中移除
updatingMap.remove(serviceName);
} else if (updatingMap.containsKey(serviceName)) {
//待更新列表中有服务对象
//保证线程安全
synchronized(serviceObj) {
try {
//等待 5 秒 等更新完成
serviceObj.wait(5000L);
} catch (InterruptedException var8) {
LogUtils.NAMING_LOGGER.error("[getServiceInfo] serviceName:" + serviceName + ", clusters:" + clusters, var8);
}
}
}
// 开启定时更新服务列表的功能 会延迟1秒执行 后面具体分析
scheduleUpdateIfAbsent(serviceName, clusters);
//返回服务信息对象
return serviceInfoMap.get(serviceObj.getKey());
}
//com.alibaba.nacos.client.naming.backups.FailoverReactor#getService
public ServiceInfo getService(String key) {
//根据 key 从服务信息 map 总获取服务信息
ServiceInfo serviceInfo = serviceMap.get(key);
if (serviceInfo == null) {
//为空 创建一个 ServiceInfo 对象 设置 name 为 key
serviceInfo = new ServiceInfo();
serviceInfo.setName(key);
}
//返回服务信息
return serviceInfo;
}
HostReactor#updateServiceNow 方法源码分析
HostReactor#updateServiceNow 方法更新服务信息，本质上其实是通过 HostReactor#updateService 方法，最终通过 NamingProxy 代理对象调用服务端的接口查询服务信息。
//com.alibaba.nacos.client.naming.core.HostReactor#updateServiceNow
private void updateServiceNow(String serviceName, String clusters) {
try {
//更新服务信息
this.updateService(serviceName, clusters);
} catch (NacosException var4) {
LogUtils.NAMING_LOGGER.error("[NA] failed to update serviceName: " + serviceName, var4);
}
}
//com.alibaba.nacos.client.naming.core.HostReactor#updateService
public void updateService(String serviceName, String clusters) throws NacosException {
//根据服务名称 和 集群名称 获取之前的 服务信息
ServiceInfo oldService = this.getServiceInfo0(serviceName, clusters);
//标识
boolean var12 = false;
try {
//标识为true
var12 = true;
//通过 NamingProxy 代理对象调用服务端的接口查询服务信息 (1.x 版本是 http 调用 2.x版本是 grpc调用)
String result = this.serverProxy.queryList(serviceName, clusters, this.pushReceiver.getUdpPort(), false);
if (StringUtils.isNotEmpty(result)) {
//调用结果不为空 处理结果
this.processServiceJson(result);
//标识设置为 false
var12 = false;
} else {
//调用结果为空 标识设置为 false 是否为空都设置为 false 那为啥有这么写 搞不懂...
var12 = false;
}
} finally {
if (var12) {
//标识为 true
if (oldService != null) {
//oldService 不为空
synchronized(oldService) {
//唤醒等待 前面有等待的线程
oldService.notifyAll();
}
}
}
}
if (oldService != null) {
//oldService 不为空
synchronized(oldService) {
//唤醒等待 前面有等待的线程
oldService.notifyAll();
}
}
}
NamingProxy#queryList 方法源码分析
NamingProxy#queryList 方法很简单，就是封装请求参数，发起 Http 调用（Nacos 1.X 是 Http 调用，2.X 使用的是 Grpc）。
//com.alibaba.nacos.client.naming.net.NamingProxy#queryList
public String queryList(String serviceName, String clusters, int udpPort, boolean healthyOnly) throws NacosException {
Map<String, String> params = new HashMap(8);
params.put("namespaceId", this.namespaceId);
params.put("serviceName", serviceName);
params.put("clusters", clusters);
params.put("udpPort", String.valueOf(udpPort));
params.put("clientIP", NetUtils.localIP());
params.put("healthyOnly", String.valueOf(healthyOnly));
//服务请求地址 分析 Nacos Server 端服务发现源码时候 这个地址对应的方法就是入口
return this.reqApi(UtilAndComs.nacosUrlBase + "/instance/list", params, "GET");
}
NamingProxy#reqApi 方法源码分析
NamingProxy#reqApi 方法源码很简单，前面分析 Nacos 注册的时候也有类似的代码，进行一些简单判断后，通过 NamingProxy#callServer 方法向 Nacos Server 发起调用，查询服务信息。
//com.alibaba.nacos.client.naming.net.NamingProxy#reqApi(java.lang.String, java.util.Map<java.lang.String,java.lang.String>, java.lang.String)
public String reqApi(String api, Map<String, String> params, String method) throws NacosException {
return this.reqApi(api, params, Collections.EMPTY_MAP, method);
}
//com.alibaba.nacos.client.naming.net.NamingProxy#reqApi(java.lang.String, java.util.Map<java.lang.String,java.lang.String>, java.util.Map<java.lang.String,java.lang.String>, java.lang.String)
public String reqApi(String api, Map<String, String> params, Map<String, String> body, String method) throws NacosException {
return this.reqApi(api, params, body, this.getServerList(), method);
}
//com.alibaba.nacos.client.naming.net.NamingProxy#reqApi(java.lang.String, java.util.Map<java.lang.String,java.lang.String>, java.util.Map<java.lang.String,java.lang.String>, java.util.List<java.lang.String>, java.lang.String)
public String reqApi(String api, Map<String, String> params, Map<String, String> body, List<String> servers, String method) throws NacosException {
//参数中加入 namespaceId
params.put("namespaceId", this.getNamespaceId());
//Nacos 服务地址是否为空
//nacosDomain nacos 域名是否为空
if (CollectionUtils.isEmpty(servers) && StringUtils.isBlank(this.nacosDomain)) {
//地址和 域名都为空 抛出异常
throw new NacosException(400, "no server available");
} else {
//异常
NacosException exception = new NacosException();
nacosDomain nacos 域名是否为空
if (StringUtils.isNotBlank(this.nacosDomain)) {
//不为空 配置的 nacos 域名
int i = 0;
//this.maxRetry 最大重试次数 默认3
while(i < this.maxRetry) {
try {
//发送请求到 Nacos Server 查询服务实例列表
return this.callServer(api, params, body, this.nacosDomain, method);
} catch (NacosException var12) {
//异常处理
exception = var12;
if (LogUtils.NAMING_LOGGER.isDebugEnabled()) {
LogUtils.NAMING_LOGGER.debug("request {} failed.", this.nacosDomain, var12);
}
//调用次数 +1
++i;
}
}
} else {
//获取 Nacos 服务数的随机数
Random random = new Random(System.currentTimeMillis());
int index = random.nextInt(servers.size());
int i = 0;
while(i < servers.size()) {
String server = (String)servers.get(index);
try {
//发送请求到 Nacos Server 查询服务实例列表
return this.callServer(api, params, body, server, method);
} catch (NacosException var13) {
//异常
exception = var13;
if (LogUtils.NAMING_LOGGER.isDebugEnabled()) {
LogUtils.NAMING_LOGGER.debug("request {} failed.", server, var13);
}
index = (index + 1) % servers.size();
//调用次数+1
++i;
}
}
}
LogUtils.NAMING_LOGGER.error("request: {} failed, servers: {}, code: {}, msg: {}", new Object[]{api, servers, exception.getErrCode(), exception.getErrMsg()});
throw new NacosException(exception.getErrCode(), "failed to req API:" + api + " after all servers(" + servers + ") tried: " + exception.getMessage());
}
}
NamingProxy#callServer 方法源码分析
NamingProxy#callServer 方法是真正向 Nacos Server 发起 Http 请求的方法，封装签名信息、header 信息，发起调用后，处理返回结果。
//com.alibaba.nacos.client.naming.net.NamingProxy#callServer(java.lang.String, java.util.Map<java.lang.String,java.lang.String>, java.util.Map<java.lang.String,java.lang.String>, java.lang.String, java.lang.String)
public String callServer(String api, Map<String, String> params, Map<String, String> body, String curServer, String method) throws NacosException {
//记录开始时间
long start = System.currentTimeMillis();
//结束时间
long end = 0L;
//签名信息
this.injectSecurityInfo(params);
//header
Header header = this.builderHeader();
//拼接请求 url
String url;
if (!curServer.startsWith("https://") && !curServer.startsWith("http://")) {
if (!IPUtil.containsPort(curServer)) {
curServer = curServer + ":" + this.serverPort;
}
url = NamingHttpClientManager.getInstance().getPrefix() + curServer + api;
} else {
url = curServer + api;
}
try {
//发送 Http 请求
HttpRestResult<String> restResult = this.nacosRestTemplate.exchangeForm(url, header, Query.newInstance().initParams(params), body, method, String.class);
end = System.currentTimeMillis();
MetricsMonitor.getNamingRequestMonitor(method, url, String.valueOf(restResult.getCode())).observe((double)(end - start));
//结果处理
if (restResult.ok()) {
return (String)restResult.getData();
} else if (304 == restResult.getCode()) {
return "";
} else {
throw new NacosException(restResult.getCode(), restResult.getMessage());
}
} catch (Exception var13) {
LogUtils.NAMING_LOGGER.error("[NA] failed to request", var13);
throw new NacosException(500, var13);
}
}
//com.alibaba.nacos.client.naming.net.NamingProxy#injectSecurityInfo
private void injectSecurityInfo(Map<String, String> params) {
if (StringUtils.isNotBlank(this.securityProxy.getAccessToken())) {
params.put("accessToken", this.securityProxy.getAccessToken());
}
String ak = this.getAccessKey();
String sk = this.getSecretKey();
params.put("app", AppNameUtils.getAppName());
if (StringUtils.isNotBlank(ak) && StringUtils.isNotBlank(sk)) {
try {
String signData = getSignData((String)params.get("serviceName"));
String signature = SignUtil.sign(signData, sk);
params.put("signature", signature);
params.put("data", signData);
params.put("ak", ak);
} catch (Exception var6) {
LogUtils.NAMING_LOGGER.error("inject ak/sk failed.", var6);
}
}
}
HostReactor#scheduleUpdateIfAbsent 方法源码分析
前面分析时候提到不管是立即更新服务列表，还是定时更新服务列表，最终都会执行 HostReactor 中的updateService()方法，而执行者就是 HostReactor#scheduleUpdateIfAbsent 方法，源码如下：
//com.alibaba.nacos.client.naming.core.HostReactor#scheduleUpdateIfAbsent
public void scheduleUpdateIfAbsent(String serviceName, String clusters) {
//判断当前服务是否已经存在了 避免重复调用
if (this.futureMap.get(ServiceInfo.getKey(serviceName, clusters)) == null) {
//synchronized 线程安全
synchronized(this.futureMap) {
//再次判断当前服务是否存在了避免重复调用浪费资源
if (this.futureMap.get(ServiceInfo.getKey(serviceName, clusters)) == null) {
//添加到调度任务中 重点关注 UpdateTask
ScheduledFuture<?> future = this.addTask(new HostReactor.UpdateTask(serviceName, clusters));
//记录当前服务已经在更新任务中了
this.futureMap.put(ServiceInfo.getKey(serviceName, clusters), future);
}
}
}
}
//com.alibaba.nacos.client.naming.core.HostReactor#addTask
public synchronized ScheduledFuture<?> addTask(HostReactor.UpdateTask task) {
return this.executor.schedule(task, 1000L, TimeUnit.MILLISECONDS);
}
public class UpdateTask implements Runnable {
long lastRefTime = 9223372036854775807L;
private final String clusters;
private final String serviceName;
private int failCount = 0;
public UpdateTask(String serviceName, String clusters) {
this.serviceName = serviceName;
this.clusters = clusters;
}
private void incFailCount() {
int limit = 6;
if (this.failCount != limit) {
++this.failCount;
}
}
private void resetFailCount() {
this.failCount = 0;
}
public void run() {
long delayTime = 1000L;
try {
//从本地缓存 serviceInfoMap 中获取服务信息
ServiceInfo serviceObj = (ServiceInfo)HostReactor.this.serviceInfoMap.get(ServiceInfo.getKey(this.serviceName, this.clusters));
if (serviceObj == null) {
//为空调用 updateService 方法 最终会向 Nacos Server 发起请求获取最新的服务信息
HostReactor.this.updateService(this.serviceName, this.clusters);
return;
}
//本地缓存中有 需要判断最后一次更新时间是否小于缓存刷新时间
if (serviceObj.getLastRefTime() <= this.lastRefTime) {
//重新从注册中心获取
HostReactor.this.updateService(this.serviceName, this.clusters);
//加入本地缓存
serviceObj = (ServiceInfo)HostReactor.this.serviceInfoMap.get(ServiceInfo.getKey(this.serviceName, this.clusters));
} else {
//刷新缓存
HostReactor.this.refreshOnly(this.serviceName, this.clusters);
}
//更新缓存刷新时间
this.lastRefTime = serviceObj.getLastRefTime();
//!HostReactor.this.notifier.isSubscribed(this.serviceName, this.clusters): 服务实例不再订阅
//!HostReactor.this.futureMap.containsKey(ServiceInfo.getKey(this.serviceName, this.clusters)) : futureMap 不包含 该服务
if (!HostReactor.this.notifier.isSubscribed(this.serviceName, this.clusters) && !HostReactor.this.futureMap.containsKey(ServiceInfo.getKey(this.serviceName, this.clusters))) {
LogUtils.NAMING_LOGGER.info("update task is stopped, service:" + this.serviceName + ", clusters:" + this.clusters);
//返回
return;
}
//服务实例为空
if (CollectionUtils.isEmpty(serviceObj.getHosts())) {
//失败次数+1
this.incFailCount();
return;
}
//延迟时间
delayTime = serviceObj.getCacheMillis();
//重置失败次数
this.resetFailCount();
} catch (Throwable var7) {
//失败次数+1
this.incFailCount();
LogUtils.NAMING_LOGGER.warn("[NA] failed to update serviceName: " + this.serviceName, var7);
} finally {
//定时任务更新缓存 没有异常的情况下是 6 秒一次 如果有异常 则最长为 1分钟
HostReactor.this.executor.schedule(this, Math.min(delayTime << this.failCount, 60000L), TimeUnit.MILLISECONDS);
}
}
}
通过源码分析，我们知道最终开启了一个定时任务，传入了 UpdateTask 对象，而 UpdateTask 实现了 Runnable 接口，我们重点关注 UpdateTask # run 方法，如下：
public void run() {
long delayTime = 1000L;
try {
//从本地缓存 serviceInfoMap 中获取服务信息
ServiceInfo serviceObj = (ServiceInfo)HostReactor.this.serviceInfoMap.get(ServiceInfo.getKey(this.serviceName, this.clusters));
if (serviceObj == null) {
//为空调用 updateService 方法 最终会向 Nacos Server 发起请求获取最新的服务信息
HostReactor.this.updateService(this.serviceName, this.clusters);
return;
}
//本地缓存中有 需要判断最后一次更新时间是否小于缓存刷新时间
if (serviceObj.getLastRefTime() <= this.lastRefTime) {
//重新从注册中心获取
HostReactor.this.updateService(this.serviceName, this.clusters);
//加入本地缓存
serviceObj = (ServiceInfo)HostReactor.this.serviceInfoMap.get(ServiceInfo.getKey(this.serviceName, this.clusters));
} else {
//刷新缓存
HostReactor.this.refreshOnly(this.serviceName, this.clusters);
}
//更新缓存刷新时间
this.lastRefTime = serviceObj.getLastRefTime();
//!HostReactor.this.notifier.isSubscribed(this.serviceName, this.clusters): 服务实例不再订阅
//!HostReactor.this.futureMap.containsKey(ServiceInfo.getKey(this.serviceName, this.clusters)) : futureMap 不包含 该服务
if (!HostReactor.this.notifier.isSubscribed(this.serviceName, this.clusters) && !HostReactor.this.futureMap.containsKey(ServiceInfo.getKey(this.serviceName, this.clusters))) {
LogUtils.NAMING_LOGGER.info("update task is stopped, service:" + this.serviceName + ", clusters:" + this.clusters);
//返回
return;
}
//服务实例为空
if (CollectionUtils.isEmpty(serviceObj.getHosts())) {
//失败次数+1
this.incFailCount();
return;
}
//延迟时间
delayTime = serviceObj.getCacheMillis();
//重置失败次数
this.resetFailCount();
} catch (Throwable var7) {
//失败次数+1
this.incFailCount();
LogUtils.NAMING_LOGGER.warn("[NA] failed to update serviceName: " + this.serviceName, var7);
} finally {
//定时任务更新缓存 没有异常的情况下是 6 秒一次 如果有异常 则最长为 1分钟
HostReactor.this.executor.schedule(this, Math.min(delayTime << this.failCount, 60000L), TimeUnit.MILLISECONDS);
}
}
整个 UpdateTask # run 方法就是服务发现（订阅）的核心，UpdateTask # run 会先判断本地缓存是否有服务信息对象，以及缓存是否过期，当不存在或者过期的时候，会去查询注册中心，获取最新实例，更新最后获取时间，处理服务信息，在最后会计算任务时间，循环执行流程，UpdateTask # run 方法的最核心点是在 finally 中，finally 中执行了定时任务，定时更新服务信息，没有异常的情况下是 6 秒执行一次，最长时间是一分钟执行一次，这个问题在 Nacos 相关面试中也是非常常见的问题，如果你没有看过源码，可能你知道是 6 秒一次，但你可能不知道有可能是一分钟执行一次，以及它是在哪里实现的。
欢迎提出建议及对错误的地方指出纠正。