1. 服务注册

服务注册的核心类如下

@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties
@ConditionalOnNacosDiscoveryEnabled
@ConditionalOnProperty(value = "spring.cloud.service-registry.auto-registration.enabled",
        matchIfMissing = true)
@AutoConfigureAfter({ AutoServiceRegistrationConfiguration.class,
        AutoServiceRegistrationAutoConfiguration.class,
        NacosDiscoveryAutoConfiguration.class })
public class NacosServiceRegistryAutoConfiguration {

    // ① 创建 Nacos 服务注册中心
    @Bean
    public NacosServiceRegistry nacosServiceRegistry(
            NacosDiscoveryProperties nacosDiscoveryProperties) {
        return new NacosServiceRegistry(nacosDiscoveryProperties);
    }

    // ② 创建 Nacos 服务注册的实例
    @Bean
    @ConditionalOnBean(AutoServiceRegistrationProperties.class)
    public NacosRegistration nacosRegistration(
            ObjectProvider<List<NacosRegistrationCustomizer>> registrationCustomizers,
            NacosDiscoveryProperties nacosDiscoveryProperties,
            ApplicationContext context) {
        return new NacosRegistration(registrationCustomizers.getIfAvailable(),
                nacosDiscoveryProperties, context);
    }

    // ③ 创建 Nacos 自动服务注册实现类
    @Bean
    @ConditionalOnBean(AutoServiceRegistrationProperties.class)
    public NacosAutoServiceRegistration nacosAutoServiceRegistration(
            NacosServiceRegistry registry,
            AutoServiceRegistrationProperties autoServiceRegistrationProperties,
            NacosRegistration registration) {
        return new NacosAutoServiceRegistration(registry,
                autoServiceRegistrationProperties, registration);
    }

}

注册过程

1. 创建 Nacos 服务注册中心 com.alibaba.cloud.nacos.registry.NacosServiceRegistry；

2. 创建 Nacos 服务注册的实例 com.alibaba.cloud.nacos.registry.NacosRegistration，判断条件为是否存在 Bean org.springframework.cloud.client.serviceregistry.AutoServiceRegistrationProperties；

3. 创建 Nacos 自动服务注册工具类 com.alibaba.cloud.nacos.registry.NacosAutoServiceRegistration，判断条件同上；

4. NacosAutoServiceRegistration 会获取 NacosRegistration 并注册到 NacosServiceRegistry

   public class NacosAutoServiceRegistration
        extends AbstractAutoServiceRegistration<Registration> {
    ...
   
    private NacosRegistration registration;
   
    ...
   
    @Override
    protected void register() {
        if (!this.registration.getNacosDiscoveryProperties().isRegisterEnabled()) {
            log.debug("Registration disabled.");
            return;
        }
        if (this.registration.getPort() < 0) {
            this.registration.setPort(getPort().get());
        }
        super.register();
    }
       ...
   }
   

2. 服务发现

服务发现核心代码：

public class NacosDiscoveryClient implements DiscoveryClient {

    private static final Logger log = LoggerFactory.getLogger(NacosDiscoveryClient.class);

    /**
     * Nacos Discovery Client Description.
     */
    public static final String DESCRIPTION = "Spring Cloud Nacos Discovery Client";

    private NacosServiceDiscovery serviceDiscovery;

    public NacosDiscoveryClient(NacosServiceDiscovery nacosServiceDiscovery) {
        this.serviceDiscovery = nacosServiceDiscovery;
    }

    @Override
    public String description() {
        return DESCRIPTION;
    }

    @Override
    public List<ServiceInstance> getInstances(String serviceId) {
        try {
            return serviceDiscovery.getInstances(serviceId);
        }
        catch (Exception e) {
            throw new RuntimeException(
                    "Can not get hosts from nacos server. serviceId: " + serviceId, e);
        }
    }

    @Override
    public List<String> getServices() {
        try {
            return serviceDiscovery.getServices();
        }
        catch (Exception e) {
            log.error("get service name from nacos server fail,", e);
            return Collections.emptyList();
        }
    }
}

public class NacosServiceDiscovery {
    ...
        
    public List<ServiceInstance> getInstances(String serviceId) throws NacosException {
        String group = discoveryProperties.getGroup();
        List<Instance> instances = namingService().selectInstances(serviceId, group,
                true);
        return hostToServiceInstanceList(instances, serviceId);
    }
    
    ...
        
    private NamingService namingService() {
        return nacosServiceManager.getNamingService(discoveryProperties.getNacosProperties());
    }
}

从以上代码可以看到，NacosDiscoveryClient 实现了 org.springframework.cloud.client.discovery.DiscoveryClient 接口，其 #getInstances() 方法从 NacosServiceDiscovery#getInstance() 获取对应的服务实例信息。

NacosServiceDiscovery 从 NacosServiceManager 获取 NameService，然后通过 NamingService#selectInstances() 获取实例信息，并转换为 org.springframework.cloud.client.ServiceInstance 类型返回给 Spring 管理。

@Configuration(proxyBeanMethods = false)
@ConditionalOnDiscoveryEnabled
@ConditionalOnBlockingDiscoveryEnabled
@ConditionalOnNacosDiscoveryEnabled
@AutoConfigureBefore({ SimpleDiscoveryClientAutoConfiguration.class,
        CommonsClientAutoConfiguration.class })
@AutoConfigureAfter(NacosDiscoveryAutoConfiguration.class)
public class NacosDiscoveryClientConfiguration {

    @Bean
    public DiscoveryClient nacosDiscoveryClient(
            NacosServiceDiscovery nacosServiceDiscovery) {
        return new NacosDiscoveryClient(nacosServiceDiscovery);
    }

    ...
}

从上面的代码可以看到，NacosDiscoveryClientConfiguration 注册了 NacosDiscoveryClient。

发现过程

1. NacosDiscoveryClient 实现了 org.springframework.cloud.client.discovery.DiscoveryClient 接口；
2. 其 getInstances 方法从 NacosServiceDiscovery#getInstance() 获取对应的服务实例信息；
3. NacosServiceDiscovery 从 discoveryProperties 获取 NamingService，然后通过 NamingService.selectInstances 获取实例信息，然后转换为 org.springframework.cloud.client.ServiceInstance 类型；
4. getServices 方法通过 NamingService.getServicesOfServer 获取 services 信息。

3. 自动装配

从以上代码可以看出，Nacos 服务注册与发现均是通过注解的方式进行判断是否存在对应的配置类，是否配置了允许等条件动态的创建对应的 Bean。

参考文档

聊聊NacosDiscoveryClient