spring cloud 服务注册、发现了解

spring cloud定义了以下几个类用于服务的注册、发现操作

• DiscoveryClient 接口代表服务发现常见的读取操作

public interface DiscoveryClient extends Ordered {
    int DEFAULT_ORDER = 0;

    String description();
    // 根据服务id获取对应的服务列表
    List<ServiceInstance> getInstances(String serviceId);
    // 获取所有服务名称
    List<String> getServices();
    default void probe() {
        this.getServices();
    }
    default int getOrder() {
        return 0;
    }
}

• EnableDiscoverClient 使用注解表示开启服务发现功能
• ServiceInstance 接口代表服务的一个实例
  在DiscoveryClient的getInstances方法中可以看到返回的list范型为ServiceInstance 每一个ServiceInstance代表的就是一个服务实例的信息

public interface ServiceInstance {
    default String getInstanceId() {
        return null;
    }
    String getServiceId();
    String getHost();
    int getPort();
    boolean isSecure();
    URI getUri();
    Map<String, String> getMetadata();
    default String getScheme() {
        return null;
    }
}

• Registration接口 代表服务注册时需要用到的实例信息，通过源码可以看到该接口即成ServiceInstance接口，并且没有任何扩展实际上就是ServiceInstance 只不过代表的意义有所区别 。
  既ServiceInstance代表在注册中心获取的实例信息 Registration代表服务注册时需要发送到注册中心的数据

public interface Registration extends ServiceInstance {
}

• ServiceRegistry 用于对服务的注册、注销等操作

public interface ServiceRegistry<R extends Registration> {
    void register(R registration);

    void deregister(R registration);

    void close();

    void setStatus(R registration, String status);

    <T> T getStatus(R registration);
}

• AutoServiceRegistration 是一个空接口代表服务的自动注册该接口有一个抽象的实现类AbstractAutoServiceRegistration 并且该抽象类实现了ApplicationListener接口 监听WebServerInitializedEvent事件，当事件被触发后AbstractAutoServiceRegistration调用ServiceRegistry.registry 进行服务的注册.
  AbstractAutoServiceRegistration关键代码如下

public abstract class AbstractAutoServiceRegistration<R extends Registration>
        implements AutoServiceRegistration, ApplicationContextAware, ApplicationListener<WebServerInitializedEvent> {
    @Override
    @SuppressWarnings("deprecation")
    public void onApplicationEvent(WebServerInitializedEvent event) {
        bind(event);
    }

    @Deprecated
    public void bind(WebServerInitializedEvent event) {
        ApplicationContext context = event.getApplicationContext();
        if (context instanceof ConfigurableWebServerApplicationContext) {
            if ("management".equals(((ConfigurableWebServerApplicationContext) context).getServerNamespace())) {
                return;
            }
        }
        this.port.compareAndSet(0, event.getWebServer().getPort());
        this.start();
    }
public void start() {
        if (!isEnabled()) {
            if (logger.isDebugEnabled()) {
                logger.debug("Discovery Lifecycle disabled. Not starting");
            }
            return;
        }

        // only initialize if nonSecurePort is greater than 0 and it isn't already running
        // because of containerPortInitializer below
        if (!this.running.get()) {
            this.context.publishEvent(new InstancePreRegisteredEvent(this, getRegistration()));
            register();
            if (shouldRegisterManagement()) {
                registerManagement();
            }
            this.context.publishEvent(new InstanceRegisteredEvent<>(this, getConfiguration()));
            this.running.compareAndSet(false, true);
        }

    }
    protected void register() {
        this.serviceRegistry.register(getRegistration());
    }

以上的类是Spring cloud定义的模型，任何服务注册中心想与Spring Cloud进行整合都需要实现上面的接口扩展对应的实体类。例如：eureka、nacos等服务注册中心，下面看下nacos的实现方式

nacos对于以上模型的实现类如下：

• NacosDiscoveryClient实现DiscoveryClient
• NacosServiceInstance实现ServiceInstance
• NacosRegistration 实现Registration
• NacosServiceRegistry 实现ServiceRegistry
• NacosAutoServiceRegistration 继承AbstractAutoServiceRegistration

在Spring-cloud-starter-alibaba-nacos-discovery 项目的spring.factories文件中可以看到EnableAutoConfiguration对应的类中有一个NacosServiceRegistryAutoConfiguration类 springboot通过工厂加载机制加载该类用于配置nacos的服务自动注册,在该配置类中加载了NacosServiceRegistry、NacosRegistration、NacosAutoServiceRegistration类

public class NacosServiceRegistryAutoConfiguration {
    public NacosServiceRegistryAutoConfiguration() {
    }

    @Bean
    public NacosServiceRegistry nacosServiceRegistry(NacosDiscoveryProperties nacosDiscoveryProperties) {
        return new NacosServiceRegistry(nacosDiscoveryProperties);
    }

    @Bean
    @ConditionalOnBean({AutoServiceRegistrationProperties.class})
    public NacosRegistration nacosRegistration(ObjectProvider<List<NacosRegistrationCustomizer>> registrationCustomizers, NacosDiscoveryProperties nacosDiscoveryProperties, ApplicationContext context) {
        return new NacosRegistration((List)registrationCustomizers.getIfAvailable(), nacosDiscoveryProperties, context);
    }

    @Bean
    @ConditionalOnBean({AutoServiceRegistrationProperties.class})
    public NacosAutoServiceRegistration nacosAutoServiceRegistration(NacosServiceRegistry registry, AutoServiceRegistrationProperties autoServiceRegistrationProperties, NacosRegistration registration) {
        return new NacosAutoServiceRegistration(registry, autoServiceRegistrationProperties, registration);
    }
}

上面说过AbstractAutoServiceRegistration实现了ApplicationListener监听WebServerInitializedEvent事件，所以在该事件触发时NacosAutoServiceRegistration自动开始实现对nacos服务注册中心进行注册

NacosDiscoveryClientConfiguration类 springboot通过工厂加载机制加载该类用于配置nacos的服务自动注册,在该配置类中加载了nacosDiscoveryClient类实现服务发现的常见读取操作