@EnableDiscoveryClient 注解如何实现服务注册与发现

@EnableDiscoveryClient 是如何实现服务注册的？
我们首先需要了解 Spring-Cloud-Commons 这个模块，Spring-Cloud-Commons 是 Spring-Cloud 官方提供的一套抽象层，类似于 JDBC 一样，提供了一套规范，具体的实现有实现厂商去根据标准实现，在Finchley版中， Spring-Cloud-Commons 共提供了６个模块标准规范。

actuator
circuitbreaker
discovery
hypermedia
loadbalancer
serviceregistry
在今天的文章中，我们一起来探讨学习一下 discovery、serviceregistry这两个模块，我们使用 alibaba 的 nacos-discovery 实现来进行学习。
@EnableDiscoveryClient 注解做了什么事？

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(EnableDiscoveryClientImportSelector.class)
public @interface EnableDiscoveryClient {

    /**
     * If true, the ServiceRegistry will automatically register the local server.
     */
    boolean autoRegister() default true;
}

从EnableDiscoveryClient源码可以看出该接口有一个autoRegister()方法默认返回值是true，它还做了一件非常重要的事，引用了EnableDiscoveryClientImportSelector类。为什么说这个类非常重要呢？我们来看看就知道了

EnableDiscoveryClientImportSelector 类做了什么事？

@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);
            }

        }

        return imports;
    }

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

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

}

将焦点聚集到selectImports()方法上，该类获取了autoRegister 的值。

当autoRegister=true 时，将AutoServiceRegistrationConfiguration类添加到自动装配中，系统就会去自动装配AutoServiceRegistrationConfiguration类，在具体的实现中自动装配类都是在这个AutoServiceRegistrationConfiguration类自动装配完成后才装配的，也就是说autoRegister=true就更够实现服务注册

当autoRegister=false时，将spring.cloud.service-registry.auto-registration.enabled 设置成了 false，这样跟注册相关的类将不会自动装配，因为自动注册相关的类都有一个条件装配@ConditionalOnProperty(value = "spring.cloud.service-registry.auto-registration.enabled", matchIfMissing = true),换句话说如果我们不想该服务注册到注册中心，只是想从注册中心拉取服务，我们只需要引导类上的注解改成@EnableDiscoveryClient(autoRegister = false)

nacos 是如何根据标准去实现服务注册的?
我们先看看在org.springframework.cloud.alibaba.nacos包下的NacosDiscoveryAutoConfiguration类

NacosDiscoveryAutoConfiguration类做了些什么？

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

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

    @Bean
    @ConditionalOnBean(AutoServiceRegistrationProperties.class)
    public NacosRegistration nacosRegistration(
            NacosDiscoveryProperties nacosDiscoveryProperties,
            ApplicationContext context) {
        return new NacosRegistration(nacosDiscoveryProperties, context);
    }

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

该类的自动装配是在AutoServiceRegistrationConfiguration之后完成，当autoRegister设置为false时，NacosDiscoveryAutoConfiguration就不会装配，也就意味着服务不会像注册中心进行注册。好了我们还是来看看NacosDiscoveryAutoConfiguration干了些啥吧，主要是装配了NacosServiceRegistry、NacosRegistration、NacosAutoServiceRegistration三个bean，来看看三个bean干了那些骚操作。

NacosServiceRegistry类做了些什么？

public class NacosServiceRegistry implements ServiceRegistry<Registration> {

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

    private final NacosDiscoveryProperties nacosDiscoveryProperties;

    private final NamingService namingService;

    public NacosServiceRegistry(NacosDiscoveryProperties nacosDiscoveryProperties) {
        this.nacosDiscoveryProperties = nacosDiscoveryProperties;
        this.namingService = nacosDiscoveryProperties.namingServiceInstance();
    }

    @Override
    public void register(Registration registration) {

        if (StringUtils.isEmpty(registration.getServiceId())) {
            log.warn("No service to register for nacos client...");
            return;
        }

        String serviceId = registration.getServiceId();

        Instance instance = new Instance();
        instance.setIp(registration.getHost());
        instance.setPort(registration.getPort());
        instance.setWeight(nacosDiscoveryProperties.getWeight());
        instance.setClusterName(nacosDiscoveryProperties.getClusterName());
        instance.setMetadata(registration.getMetadata());

        try {
            namingService.registerInstance(serviceId, instance);
            log.info("nacos registry, {} {}:{} register finished", serviceId,
                    instance.getIp(), instance.getPort());
        }
        catch (Exception e) {
            log.error("nacos registry, {} register failed...{},", serviceId,
                    registration.toString(), e);
        }
    }

    @Override
    public void deregister(Registration registration) {

        log.info("De-registering from Nacos Server now...");

        if (StringUtils.isEmpty(registration.getServiceId())) {
            log.warn("No dom to de-register for nacos client...");
            return;
        }

        NamingService namingService = nacosDiscoveryProperties.namingServiceInstance();
        String serviceId = registration.getServiceId();

        try {
            namingService.deregisterInstance(serviceId, registration.getHost(),
                    registration.getPort(), nacosDiscoveryProperties.getClusterName());
        }
        catch (Exception e) {
            log.error("ERR_NACOS_DEREGISTER, de-register failed...{},",
                    registration.toString(), e);
        }

        log.info("De-registration finished.");
    }

    @Override
    public void close() {

    }

    @Override
    public void setStatus(Registration registration, String status) {
        // nacos doesn't support set status of a particular registration.
    }

    @Override
    public <T> T getStatus(Registration registration) {
        // nacos doesn't support query status of a particular registration.
        return null;
    }

}

该类实现了 spring-cloud-commons 提供的 ServiceRegistry 接口，重写了register、deregister两个方法，在register
方法中主要是将配置文件装换成Instance实例，调用了namingService.registerInstance(serviceId, instance);方法，这个方法应该是服务端提供的服务注册方法。这一顿操作之后，服务就注册好了。

NacosRegistration类做了些什么?

public class NacosRegistration implements Registration, ServiceInstance {

    public static final String MANAGEMENT_PORT = "management.port";
    public static final String MANAGEMENT_CONTEXT_PATH = "management.context-path";
    public static final String MANAGEMENT_ADDRESS = "management.address";
    public static final String MANAGEMENT_ENDPOINT_BASE_PATH = "management.endpoints.web.base-path";

    private NacosDiscoveryProperties nacosDiscoveryProperties;

    private ApplicationContext context;

    public NacosRegistration(NacosDiscoveryProperties nacosDiscoveryProperties,
            ApplicationContext context) {
        this.nacosDiscoveryProperties = nacosDiscoveryProperties;
        this.context = context;
    }

    @PostConstruct
    public void init() {

        Map<String, String> metadata = nacosDiscoveryProperties.getMetadata();
        Environment env = context.getEnvironment();

        String endpointBasePath = env.getProperty(MANAGEMENT_ENDPOINT_BASE_PATH);
        if (!StringUtils.isEmpty(endpointBasePath)) {
            metadata.put(MANAGEMENT_ENDPOINT_BASE_PATH, endpointBasePath);
        }

        Integer managementPort = ManagementServerPortUtils.getPort(context);
        if (null != managementPort) {
            metadata.put(MANAGEMENT_PORT, managementPort.toString());
            String contextPath = env
                    .getProperty("management.server.servlet.context-path");
            String address = env.getProperty("management.server.address");
            if (!StringUtils.isEmpty(contextPath)) {
                metadata.put(MANAGEMENT_CONTEXT_PATH, contextPath);
            }
            if (!StringUtils.isEmpty(address)) {
                metadata.put(MANAGEMENT_ADDRESS, address);
            }
        }
    }

    @Override
    public String getServiceId() {
        return nacosDiscoveryProperties.getService();
    }

    @Override
    public String getHost() {
        return nacosDiscoveryProperties.getIp();
    }

    @Override
    public int getPort() {
        return nacosDiscoveryProperties.getPort();
    }

    public void setPort(int port) {
        this.nacosDiscoveryProperties.setPort(port);
    }

    @Override
    public boolean isSecure() {
        return nacosDiscoveryProperties.isSecure();
    }

    @Override
    public URI getUri() {
        return DefaultServiceInstance.getUri(this);
    }

    @Override
    public Map<String, String> getMetadata() {
        return nacosDiscoveryProperties.getMetadata();
    }

    public boolean isRegisterEnabled() {
        return nacosDiscoveryProperties.isRegisterEnabled();
    }

    public String getCluster() {
        return nacosDiscoveryProperties.getClusterName();
    }

    public float getRegisterWeight() {
        return nacosDiscoveryProperties.getWeight();
    }

    public NacosDiscoveryProperties getNacosDiscoveryProperties() {
        return nacosDiscoveryProperties;
    }

    public NamingService getNacosNamingService() {
        return nacosDiscoveryProperties.namingServiceInstance();
    }

    @Override
    public String toString() {
        return "NacosRegistration{" + "nacosDiscoveryProperties="
                + nacosDiscoveryProperties + '}';
    }
}

该类主要是装配了一些management管理类的配置信息

NacosAutoServiceRegistration类做了些什么事情?

public class NacosAutoServiceRegistration
        extends AbstractAutoServiceRegistration<Registration> {
    private static final Logger log = LoggerFactory
            .getLogger(NacosAutoServiceRegistration.class);

    private NacosRegistration registration;

    public NacosAutoServiceRegistration(ServiceRegistry<Registration> serviceRegistry,
            AutoServiceRegistrationProperties autoServiceRegistrationProperties,
            NacosRegistration registration) {
        super(serviceRegistry, autoServiceRegistrationProperties);
        this.registration = registration;
    }

    @Deprecated
    public void setPort(int port) {
        getPort().set(port);
    }

    @Override
    protected NacosRegistration getRegistration() {
        if (this.registration.getPort() < 0 && this.getPort().get() > 0) {
            this.registration.setPort(this.getPort().get());
        }
        Assert.isTrue(this.registration.getPort() > 0, "service.port has not been set");
        return this.registration;
    }

    @Override
    protected NacosRegistration getManagementRegistration() {
        return null;
    }

    @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();
    }

    @Override
    protected void registerManagement() {
        if (!this.registration.getNacosDiscoveryProperties().isRegisterEnabled()) {
            return;
        }
        super.registerManagement();

    }

    @Override
    protected Object getConfiguration() {
        return this.registration.getNacosDiscoveryProperties();
    }

    @Override
    protected boolean isEnabled() {
        return this.registration.getNacosDiscoveryProperties().isRegisterEnabled();
    }

    @Override
    @SuppressWarnings("deprecation")
    protected String getAppName() {
        String appName = registration.getNacosDiscoveryProperties().getService();
        return StringUtils.isEmpty(appName) ? super.getAppName() : appName;
    }
}

这个类主要是调用NacosServiceRegistry的register()方法，我们来关注一下他的父类AbstractAutoServiceRegistration的start()，这个才是启动方法。

    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()) {
            register();
            if (shouldRegisterManagement()) {
                registerManagement();
            }
            this.context.publishEvent(
                    new InstanceRegisteredEvent<>(this, getConfiguration()));
            this.running.compareAndSet(false, true);
        }

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

在start()方法里调用了NacosAutoServiceRegistration.register方法，NacosAutoServiceRegistration.register的方法里又调用了父类AbstractAutoServiceRegistration.register方法，在父类AbstractAutoServiceRegistration.register方法里调用了NacosServiceRegistry.register方法，实现了服务注册。

服务注册大概经历了这么多，有兴趣的可以自己出看看源码，相信你肯定比我理解的更好。下面是个人学习 Nacos 服务注册的源码阅读流程图,其他的实现也差不多，主要是要理解 Spring-Cloud-Commons 的规范。

@EnableDiscoveryClient 是如何实现服务发现？

通过上面我们知道了 Spring-Cloud-Commons 模块实现了一套规范，我们直接去看在服务发现的规范是什么？我们能够找到DiscoveryClient接口。

public interface DiscoveryClient {

    /**
     * A human readable description of the implementation, used in HealthIndicator
     * @return the description
     */
    String description();

    /**
     * Get all ServiceInstances associated with a particular serviceId
     * @param serviceId the serviceId to query
     * @return a List of ServiceInstance
     */
    List<ServiceInstance> getInstances(String serviceId);

    /**
     * @return all known service ids
     */
    List<String> getServices();

}

里面就提供了三个接口，我们接下来看看nacos是如何实现的?

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 NacosDiscoveryProperties discoveryProperties;

    public NacosDiscoveryClient(NacosDiscoveryProperties discoveryProperties) {
        this.discoveryProperties = discoveryProperties;
    }

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

    @Override
    public List<ServiceInstance> getInstances(String serviceId) {
        try {
            List<Instance> instances = discoveryProperties.namingServiceInstance()
                    .selectInstances(serviceId, true);
            return hostToServiceInstanceList(instances, serviceId);
        }
        catch (Exception e) {
            throw new RuntimeException(
                    "Can not get hosts from nacos server. serviceId: " + serviceId, e);
        }
    }

    private static ServiceInstance hostToServiceInstance(Instance instance,
            String serviceId) {
        NacosServiceInstance nacosServiceInstance = new NacosServiceInstance();
        nacosServiceInstance.setHost(instance.getIp());
        nacosServiceInstance.setPort(instance.getPort());
        nacosServiceInstance.setServiceId(serviceId);

        Map<String, String> metadata = new HashMap<>();
        metadata.put("nacos.instanceId", instance.getInstanceId());
        metadata.put("nacos.weight", instance.getWeight() + "");
        metadata.put("nacos.healthy", instance.isHealthy() + "");
        metadata.put("nacos.cluster", instance.getClusterName() + "");
        metadata.putAll(instance.getMetadata());
        nacosServiceInstance.setMetadata(metadata);

        if (metadata.containsKey("secure")) {
            boolean secure = Boolean.parseBoolean(metadata.get("secure"));
            nacosServiceInstance.setSecure(secure);
        }
        return nacosServiceInstance;
    }

    private static List<ServiceInstance> hostToServiceInstanceList(
            List<Instance> instances, String serviceId) {
        List<ServiceInstance> result = new ArrayList<>(instances.size());
        for (Instance instance : instances) {
            result.add(hostToServiceInstance(instance, serviceId));
        }
        return result;
    }

    @Override
    public List<String> getServices() {

        try {
            ListView<String> services = discoveryProperties.namingServiceInstance()
                    .getServicesOfServer(1, Integer.MAX_VALUE);
            return services.getData();
        }
        catch (Exception e) {
            log.error("get service name from nacos server fail,", e);
            return Collections.emptyList();
        }
    }
}

这里面的逻辑非常简单，就不过多赘述了，有兴趣的小伙伴，可以自行去研究喔。

整个服务注册与发现差不多就是这样子，因为涉及的内容非常多，在很多地方小弟我确实看不懂，在上述中肯定有非常多的错误还请大神们多多指教。