@EnableDiscoveryClient

@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;
}

@Import注解：支持导入普通的java类,并将其声明成一个bean
现在看EnableDiscoveryClientImportSelector类实现

EnableDiscoveryClientImportSelector

@Order(Ordered.LOWEST_PRECEDENCE - 100) //指定实例化的顺序
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;
	}
}

SpringFactoryImportSelector

	@Override
	public String[] selectImports(AnnotationMetadata metadata) {
		if (!isEnabled()) {
			return new String[0];
		}
		AnnotationAttributes attributes = AnnotationAttributes.fromMap(
				metadata.getAnnotationAttributes(this.annotationClass.getName(), true));

		Assert.notNull(attributes, "No " + getSimpleName() + " attributes found. Is "
				+ metadata.getClassName() + " annotated with @" + getSimpleName() + "?");

		//重点 寻找到所有的自动装备的类，并且去重
		List<String> factories = new ArrayList<>(new LinkedHashSet<>(SpringFactoriesLoader
				.loadFactoryNames(this.annotationClass, this.beanClassLoader)));

		if (factories.isEmpty() && !hasDefaultFactory()) {
			throw new IllegalStateException("Annotation @" + getSimpleName()
					+ " found, but there are no implementations. Did you forget to include a starter?");
		}

		if (factories.size() > 1) {
			// there should only ever be one DiscoveryClient, but there might be more than
			// one factory
			log.warn("More than one implementation " + "of @" + getSimpleName()
					+ " (now relying on @Conditionals to pick one): " + factories);
		}

		return factories.toArray(new String[factories.size()]);
	}

SpringFactoriesLoader调用loadFactoryNames其实加载META-INF/spring.factories下的class。

javadoc中有

区别

• @EnableEurekaClient和@EnableDiscoveryClient的区别就是@EnableEurekaClient是针对于eureka的服务发现的注解，而@EnableDiscoveryClient是对于cloud所有服务发现的注解；
• 两个的实现有在Spring boot 2.0前后有差别，Spring boot 2.0之后，@EnableEurekaClient已经不是由@EnableDiscoveryClient实现的了

参考

• SpringCloud服务发现注解之@EnableDiscoveryClient与@EnableEurekaClient