大家知道，將一個普通的springboot 應用註冊到Eureka Server只需要兩步

1. 在應用主類新增@EnableDiscoveryClient註解
2. 在application.properties中配置eureka.client.serviceUrl.defaultZone

順著這條思路，首先看下@EnableDiscoveryClient有什麼類容

/**
 * Annotation to enable a DiscoveryClient implementation.
 * @author Spencer Gibb
 */
@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;
}
 

從註釋就能知道，該註解主要是開啟DiscoveryClient的例項。梳理關係後可得到下圖

其中，左邊的org.springframework.cloud.client.discovery.DiscoveryClient是spring cloud的介面，定義了用來發現服務的常用抽象方法。org.springframework.cloud.netflix.eureka.EurekaDiscoveryClient是對介面的實現，從命名就可以知道，是對Eureka的發現服務的封裝，所以依賴了com.netflix.discovery.EurekaClient介面，EurekaClient又繼承於LookupService，主要是定義了發現服務的抽象方法，而真正去實現發現服務功能的類是com.netflix.discovery.DiscoveryClient。

要看一個類，首先就看類頭部定義的註釋

/**
 * The class that is instrumental for interactions with <tt>Eureka Server</tt>.
 *
 * <p>
 * <tt>Eureka Client</tt> is responsible for a) <em>Registering</em> the
 * instance with <tt>Eureka Server</tt> b) <em>Renewal</em>of the lease with
 * <tt>Eureka Server</tt> c) <em>Cancellation</em> of the lease from
 * <tt>Eureka Server</tt> during shutdown
 * <p>
 * d) <em>Querying</em> the list of services/instances registered with
 * <tt>Eureka Server</tt>
 * <p>
 *
 * <p>
 * <tt>Eureka Client</tt> needs a configured list of <tt>Eureka Server</tt>
 * {@link java.net.URL}s to talk to.These {@link java.net.URL}s are typically amazon elastic eips
 * which do not change. All of the functions defined above fail-over to other
 * {@link java.net.URL}s specified in the list in the case of failure.
 * </p>
 *
 * @author Karthik Ranganathan, Greg Kim
 * @author Spencer Gibb
 *
 */
@Singleton
 

這個類用於Eureka Server相互協作；

Eureka Client主要負責下面的任務：

• 向Eureka Server註冊服務例項
• 向Eureka Server獲取服務租約
• 在服務關閉時，向Eureka Server取消租約
• 查詢Eureka Server中的服務例項列表

Eureka Client還要配置Eureka Server的URL列表 -----------------------------------------------------------------------------------------------------------------------------

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