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要把一个SpringBoot应用注册到Eureka Server或者是从Eureka Server上获取服务列表，主要做了一下两件事：
1.在应用主类中配置@EnableDiscoveryClient注解。
2.在application.yml中配置eureka.client.service-url.defaultZone指定注册中心的位置。
那么@EnableDiscoveryClient背后封装了那些逻辑呢？

@EnableDiscoveryClient源码分析

先进入@EnableDiscoveryClient注解看一下它的源码和注释。

/*
 * Copyright 2012-2019 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.cloud.client.discovery;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import org.springframework.context.annotation.Import;

/**
 * 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.
     * @return - {@code true} if you want to automatically register.
     */
    boolean autoRegister() default true;

}

注释写的很清楚：Annotation to enable a DiscoveryClient implementation.这个注解使用来开启DiscoveryClient这个接口的实现的。
全局搜索一下DiscoveryClient，发现除了接口，还有一个类也叫这个名字。这个类很重要，但是先不看这个类，先看DiscoveryClient接口。DiscoveryClient接口和@EnableDiscoveryClient在一个包里。

DiscoveryClient接口.png

查看DiscoveryClient接口的实现，发现有EurekaDiscoveryClient这个类。看一下EurekaDiscoveryClient这个类的源码。

/*
 * Copyright 2013-2019 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.cloud.netflix.eureka;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

import com.netflix.appinfo.EurekaInstanceConfig;
import com.netflix.appinfo.InstanceInfo;
import com.netflix.discovery.EurekaClient;
import com.netflix.discovery.EurekaClientConfig;
import com.netflix.discovery.shared.Application;
import com.netflix.discovery.shared.Applications;

import org.springframework.cloud.client.ServiceInstance;
import org.springframework.cloud.client.discovery.DiscoveryClient;
import org.springframework.core.Ordered;

/**
 * A {@link DiscoveryClient} implementation for Eureka.
 *
 * @author Spencer Gibb
 * @author Tim Ysewyn
 */
public class EurekaDiscoveryClient implements DiscoveryClient {

    /**
     * Client description {@link String}.
     */
    public static final String DESCRIPTION = "Spring Cloud Eureka Discovery Client";

    private final EurekaClient eurekaClient;

    private final EurekaClientConfig clientConfig;

    @Deprecated
    public EurekaDiscoveryClient(EurekaInstanceConfig config, EurekaClient eurekaClient) {
        this(eurekaClient, eurekaClient.getEurekaClientConfig());
    }

    public EurekaDiscoveryClient(EurekaClient eurekaClient,
            EurekaClientConfig clientConfig) {
        this.clientConfig = clientConfig;
        this.eurekaClient = eurekaClient;
    }

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

    @Override
    public List<ServiceInstance> getInstances(String serviceId) {
        List<InstanceInfo> infos = this.eurekaClient.getInstancesByVipAddress(serviceId,
                false);
        List<ServiceInstance> instances = new ArrayList<>();
        for (InstanceInfo info : infos) {
            instances.add(new EurekaServiceInstance(info));
        }
        return instances;
    }

    @Override
    public List<String> getServices() {
        Applications applications = this.eurekaClient.getApplications();
        if (applications == null) {
            return Collections.emptyList();
        }
        List<Application> registered = applications.getRegisteredApplications();
        List<String> names = new ArrayList<>();
        for (Application app : registered) {
            if (app.getInstances().isEmpty()) {
                continue;
            }
            names.add(app.getName().toLowerCase());

        }
        return names;
    }

    @Override
    public int getOrder() {
        return clientConfig instanceof Ordered ? ((Ordered) clientConfig).getOrder()
                : DiscoveryClient.DEFAULT_ORDER;
    }

    /**
     * An Eureka-specific {@link ServiceInstance} implementation. Extends
     * {@link org.springframework.cloud.netflix.eureka.EurekaServiceInstance} for
     * backwards compatibility.
     *
     * @deprecated In favor of
     * {@link org.springframework.cloud.netflix.eureka.EurekaServiceInstance}.
     */
    @Deprecated
    public static class EurekaServiceInstance
            extends org.springframework.cloud.netflix.eureka.EurekaServiceInstance {

        public EurekaServiceInstance(InstanceInfo instance) {
            super(instance);
        }

    }

}

EurekaDiscoveryClient这个类自己好像也没干什么，所有的操作几乎都是它的成员变量EurekaClient来实现的。那就再去EurekaClient的源码里看一看。

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package com.netflix.discovery;

import com.google.inject.ImplementedBy;
import com.netflix.appinfo.ApplicationInfoManager;
import com.netflix.appinfo.HealthCheckCallback;
import com.netflix.appinfo.HealthCheckHandler;
import com.netflix.appinfo.InstanceInfo;
import com.netflix.appinfo.InstanceInfo.InstanceStatus;
import com.netflix.discovery.shared.Applications;
import com.netflix.discovery.shared.LookupService;
import java.util.List;
import java.util.Set;
import javax.annotation.Nullable;

@ImplementedBy(DiscoveryClient.class)
public interface EurekaClient extends LookupService {
    Applications getApplicationsForARegion(@Nullable String var1);

    Applications getApplications(String var1);

    List<InstanceInfo> getInstancesByVipAddress(String var1, boolean var2);

    List<InstanceInfo> getInstancesByVipAddress(String var1, boolean var2, @Nullable String var3);

    List<InstanceInfo> getInstancesByVipAddressAndAppName(String var1, String var2, boolean var3);

    Set<String> getAllKnownRegions();

    InstanceStatus getInstanceRemoteStatus();

    /** @deprecated */
    @Deprecated
    List<String> getDiscoveryServiceUrls(String var1);

    /** @deprecated */
    @Deprecated
    List<String> getServiceUrlsFromConfig(String var1, boolean var2);

    /** @deprecated */
    @Deprecated
    List<String> getServiceUrlsFromDNS(String var1, boolean var2);

    /** @deprecated */
    @Deprecated
    void registerHealthCheckCallback(HealthCheckCallback var1);

    void registerHealthCheck(HealthCheckHandler var1);

    void registerEventListener(EurekaEventListener var1);

    boolean unregisterEventListener(EurekaEventListener var1);

    HealthCheckHandler getHealthCheckHandler();

    void shutdown();

    EurekaClientConfig getEurekaClientConfig();

    ApplicationInfoManager getApplicationInfoManager();
}

我们发现EurekaClient是一个接口，所有操作都是由它的实现类来完成的。那么它的实现类是什么呢？还记得前面说叫DiscoveryClient的除了有一个接口还有一个类么？是的，那个DiscoveryClient就是EurekaClient的实现类。看一下这个类的注解。

/**
 * 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
 *
 */

意思是这个类用于和Eureka Server合作，负责向Eureka Server注册服务实例；向Eureka Server服务续约；当服务关闭时向Eureka Server取消租约；查询Eureka Server中的服务实例列表。最后一句“Eureka Client还需要配置一个Eureka Server列表”，嗯，正好和开头说的“在application.yml中配置eureka.client.service-url.defaultZone指定注册中心的位置”呼应上了。
至于DiscoveryClient这个类里的方法都是怎么实现的，我们下次再看。