原文：https://xsxy007.github.io/2019/10/SpringCloud-Eureka/
Eureka 是Netflix开源的一款提供服务注册和发现的产品，提供了完整的Service Registery和Service Discovery实现，也是SpringCloud体系中最核心的组件之一

Eureka

由上边的图可以看出，Eureka由客户端和服务端组成，服务用用于服务的注册服务器，客户端用作服务的提供者和发现者

案例

git@github.com:xsxy007/springcloud-demo.git

Eureka Server

springboot 已经很好的支持了Eureka，只需要在pom中加入Eureka依赖，并在Application启动类中加上相关注解即可

1、pom依赖添加

本次使用的springboot版本为2.1.9；springcloud版本为Greenwich.RELEASE

以下列出了所有的依赖（ps：一开始因为springcloud和springboot的版本依赖问题，导致项目一直启动报错）

    <properties><project.build.sourceEncoding>UTF-8</project.build.sourceEncoding><project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding><java.version>1.8</java.version><spring-cloud.version>Greenwich.RELEASE</spring-cloud.version></properties><dependencies><dependency><groupId>org.springframework.cloud</groupId><artifactId>spring-cloud-starter-netflix-eureka-server</artifactId></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-test</artifactId><scope>test</scope></dependency></dependencies><dependencyManagement><dependencies><dependency><groupId>org.springframework.cloud</groupId><artifactId>spring-cloud-dependencies</artifactId><version>${spring-cloud.version}</version><type>pom</type><scope>import</scope></dependency></dependencies></dependencyManagement><build><plugins><plugin><groupId>org.springframework.boot</groupId><artifactId>spring-boot-maven-plugin</artifactId></plugin></plugins></build>

2、在启动类上加注解@EnableEurekaServer注解

@SpringBootApplication
@EnableEurekaServer
public class SpringcloudDemoApplication {public static void main(String[] args) {SpringApplication.run(SpringcloudDemoApplication.class, args);}
}

3、配置文件

在默认配置下，该注册中心会将自己作为客户端尝试注册自己，因此在单机的注册中心时，需要将此机制关闭掉

server:port: 8761 # 使用的是默认的端口
spring:application:name: eureka-server
eureka:client:register-with-eureka: falsefetch-registry: falseservice-url:defaultZone: http://localhost:${server.port}/eureka/

eureka.client.register-with-eureka：表示是否将自己注册的eureka server，默认为true
eureka.client.fetch-registry：表示是否从eureka server上拉取注册信息，默认为true
eureka.client.service-url.defaultZone：表示注册中心的地址（默认为localhost:8761/eureka/）

4、启动项目

启动项目后访问localhost:8761 即可显示如下页面，此时 Instances current Registers里边并没有内容

集群配置

简单的集群，只要将各个注册中心相互配置即可
eureka.client.serviceUrl.defaultZone=http://域名:/${server.port}/eureka/,http://域名:/${server.port}/eureka/ 请其他注册中心的地址配置的defaultZone中

源码分析

从启动类注解开始 @EnableEurekaServer

• 可以注意注释中的EurekaServerAutoConfiguration，该类就是EurekaServer的自动配置类

/*** Annotation to activate Eureka Server related configuration {@link EurekaServerAutoConfiguration}* @author Dave Syer* @author Biju Kunjummen*/
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(EurekaServerMarkerConfiguration.class)
public @interface EnableEurekaServer {}

• 再看上边注解类上有一个 @Import(EurekaServerMarkerConfiguration.class)，进入该类查看

// 可以看到，该类并没有什么实现，该类的作用：是为了控制 `EurekaServerAutoConfiguration` 类是否加载
@Configuration
public class EurekaServerMarkerConfiguration {@Beanpublic Marker eurekaServerMarkerBean() {return new Marker();}class Marker {}
}

• 查看 EurekaServerAutoConfiguration（该类是在通过Springboot的@EnableAutoconfiguration 来加载spring.factories中的类的）

// 省略了部分代码
@Configuration
@Import(EurekaServerInitializerConfiguration.class)
@ConditionalOnBean(EurekaServerMarkerConfiguration.Marker.class) // 当存在 EurekaServerMarkerConfiguration 类是才初始化当前文件
@EnableConfigurationProperties({ EurekaDashboardProperties.class,InstanceRegistryProperties.class })
@PropertySource("classpath:/eureka/server.properties")
public class EurekaServerAutoConfiguration extends WebMvcConfigurerAdapter {// 实例化一个配置类，该配置类在 EurekaServerInitializerConfiguration 中会使用到@Configurationprotected static class EurekaServerConfigBeanConfiguration {@Bean@ConditionalOnMissingBeanpublic EurekaServerConfig eurekaServerConfig(EurekaClientConfig clientConfig) {EurekaServerConfigBean server = new EurekaServerConfigBean();if (clientConfig.shouldRegisterWithEureka()) {// Set a sensible default if we are supposed to replicateserver.setRegistrySyncRetries(5);}return server;}}// eurekaServer的上下文@Beanpublic EurekaServerContext eurekaServerContext(ServerCodecs serverCodecs,PeerAwareInstanceRegistry registry, PeerEurekaNodes peerEurekaNodes) {return new DefaultEurekaServerContext(this.eurekaServerConfig, serverCodecs,registry, peerEurekaNodes, this.applicationInfoManager);}// eurekaServer的启动@Beanpublic EurekaServerBootstrap eurekaServerBootstrap(PeerAwareInstanceRegistry registry,EurekaServerContext serverContext) {return new EurekaServerBootstrap(this.applicationInfoManager,this.eurekaClientConfig, this.eurekaServerConfig, registry,serverContext);}
}

• 注意 EurekaServerInitializerConfiguration 类，该类实现了lifecycle接口，所以会被spring容器毁掉start方法

    @Overridepublic void start() {new Thread(new Runnable() {@Overridepublic void run() {try {//TODO: is this class even needed now?eurekaServerBootstrap.contextInitialized(EurekaServerInitializerConfiguration.this.servletContext);log.info("Started Eureka Server");publish(new EurekaRegistryAvailableEvent(getEurekaServerConfig()));EurekaServerInitializerConfiguration.this.running = true;publish(new EurekaServerStartedEvent(getEurekaServerConfig()));}catch (Exception ex) {// Help!log.error("Could not initialize Eureka servlet context", ex);}}}).start(); // 调用 start 方法启动线程}

参考： https://blog.csdn.net/cuixhao110/article/details/88353714