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: false
    fetch-registry: false
    service-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 ```java /**
• 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
```java
// 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 {
	@Bean
	public 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
    @Configuration
    protected static class EurekaServerConfigBeanConfiguration {
        @Bean
        @ConditionalOnMissingBean
        public EurekaServerConfig eurekaServerConfig(EurekaClientConfig clientConfig) {
            EurekaServerConfigBean server = new EurekaServerConfigBean();
            if (clientConfig.shouldRegisterWithEureka()) {
                // Set a sensible default if we are supposed to replicate
                server.setRegistrySyncRetries(5);
            }
            return server;
        }
    }
  
    // eurekaServer的上下文
    @Bean
    public EurekaServerContext eurekaServerContext(ServerCodecs serverCodecs,
            PeerAwareInstanceRegistry registry, PeerEurekaNodes peerEurekaNodes) {
        return new DefaultEurekaServerContext(this.eurekaServerConfig, serverCodecs,
                registry, peerEurekaNodes, this.applicationInfoManager);
    }
  
    // eurekaServer的启动
    @Bean
    public 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

    @Override
    public void start() {
        new Thread(new Runnable() {
            @Override
            public 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