1、Eureka 简述

本文主要从应用角度体验一下注册中心的搭建和使用，后文会由浅入深学习Spring Cloud Eureka 的各种原理和机制。

Spring Cloud Eureka 是 Spring Cloud Netflix 组件的一部分，而 Spring Cloud Netflix 又是基于 netflix 的 eureka 封装，所以最终的源码实现在于 eureka 中。废话不多说，先上手体验一下，后文在一步步深入了解。

2、Eureka Server 服务端搭建

1、创建项目，添加 maven 依赖

<modelVersion>4.0.0</modelVersion>

    <artifactId>lkf-cloud-eureka</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<packaging>jar</packaging>

	<name>lkf-cloud-eureka</name>
    
 <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.4.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>

    <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>Finchley.SR1</spring-cloud.version>
    </properties>

<dependencies>
		<dependency>
			<groupId>org.springframework.cloud</groupId>
			<artifactId>spring-cloud-starter-netflix-eureka-server</artifactId>
		</dependency>
		<dependency>
			<groupId>org.springframework.cloud</groupId>
			<artifactId>spring-cloud-config-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>

2、创建启动类，启用注册中心服务 @EnableEurekaServer

@SpringBootApplication
@EnableEurekaServer
public class EurekaApplication {

	public static void main(String[] args) {		SpringApplication.run(EurekaApplication.class, args);
	}
}

3、添加配置文件 appication

server.port=8888
eureka.instance.hostname=localhost
#由于该应用为注册中心,所以设置为false,代表不向注册中心注册自己
eureka.client.register-with-eureka=false
#由于注册中心的职责就是维护服务实例,它并不需要去检索服务,所以也设置为false
eureka.client.fetch-registry=false
eureka.client.service-url.defaultZone=http://${eureka.instance.hostname}:${server.port}/eureka/
eureka.server.wait-time-in-ms-when-sync-empty=0
#eureka.instance.prefer-ip-address=true
#eureka.instance.ip-address=127.0.0.1

spring.application.name=lkf-cloud-eureka
spring.cloud.config.uri=${CONFIG_SERVER_URL:http://localhost:8888}

4、启动 Eureka Server 服务

启动应用，访问http://localhost:8888/，可以看到 Eureka Server 自带的 UI 管理界面。

3、Eureka Client 客户端搭建

1、创建项目 lkf-eureka-client，添加maven 依赖

<modelVersion>4.0.0</modelVersion>

    <artifactId>lkf-eureka-client</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>lkf-eureka-client</name>
    
     <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.4.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    
      <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>Finchley.SR1</spring-cloud.version>
    </properties>
    
     <dependencies>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </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>

这里如果没有引用 spring-boot-starter-web 会报以下错误：

Destroy method on bean with name 'scopedTarget.eurekaClient' threw an exception: org.springframework.beans.factory.BeanCreationNotAllowedException: Error creating bean with name 'eurekaInstanceConfigBean': Singleton bean creation not allowed while singletons of this factory are in destruction (Do not request a bean from a BeanFactory in a destroy method implementation!)

2、创建启动类，启用 @EnableDiscoveryClient 或 @EnableEurekaClient

/**
 * @author kaifeng
 */
@SpringBootApplication
@EnableDiscoveryClient
public class EurekaClientApplication {

    public static void main(String[] args) {
    SpringApplication.run(EurekaClientApplication.class, args);
    }
}

3、添加配置文件

server.port=8001
spring.application.name=lkf-eureka-client
eureka.client.serviceUrl.defaultZone=http://localhost:8888/eureka/

4、启动服务，访问注册中心