作为微服务架构中最为核心和基础的服务治理，注册中心提供了微服务实例的自动化注册与发现。而作为一个服务注册中心，eureka的作用与传统的zk、etcd的作用是一样的，同样也支持高可用（集群）。不同之处在于它不是作为一个独立的服务器或者服务器集群存在，而是作为一个组件（或者说服务）存在，它分为服务端组件、客户端组件。可以把服务端组件看成是zk或者etcd（注册中心），其他注册到服务端的组件都可以看成是把客户端组件，比如服务提供方、消费方的微服务等。另外一个明显的特征是eureka是由java代码实现的，因此主要使用对象是JVM兼容的分布式系统。

　　搭建单例eureka服务端很简单，三板斧即可：

　　1、pom.xml引入最新版本Greenwich.SR2的spring-cloud-starter-netflix-eureka-server：

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">

    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>

    <artifactId>eureka-service</artifactId>

    <version>0.0.1-SNAPSHOT</version>

    <packaging>jar</packaging>

    <parent>

        <groupId>org.springframework.boot</groupId>

        <artifactId>spring-boot-starter-parent</artifactId>

        <version>2.1.6.RELEASE</version>

        <relativePath/> <!-- lookup parent from repository -->

    </parent>

    <properties>

        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>

        <java.version>1.8</java.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>Greenwich.SR2</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>

</project>

　　2、application.properties配置端口、单机的话无需让注册中心注册自己、也无需去注册中心查询服务（集群时需要）：

#本机端口

server.port=8888

#是否向注册中心注册自己

eureka.client.register-with-eureka=false

#是否从注册中心查询服务

eureka.client.fetch-registry=false

#注册中心地址

eureka.client.service-url.defaultZone=http://localhost:8888/eureka/

　　3、最后写一个主类，只要加上@EnableEurekaServer证明它是注册中心：

package hello;

import org.springframework.boot.SpringApplication;

import org.springframework.boot.autoconfigure.SpringBootApplication;

import org.springframework.cloud.netflix.eureka.server.EnableEurekaServer;

@EnableEurekaServer

@SpringBootApplication

public class EurekaServiceApplication {

    public static void main(String[] args) {

        SpringApplication.run(EurekaServiceApplication.class, args);

    }

}

　　客户端分服务提供方、服务消费方，先看服务提供方，还是三板斧：

　　1、pom需要把eureka客户端jar引入spring-cloud-starter-netflix-eureka-client：

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">

    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>

    <artifactId>eureka-client-service-provider</artifactId>

    <version>0.0.1-SNAPSHOT</version>

    <packaging>jar</packaging>

    <parent>

        <groupId>org.springframework.boot</groupId>

        <artifactId>spring-boot-starter-parent</artifactId>

        <version>2.1.6.RELEASE</version>

        <relativePath/> <!-- lookup parent from repository -->

    </parent>

    <properties>

        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>

        <java.version>1.8</java.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-web</artifactId>

        </dependency>

        <dependency>

            <groupId>org.springframework.boot</groupId>

            <artifactId>spring-boot-starter-test</artifactId>

            <scope>test</scope>

        </dependency>

        <dependency>

            <groupId>org.springframework.cloud</groupId>

            <artifactId>spring-cloud-starter-netflix-eureka-server</artifactId>

            <scope>test</scope>

        </dependency>

    </dependencies>

    <dependencyManagement>

        <dependencies>

            <dependency>

                <groupId>org.springframework.cloud</groupId>

                <artifactId>spring-cloud-dependencies</artifactId>

                <version>Greenwich.SR2</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>

</project>

　　2、application配置：

#本机端口

server.port=8762
#本机服务名
spring.application.name=a-bootiful-client
#注册中心地址

eureka.client.service-url.defaultZone=http://localhost:8888/eureka/

　　3、主类加上注解@EnableDiscoveryClient用于服务发现：

package hello;

import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.beans.factory.annotation.Value;

import org.springframework.boot.SpringApplication;

import org.springframework.boot.autoconfigure.SpringBootApplication;

import org.springframework.cloud.client.ServiceInstance;

import org.springframework.cloud.client.discovery.DiscoveryClient;

import org.springframework.cloud.client.discovery.EnableDiscoveryClient;

import org.springframework.web.bind.annotation.*;

import java.util.List;

@EnableDiscoveryClient

@SpringBootApplication

public class EurekaClientServiceProviderApplication {

    public static void main(String[] args) {

        SpringApplication.run(EurekaClientServiceProviderApplication.class, args);

    }

}

@RestController

class ServiceInstanceRestController {

    @Value("${server.port}")

    private String port;

    @Autowired

    private DiscoveryClient discoveryClient;

    @RequestMapping("/service-instances/{applicationName}")

    public List<ServiceInstance> serviceInstancesByApplicationName(

            @PathVariable String applicationName) {

        return this.discoveryClient.getInstances(applicationName);

    }

    @RequestMapping("/hello")

    public String hello(@RequestParam(value = "name", defaultValue = "World") String name) {

        return "Hello, " + name + "---------port: " + port;

    }

}

　　服务消费方还是老套路：

　　1、pom.xml

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">

    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>

    <artifactId>eureka-client-service-client</artifactId>

    <version>0.0.1-SNAPSHOT</version>

    <packaging>jar</packaging>

    <parent>

        <groupId>org.springframework.boot</groupId>

        <artifactId>spring-boot-starter-parent</artifactId>

        <version>2.1.6.RELEASE</version>

        <relativePath/> <!-- lookup parent from repository -->

    </parent>

    <properties>

        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>

        <java.version>1.8</java.version>

    </properties>

    <dependencies>

        <dependency>

            <groupId>org.springframework.cloud</groupId>

            <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>

        </dependency>

        <dependency>

            <groupId>org.springframework.cloud</groupId>

            <artifactId>spring-cloud-starter-netflix-ribbon</artifactId>

        </dependency>

        <dependency>

            <groupId>org.springframework.boot</groupId>

            <artifactId>spring-boot-starter-web</artifactId>

        </dependency>

        <dependency>

            <groupId>org.springframework.boot</groupId>

            <artifactId>spring-boot-starter-test</artifactId>

            <scope>test</scope>

        </dependency>

        <dependency>

            <groupId>org.springframework.cloud</groupId>

            <artifactId>spring-cloud-starter-netflix-eureka-server</artifactId>

            <scope>test</scope>

        </dependency>

    </dependencies>

    <dependencyManagement>

        <dependencies>

            <dependency>

                <groupId>org.springframework.cloud</groupId>

                <artifactId>spring-cloud-dependencies</artifactId>

                <version>Greenwich.SR2</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>

</project>

　　2、application，这里配置了app.service.url是服务方实例名，方便调用时指定，当然你也可以在调用服务提供方的代码里硬编码指定:

#本机端口

server.port=8763
#本机服务名
spring.application.name=a-beautiful-client
#注册中心地址

eureka.client.service-url.defaultZone=http://localhost:8888/eureka/

#服务提供者实例名称

app.service.url=http://A-BOOTIFUL-CLIENT/

　　3、主类同样需要加上：

package hello;

import hello.service.ConsumerService;

import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.boot.SpringApplication;

import org.springframework.boot.autoconfigure.SpringBootApplication;

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

import org.springframework.cloud.client.discovery.EnableDiscoveryClient;

import org.springframework.cloud.client.loadbalancer.LoadBalanced;import org.springframework.context.annotation.Bean;

import org.springframework.web.bind.annotation.*;

import org.springframework.web.client.RestTemplate;

import java.util.List;

@EnableDiscoveryClient

@SpringBootApplication

public class EurekaClientServiceClientApplication {

    @LoadBalanced

    @Bean

    public RestTemplate restTemplate() {

        return new RestTemplate();

    }

    public static void main(String[] args) {

        SpringApplication.run(EurekaClientServiceClientApplication.class, args);

    }

}

@RestController

class ServiceInstanceRestController {

    @Autowired

    private ConsumerService consumerService;

    @Autowired

    private DiscoveryClient discoveryClient;

    @RequestMapping("/sayHello")

    public String sayHello(@RequestParam(value = "name", defaultValue = "Spring Cloud") String name) {

        return consumerService.call(name);

    }

    @RequestMapping("/service-instances/{applicationName}")

    public List<ServiceInstance> serviceInstancesByApplicationName(

            @PathVariable String applicationName) {

        return this.discoveryClient.getInstances(applicationName);

    }

}

　　ConsumerService类：

package hello.service;

public interface ConsumerService {

    String call(String name);

}

　　ConusmerServiceImpl类：

package hello.service.impl;

import hello.service.ConsumerService;

import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.beans.factory.annotation.Value;

import org.springframework.http.ResponseEntity;

import org.springframework.stereotype.Service;

import org.springframework.web.client.RestTemplate;

@Service

public class ConusmerServiceImpl implements ConsumerService {

    @Value("${app.service.url}")

    private String appServiceUrl;

    @Autowired

    private RestTemplate restTemplate;

    @Override

    public String call(String name) {

        ResponseEntity resultResponseEntity = restTemplate.postForEntity(appServiceUrl + "hello?name=" + name, null, String.class);

        if (resultResponseEntity != null && resultResponseEntity.getBody() != null) {

            return name + " says: " + resultResponseEntity.getBody().toString();

        }

        return null;

    }

}

　　好了，接下来依次启动注册中心、服务提供方、服务消费方，浏览器上输入http://localhost:8763/sayHello?name=world，可以看到消费方通过注册中心发现了提供方，然后消费方调用了提供方的/hello：

　　直接调提供方：

　　这个是注册中心：

　　如图，服务端（a-bootiful-client）可以看到有两个实例，为何起两个呢？用来测试ribbon的负载均衡，这块内容详见Greenwich.SR2版本的Spring Cloud Ribbon实例。

　　

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