Greenwich.SR2版本的Spring Cloud Eureka实例

　　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。