一，問題

採取eureka叢集、客戶端通過Ribbon呼叫服務，Ribbon端報下列異常

java.net.UnknownHostException: SERVICE-HI

java.lang.IllegalStateException: No instances available for SERVICE-HI

java.lang.IllegalStateException: Request URI does not contain a valid hostname: http://SERVICE-HI

com.netfix.discovery.shared.taransport.TransportException: Cannot execute request on any known server

Spring Cloud版本比較亂，版本關聯引用更是亂，最終我切換到 <spring-cloud.version>Greenwich.SR1</spring-cloud.version> 異常為：No instances available for SERVICE-HI

二、尋找答案 

網上答案千奇百怪

1，Spring Cloud 官網，RestTemplate bean配置中新增負載均衡註解@LoadBalanced，我添加了

@Bean
@LoadBalanced
public RestTemplate getRestTemplate(){
    return new RestTemplate();
}

結果無效仍然一樣報錯

2，訪問的服務名名稱不能有下劃線：　　

我的名稱是“SERVICE-HI”本身就不存在下劃線，所以不考慮這條。

3，主機名稱沒在系統檔案hosts中配置，ping不通你服務名：

很扯的答案，為什麼要配host，負載多臺機器讓主機名指向誰？不考慮此答案

三，分析問題

百度不到，自己分析原因，發現ribbon伺服器沒有註冊到 eureka server中

分析原理：我的客戶端服務“SERVICE-HI”已經成功註冊到eureka server中了，如果ribbon伺服器不在eureka server中註冊，是不會知道客戶端服務“SERVICE-HI”的存在以及它存在的位置，那麼結論就是，因為ribbon伺服器沒有在eureka server中註冊成功，所以不能識別主機名稱。

四，解決問題

配置檔案

eureka:
  client:
    serviceUrl:
      defaultZone: http://localhost:8761/eureka/
server:
  port: 8764
spring:
  application:
    name: service-ribbon

依賴匯入

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-netflix-ribbon</artifactId>
    </dependency>
</dependencies>

主程式註釋

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

}

有問題，最終發現@EnableDiscoveryClient標籤無法註冊到註冊中心，百度@EnableDiscoveryClient，得到的結論是

@EnableDiscoveryClient和@EnableEurekaClient一樣，能夠讓註冊中心能夠發現，掃描到改服務，不同點：@EnableEurekaClient只適用於Eureka作為註冊中心，@EnableDiscoveryClient 可以是Eureka或其他（consul、zookeeper等）註冊中心。

具體原因不去分析，這裡先直接切換為@EnableEurekaClient註釋

@EnableEurekaClient在哪個包裡簡直是迷一樣的存在，不同版本的spring cloud 中位置不同，我使用Greenwich.SR1，需要引入下面的包

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
</dependency>

修改主程式註釋

import org.springframework.cloud.netflix.eureka.EnableEurekaClient;
import org.springframework.cloud.netflix.hystrix.EnableHystrix;


@SpringBootApplication
@EnableEurekaClient
@EnableHystrix //我開啟了段容器
public class ServiceRibbonApplication {

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

}

這裡提一句在 Greenwich.SR1中段容器在下面包中

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-netflix-hystrix</artifactId>
</dependency>

重新啟動ribbon，發現控制檯輸入

2019-06-15 13:08:06.668  INFO 14796 --- [           main] com.netflix.discovery.DiscoveryClient    : Getting all instance registry info from the eureka server
2019-06-15 13:08:06.878  INFO 14796 --- [           main] com.netflix.discovery.DiscoveryClient    : The response status is 200
2019-06-15 13:08:06.882  INFO 14796 --- [           main] com.netflix.discovery.DiscoveryClient    : Starting heartbeat executor: renew interval is: 30
2019-06-15 13:08:06.886  INFO 14796 --- [           main] c.n.discovery.InstanceInfoReplicator     : InstanceInfoReplicator onDemand update allowed rate per min is 4
2019-06-15 13:08:06.891  INFO 14796 --- [           main] com.netflix.discovery.DiscoveryClient    : Discovery Client initialized at timestamp 1560575286889 with initial instances count: 2
2019-06-15 13:08:06.894  INFO 14796 --- [           main] o.s.c.n.e.s.EurekaServiceRegistry        : Registering application SERVICE-RIBBON with eureka with status UP
2019-06-15 13:08:06.896  INFO 14796 --- [           main] com.netflix.discovery.DiscoveryClient    : Saw local status change event StatusChangeEvent [timestamp=1560575286896, current=UP, previous=STARTING]
2019-06-15 13:08:06.900  INFO 14796 --- [nfoReplicator-0] com.netflix.discovery.DiscoveryClient    : DiscoveryClient_SERVICE-RIBBON/DESKTOP-FJQITE3:service-ribbon:8764: registering service...
2019-06-15 13:08:06.958  INFO 14796 --- [nfoReplicator-0] com.netflix.discovery.DiscoveryClient    : DiscoveryClient_SERVICE-RIBBON/DESKTOP-FJQITE3:service-ribbon:8764 - registration status: 204
2019-06-15 13:08:06.961  INFO 14796 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8764 (http) with context path ''
2019-06-15 13:08:06.963  INFO 14796 --- [           main] .s.c.n.e.s.EurekaAutoServiceRegistration : Updating port to 8764
2019-06-15 13:08:06.967  INFO 14796 --- [           main] cn.meylink.ServiceRibbonApplication      : Started ServiceRibbonApplication in 5.868 seconds (JVM running for 7.204)

檢視Eureka

瀏覽器測試訪問成功！！！

五，附件：Greenwich.SR1 版中常用依賴

有好多問題都是因為 不同版本中引入不正確的依賴導致，這裡列出 Greenwich.SR1 版中常用依賴，這裡都不需要指定版本號

<dependencies>
    <!-- eureka client -->
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
    </dependency>
    <!-- eureka server -->
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-netflix-eureka-server</artifactId>
    </dependency>
    <!-- 段容器 -->
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-netflix-hystrix</artifactId>
    </dependency>
    <!-- ribbon -->
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-netflix-ribbon</artifactId>
    </dependency>
    <!-- feign -->
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-openfeign</artifactId>
    </dependency>
    <!-- config server -->
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-config-server</artifactId>
    </dependency>
    <!-- config client -->
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-config</artifactId>
    </dependency>
    <!-- zuul -->
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-netflix-zuul</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
</dependencies>

&n