详解SpringCloud Ribbon 负载均衡通过服务器名无法连接的神坑

一，问题

采取eureka集群、客户端通过Ribbon调用服务，Ribbon端报下列异常

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版本比较乱，版本关联引用更是乱，最终我切换到 <spring-cloud.version> Greenwich.SR1 </spring-cloud.version> 异常为： No instances available for SERVICE-HI

二、寻找答案 

网上答案千奇百怪

1，Spring Cloud 官网，RestTemplate bean配置中添加负载均衡注解@LoadBalanced，我添加了

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

结果无效仍然一样报错

2，访问的服务名名称不能有下划线：

我的名称是“SERVICE-HI”本身就不存在下划线，所以不考虑这条。

3，主机名称没在系统文件hosts中配置，ping不通你服务名：

很扯的答案，为什么要配host，负载多台机器让主机名指向谁？不考虑此答案

三，分析问题

百度不到，自己分析原因，发现ribbon服务器没有注册到 eureka server中

分析原理：我的客户端服务“SERVICE-HI”已经成功注册到eureka server中了，如果ribbon服务器不在eureka server中注册，是不会知道客户端服务“SERVICE-HI”的存在以及它存在的位置，那么结论就是，因为ribbon服务器没有在eureka server中注册成功，所以不能识别主机名称。

四，解决问题

配置文件

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

依赖导入

<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>

主程序注释

@SpringBootApplication
@EnableDiscoveryClient
public class ServiceRibbonApplication {

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

}

有问题，最终发现@EnableDiscoveryClient标签无法注册到注册中心，百度@EnableDiscoveryClient，得到的结论是

@EnableDiscoveryClient和@EnableEurekaClient一样，能够让注册中心能够发现，扫描到改服务，不同点：@EnableEurekaClient只适用于Eureka作为注册中心，@EnableDiscoveryClient 可以是Eureka或其他（consul、zookeeper等）注册中心 。

具体原因不去分析，这里先直接切换为@EnableEurekaClient注释

@EnableEurekaClient在哪个包里简直是迷一样的存在，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>

修改主程序注释

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

@SpringBootApplication
@EnableEurekaClient
@EnableHystrix //我开启了段容器
public class ServiceRibbonApplication {

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

}

这里提一句在 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>

重新启动ribbon，发现控制台输入

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)

查看Eureka

浏览器测试访问成功！！！

五，附件：Greenwich.SR1 版中常用依赖

有好多问题都是因为 不同版本中引入不正确的依赖导致，这里列出 Greenwich.SR1 版中常用依赖，这里都不需要指定版本号

<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>

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