eureka的client注冊到server時默認是使用hostname而不是ip。
這樣的話使用feign client通過eureka進行服務間相互調用時也會使用hostname進行調用，從而調用失敗。
報錯如下 ：
Caused by: java.net.UnknownHostException: xxxx-hostname
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:184)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:589)
at sun.net.NetworkClient.doConnect(NetworkClient.java:175)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:463)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:558)
at sun.net.www.http.HttpClient.<init>(HttpClient.java:242)
at sun.net.www.http.HttpClient.New(HttpClient.java:339)
at sun.net.www.http.HttpClient.New(HttpClient.java:357)
at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:1220)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1156)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1050)
at sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:984)
at sun.net.www.protocol.http.HttpURLConnection.getOutputStream0(HttpURLConnection.java:1334)
at sun.net.www.protocol.http.HttpURLConnection.getOutputStream(HttpURLConnection.java:1309)
at feign.Client$Default.convertAndSend(Client.java:133)
at feign.Client$Default.execute(Client.java:73)
at org.springframework.cloud.sleuth.instrument.web.client.feign.TraceFeignClient.execute(TraceFeignClient.java:80)
at org.springframework.cloud.netflix.feign.ribbon.FeignLoadBalancer.execute(FeignLoadBalancer.java:77)
at org.springframework.cloud.netflix.feign.ribbon.FeignLoadBalancer.execute(FeignLoadBalancer.java:45)
at com.netflix.client.AbstractLoadBalancerAwareClient$1.call(AbstractLoadBalancerAwareClient.java:109)
at com.netflix.loadbalancer.reactive.LoadBalancerCommand$3$1.call(LoadBalancerCommand.java:303)
at com.netflix.loadbalancer.reactive.LoadBalancerCommand$3$1.call(LoadBalancerCommand.java:287)
at rx.internal.util.ScalarSynchronousObservable$3.call(ScalarSynchronousObservable.java:231)
at rx.internal.util.ScalarSynchronousObservable$3.call(ScalarSynchronousObservable.java:228)
at rx.Observable.unsafeSubscribe(Observable.java:10211)
at rx.internal.operators.OnSubscribeConcatMap$ConcatMapSubscriber.drain(OnSubscribeConcatMap.java:286)
at rx.internal.operators.OnSubscribeConcatMap$ConcatMapSubscriber.onNext(OnSubscribeConcatMap.java:144)
at com.netflix.loadbalancer.reactive.LoadBalancerCommand$1.call(LoadBalancerCommand.java:185)
at com.netflix.loadbalancer.reactive.LoadBalancerCommand$1.call(LoadBalancerCommand.java:180)
at rx.Observable.unsafeSubscribe(Observable.java:10211)
解決方案：
1、使用ip來服務到eureka-server上，需要在eureka的client增加配置如下：eureka.instance.prefer-ip-address=true 或 eureka.instance.prefer-ip-address=true
eureka.instance.instance-id=${spring.cloud.client.ipAddress}:${server.port} ##此行可加可不加，只是用於實例的顯示
feign接口示例：
@FeignClient("xxx-business-service-name")
public interface MyFeignClient {
......
}
2、使用域名方式調用，通過注解注入服務的域名，此處域名使用配置注入，是為了多環境適配
feign接口示例，：
@FeignClient(name = "xxx-business-service-name", url = "http://${xxx.business.service.domain}")
public interface MyFeignClient {
......
}