无法在任何已知服务器上执行请求。
原因是注册中心未启动。在使用springcloud搭建微服务时，一定要先启动注册中心，再启动服务端和客户端。具体报错如下：
com.netflix.discovery.shared.transport.TransportException: Cannot execute request on any known server
at com.netflix.discovery.shared.transport.decorator.RetryableEurekaHttpClient.execute(RetryableEurekaHttpClient.java:111) ~[eureka-client-1.6.2.jar:1.6.2]
at com.netflix.discovery.shared.transport.decorator.EurekaHttpClientDecorator.register(EurekaHttpClientDecorator.java:56) ~[eureka-client-1.6.2.jar:1.6.2]
at com.netflix.discovery.shared.transport.decorator.EurekaHttpClientDecorator$1.execute(EurekaHttpClientDecorator.java:59) ~[eureka-client-1.6.2.jar:1.6.2]
at com.netflix.discovery.shared.transport.decorator.SessionedEurekaHttpClient.execute(SessionedEurekaHttpClient.java:77) ~[eureka-client-1.6.2.jar:1.6.2]
at com.netflix.discovery.shared.transport.decorator.EurekaHttpClientDecorator.register(EurekaHttpClientDecorator.java:56) ~[eureka-client-1.6.2.jar:1.6.2]
at com.netflix.discovery.DiscoveryClient.register(DiscoveryClient.java:798) ~[eureka-client-1.6.2.jar:1.6.2]
at com.netflix.discovery.InstanceInfoReplicator.run(InstanceInfoReplicator.java:104) ~[eureka-client-1.6.2.jar:1.6.2]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) [na:1.8.0_131]
at java.util.concurrent.FutureTask.run(FutureTask.java:266) [na:1.8.0_131]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180) [na:1.8.0_131]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293) [na:1.8.0_131]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_131]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_131]
at java.lang.Thread.run(Thread.java:748) [na:1.8.0_131]