使用RestTemplate 来调用服务，配置类有

    @Bean
    @LoadBalanced
    RestTemplate restTemplate() {
       return new RestTemplate();
    }

EUREKA-CLIENT调用EUREKA-SERVER报错如下。

2022-03-02 18:04:36.801 ERROR 17956 --- [nio-8083-exec-7] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.web.client.ResourceAccessException: I/O error on GET request for "http://EUREKA-SERVER/back/getmessage": EUREKA-SERVER; nested exception is java.net.UnknownHostException: EUREKA-SERVER] with root cause

java.net.UnknownHostException: EUREKA-SERVER
	at java.net.InetAddress.getAllByName0(InetAddress.java:1281) ~[na:1.8.0_281]
	at java.net.InetAddress.getAllByName(InetAddress.java:1193) ~[na:1.8.0_281]
	at java.net.InetAddress.getAllByName(InetAddress.java:1127) ~[na:1.8.0_281]
	at org.apache.http.impl.conn.SystemDefaultDnsResolver.resolve(SystemDefaultDnsResolver.java:45) ~[httpclient-4.5.12.jar:4.5.12]
	at org.apache.http.impl.conn.DefaultHttpClientConnectionOperator.connect(DefaultHttpClientConnectionOperator.java:112) ~[httpclient-4.5.12.jar:4.5.12]
	at org.apache.http.impl.conn.PoolingHttpClientConnectionManager.connect(PoolingHttpClientConnectionManager.java:376) ~[httpclient-4.5.12.jar:4.5.12]
	at org.apache.http.impl.execchain.MainClientExec.establishRoute(MainClientExec.java:393) ~[httpclient-4.5.12.jar:4.5.12]
	at org.apache.http.impl.execchain.MainClientExec.execute(MainClientExec.java:236) ~[httpclient-4.5.12.jar:4.5.12]
	at org.apache.http.impl.execchain.ProtocolExec.execute(ProtocolExec.java:186) ~[httpclient-4.5.12.jar:4.5.12]
	at org.apache.http.impl.execchain.RetryExec.execute(RetryExec.java:89) ~[httpclient-4.5.12.jar:4.5.12]
	at org.apache.http.impl.execchain.RedirectExec.execute(RedirectExec.java:110) ~[httpclient-4.5.12.jar:4.5.12]
	at org.apache.http.impl.client.InternalHttpClient.doExecute(InternalHttpClient.java:185) ~[httpclient-4.5.12.jar:4.5.12]
	at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:83) ~[httpclient-4.5.12.jar:4.5.12]
	at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:56) ~[httpclient-4.5.12.jar:4.5.12]
	at org.springframework.http.client.HttpComponentsClientHttpRequest.executeInternal(HttpComponentsClientHttpRequest.java:87) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.http.client.AbstractBufferingClientHttpRequest.executeInternal(AbstractBufferingClientHttpRequest.java:48) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.http.client.AbstractClientHttpRequest.execute(AbstractClientHttpRequest.java:53) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.http.client.InterceptingClientHttpRequest$InterceptingRequestExecution.execute(InterceptingClientHttpRequest.java:109) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.cloud.client.loadbalancer.LoadBalancerRequestFactory.lambda$createRequest$0(LoadBalancerRequestFactory.java:61) ~[spring-cloud-commons-2.2.4.RELEASE.jar:2.2.4.RELEASE]
	at org.springframework.cloud.netflix.ribbon.RibbonLoadBalancerClient.execute(RibbonLoadBalancerClient.java:144) ~[spring-cloud-netflix-ribbon-2.2.4.RELEASE.jar:2.2.4.RELEASE]
	at org.springframework.cloud.client.loadbalancer.RetryLoadBalancerInterceptor.lambda$intercept$0(RetryLoadBalancerInterceptor.java:81) ~[spring-cloud-commons-2.2.4.RELEASE.jar:2.2.4.RELEASE]
	at org.springframework.retry.support.RetryTemplate.doExecute(RetryTemplate.java:287) ~[spring-retry-1.2.5.RELEASE.jar:na]
	at org.springframework.retry.support.RetryTemplate.execute(RetryTemplate.java:180) ~[spring-retry-1.2.5.RELEASE.jar:na]
	at org.springframework.cloud.client.loadbalancer.RetryLoadBalancerInterceptor.intercept(RetryLoadBalancerInterceptor.java:71) ~[spring-cloud-commons-2.2.4.RELEASE.jar:2.2.4.RELEASE]
	at org.springframework.http.client.InterceptingClientHttpRequest$InterceptingRequestExecution.execute(InterceptingClientHttpRequest.java:93) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at com.rongsoft.eurekaclient.interceptor.RestTemplateHeaderInterceptor.intercept(RestTemplateHeaderInterceptor.java:40) ~[classes/:na]
	at org.springframework.http.client.InterceptingClientHttpRequest$InterceptingRequestExecution.execute(InterceptingClientHttpRequest.java:93) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.http.client.InterceptingClientHttpRequest.executeInternal(InterceptingClientHttpRequest.java:77) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.http.client.AbstractBufferingClientHttpRequest.executeInternal(AbstractBufferingClientHttpRequest.java:48) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.http.client.AbstractClientHttpRequest.execute(AbstractClientHttpRequest.java:53) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:739) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:674) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.web.client.RestTemplate.getForEntity(RestTemplate.java:342) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at com.rongsoft.eurekaclient.controller.CardBinController.getById(CardBinController.java:36) ~[classes/:na]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_281]
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_281]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_281]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_281]
	at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:190) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:138) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:105) ~[spring-webmvc-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:878) ~[spring-webmvc-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:792) ~[spring-webmvc-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1040) ~[spring-webmvc-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:943) ~[spring-webmvc-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) ~[spring-webmvc-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898) ~[spring-webmvc-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:626) ~[tomcat-embed-core-9.0.37.jar:4.0.FR]
	at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) ~[spring-webmvc-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:733) ~[tomcat-embed-core-9.0.37.jar:4.0.FR]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) ~[tomcat-embed-websocket-9.0.37.jar:9.0.37]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.springframework.boot.actuate.metrics.web.servlet.WebMvcMetricsFilter.doFilterInternal(WebMvcMetricsFilter.java:93) ~[spring-boot-actuator-2.3.3.RELEASE.jar:2.3.3.RELEASE]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.2.8.RELEASE.jar:5.2.8.RELEASE]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202) ~[tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541) [tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139) [tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) [tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) [tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) [tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:373) [tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) [tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868) [tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1589) [tomcat-embed-core-9.0.37.jar:9.0.37]
	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-9.0.37.jar:9.0.37]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_281]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_281]
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-9.0.37.jar:9.0.37]
	at java.lang.Thread.run(Thread.java:748) [na:1.8.0_281]

先来看部分日志。Server stats: [[Server:EUREKA-SERVER:8082;

2022-03-02 19:15:27.759  INFO 19300 --- [nio-8083-exec-1] c.n.l.DynamicServerListLoadBalancer      : DynamicServerListLoadBalancer for client EUREKA-SERVER initialized: DynamicServerListLoadBalancer:{NFLoadBalancer:name=EUREKA-SERVER,current list of Servers=[EUREKA-SERVER:8082],Load balancer stats=Zone stats: {defaultzone=[Zone:defaultzone;	Instance count:1;	Active connections count: 0;	Circuit breaker tripped count: 0;	Active connections per server: 0.0;]
},Server stats: [[Server:EUREKA-SERVER:8082;	Zone:defaultZone;	Total Requests:0;	Successive connection failure:0;	Total blackout seconds:0;	Last connection made:Thu Jan 01 08:00:00 CST 1970;	First connection made: Thu Jan 01 08:00:00 CST 1970;	Active Connections:0;	total failure count in last (1000) msecs:0;	average resp time:0.0;	90 percentile resp time:0.0;	95 percentile resp time:0.0;	min resp time:0.0;	max resp time:0.0;	stddev resp time:0.0]
]}ServerList:org.springframework.cloud.netflix.ribbon.eureka.DomainExtractingServerList@1763e375

这里的Server并不是显示的是IP地址，说明Ribbon并没有问题，可能是配置错误，检查参数，搜索果然发现有这么一段参数。

eureka.instance.hostname=EUREKA-SERVER

删除这个参数并重新启动，测试通过，果然是这一问题。

RestTemplate是基于Ribbon的实现了负载均衡功能，我们来看LoadBalancerClient的实现类RibbonLoadBalancerClient。

 RibbonLoadBalancerClient实现了ServiceInstanceChooser接口，可以选择服务器。通过实现IRule接口实现负载均衡策略。

而RestTemplate继承了抽象类InterceptingHttpAccessor，可以设置实现了ClientHttpRequestInterceptor的拦截器。

RestTemplate还实现了RestOperations接口，拥有了执行request的各种方法。

如果是默认的RestTemplate执行了InterceptingClientHttpRequest，InterceptingClientHttpRequest包装了ClientHttpRequest类，为了支持ClientHttpRequestInterceptor拦截器。

/**
 * Wrapper for a {@link ClientHttpRequest} that has support for {@link ClientHttpRequestInterceptor
 * ClientHttpRequestInterceptors}.
 */

InterceptingClientHttpRequest的executeInternal方法创建了一个执行对象InterceptingRequestExecution这个执行对象会执行默认的拦截器以及手动添加的拦截器。

这里默认执行了RetryLoadBalancerInterceptor拦截器，上面讲到RibbonLoadBalancerClient能选择服务器，org.springframework.cloud.netflix.ribbon.RibbonLoadBalancerClient#reconstructURI方法能将选好的服务器的IP和端口替换到请求URL中的serviceName。

这样就能像访问IP和端口一样访问服务器了。