org.springframework.web.client.ResourceAccessException: I/O error on POST request for "http://localhost:9411/api/v2/spans": connect timed out; nested exception is java.net.SocketTimeoutException: connect timed out

报错的原因可能是没有启动 redis 服务器, 启动 redis 方法  打开 redis 文件夹，找到 redis-server.exe 双击即可打开，成功打开：

若有如下报错

org.springframework.web.client.ResourceAccessException: I/O error on POST request for "http://localhost:9411/api/v2/spans": connect timed out; nested exception is java.net.SocketTimeoutException: connect timed out
at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:751) ~[spring-web-5.2.2.RELEASE.jar:5.2.2.RELEASE]
at org.springframework.cloud.sleuth.zipkin2.sender.ZipkinRestTemplateWrapper.doExecute(ZipkinRestTemplateSenderConfiguration.java:228) ~[spring-cloud-sleuth-zipkin-2.2.1.RELEASE.jar:2.2.1.RELEASE]
at org.springframework.web.client.RestTemplate.exchange(RestTemplate.java:644) ~[spring-web-5.2.2.RELEASE.jar:5.2.2.RELEASE]
at org.springframework.cloud.sleuth.zipkin2.sender.RestTemplateSender.post(RestTemplateSender.java:129) ~[spring-cloud-sleuth-zipkin-2.2.1.RELEASE.jar:2.2.1.RELEASE]
at org.springframework.cloud.sleuth.zipkin2.sender.RestTemplateSender$HttpPostCall.doExecute(RestTemplateSender.java:142) ~[spring-cloud-sleuth-zipkin-2.2.1.RELEASE.jar:2.2.1.RELEASE]
at org.springframework.cloud.sleuth.zipkin2.sender.RestTemplateSender$HttpPostCall.doExecute(RestTemplateSender.java:132) ~[spring-cloud-sleuth-zipkin-2.2.1.RELEASE.jar:2.2.1.RELEASE]
at zipkin2.Call$Base.execute(Call.java:380) ~[zipkin-2.19.0.jar:na]
at zipkin2.reporter.AsyncReporter$BoundedAsyncReporter.flush(AsyncReporter.java:285) ~[zipkin-reporter-2.11.0.jar:na]
at zipkin2.reporter.AsyncReporter$Flusher.run(AsyncReporter.java:354) [zipkin-reporter-2.11.0.jar:na]
at java.lang.Thread.run(Thread.java:750) [na:1.8.0_321]
Caused by: java.net.SocketTimeoutException: connect timed out
at java.net.DualStackPlainSocketImpl.waitForConnect(Native Method) ~[na:1.8.0_321]
at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:81) ~[na:1.8.0_321]
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:476) ~[na:1.8.0_321]
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:218) ~[na:1.8.0_321]
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:200) ~[na:1.8.0_321]
at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:162) ~[na:1.8.0_321]
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:394) ~[na:1.8.0_321]
at java.net.Socket.connect(Socket.java:606) ~[na:1.8.0_321]
at sun.net.NetworkClient.doConnect(NetworkClient.java:175) ~[na:1.8.0_321]
at sun.net.www.http.HttpClient.openServer(HttpClient.java:463) ~[na:1.8.0_321]
at sun.net.www.http.HttpClient.openServer(HttpClient.java:558) ~[na:1.8.0_321]
at sun.net.www.http.HttpClient.<init>(HttpClient.java:242) ~[na:1.8.0_321]
at sun.net.www.http.HttpClient.New(HttpClient.java:339) ~[na:1.8.0_321]
at sun.net.www.http.HttpClient.New(HttpClient.java:357) ~[na:1.8.0_321]
at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:1240) ~[na:1.8.0_321]
at sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1167) ~[na:1.8.0_321]
at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1061) ~[na:1.8.0_321]
at sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:995) ~[na:1.8.0_321]
at org.springframework.http.client.SimpleBufferingClientHttpRequest.executeInternal(SimpleBufferingClientHttpRequest.java:76) ~[spring-web-5.2.2.RELEASE.jar:5.2.2.RELEASE]
at org.springframework.http.client.AbstractBufferingClientHttpRequest.executeInternal(AbstractBufferingClientHttpRequest.java:48) ~[spring-web-5.2.2.RELEASE.jar:5.2.2.RELEASE]
at org.springframework.http.client.AbstractClientHttpRequest.execute(AbstractClientHttpRequest.java:53) ~[spring-web-5.2.2.RELEASE.jar:5.2.2.RELEASE]
at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:742) ~[spring-web-5.2.2.RELEASE.jar:5.2.2.RELEASE]
... 9 common frames omitted

是因为导入了 zipkin  依赖 (如下) 但没有配置端口等信息

<!-- zipkin-->
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-zipkin</artifactId>
</dependency>
解决方式：找到 zipkin  的 jar 包 (我这里的 jar 包是 zipkin-server-2.12.9-exec.jar) 所在文件夹，
在文件路径下单击输入 cmd, 然后输入 java -jar zipkin-server-2.12.9-exec.jar 即可启动 zipkin

此时即可启动项目成功 此外还可以去浏览器输入 Zipkin - Index 打开 zipkin 的 web 页面

注：新人写博客，多有不足，望朋友们包涵。如有建议欢迎大家提出，我会继续改进！谢谢