上週五夜間，服務端程式發版期間，客戶端程式在通過http呼叫服務端介面時出現一堆 “拒絕連線 (Connection refused) ” 異常。

2022-04-08 22:22:28,676 ERROR [2022040822222771523885] ? (com.cn.yft.pay.util.HttpClientHelper:) - 請求urlParam 異常：
org.apache.http.conn.HttpHostConnectException: Connect to 123.56.223.21:8280 [/123.56.223.21] failed: 拒絕連線 (Connection refused)
 

        at org.apache.http.impl.conn.DefaultHttpClientConnectionOperator.connect(DefaultHttpClientConnectionOperator.java:158) ~[httpclient-4.5.2.jar:4.5.2]
        at org.apache.http.impl.conn.PoolingHttpClientConnectionManager.connect(PoolingHttpClientConnectionManager.java:353) ~[httpclient-4.5.2.jar:4.5.2]
        at org.apache.http.impl.execchain.MainClientExec.establishRoute(MainClientExec.java:
 
380) ~[httpclient-4.5.2.jar:4.5.2]
        at org.apache.http.impl.execchain.MainClientExec.execute(MainClientExec.java:236) ~[httpclient-4.5.2.jar:4.5.2]
        at org.apache.http.impl.execchain.ProtocolExec.execute(ProtocolExec.java:184) ~[httpclient-4.5.2.jar:4.5.2]
        at org.apache.http.impl.execchain.RetryExec.execute(RetryExec.java:
 
88) ~[httpclient-4.5.2.jar:4.5.2]
        at org.apache.http.impl.execchain.RedirectExec.execute(RedirectExec.java:110) ~[httpclient-4.5.2.jar:4.5.2]
        at org.apache.http.impl.client.InternalHttpClient.doExecute$original$szvaDZwu(InternalHttpClient.java:184) ~[httpclient-4.5.2.jar:4.5.2]
        at org.apache.http.impl.client.InternalHttpClient.doExecute$original$szvaDZwu$accessor$2wpTSoQJ(InternalHttpClient.java) ~[httpclient-4.5.2.jar:4.5.2]
        at org.apache.http.impl.client.InternalHttpClient$auxiliary$f5vLomHg.call(Unknown Source) ~[httpclient-4.5.2.jar:4.5.2]
        at org.apache.skywalking.apm.agent.core.plugin.interceptor.enhance.InstMethodsInter.intercept(InstMethodsInter.java:86) ~[skywalking-agent.jar:8.6.0]
        at org.apache.http.impl.client.InternalHttpClient.doExecute(InternalHttpClient.java) ~[httpclient-4.5.2.jar:4.5.2]
        at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:82) ~[httpclient-4.5.2.jar:4.5.2]
        at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:107) ~[httpclient-4.5.2.jar:4.5.2]
        at com.cn.yft.pay.util.HttpClientHelper.httpClientPost(HttpClientHelper.java:506) [pctrans-core-0.0.1-SNAPSHOT.jar:?]
        at com.cn.yft.bank.p600001.own.PaymentTransByOWN.doBusiness$original$a5alvy8U(PaymentTransByOWN.java:150) [pctrans-core-0.0.1-SNAPSHOT.jar:?]
        at com.cn.yft.bank.p600001.own.PaymentTransByOWN.doBusiness$original$a5alvy8U$accessor$RlW8FGim(PaymentTransByOWN.java) [pctrans-core-0.0.1-SNAPSHOT.jar:?]
        at com.cn.yft.bank.p600001.own.PaymentTransByOWN$auxiliary$GTU0SZKs.call(Unknown Source) [pctrans-core-0.0.1-SNAPSHOT.jar:?]
        at org.apache.skywalking.apm.agent.core.plugin.interceptor.enhance.InstMethodsInter.intercept(InstMethodsInter.java:86) [skywalking-agent.jar:8.6.0]
        at com.cn.yft.bank.p600001.own.PaymentTransByOWN.doBusiness(PaymentTransByOWN.java) [pctrans-core-0.0.1-SNAPSHOT.jar:?]
        at com.cn.yft.pay.service.PCPaymentRunnable.run(PCPaymentRunnable.java:50) [pctrans-core-0.0.1-SNAPSHOT.jar:?]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_191]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_191]
        at java.lang.Thread.run(Thread.java:748) [?:1.8.0_191]
Caused by: java.net.ConnectException: 拒絕連線 (Connection refused)
        at java.net.PlainSocketImpl.socketConnect(Native Method) ~[?:1.8.0_191]
        at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350) ~[?:1.8.0_191]
        at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206) ~[?:1.8.0_191]
        at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188) ~[?:1.8.0_191]
        at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392) ~[?:1.8.0_191]
        at java.net.Socket.connect(Socket.java:589) ~[?:1.8.0_191]
        at org.apache.http.conn.socket.PlainConnectionSocketFactory.connectSocket(PlainConnectionSocketFactory.java:74) ~[httpclient-4.5.2.jar:4.5.2]
        at org.apache.http.impl.conn.DefaultHttpClientConnectionOperator.connect(DefaultHttpClientConnectionOperator.java:141) ~[httpclient-4.5.2.jar:4.5.2]
        ... 23 more

參考csdn上網友的Connection refused: connect問題解決方案集合，整理一下http調用出現connection refused異常的各種情況：

客戶端：

• 寫錯了主機和埠
• 通訊協議不正確，http://，https://，rmi://
• 客戶端服務不在網路中。它可能沒有連線到LAN或網際網路或任何其他網路。
• 客戶端網路防火牆攔截請求

服務端：

• 服務掛掉了 / 服務未執行
• 服務在重啟中 / 服務處於kill -15過程中
• 服務端網路防火牆攔截請求
• 服務端服務不在網路中。它可能沒有連線到LAN或網際網路或任何其他網路。
• 伺服器正在執行但沒有偵聽埠，或者伺服器正在執行但是正在偵聽不同的埠。

簡單測試一下上面的幾種情況。結論是：上面總結的幾種情況並不都返回connection refused異常。至少如下幾種不是。

我司公司網路限制訪問淘寶、優酷等視訊購物網站。就拿訪問淘寶網（https://www.taobao.com）來說， 我們來 curl https://www.taobao.com 或 curl http://www.taobao.com

C:\Users\zhangguozhan>curl https://www.taobao.com
curl: (35) schannel: next InitializeSecurityContext failed: SEC_E_INVALID_TOKEN (0x80090308) - 給函式提供的標誌無效

<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" /
><title>Forbidden</title>
</head>
<body>
你被禁止訪問這個網站，請聯絡網站管理員！
</body>
</html>

把網路切換成自己手機的4G，再 curl https://www.taobao.com/，  此時可以正常返回響應結果。

然後，自己手機4G情況下，把上面的https換成http， 即， 此時，返回的是我們熟知的301（重定向）錯誤。 

C:\Users\zhangguozhan>curl http://www.taobao.com
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html>
<head><title>301 Moved Permanently</title></head>
<body>
<h1>301 Moved Permanently</h1>
<p>The requested resource has been assigned a new permanent URI.</p>
<hr/>Powered by Tengine</body>
</html>

以上，做個記錄，方便日後工作中查證。