在服务器级别的最后一周，我们已将GCM端点（https://android.googleapis.com/gcm/send）更改为FCM端点（https://fcm.googleapis.com/fcm/send）URL，但未更改Google论坛建议的任何代码逻辑。 对于某些请求，我们能够从服务器级别成功将Android PUSH通知发送到设备令牌。 但是有时它会给我们以下提到的错误：
2019-03-29 10:41:48 ERROR Error:428 - fcm.googleapis.com
java.net.UnknownHostException: fcm.googleapis.com
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.security.ssl.SSLSocketImpl.connect(SSLSocketImpl.java:668)
at sun.security.ssl.BaseSSLSocketImpl.connect(BaseSSLSocketImpl.java:173)
at sun.net.NetworkClient.doConnect(NetworkClient.java:180)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:432)
at sun.net.www.http.HttpClient.openServer(HttpClient.java:527)
at sun.net.www.protocol.https.HttpsClient.<init>(HttpsClient.java:264)
at sun.net.www.protocol.https.HttpsClient.New(HttpsClient.java:367)
at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.getNewHttpClient(AbstractDelegateHttpsURLConnection.java:191)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1138)
at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1032)
at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:177)
at sun.net.www.protocol.http.HttpURLConnection.getOutputStream0(HttpURLConnection.java:1316)
at sun.net.www.protocol.http.HttpURLConnection.getOutputStream(HttpURLConnection.java:1291)
at sun.net.www.protocol.https.HttpsURLConnectionImpl.getOutputStream(HttpsURLConnectionImpl.java:250)
at com.google.android.gcm.server.Sender.post(Sender.java:740)
at com.google.android.gcm.server.Sender.post(Sender.java:717)
at com.google.android.gcm.server.Sender.sendNoRetry(Sender.java:410)
I have checked with proxy , dns team. There is no blocking .
Why this issue is coming . Please suggest.