Java 在服务器中调用FCM后端服务时出错

我在服务器上部署了Springboot，当我尝试向特定用户发送消息时，错误是这样的

Caused by: java.net.UnknownHostException: oauth2.googleapis.com
    at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:184) ~[na:1.8.0_212]
    at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392) ~[na:1.8.0_212]
    at java.net.Socket.connect(Socket.java:589) ~[na:1.8.0_212]
    at sun.security.ssl.SSLSocketImpl.connect(SSLSocketImpl.java:666) ~[na:1.8.0_212]
    at sun.net.NetworkClient.doConnect(NetworkClient.java:175) ~[na:1.8.0_212]
    at sun.net.www.http.HttpClient.openServer(HttpClient.java:463) ~[na:1.8.0_212]
    at sun.net.www.http.HttpClient.openServer(HttpClient.java:558) ~[na:1.8.0_212]
    at sun.net.www.protocol.https.HttpsClient.<init>(HttpsClient.java:264) ~[na:1.8.0_212]
    at sun.net.www.protocol.https.HttpsClient.New(HttpsClient.java:367) ~[na:1.8.0_212]
    at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.getNewHttpClient(AbstractDelegateHttpsURLConnection.java:191) ~[na:1.8.0_212]
    at sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1156) ~[na:1.8.0_212]
    at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1050) ~[na:1.8.0_212]
    at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:177) ~[na:1.8.0_212]
    at sun.net.www.protocol.http.HttpURLConnection.getOutputStream0(HttpURLConnection.java:1334) ~[na:1.8.0_212]
    at sun.net.www.protocol.http.HttpURLConnection.getOutputStream(HttpURLConnection.java:1309) ~[na:1.8.0_212]
    at sun.net.www.protocol.https.HttpsURLConnectionImpl.getOutputStream(HttpsURLConnectionImpl.java:259) ~[na:1.8.0_212]
    at com.google.api.client.http.javanet.NetHttpRequest.execute(NetHttpRequest.java:108) ~[google-http-client-1.30.1.jar!/:na]
    at com.google.api.client.http.javanet.NetHttpRequest.execute(NetHttpRequest.java:79) ~[google-http-client-1.30.1.jar!/:na]
    at com.google.api.client.http.HttpRequest.execute(HttpRequest.java:995) ~[google-http-client-1.30.1.jar!/:na]
    at com.google.auth.oauth2.ServiceAccountCredentials.refreshAccessToken(ServiceAccountCredentials.java:429) ~[google-auth-library-oauth2-http-0.17.1.jar!/:na]
    ... 17 common frames omitted

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似乎是某种证书问题。看起来您的程序无法访问google API。试着沿着这些线检查。你是说firebase证书？我在firebase中有一个帐户，并且已经将“serviceAccountKey”放入我的程序中。或者我的机器没有证书@德里尔托马斯诺。不是firebase证书。我认为您的机器没有相关的客户端/ssl证书来与google API通信。我对这方面的知识不多，因此我添加了一些新的标签，可能有助于将其传达给正确的受众。。。是的，我的服务器是http，不是https。看起来是因为我没有ssl。顺便说一句，谢谢你的帮助，伙计DUnknownHostException意味着您的DNS服务器不知道Googleapis.com

Caused by: java.io.IOException: Error getting access token for service account: Unrecognized SSL message, plaintext connection?
    at com.google.auth.oauth2.ServiceAccountCredentials.refreshAccessToken(ServiceAccountCredentials.java:432) ~[google-auth-library-oauth2-http-0.17.1.jar!/:na]
    at com.google.auth.oauth2.OAuth2Credentials.refresh(OAuth2Credentials.java:157) ~[google-auth-library-oauth2-http-0.17.1.jar!/:na]
    at com.google.auth.oauth2.OAuth2Credentials.getRequestMetadata(OAuth2Credentials.java:145) ~[google-auth-library-oauth2-http-0.17.1.jar!/:na]
    at com.google.auth.http.HttpCredentialsAdapter.initialize(HttpCredentialsAdapter.java:91) ~[google-auth-library-oauth2-http-0.17.1.jar!/:na]
    at com.google.firebase.internal.FirebaseRequestInitializer.initialize(FirebaseRequestInitializer.java:55) ~[firebase-admin-6.12.0.jar!/:na]
    at com.google.api.client.http.HttpRequestFactory.buildRequest(HttpRequestFactory.java:88) ~[google-http-client-1.30.1.jar!/:na]
    at com.google.api.client.http.HttpRequestFactory.buildPostRequest(HttpRequestFactory.java:128) ~[google-http-client-1.30.1.jar!/:na]
    at com.google.firebase.messaging.FirebaseMessagingClientImpl.sendSingleRequest(FirebaseMessagingClientImpl.java:148) ~[firebase-admin-6.12.0.jar!/:na]
    at com.google.firebase.messaging.FirebaseMessagingClientImpl.send(FirebaseMessagingClientImpl.java:126) ~[firebase-admin-6.12.0.jar!/:na]