Failed to send messages to FCM.

从 Java server-side sample 开始，检查 Messaging.java 中的 getAccessToken（第 50 行）、getConnection（第 68 行）、sendMessage（第 83 行）和 sendCommonMessage（第 191 行）。使用报告中的 project_id 重现该 sample，并跟踪以 Connection reset 结束的 token 请求。确定原因并记录经过验证的解决方案后，即视为完成。

  I  use the  java server side sample then change the my  `project_id` variable in `com.google.firebase.quickstart.Messaging.java' . The following logs show in my project. I  don't know why.

java.net.SocketException: Connection reset
at java.net.SocketInputStream.read(SocketInputStream.java:210)
at java.net.SocketInputStream.read(SocketInputStream.java:141)
at sun.security.ssl.InputRecord.readFully(InputRecord.java:465)
at sun.security.ssl.InputRecord.read(InputRecord.java:503)
at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:973)
at sun.security.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1375)
at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1403)
at sun.security.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1387)
at sun.net.www.protocol.https.HttpsClient.afterConnect(HttpsClient.java:559)
at sun.net.www.protocol.https.AbstractDelegateHttpsURLConnection.connect(AbstractDelegateHttpsURLConnection.java:185)
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.api.client.http.javanet.NetHttpRequest.execute(NetHttpRequest.java:77)
at com.google.api.client.http.HttpRequest.execute(HttpRequest.java:981)
at com.google.api.client.auth.oauth2.TokenRequest.executeUnparsed(TokenRequest.java:283)
at com.google.api.client.auth.oauth2.TokenRequest.execute(TokenRequest.java:307)
at com.google.api.client.googleapis.auth.oauth2.GoogleCredential.executeRefreshToken(GoogleCredential.java:384)
at com.google.api.client.auth.oauth2.Credential.refreshToken(Credential.java:489)
at jp.co.kyocera.thp.sendNotice.service.Messaging.getAccessToken(Messaging.java:50)
at jp.co.kyocera.thp.sendNotice.service.Messaging.getConnection(Messaging.java:68)
at jp.co.kyocera.thp.sendNotice.service.Messaging.sendMessage(Messaging.java:83)
at jp.co.kyocera.thp.sendNotice.service.Messaging.sendCommonMessage(Messaging.java:191)