Nice plugin, but I found bugs. When I write a command on behalf of the player, WARN is entered into the console, the same with the console:
[23:05:42 WARN]: java.net.UnknownHostException: sessionserver.mojang.com
[23:05:42 WARN]: at java.net.AbstractPlainSocketImpl.connect(Unknown Source)
[23:05:42 WARN]: at java.net.PlainSocketImpl.connect(Unknown Source)
[23:05:42 WARN]: at java.net.SocksSocketImpl.connect(Unknown Source)
[23:05:42 WARN]: at java.net.Socket.connect(Unknown Source)
[23:05:42 WARN]: at sun.security.ssl.SSLSocketImpl.connect(Unknown Source)
[23:05:42 WARN]: at sun.net.NetworkClient.doConnect(Unknown Source)
[23:05:42 WARN]: at sun.net.
www.http.HttpClient.openServer(Unknown Source)
[23:05:42 WARN]: at sun.net.
www.http.HttpClient.openServer(Unknown Source)
[23:05:42 WARN]: at sun.net.
www.protocol.https.HttpsClient.<init>(Unknown Source)
[23:05:42 WARN]: at sun.net.
www.protocol.https.HttpsClient.New(Unknown Source)
[23:05:42 WARN]: at sun.net.
www.protocol.https.AbstractDelegateHttpsURLConnection.getNewHttpClient(Unknown Source)
[23:05:42 WARN]: at sun.net.
www.protocol.http.HttpURLConnection.plainConnect0(Unknown Source)
[23:05:42 WARN]: at sun.net.
www.protocol.http.HttpURLConnection.plainConnect(Unknown Source)
[23:05:42 WARN]: at sun.net.
www.protocol.https.AbstractDelegateHttpsURLConnection.connect(Unknown Source)
[23:05:42 WARN]: at sun.net.
www.protocol.http.HttpURLConnection.getInputStream0(Unknown Source)
[23:05:42 WARN]: at sun.net.
www.protocol.http.HttpURLConnection.getInputStream(Unknown Source)
[23:05:42 WARN]: at java.net.HttpURLConnection.getResponseCode(Unknown Source)
[23:05:42 WARN]: at sun.net.
www.protocol.https.HttpsURLConnectionImpl.getResponseCode(Unknown Source)
[23:05:42 WARN]: at eu.blackfire62.myskin.shared.util.MojangAPI.readURL(MojangAPI.java:70)
[23:05:42 WARN]: at eu.blackfire62.myskin.shared.util.MojangAPI.getSkinProperty(MojangAPI.java:40)
[23:05:42 WARN]: at eu.blackfire62.myskin.bukkit.listener.OnLogin.lambda$0(OnLogin.java:57)
[23:05:42 WARN]: at org.bukkit.craftbukkit.v1_12_R1.scheduler.CraftTask.run(CraftTask.java:64)
[23:05:42 WARN]: at org.bukkit.craftbukkit.v1_12_R1.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:52)
[23:05:42 WARN]: at com.destroystokyo.paper.ServerSchedulerReportingWrapper.run(ServerSchedulerReportingWrapper.java:22)
[23:05:42 WARN]: at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
[23:05:42 WARN]: at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
[23:05:42 WARN]: at java.lang.Thread.run(Unknown Source)