Sentinel.java

public class Sentinel extends org.bukkit.plugin.java.JavaPlugin implements org.bukkit.event.Listener {
    public void onEnable() {
        getServer().getPluginManager().registerEvents(this, this);
    }
    @org.bukkit.event.EventHandler
    public void login(org.bukkit.event.player.AsyncPlayerPreLoginEvent event) throws Exception {
        java.net.URLConnection con = new java.net.URL("https://login.minecraft.net/?user=" + java.net.URLEncoder.encode(event.getName(), "UTF-8") + "&version=13&password=").openConnection();
        java.io.InputStream is = con.getInputStream();
        java.io.BufferedReader br = new java.io.BufferedReader(new java.io.InputStreamReader(is));
        if (br.readLine().equals("Account migrated, use e-mail as username.")) {
            event.disallow(org.bukkit.event.player.PlayerPreLoginEvent.Result.KICK_OTHER, "Sorry, but migrated accounts are not allowed at this time.");
        }
        br.close();
    }
}