presto-plugin-toolkit/src/main/java/io/prestosql/plugin/base/authentication/CachingKerberosAuthentication.java/CachingKerberosAuthentication/getSubject
class CachingKerberosAuthentication:    public synchronized Subject getSubject()
    {
        if (subject == null || nextRefreshTime < System.currentTimeMillis()) {
            subject = requireNonNull(kerberosAuthentication.getSubject(), "kerberosAuthentication.getSubject() is null");
            KerberosTicket tgtTicket = getTicketGrantingTicket(subject);
            nextRefreshTime = KerberosTicketUtils.getRefreshTime(tgtTicket);
        }
        return subject;
    }