private KerberosTicket getTGT() {
        Set<KerberosTicket> tickets = subject.getPrivateCredentials(KerberosTicket.class);
        for (KerberosTicket ticket : tickets) {
            KerberosPrincipal server = ticket.getServer();
            if (server.getName().equals("krbtgt/" + server.getRealm() + "@" + server.getRealm())) {
                log.debug("Found TGT with client principal '{}' and server principal '{}'.", ticket.getClient().getName(),
                        ticket.getServer().getName());
                return ticket;
            }
        }
        return null;
    }
Performs login for each login module specified for the login context of this instance and starts the thread used
to periodically re-login to the Kerberos Ticket Granting Server.
clients/src/main/java/org/apache/kafka/common/security/kerberos/KerberosLogin.java