@Override
    public void configure(Map<String, ?> configs, String saslMechanism, List<AppConfigurationEntry> jaasConfigEntries) {
        if (!saslMechanism.equals(SaslConfigs.GSSAPI_MECHANISM))
            throw new IllegalStateException("Kerberos callback handler should only be used with GSSAPI");
    }
Callback handler for SASL/GSSAPI clients.
clients/src/main/java/org/apache/kafka/common/security/kerberos/KerberosClientCallbackHandler.java