import jcifs.smb.NtlmChallenge; //导入依赖的package包/类
private synchronized NtlmChallenge getChallengeForDomain ( String domain ) throws UnknownHostException, ServletException {
    if ( domain == null ) {
        throw new ServletException("A domain was not specified");
    }
    long now = System.currentTimeMillis();
    int retry = 1;

    do {
        if ( this.dcListExpiration < now ) {
            Address[] list = getTransportContext().getNameServiceClient().getNbtAllByName(domain, 0x1C, null, null);
            this.dcListExpiration = now + this.netbiosCacheTimeout * 1000L;
            if ( list != null && list.length > 0 ) {
                this.dcList = list;
            }
            else { /* keep using the old list */
                this.dcListExpiration = now + 1000 * 60 * 15; /* 15 min */
                log.warn("Failed to retrieve DC list from WINS");
            }
        }

        int max = Math.min(this.dcList.length, this.netbiosLookupRespLimit);
        for ( int j = 0; j < max; j++ ) {
            int i = dcListCounter++ % max;
            if ( this.dcList[ i ] != null ) {
                try {
                    return interrogate(getTransportContext(), this.dcList[ i ]);
                }
                catch ( SmbException se ) {
                    log.warn("Failed validate DC: " + this.dcList[ i ], se);
                }
                this.dcList[ i ] = null;
            }
        }

        /*
         * No DCs found, for retieval of list by expiring it and retry.
         */
        this.dcListExpiration = 0;
    }
    while ( retry-- > 0 );

    this.dcListExpiration = now + 1000 * 60 * 15; /* 15 min */
    throw new UnknownHostException("Failed to negotiate with a suitable domain controller for " + domain);
}