>     void JsonStorage::save(bool bIgnoreLength) {
        if (nullptr == _document) {
            assert(0);
            return;
        }

        SYNCHRONIZED();
        std::string jsonString = rapidjson::getReprsentedString(*_document);
        if (jsonString.length() > MIN_JSON_LENGTH || bIgnoreLength) {
            //GCL-3613, directly write to original file will clear the file first, if crash happend in this step, the file contents will lost;
            // so next time re-launch app will be not in signin status;
            // using a more safe method to save file;
            std::string tempFileName = _filename + "_temp";
#ifdef _MSC_VER
            FILE *fp = fopen(tempFileName.c_str(), "wb");
            if (fp) {
                fwrite(jsonString.c_str(), 1, jsonString.length(), fp);
                fclose(fp);

                remove(_filename.c_str());

#else
            int fp = ::open(tempFileName.c_str(), O_WRONLY | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH);
            if (fp != -1) {
                write(fp, jsonString.c_str(), jsonString.length());
                ::close(fp);
#endif
                
                // move temp file to overwitten original one;
                rename(tempFileName.c_str(), _filename.c_str());
            }
        }
    }