if (keyList == null || keyList.isEmpty()) {
            throw new CrawlerSystemException("The key list is empty.");
        }
        keyList.stream().distinct().forEach(key -> addExtractor(key, extractor));
    }

    /**
     * Retrieves an extractor for the specified key.
     * If multiple extractors are associated with the key, returns a composite extractor

        this.clientMap = clientMap;
    }

    /**
     * Closes all clients in the factory.
     */
    @Override
    public void close() {
        clientMap.values().stream().distinct().forEach(client -> {
            try {
                client.close();
            } catch (final Exception e) {
                logger.warn("Failed to close {}.", client.getClass().getCanonicalName(), e);

                concatValues(existingValues, readings[i]);
                map.put(FieldNames.READING_PREFIX + i, existingValues.stream().distinct().toList());
            } else {
                final String[] values = readings[i] == null ? null : Arrays.stream(readings[i]).distinct().toArray(n -> new String[n]);
                map.put(FieldNames.READING_PREFIX + i, values);
            }
        }