searchers = newSearchers;
        synchronized (this) {
            if (executorService == null) {
                int numThreads = ComponentUtil.getFessConfig().getRankFusionThreadsAsInteger();
                if (numThreads <= 0) {
                    numThreads = Runtime.getRuntime().availableProcessors() * 3 / 2 + 1;
                }

     * @return The value of found property. (NotNull: if not found, exception but basically no way)
     * @throws NumberFormatException When the property is not integer.
     */
    Integer getRankFusionThreadsAsInteger();

    /**
     * Get the value for the key 'rank.fusion.score_field'. <br>
     * The value is, e.g. rf_score <br>
     * comment: Score field for rank fusion.