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