synchronized (mutex) {
        return delegate().get(key);
      }
    }

    @Override
    @CheckForNull
    public V getOrDefault(@CheckForNull Object key, @CheckForNull V defaultValue) {
      synchronized (mutex) {
        return delegate().getOrDefault(key, defaultValue);
      }
    }

    @Override
    public boolean isEmpty() {
      synchronized (mutex) {

        }
    }

    protected void sendConfigFiles(final String index) {
        final FessConfig fessConfig = ComponentUtil.getFessConfig();
        configListMap.getOrDefault(index, Collections.emptyList()).forEach(path -> {
            String source = null;
            final String filePath = indexConfigPath + "/" + index + "/" + path;
            final String dictionaryPath;