private void clearCache(Presence presence) {
          String rjid = presence.getFrom();

          if (rjid == null) {
            LOG.error("presence.getFrom() is null");
            return;
          }

          DiscoverInfoWrapper infoWrapper = cache.remove(rjid);

          if (infoWrapper != null) {
            if (infoWrapper.isAvailable()) {
              LOG.debug(
                  "clearing cache entry of contact "
                      + rjid
                      + ": "
                      + infoWrapper.item.getChildElementXML());
            } else {
              LOG.debug(
                  "clearing cache entry of contact "
                      + rjid
                      + " but cache entry is empty (a discovery is "
                      + "still running or the last one failed)");
            }
          }
        }
  /**
   * Perform a service discovery and check if the given feature is among the features supported by
   * the given recipient. All registered listeners will be notified about the result.
   *
   * @param jid A RQ-JID (user@host/resource) of the user to query support for or a non RQ-JID to
   *     query all presences for this JID.
   * @blocking This method blocks until the ServiceDiscovery returns.
   * @reentrant This method can be called concurrently.
   * @caching If results are available in the cache, they are used instead of querying the server.
   */
  private Boolean queryFeatureSupport(JID jid, String namespace) {

    Boolean supported = null;

    checkJID(jid);

    DiscoverInfoWrapper wrapper;

    final List<JID> jidsToQuery = new ArrayList<JID>();

    if (jid.isBareJID()) jidsToQuery.addAll(rosterTracker.getAvailablePresences(jid));
    else jidsToQuery.add(jid);

    for (JID rqJID : jidsToQuery) {

      // add dummy
      synchronized (cache) {
        wrapper = cache.get(rqJID.toString());
        if (wrapper == null) {
          wrapper = new DiscoverInfoWrapper();
          cache.put(rqJID.toString(), wrapper);
        }
      }

      DiscoverInfo disco = null;

      // wait if there is already a discovery for the JID in progress
      synchronized (wrapper) {
        if (wrapper.isAvailable()) disco = wrapper.item;
        else {
          disco = wrapper.item = performServiceDiscovery(rqJID);
          if (disco != null) LOG.debug("Inserted DiscoveryInfo into Cache for: " + rqJID);
        }
      }

      // Null means that the discovery failed
      if (disco == null) {
        // and so we do not know if the feature is supported
        // notifyFeatureSupportUpdated(jid, namespace, false);
        continue;
      }

      notifyFeatureSupportUpdated(rqJID, namespace, disco.containsFeature(namespace));

      /*
       * loop through all presence regardless if we already know that the
       * feature is supported to notify the listener for every current
       * presence
       */
      if (supported != null) supported |= disco.containsFeature(namespace);
      else supported = disco.containsFeature(namespace);
    }

    return supported;
  }