@Override
    public void execute(@Nonnull final SessionData data, Element command) {
        final Locale preferredLocale = SessionManager.getInstance().getLocaleForSession(data.getOwner());

        int maxItems = -1;
        final List<String> max_items_data = data.getData().get("max_items");
        if (max_items_data != null && !max_items_data.isEmpty()) {
            String max_items = max_items_data.get(0);
            if (max_items != null && !"none".equals(max_items)) {
                try {
                    maxItems = Integer.parseInt(max_items);
                } catch (NumberFormatException e) {
                    // Do nothing. Assume that all users are being requested
                }
            }
        }

        DataForm form = new DataForm(DataForm.Type.result);

        FormField field = form.addField();
        field.setType(FormField.Type.hidden);
        field.setVariable("FORM_TYPE");
        field.addValue("http://jabber.org/protocol/admin");

        field = form.addField();
        field.setType(FormField.Type.jid_multi);
        field.setLabel(LocaleUtils.getLocalizedString("commands.admin.getlistactiveusers.form.field.activeuserjids.label", preferredLocale));
        field.setVariable("activeuserjids");

        // Get list of users (i.e. bareJIDs) that are connected to the server
        Collection<ClientSession> sessions = SessionManager.getInstance().getSessions();
        Set<String> users = new HashSet<>(sessions.size());
        for (ClientSession session : sessions) {
            if (session.getPresence().isAvailable()) {
                users.add(session.getAddress().toBareJID());
            }
            if (maxItems > 0 && users.size() >= maxItems) {
                break;
            }
        }
        // Add users to the result
        for (String user : users) {
            field.addValue(user);
        }
        command.add(form.getElement());
    }
Command that allows to retrieve a list of all active users.

@author Gaston Dombiak
@see <a href="https://xmpp.org/extensions/xep-0133.html#get-active-users-list">XEP-0133 Service Administration: Get List of Active Users</a>
xmppserver/src/main/java/org/jivesoftware/openfire/commands/admin/GetListActiveUsers.java