code: public void addTangoUserListener(ITangoUserListener listener, String attrName, String[] filters, boolean stateless)
              throws DevFailed {
          TangoUser tangoUser;
          String key = deviceName+"/"+attrName;
          if ((tangoUser = tango_user_source.get(key)) == null) {
              tangoUser = new TangoUser(deviceProxy, attrName, filters);
              tango_user_source.put(key, tangoUser);
          }
  
          synchronized (moni) {
              tangoUser.addTangoUserListener(listener, stateless);
          }
      }
docs: =======================================================================