code: public void addTangoDataReadyListener(ITangoDataReadyListener listener, String attrName, String[] filters, boolean stateless)
              throws DevFailed {
          TangoDataReady tangoDataReady;
          String key = deviceName+"/"+attrName;
          if ((tangoDataReady = tango_data_ready_source.get(key)) == null) {
              tangoDataReady = new TangoDataReady(deviceProxy, attrName, filters);
              tango_data_ready_source.put(key, tangoDataReady);
          }
  
          synchronized (moni) {
              tangoDataReady.addTangoDataReadyListener(listener, stateless);
          }
      }
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