when we set custom attribute for stream in web UI. how to get those custom attrbute value in java (in SCM )

I am checking the RTC 6.0.5 feature.When in the Web UI Project Area configuration page, on the left hand navigator, click 'Source Control' --> 'Attribute Definition' and configure custom attribute value for a Stream as below figure.

Now i am trying to get the custom attribute via java code for server side extension(below is plugin code),but  I am not able to get custom attribute value, what i set in the UI (mention in above figure).

public void run(final AdvisableOperation operation, final IProcessConfigurationElement participantConfig,
      final IParticipantInfoCollector collector, final IProgressMonitor monitor) throws TeamRepositoryException {
    try {
      Object operationData = operation.getOperationData();
      DeliverOperationData deliveryData = null;
      IScmDeltaSource scmDeltaSrc = null;
      if (operationData instanceof DeliverOperationData) {
        deliveryData = (DeliverOperationData) operationData;
      }
      // Get Service I need
      IScmService scmService = getService(IScmService.class);
      if (deliveryData != null) {
         System.out.println(deliveryData.getSourceWorkspace().getName());
         System.out.println(deliveryData.getDestWorkspace().getName());
        IWorkspace destWorkspace = deliveryData.getDestWorkspace();
        String streamName = destWorkspace.getName();
        if (destWorkspace.isStream()) {
          System.out.println(streamName);
          System.out.println(destWorkspace.isImmutable());
        }
         // to get the custom attributes of the stream
        Map<String, Object> customAttributes = destWorkspace.getCustomAttributes();
        Set<String> keySet = customAttributes.keySet();
        for (String key : keySet) {
          System.out.println(key + ": " + customAttributes.get(key));
          }
          }
    catch (Exception e) {
      e.printStackTrace();
      collector.addInfo(collector.createExceptionInfo("Exception Occurs in follow-up action: " + e.getMessage(), e));
    }