code: @Reference(cardinality = ReferenceCardinality.MULTIPLE,
                 policyOption = ReferencePolicyOption.GREEDY)
      protected synchronized void setCustomUserRegistry(com.ibm.websphere.security.UserRegistry externalUserRegistry, Map<String, Object> props) {
          String id = getId(props);
          customUserRegistries.put(id, externalUserRegistry);
      }
docs: Method will be called for each com.ibm.websphere.security.UserRegistry that is
  registered in the OSGi service registry. We maintain an internal set
  of these for easy access.
  
  @param ref Reference to a registered com.ibm.websphere.security.UserRegistry