code: public static Namespace findByQualifier(EntityManager em, String qualifier) {
          SystemAssert.requireArgument(em != null, "EntityManager cannot be null.");
          SystemAssert.requireArgument(qualifier != null && !qualifier.isEmpty(), "Namespace qualifier cannot be null or empty.");
  
          TypedQuery<Namespace> query = em.createNamedQuery("Namespace.findByQualifier", Namespace.class);
  
          try {
              return query.setParameter("qualifier", qualifier).getSingleResult();
          } catch (NoResultException ex) {
              return null;
          }
      }
docs: Returns a namespace entity using the provided namespace qualifier.
  
  @param   em         The entity manager.  Cannot be null.
  @param   qualifier  The qualifier.  Cannot be null or empty.
  
  @return  The namespace or null if no corresponding namespace is found.