@Override
 public void readyReferencesById(List<Long> referenceIds) {
   for (Long referenceId : referenceIds) {
     Reference reference = referenceDAO.findById(referenceId);
     reference.setState(Reference.ReferenceState.READY);
     referenceDAO.update(reference);
   }
 }
 @Override
 public void printReferencesById(List<Long> referenceIds) {
   for (Long referenceId : referenceIds) {
     Reference reference = referenceDAO.findById(referenceId);
     reference.setState(Reference.ReferenceState.PROCESSED);
     referenceDAO.update(reference);
   }
 }
 @Override
 public void issueReferencesById(List<Long> referenceIds) {
   for (Long referenceId : referenceIds) {
     Reference reference = referenceDAO.findById(referenceId);
     reference.setIssuedDate(new Date());
     reference.setState(Reference.ReferenceState.ISSUED);
     referenceDAO.update(reference);
   }
 }