public String getProperty(String s){
        try{
            PersistentEntry persistentEntry = (PersistentEntry)
                    em.createNamedQuery(PersistentEntry.FIND_BY_CATEGORY_AND_KEY).
                            setParameter("category", getPropertiesFilename()).
                            setParameter("key", s).getSingleResult();

            return persistentEntry.getValue();
        } catch (NoResultException e){
            return null;
        }
    }