java - Abstracting named queries in an abstract JPA DAO

Question

I have an abstract DAO class which uses parameterized types E (Entity) and K (Primary Key). In every entity I have a @NamedQuery. I want to dynamically invoke this named query without knowing its exact name and parameter name.

As an example, imagine the following entity City

@Entity(name="CITY")
@NamedQuery(
    name="findCityByname",
    query="FROM CITY c WHERE name = :CityName"
)
public class City { 
    // ...
}

and this CityDao

public class CityDao extends AbstractDao<City, Long> {
    public CityDao() {
        super(City.class);
    }   
}

How should I implement the findByName() method in AbstractDao so that I don't need to know the exact name and parameter name?

public abstract class AbstractDao<E, K> implements Dao<E, K> {

    @PersistenceContext
    protected EntityManager entityManager;
    protected Class<E> entityClass;

    protected AbstractDao(Class<E> entityClass) {
        this.entityClass = entityClass; 
    }

    @Override
    public E findByName(String name) {
        try {
            return (E) entityManager
                .createNamedQuery("findCityByName")
                .setParameter("CityName", name)
                .getSingleResult();
        } catch(Exception e) {
            return null;
        }
    }

    // ...
}

Answer 1

The naming convention for named queries is usually <Entity Name>.findBy<PropertyAndAnotherProperty>, "City.findByName" in your example, so I would try to change the named queries to follow this pattern. The parameter to this query should then also have the same name, or you could use positional parameters. Your find method would then turn into

@Override
public E findByName(String name) {
    E entity = null;
    try {
        return (E)entityManager.createNamedQuery(myClass.getSimpleName() + ".findByName")
                               .setParameter("name", name)
                               .getSingleResult();
    } catch (Exception ex) {
        return null;
    }
}