Playing Around with JPA Criteria API

It's still about multiselect like the one discussed in 005 - JPA Projective Query in EL & Hibernate, but instead of using named query, I will use the Criteria API instead.

Let's use the same query as the other article:

SELECT new com.arizal.flight.FlightNumber" +

          "(f.number, f.airline) " +
           " FROM Flight f WHERE f.departure.code='SYD'")

This can be expressed using Criteria as follow:

public static void main(String args[]) {     EntityManagerFactory emf =        Persistence.createEntityManagerFactory("jpa");     EntityManager em = emf.createEntityManager();     try {       CriteriaBuilder cb = em.getCriteriaBuilder();       CriteriaQuery<FlightNumber> criteriaQ =           cb.createQuery(FlightNumber.class);       Root<Flight> flight = criteriaQ.from(Flight.class);       criteriaQ.select(         cb.construct(             FlightNumber.class,              flight.get("number"),              flight.get("airline"))).       where(cb.equal(flight.get("departure").get("code"), "SYD"));       TypedQuery<FlightNumber> query = em.createQuery(criteriaQ);       List<FlightNumber> fnList = query.getResultList();       for (FlightNumber fn : fnList) {         System.out.println(fn.getAirline() + " " + fn.getNumber());       }     } finally {       em.close();       emf.close();     }

Pretty simple, isn't it ?

Yeah, but let's see closer. Unlike SQL where we do first the selection followed by FROM (e.g. SELECT number, airline FROM flight), CriteriaBuilder imposes the use of FROM first (Root<Flight> flight = criteriaQ.from(Flight.class);) followed by select.

We do then: FROM flight SELECT number, airline. 

Could have been better, don't you think ?

Oh, yes. The code works for both Hibernate and EclipseLink. Hibernate does not suffer from this bug: http://opensource.atlassian.com/projects/hibernate/browse/HHH-5348