Best practice for JPA with Java8's Optional return?

Question

I love Java8's semantics. I use a lot of such code in my DAOs :

  public Optional<User> findBy(String username) {
    try {
      return Optional.of(
        emp.get().createQuery("select u from User u where u.username = :username" , User.class)
        .setParameter("username" , username)
        .setMaxResults(1)
        .getSingleResult()
      );
    } catch (NoResultException e) {
      return Optional.empty();
    }
  }

It works well , but such code (try catch NoResultException ) scatters over my DAOs. And I have to catch Exception , which somehow lowers performance .

I wonder if it the best solution ? or any better solution , without try-catch ?

If it is not possible (because NoResultException is defined in JPA) , any shortcut to 'templatize' such workflow ?

Thanks.

Answer 1

If course you can templatize it, using the magic of lambdas!

Start with an @FunctionalInterface to define the lambda's contract:

@FunctionalInterface
public interface DaoRetriever<T> {
    T retrieve() throws NoResultException;
}

This is a Single Method Interface (or SMI) that will encapsulate the behaviour of your method.

Now create a utility method to use the SMI:

public static <T> Optional<T> findOrEmpty(final DaoRetriever<T> retriever) {
    try {
        return Optional.of(retriever.retrieve());
    } catch (NoResultException ex) {
        //log
    }
    return Optional.empty();
}

Now, using an import static in your calling code, your method above becomes:

public Optional<User> findBy(String username) {
    return findOrEmpty(() ->
            emp.get().createQuery("select u from User u where u.username = :username", User.class)
                    .setParameter("username", username)
                    .setMaxResults(1)
                    .getSingleResult());
}

So here, () -> emp.get()... is a lambda that captures the retrieval behaviour. The interface DaoRetriever is allowed to throw a NoResultException so the lambda is too.

Alternatively, I would use the other method of TypedQuery - getResultList - and change the code as follows:

public Optional<User> findBy(String username) {
    return emp.get().createQuery("select u from User u where u.username = :username", User.class)
            .setParameter("username", username)
            .setMaxResults(1)
            .getResultList()
            .stream()
            .findFirst();
}

This has the advantage of being simpler, but the disadvantage of simply discarding other results if there are any.