How to perform a join fetch in JPA Criteria without unchecked casting?

问题

I need to do a JOIN FETCH in JPA Criteria using a static metamodel, however I'm at a loss on how to do it without getting an unchecked exception warning.

Suppose we have a Thing entity with a lazily-initialized Other entity inside it. I want to retrieve Things with fetched Others, where other.someField="someValue". This is roughly how I would do it:

public List<Thing> getThings() {
    CriteriaBuilder cb = em.getCriteriaBuilder();
    CriteriaQuery<Thing> cq = cb.createQuery(Thing.class);
    Root root = cq.from(Thing.class);

    // This gives an unchecked cast warning:
    Join<Thing, Other> other = (Join<Thing, Other>) root.fetch(Thing_.other);

    cq.select(root).where(cb.equal(other.get(Other_.someField), "someValue"));

    return em.createQuery(cq).getResultList();
}

However, since the Join and Fetch interfaces have nothing in common, I get an "unchecked cast" warning. I would like to get rid of that warning (without using @SuppressWarnings).

I suppose I could do it this way:

cb.equal(root.get(This_.THING).get(Other_.SOMEFIELD), "someValue"))

But I'm doubtful whether it's better, seeing as I'm operating on Strings here (so no type safety).

What is a cleaner way of achieving the above?

回答1:

Fetch method is not supposed for creating JPA joins. In JPA join is used to create where conditions, not for loading data. This is different from native SQL. For JOIN use:

Join<Thing, Other> other = root.join(Thing_.other);

Independently collection can be loaded with or without calling join():

root.fetch(Thing_.other);

来源：https://stackoverflow.com/questions/55450323/how-to-perform-a-join-fetch-in-jpa-criteria-without-unchecked-casting