What is the "proper" way to cast Hibernate Query.list() to List<Type>?

85

I'm a newbie with Hibernate, and I'm writing a simple method to return a list of objects matching a specific filter. List<Foo> seemed a natural return type.

Whatever I do, I can't seem to make the compiler happy, unless I employ an ugly @SuppressWarnings.

import java.util.List;
import org.hibernate.Query;
import org.hibernate.Session;

public class Foo {

    public Session acquireSession() {
        // All DB opening, connection etc. removed,
        // since the problem is in compilation, not at runtime.
        return null;
    }

    @SuppressWarnings("unchecked") /* <----- */

    public List<Foo> activeObjects() {
        Session s = acquireSession();
        Query   q = s.createQuery("from foo where active");
        return (List<Foo>) q.list();
    }
}

I would like to get rid of that SuppressWarnings. But if I do, I get the warning

Warning: Unchecked cast from List to List<Foo>

(I can ignore it, but I'd like to not get it in the first place), and if I remove the generic to conform to .list() return type, I get the warning

Warning: List is a raw type. References to generic type List<E>
should be parameterized.

I noticed that org.hibernate.mapping does declare a List; but it is a different type altogether - Query returns a java.util.List, as a raw type. I find it odd that a recent Hibernate (4.0.x) would not implement parameterized types, so I suspect that it's me instead doing something wrong.

It looks very much like Cast Hibernate result to a list of objects, but here I have no "hard" errors (the system knows type Foo, and I'm not using a SQLQuery but a straight Query). So no joy.

I have also looked at Hibernate Class Cast Exception since it looked promising, but then I realized that I do not actually get any Exception... my problem is just that of a warning - a coding style, if you will.

Documentation on jboss.org, Hibernate manuals and several tutorials do not seem to cover the topic in such detail (or I didn't search in the right places?). When they do enter into detail, they use on-the-fly casting - and this on tutorials that weren't on the official jboss.org site, so I'm a bit wary.

The code, once compiled, runs with no apparent problem... that I know of... yet; and the results are the expected ones.

So: am I doing this right? Am I missing something obvious? Is there an "official" or "recommended" Way To Do It?

This question is tagged with java hibernate generics

~ Asked on 2013-04-09 21:44:34

The Best Answer is


101

Short answer @SuppressWarnings is the right way to go.

Long answer, Hibernate returns a raw List from the Query.list method, see here. This is not a bug with Hibernate or something the can be solved, the type returned by the query is not known at compile time.

Therefore when you write

final List<MyObject> list = query.list();

You are doing an unsafe cast from List to List<MyObject> - this cannot be avoided.

There is no way you can safely carry out the cast as the List could contain anything.

The only way to make the error go away is the even more ugly

final List<MyObject> list = new LinkedList<>();
for(final Object o : query.list()) {
    list.add((MyObject)o);
}

~ Answered on 2013-04-09 21:51:03


27

The resolution is to use TypedQuery instead. When creating a query from the EntityManager instead call it like this:

TypedQuery<[YourClass]> query = entityManager.createQuery("[your sql]", [YourClass].class);
List<[YourClass]> list = query.getResultList(); //no type warning

This also works the same for named queries, native named queries, etc. The corresponding methods have the same names as the ones that would return the vanilla query. Just use this instead of a Query whenever you know the return type.

~ Answered on 2016-03-09 18:28:06


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