How to use ScrollableResults with Hibernate

Supposing you have to fetch a large resultset and update the single objects. By using the standard Query Object you would retrieve the whole set of Objects in Memory:

Query q = session.createQuery("from ABC");
List l = q.list();

On the other hand, if you need to operate on an online cursor, then you can use the ScrollableResults in your Query as you can see in this example:

public class ScrollableResultsWithQuery {
    public static void main(String[] args) {
        Session session = HibernateUtil.getSessionFactory().openSession();
        ScrollableResults scResults = session.createQuery("from Customer")
               .scroll(ScrollMode.SCROLL_INSENSITIVE);
        while(scResults.next()) {
            Customer c = (Customer)scResults.get(0);
            System.out.println(g.getId()+", "+g.getName());
            scResults.afterLast();
        }
        session.close();
    }
}

ScrollableResults requires the underlying Hibernate API for the scrolling but you can use all the features of JPA querying. 

There are different modes available in ScrollableResults:

FORWARD_ONLY : It requests ScrollableResults that scrolls forward only.
SCROLL_SENSITIVE : ScrollableResults that is sensitive to changes in underlying data.
SCROLL_INSENSITIVE : Insensitive to changes in underlying data.

You are not restricted to using ScrollableResults with Query, you can use it also with the Criteria API:

public class ScrollableResultsWithCriteria {
    public static void main(String[] args) {
        Session session = HibernateUtil.getSessionFactory().openSession();
        ScrollableResults scResults = session.createCriteria(Customer.class)
                     .scroll(ScrollMode.FORWARD_ONLY);
        while(scResults.next()) {
            Customer c = (Customer)scResults.get(0);
            System.out.println(c.getId()+", "+c.getName());
        }
        //Set the location at index 2
        System.out.println("After setting location");
        scResults.setRowNumber(1);
        while(scResults.next()) {
            Customer c = (Customer)scResults.get(0);
            System.out.println(c.getId()+", "+c.getName());
        }
        session.close();
    }
}

Scrolling in JPA

There is no equivalent API in JPA to scroll through an open cursor. Still you can use as alternative the methods setMaxResult()/setFirstResult() as discussed in this tutorial: How to paginate your Entity data

Query query = em.createQuery("From Account");
query.setFirstResult(0);
query.setMaxResults(10);
List<Account> list = query.list();

In terms of performance, however, consider that using setMaxResult()/setFirstResult() will fire a new query every time, and the overhead really adds up. Plus, you need to sort on something for reliable paging which also adds to query times.