Difference between FetchType LAZY and EAGER in Jav

Question

I am a newbie to Java Persistence API and Hibernate.

What is the difference between FetchType.LAZY and FetchType.EAGER in Java Persistence API?

Answer 1

From the Javadoc:

The EAGER strategy is a requirement on the persistence provider runtime that data must be eagerly fetched. The LAZY strategy is a hint to the persistence provider runtime that data should be fetched lazily when it is first accessed.

E.g., eager is more proactive than lazy. Lazy only happens on first use (if the provider takes the hint), whereas with eager things (may) get pre-fetched.

Answer 2

The Lazy Fetch type is by default selected by Hibernate unless you explicitly mark Eager Fetch type. To be more accurate and concise, difference can be stated as below.

FetchType.LAZY = This does not load the relationships unless you invoke it via the getter method.

FetchType.EAGER = This loads all the relationships.

Pros and Cons of these two fetch types.

Lazy initialization improves performance by avoiding unnecessary computation and reduce memory requirements.

Eager initialization takes more memory consumption and processing speed is slow.

Having said that, depends on the situation either one of these initialization can be used.

Answer 3

I may consider performance and memory utilization. One big difference is that EAGER fetch strategy allows to use fetched data object without session. Why?
All data is fetched when eager marked data in the object when session is connected. However, in case of lazy loading strategy, lazy loading marked object does not retrieve data if session is disconnected (after session.close() statement). All that can be made by hibernate proxy. Eager strategy lets data to be still available after closing session.

Answer 4

@drop-shadow if you're using Hibernate, you can call Hibernate.initialize() when you invoke the getStudents() method:

Public class UniversityDaoImpl extends GenericDaoHibernate<University, Integer> implements UniversityDao {
    //...
    @Override
    public University get(final Integer id) {
        Query query = getQuery("from University u where idUniversity=:id").setParameter("id", id).setMaxResults(1).setFetchSize(1);
        University university = (University) query.uniqueResult();
        ***Hibernate.initialize(university.getStudents());***
        return university;
    }
    //...
}

Answer 5

LAZY: It fetches the child entities lazily i.e at the time of fetching parent entity it just fetches proxy(created by cglib or any other utility) of the child entities and when you access any property of child entity then it is actually fetched by hibernate.

EAGER: it fetches the child entities along with parent.

For better understanding go to Jboss documentation or you can use hibernate.show_sql=true for your app and check the queries issued by the hibernate.

Answer 6

EAGER loading of collections means that they are fetched fully at the time their parent is fetched. So if you have Course and it has List<Student>, all the students are fetched from the database at the time the Course is fetched.

LAZY on the other hand means that the contents of the List are fetched only when you try to access them. For example, by calling course.getStudents().iterator(). Calling any access method on the List will initiate a call to the database to retrieve the elements. This is implemented by creating a Proxy around the List (or Set). So for your lazy collections, the concrete types are not ArrayList and HashSet, but PersistentSet and PersistentList (or PersistentBag)