Difference between FetchType LAZY and EAGER in Java Persistence API?

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

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, 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)