What is the use of the Hibernate @LazyCollection annotation

Question

I have 2 entities as Parent and Child as OneToMany relation as

@Entity public class Parent {  @Id @GeneratedValue(strategy = GenerationType.AUTO) private Integer id;  private String name;  @OneToMany(mappedBy = "parent", fetch = FetchType.LAZY) @IndexColumn(name = "index", base = 1) @Cascade(org.hibernate.annotations.CascadeType.ALL) @LazyCollection(LazyCollectionOption.EXTRA) private List<Child> childs = new ArrayList<Child>(); // getter and setter  } 

So here what is use of @LazyCollection(LazyCollectionOption.EXTRA) and when does it will come in picture, like for which operation with child list, it will be beneficial ?

Answer 1

EXTRA = .size() and .contains() won't initialize the whole collection

TRUE = initialize the whole collection on first access

FALSE = Eager-Loading

Answer 2

There's actually no reason to use @LazyCollection.

The TRUE and FALSE values are not needed since the same behavior can be obtained with the JPA FetchType.LAZY or FetchType.EAGER.

The EXTRA value has no equivalent in JPA and was designed for very large collections. When you access an EXTRA lazy collection for the first time, the collection is not entirely loaded, as it's usually the case with any JPA collection.

Instead, each element is fetched one by one, using a secondary SELECT. This might sound like an optimization, but it's not because EXTRA lazy collections are prone to N+1 query issues.

Note that this only works for ordered collections, either List(s) that are annotated with @OrderColumn or Map(s). For bags (e.g. regular List(s) of entities that do not preserve any certain ordering), the @LazyCollection(LazyCollectionOption.EXTRA) behaves just like any other LAZY collection (the collection is fetched entirely upon being accessed for the first time).

If you have a very large collection, then you should not map it at all. Instead, you should map only the @ManyToOne side, and, instead of a parent-side collection, you should use a paginated JPQL query.

JPQL queries are much easier to tune because you can apply any filtering criteria, and you can paginate the result set.