difference between FetchMode and FetchType

Question

what is the difference in specifying lazy = "true" and using fetch = "select" or "join" ? which one is preferred over the other ?

regards jayendra

Answer 1

Let's say we have entities like this:

@Entity
@Table
public class Parent {
    @Id
    private Long id;

    @OneToMany(mappedBy="parent", fetch = FetchType.EAGER)
    @Fetch(FetchMode.JOIN)
    private List<Child> child;    
    //getter setters
}


@Entity
@Table
public class Child {    
    @Id
    private Long id;
    
    @ManyToOne(fetch = FetchType.LAZY)
    private Parent parent;

    //getter setter
}

In the example above, when getting Parent entity, hibernate will automatically load all child entities eagerly using join. On the other hand, when you fetch Child, Parent entity won't be selected unless you call it explicitly in your code child.getParent().

FetchType (Lazy/Eager) tells whether we want entity to be loaded eagerly or lazy, when there's call in code.

FetchMode (Select/Join) tells whether we want our entity to be loaded with additional select or in one query with join or subselect.

Answer 2

FetchMode : It defines how hibernate (using which strategy, e.g. Join, SubQuery etc) will fetch data from database.

FetchType : It defines whether hibernate will fetch the data or not.

UPDATES (as per suggestions from comments)

FetchMode isn't only applicable with FetchType.EAGER. The rules are as follows: a) if you don't specify FetchMode, the default is JOIN and FetchType works normal, b) if you specify FetchMode.JOIN explicitly, FetchType is ignored and a query is always eager, c) if you specify FetchMode.SELECT or FetchMode.SUBSELECT, FetchType.Type works normal