Java: What is the difference between @OneToOne and @ManyToOne?

@OneToOne — one-to-one relationship. @ManyToOne — many-to-one relationship.

@OneToOne:

• Each entity references exactly one other.
• 1:1 cardinality.
• Shared primary key or foreign key.
• Can cause issues if not careful with cascading.

1@Entity
2public class User {
3    @Id
4    private Long id;
5
6    @OneToOne(cascade = CascadeType.ALL)
7    @JoinColumn(name = "profile_id")
8    private Profile profile;
9}

@ManyToOne:

• Many entities reference one.
• N:1 cardinality.
• Foreign key.
• Most common relationship type.

1@Entity
2public class Order {
3    @Id
4    private Long id;
5
6    @ManyToOne
7    @JoinColumn(name = "user_id")
8    private User user;
9}

How it works:

1. @OneToOne maps a one-to-one relationship between two entities.
2. @ManyToOne maps a many-to-one relationship.
3. Both can be bidirectional with @OneToMany or @OneToOne on the other side.
4. The owning side controls the foreign key.

Key difference:

• @OneToOne — 1:1.
• @ManyToOne — N:1.

Best practice:

• Use @OneToOne for 1:1 relationships (e.g., User-Profile).
• Use @ManyToOne for N:1 relationships (e.g., Order-User).
• Always define the owning side explicitly.