The @OneToOne annotation is used to specify a one-to-one database relationship.

The @OneToOne association can either be unidirectional or bidirectional. A unidirectional association follows the relational database foreign key semantics, the client-side owning the relationship.

A bidirectional association features a mappedBy @OneToOne parent side too.

Unidirectional @OneToOne Example

In this example, we create Phone and PhoneDetails entities and we make a unidirectional one-to-one mapping between them.

Phone.java

@Entity(name = "Phone")
public static class Phone {

	@Id
	@GeneratedValue
	private Long id;

	@Column(name = "`number`")
	private String number;

	@OneToOne
	@JoinColumn(name = "details_id")
	private PhoneDetails details;

	//Getters and setters are omitted for brevity

}

PhoneDetails.java

@Entity(name = "PhoneDetails")
public static class PhoneDetails {

	@Id
	@GeneratedValue
	private Long id;

	private String provider;

	private String technology;

	//Getters and setters are omitted for brevity

}

Here are the SQL tables:

CREATE TABLE Phone (
    id BIGINT NOT NULL ,
    number VARCHAR(255) ,
    details_id BIGINT ,
    PRIMARY KEY ( id )
)

CREATE TABLE PhoneDetails (
    id BIGINT NOT NULL ,
    provider VARCHAR(255) ,
    technology VARCHAR(255) ,
    PRIMARY KEY ( id )
)

ALTER TABLE Phone
ADD CONSTRAINT FKnoj7cj83ppfqbnvqqa5kolub7
FOREIGN KEY (details_id) REFERENCES PhoneDetails

Bidirectional @OneToOne Example

In this example, we create Phone and PhoneDetails entities and we make a bidirectional one-to-one mapping between them.

Phone.java

@Entity(name = "Phone")
public static class Phone {

	@Id
	@GeneratedValue
	private Long id;

	@Column(name = "`number`")
	private String number;

	@OneToOne(
		mappedBy = "phone",
		cascade = CascadeType.ALL,
		orphanRemoval = true,
		fetch = FetchType.LAZY
	)
	private PhoneDetails details;

	//Getters and setters are omitted for brevity

	public void addDetails(PhoneDetails details) {
		details.setPhone( this );
		this.details = details;
	}

	public void removeDetails() {
		if ( details != null ) {
			details.setPhone( null );
			this.details = null;
		}
	}
}

PhoneDetails.java

@Entity(name = "PhoneDetails")
public static class PhoneDetails {

	@Id
	@GeneratedValue
	private Long id;

	private String provider;

	private String technology;

	@OneToOne(fetch = FetchType.LAZY)
	@JoinColumn(name = "phone_id")
	private Phone phone;

	//Getters and setters are omitted for brevity

}

Here are the SQL tables:

CREATE TABLE Phone (
    id BIGINT NOT NULL ,
    number VARCHAR(255) ,
    PRIMARY KEY ( id )
)

CREATE TABLE PhoneDetails (
    id BIGINT NOT NULL ,
    provider VARCHAR(255) ,
    technology VARCHAR(255) ,
    phone_id BIGINT ,
    PRIMARY KEY ( id )
)

ALTER TABLE PhoneDetails
ADD CONSTRAINT FKeotuev8ja8v0sdh29dynqj05p
FOREIGN KEY (phone_id) REFERENCES Phone