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

    @column
    private String parentAttribute;

    @OneToOne(mappedBy = "parent", optional = false)
    @JsonBackReference
    private Child child;

@Entity
public class Child {
    @Id
    @GeneratedValue
    private Long id;

    @column
    private String childAttribute;

    @OneToOne(optional = false, cascade = CascadeType.ALL)
    @JsonManagedReference
    private Parent parent;

public interface ParentRepository extends CrudRepository<Parent> {

    @EntityGraph(attributePaths = { "child" })
    //a hack to use findAll with default lazy/eager mapping
    Collection<Parent> findByIdNotNull();
}

Hibernate:
    select
        parent0_.id as id1_33_0_,
        child1_.id as id1_32_1_,
        parent0_.parent_attribute as parent_attribute2_33_0_,
        child1_.child_attribute as child_attribute2_32_1_,
    from
        test.parent parent0_
    left outer join
        test.child child1_
            on parent0_.id=child1_.parent_id
    where
        parent0_.id is not null

[ {
    "id": 1
    "parentAttribute": "I am the parent"
    } ]