与使用id相比,使用实体+“mapped by”有什么优势吗?

这种方法有什么大优势吗

@Entity
public class A {
    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name = "ID_B")
    private B b;
}

@Entity
public class B {
    @OneToMany(mappedBy = "a", cascade = CascadeType.ALL)
    private List<A> aList;
}

...而不是这个?。。。

@Entity
public class A {
    @Column(name = "ID_B")
    private Long idB;
}

@Entity
public class B {
    //no reference to A
}