hibernate 使用注解 一对多双向关联，自建表中生成了两个外键

@Entity
@Table(name = "t_classes3")
public class Classes3 {
@Id
@Column(name = "id")
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int cid;
@Column(name = "name")
private String className;
@OneToMany(mappedBy = "classes3", cascade = CascadeType.ALL)
private Set students;
public Classes3() {
super();
}

@Entity
@Table(name = "t_student3")
public class Student3 {
@Id
@Column(name = "id")
@GeneratedValue(strategy = GenerationType.IDENTITY)
private int sid;
@Column(name = "name")
private String studentName;
@ManyToOne(cascade = CascadeType.ALL)
@JoinColumn(name = "classesID")
private Classes3 classes3;

测试代码：
            Classes3 classes = new Classes3();
        classes.setClassName("一班");
        classes.setStudents(new HashSet<Student3>());
        Student3 student1 = new Student3();
        student1.setStudentName("安德森");
        Student3 student2 = new Student3();
        student2.setStudentName("斯蒂芬");
        classes.getStudents().add(student1);
        classes.getStudents().add(student2);
        session.save(classes);
        transaction.commit();

自动建表时添加了两个外键
Hibernate:
alter table t_student3
add constraint FKd9jbqete9tuogxlrtehh2a8c3
foreign key (cid)
references t_classes3 (id)
Hibernate:
alter table t_student3
add constraint FK67p9rnr4sov80e8iyoxaamquu
foreign key (classesID)
references t_classes3 (id)

这是为什么？我只需要classesID这个外键

add constraint FKd9jbqete9tuogxlrtehh2a8c3
foreign key (cid) 这边不是添加了 cid外键