Hibernate之关于一对多，多对一双向关联映射

@Entity
@Table(name="t_group")
publicclass Group {
    private Integer id;
    private String name;
    private Set<Person> persons=newHashSet<Person>();//set不允许重复，最适合数据库模型
    @Id
    @GeneratedValue
    public Integer getId() {
       returnid;
    }
    publicvoid setId(Integerid) {
       this.id = id;
    }
    @Column(name="t_name")
    public String getName() {
       returnname;
    }
    publicvoid setName(Stringname) {
       this.name = name;
    }
    @OneToMany(mappedBy="group")//意识是告诉Hibernate的关联关系应该设在Person，多的一方
//  只有OneToOne,OneToMany,ManyToMany上才有mappedBy属性，ManyToOne不存在该属性；
    public Set<Person>getPersons() {
       returnpersons;
    }
    publicvoidsetPersons(Set<Person> persons) {
       this.persons = persons;
    }
}

@Entity
@Table(name="t_person")
publicclass Person {
    private Integer id;
    private String name;
    private Integer age;
    private Group group;
     
    @ManyToOne
    public Group getGroup() {
       returngroup;
    }
    publicvoid setGroup(Groupgroup) {
       this.group = group;
    }
    @Id
    @GeneratedValue
    public Integer getId() {
       returnid;
    }
    publicvoid setId(Integerid) {
       this.id = id;
    }
    @Column(name="p_name")
    public String getName() {
       returnname;
    }
    publicvoid setName(Stringname) {
       this.name = name;
    }
    @Column(name="p_age")
    public Integer getAge() {
       returnage;
    }
    publicvoid setAge(Integerage) {
       this.age = age;
    }
}

XML配置

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="csg.hibernate.entity">
    <class name="Group" table="t_group">
       <id name="id">
           <column name="id"/>
           <generator class="native" />
       </id>
       <property name="name" />
       <set name="persons">
           <key column="groupId"/>
           <one-to-many class="csg.hibernate.entity.Person"/>
       </set>
    </class>
</hibernate-mapping>

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="csg.hibernate.entity">
    <class name="Person" table="t_person">
       <id name="id">
           <column name="id"/>
           <generator class="native" />
       </id>
       <property name="name" />
       <property name="age" />
       <many-to-one name="group" column="groupId"/>
    </class>
</hibernate-mapping>

由上大家都可以得出疑问？

为什么双向关联在Group中key需要指定column而在Person中也需要指定column呢？

如果其中一个不指定会有什么后果？

如果指定的column不一致又会有什么后果？

大家可以试一下！