【hibernate】之关于一对一单向，双向关联映射
首先我们来看，Hibernate官方对于一对一单向关联的解释:
基于外键关联的单向一对一关联和单向多对一关联几乎是一样的。唯一的不同就是单向一对一关
联中的外键字段具有唯一性约束。
ok,那我们也可以这样理解，一对一其实就是多对一关联的特殊形式，我们知道Hibernate的配置方式有两种，分别是Annotations，XML两种配置方式！
Annotations的一对一单向关联映射
人(User)和***号(Card)
@Entity
@Table(name="t_user")
publicclass User {
private Integer id;
private String name;
private Card card;
@OneToOne
@JoinColumn(name="card_id",unique=true)//name是自定义关联外键的列名
public Card getCard() {
returncard;
}
publicvoid setCard(Card card) {
this.card = card;
}
@Id
@GeneratedValue
public Integer getId() {
returnid;
}
publicvoid setId(Integerid) {
this.id = id;
}
@Column(name="name")
public String getName() {
returnname;
}
publicvoid setName(Stringname) {
this.name = name;
}@Entity
@Table(name="t_card")
publicclass Card {
private Integer id;
private String num;
@Id
@GeneratedValue
public Integer getId() {
returnid;
}
publicvoid setId(Integerid) {
this.id = id;
}
@Column(name="card_id")
public String getNum() {
returnnum;
}
publicvoid setNum(Stringnum) {
this.num = num;
}
}
XML一对一单向关联
Person和Group，一个人只能在一个组
publicclass Group {
private Integer id;
private String name;
public Integer getId() {
returnid;
}
publicvoid setId(Integerid) {
this.id = id;
}
public String getName() {
returnname;
}
publicvoid setName(Stringname) {
this.name = name;
}
}publicclass Person {
private Integer id;
private String name;
private Integer age;
private Group group;
public Group getGroup() {
returngroup;
}
publicvoid setGroup(Groupgroup) {
this.group = group;
}
public Integer getId() {
returnid;
}
publicvoid setId(Integerid) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(Stringname) {
this.name = name;
}
public Integer getAge() {
return age;
}
publicvoid setAge(Integer age){
this.age = age;
}
}<?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" /> </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="group_id" unique="true" not-null="true" /> </class> </hibernate-mapping>
Ok,到这里，大家就有点疑惑，为什么我们的Annotaions配置采用的是one-to-one而我们的xml配置采用的是many-to-one呢？
当然XML配置是我参考Hibernate的文档而来，因为一对一本身就是多对一的特殊形式，
但是配置many-to-one又给我们一种暗示，多个***号对应一个人，所以Hibernate根据这种情况提供unique唯一性来确认!
Annotations一对一双向关联映射
User和Card
@Entity
@Table(name="t_user")
publicclass User {
private Integer id;
private String name;
private Card card;
@OneToOne
@JoinColumn(name="card_id",unique=true)//name主要是自定义关联外键的列名
public Card getCard() {
returncard;
}
publicvoid setCard(Cardcard) {
this.card = card;
}
@Id
@GeneratedValue
public Integer getId() {
returnid;
}
publicvoid setId(Integerid) {
this.id = id;
}
@Column(name="name")
public String getName() {
returnname;
}
publicvoid setName(Stringname) {
this.name = name;
}
}@Entity
@Table(name="t_card")
publicclass Card {
private Integer id;
private String num;
private User user;
@OneToOne(mappedBy="card")//mappedBy的意思是指定User中的card作为关联外键，否则User和Card都会出现外键
public User getUser() {
returnuser;
}
publicvoid setUser(Useruser) {
this.user = user;
}
@Id
@GeneratedValue
public Integer getId() {
returnid;
}
publicvoid setId(Integerid) {
this.id = id;
}
@Column(name="card_id")
public String getNum() {
returnnum;
}
publicvoid setNum(Stringnum) {
this.num = num;
}
}Ok？那么我们可以看到在Annotaions中的配置都是ont-to-one，那么我们在xml中的配置呢？
<?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="group_id" unique="true" not-null="true" /> </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="Group" table="t_group"> <id name="id"> <column name="id"/> <generator class="native" /> </id> <property name="name" /> <!-- many-to-one这种配置会分别在两个表中都产生外键，造成数据的多余，通常我们采用one-to-one的形式在xml中配置 --> <many-to-one name="Person" column="person_id" unique="true" /> <!-- <one-to-onename="person"property-ref="group"/> --> </class> </hibernate-mapping>
ok，到这里一对一的单双向关联映射基本上就是这样，随笔之作，有问题，请留言！
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原文地址：http://1936625305.blog.51cto.com/6410597/1567291
