【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差点儿是一样的。
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,那我们也能够这样理解，一对一事实上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，各自是Annotations，XML两种配置方式！
Annotations的一对一单向关联映射
人(User)和身 份 证号(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是自己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，一个人仅仅能在一个组
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配置採用的是one-to-one而我们的xml配置採用的是many-to-one呢？
当然XML配置是我參考Hibernate的文档而来，由于一对一本身就是多对一的特殊形式，
可是配置many-to-one又给我们一种暗示。多个***号相应一个人。所以Hibernate依据这样的情况提供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主要是自己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？那么我们能够看到在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这样的配置会分别在两个表中都产生外键，造成数据的多余，通常我们採用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！