Hibernate关联映射1:一对一主键关联
2张表之间通过主键形成一对一映射关系，如一个人只能有一张身份证：
t_identity_card表建表语句：
CREATE TABLE `t_identity_card` ( `id` int(11) NOT NULL, `identity` int(11) NOT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=gb2312;
t_person表建表语句：
CREATE TABLE `t_person` ( `id` int(11) NOT NULL, `name` varchar(255) NOT NULL, `age` int(11) NOT NULL, PRIMARY KEY (`id`), CONSTRAINT `FK_ID` FOREIGN KEY (`id`) REFERENCES `t_identity_card` (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=gb2312;
hibernate.cfg.xml:
<?xml version='1.0' encoding='utf-8'?> <!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"> <hibernate-configuration> <session-factory> <property name="dialect">org.hibernate.dialect.MySQLDialect</property> <property name="connection.driver_class">com.mysql.jdbc.Driver</property> <property name="connection.url">jdbc:mysql://localhost:3306/mydb</property> <property name="connection.username">root</property> <property name="connection.password">196428</property> <property name="hibernate.show_sql">true</property> <mapping resource="com/po/IdentityCard.hbm.xml" /> <mapping resource="com/po/Person.hbm.xml" /> </session-factory> </hibernate-configuration>
单向一对一主键关联：
t_identity_card表：
public class IdentityCard implements Serializable {
private static final long serialVersionUID = 1L;
private int id;
private int identity;
private Person person;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public int getIdentity() {
return identity;
}
public void setIdentity(int identity) {
this.identity = identity;
}
public Person getPerson() {
return person;
}
public void setPerson(Person person) {
this.person = person;
}
}
由于t_identity_card表使用foreign主键生成策略，故在t_identity_card表的配置文件中必须配置主键生成表t_person，这样在向t_identity_card表中插入数据时才能找到所需的主键值，所以单向一对一主键关联中只有t_person<-t_identity_card，没有t_person->t_identity_card
<?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>
<class name="com.po.IdentityCard" table="t_identity_card">
<id name="id" column="id" type="integer">
<generator class="foreign">
<param name="property">person</param>
</generator>
</id>
<property name="identity" column="identity" type="integer"/>
<one-to-one name="person" class="com.po.Person"/>
</class>
</hibernate-mapping>
t_person表：
public class Person implements Serializable {
private static final long serialVersionUID = 1L;
private int id;
private String name;
private int age;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int 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>
<class name="com.po.Person" table="t_person">
<id name="id" column="id" type="integer">
<generator class="assigned"/>
</id>
<property name="name" column="name" type="string"/>
<property name="age" column="age" type="integer"/>
</class>
</hibernate-mapping>
测试一下：
public class Test {
public static void main(String[] args) {
Configuration conf = new Configuration();
SessionFactory sessionFactory = conf.configure().buildSessionFactory();
Session session = sessionFactory.openSession();
IdentityCard identityCard = new IdentityCard();
identityCard.setIdentity(123456);
Person person = new Person();
person.setId(1);
person.setName("sean");
person.setAge(25);
identityCard.setPerson(person);
Transaction tran = session.beginTransaction();
session.save(identityCard);
tran.commit();
IdentityCard identityCard2 = (IdentityCard)session.get(IdentityCard.class, 1);
Person person2 = identityCard2.getPerson();
System.out.println(person2.getName());
session.close();
}
}
测试结果为：
Hibernate: insert into t_identity_card (identity, id) values (?, ?) sean
只向t_identity_card表中插入了数据，t_person表为空，由于没有设置关联关系的cascade属性，在持久化一张表的时候，并不会级联的持久化另一张表
将t_identity_card表的映射文件中的关联关系修改为：<one-to-one name="person" class="com.po.Person"cascade="all" />
Hibernate: select person_.id, person_.name as name1_, person_.age as age1_ from t_person person_ where person_.id=? Hibernate: insert into t_identity_card (identity, id) values (?, ?) Hibernate: insert into t_person (name, age, id) values (?, ?, ?) sean
双向一对一主键关联：
这时需要对t_person表的配置做一些修改：
public class Person implements Serializable {
private static final long serialVersionUID = 1L;
private int id;
private String name;
private int age;
private IdentityCard identityCard;//新增
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public IdentityCard getIdentityCard() {
return identityCard;
}
public void setIdentityCard(IdentityCard identityCard) {
this.identityCard = identityCard;
}
}<?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>
<class name="com.po.Person" table="t_person">
<id name="id" column="id" type="integer">
<generator class="native"/>
</id>
<property name="name" column="name" type="string"/>
<property name="age" column="age" type="integer"/>
<!--新增-->
<one-to-one name="identityCard" class="com.po.IdentityCard" cascade="all" />
</class>
</hibernate-mapping>
测试一下：
public class Test {
public static void main(String[] args) {
Configuration conf = new Configuration();
SessionFactory sessionFactory = conf.configure().buildSessionFactory();
Session session = sessionFactory.openSession();
IdentityCard identityCard = new IdentityCard();
identityCard.setIdentity(123456);
Person person = new Person();
person.setId(1);
person.setName("sean");
person.setAge(25);
identityCard.setPerson(person);
person.setIdentityCard(identityCard);
Transaction tran = session.beginTransaction();
session.save(identityCard);
tran.commit();
Person person_tmp = (Person)session.get(Person.class, 1);
IdentityCard id_card_tmp = person_tmp.getIdentityCard();
System.out.println(id_card_tmp.getIdentity());
id_card_tmp = (IdentityCard)session.get(IdentityCard.class, 1);
person_tmp = id_card_tmp.getPerson();
System.out.println(person_tmp.getName());
session.close();
}
}
测试结果为：
Hibernate: select person_.id, person_.name as name1_, person_.age as age1_ from t_person person_ where person_.id=? Hibernate: insert into t_identity_card (identity, id) values (?, ?) Hibernate: insert into t_person (name, age, id) values (?, ?, ?) 123456 sean
Hibernate首先持久化了identityCard对象，将测试代码中的session.save(identityCard)修改为session.save(person)，再次测试：
Hibernate: insert into t_person (name, age, id) values (?, ?, ?) Exception in thread "main" org.hibernate.exception.ConstraintViolationException: Could not execute JDBC batch update ...... Caused by: java.sql.BatchUpdateException: Cannot add or update a child row: a foreign key constraint fails (`mydb`.`t_person`, CONSTRAINT `FK_ID` FOREIGN KEY (`id`) REFERENCES `t_identity_card` (`id`)) ......
Hibernate这次首先持久化了person对象，由于t_person表中的id字段建立了外键关系，故持久化失败，删除掉t_person表中id字段的外键关系之后，插入正常：
Hibernate: insert into t_person (name, age, id) values (?, ?, ?) Hibernate: insert into t_identity_card (identity, id) values (?, ?) 123456 sean
可见Hibernate有时不是很智能，save的对象将会首先被持久化，这时要特别注意表上建立的关联关系
