hibernate 学习之第六篇

关键字: hibernate 一对一 关联

基于主键的一对一关联

Person类和IdCard，一对一映射。
person的属性为：id，name，idCard
IdCard的属性为：id，usefulLife，person

由于身份证和人是一对一的，身份证的id和人的id可以相同。可以使得idcard的id的产生与person保持一致。主从对象通过主键关联起来。

类Person:

public class Person {
private int id;
private String name;
private IdCard idCard;
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public IdCard getIdCard() {
        return idCard;
    }

    public void setIdCard(IdCard idCard) {
        this.idCard = idCard;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

public class Person {
private int id;
private String name;
private IdCard idCard;
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public IdCard getIdCard() {
        return idCard;
    }

    public void setIdCard(IdCard idCard) {
        this.idCard = idCard;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

 

类IdCard:

public class IdCard {
private int id;
private Date usefulLife;
private Person person;
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public Person getPerson() {
        return person;
    }

    public void setPerson(Person person) {
        this.person = person;
    }

    public Date getUsefulLife() {
        return usefulLife;
    }

    public void setUsefulLife(Date usefulLife) {
        this.usefulLife = usefulLife;
    }

}

public class IdCard {
private int id;
private Date usefulLife;
private Person person;
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public Person getPerson() {
        return person;
    }

    public void setPerson(Person person) {
        this.person = person;
    }

    public Date getUsefulLife() {
        return usefulLife;
    }

    public void setUsefulLife(Date usefulLife) {
        this.usefulLife = usefulLife;
    }

}

 

映射文件person.hbm.xml

 

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="hibernatetest01">
    <class name="Person" table="t_person">
        <id name="id" >
            <generator class="native"></generator>
        </id>
        <property name="name"></property>
       <one-to-one name="idCard"/>   //person和idcard是一对一映射
    </class>
</hibernate-mapping>

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="hibernatetest01">
    <class name="Person" table="t_person">
        <id name="id" >
            <generator class="native"></generator>
        </id>
        <property name="name"></property>
       <one-to-one name="idCard"/>   //person和idcard是一对一映射
    </class>
</hibernate-mapping>

 

映射文件idcard.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="hibernatetest01">
    <class name="IdCard" table="t_idcard">
        <id name="id" >
            <generator class="foreign">
                <param name="property">person</param>
            </generator>
        </id>
        <property name="usefulLife"></property>
       <one-to-one name="person"  constrained="true"/> //加上约束，保证了person必须有idcard与之对应，未加时，保存person可以没有idcard
    </class>
</hibernate-mapping>

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="hibernatetest01">
    <class name="IdCard" table="t_idcard">
        <id name="id" >
            <generator class="foreign">    
                <param name="property">person</param>
            </generator>
        </id>
        <property name="usefulLife"></property>
       <one-to-one name="person"  constrained="true"/> //加上约束，保证了person必须有idcard与之对应，未加时，保存person可以没有idcard
    </class>
</hibernate-mapping>

 

注： 采用外键产生方式，该类有一个属性参数，需要在配置文件中指定给他，他需要知道利用什么来产生idcard的主键。在此指定person，与下面的 <one-to-one name="person"/>对应。

测试类：

public class Main {
    public static void main(String[] args){
    add();
    }

    static Person add(){
        IdCard idCard = new IdCard();
        idCard.setUsefulLife(new Date());
        Person  p = new Person();
        p.setName("jianchen");
        p.setIdCard(idCard);
        idCard.setPerson(p);
        Session s = null;
        Transaction tx = null;
        s = HibernateUtil.getSession();
        tx = s.beginTransaction();
        s.save(p);
         s.save(idCard);
        tx.commit();
       return p;
    }
}

public class Main {
    public static void main(String[] args){
    add();
    }

    static Person add(){
        IdCard idCard = new IdCard();
        idCard.setUsefulLife(new Date());
        Person  p = new Person();
        p.setName("jianchen");
        p.setIdCard(idCard);
        idCard.setPerson(p);
        Session s = null;
        Transaction tx = null;
        s = HibernateUtil.getSession();
        tx = s.beginTransaction();
        s.save(p);
         s.save(idCard);
        tx.commit();
       return p;
    }
}

 

一对一主键关联关系的检索：

static void query(int id){
    Session s = HibernateUtil.getSession();
    Transaction tx = s.beginTransaction();
    Person p = (Person)s.get(Person.class, id);
    System.out.println(p.getIdCard().getUsefulLife());
    tx.commit();
   }

 static void query(int id){
     Session s = HibernateUtil.getSession();
     Transaction tx = s.beginTransaction();
     Person p = (Person)s.get(Person.class, id);
     System.out.println(p.getIdCard().getUsefulLife());
     tx.commit();
    }

 

在主函数中调用query（1），person和idcard是主从关系。对主类进行查询，会把从类也查询出来，体现在查询使用左外连接，一次性的查询。

Hibernate: select person0_.id as id1_1_, person0_.name as name1_1_, idcard1_.id as id2_0_, idcard1_.usefulLife as usefulLife2_0_ from t_person person0_ left outer join t_idcard idcard1_ on person0_.id=idcard1_.id where person0_.id=?
2009-02-04 15:51:20.0

 

就是把打印的那句话注释掉，同样会一次性的查出主从类 。查询语句如下：

 

Hibernate: select person0_.id as id1_1_, person0_.name as name1_1_, idcard1_.id as id2_0_, idcard1_.usefulLife as usefulLife2_0_ from t_person person0_ left outer join t_idcard idcard1_ on person0_.id=idcard1_.id where person0_.id=?

 

如果是查询从类，

static void query(int id){
     Session s = HibernateUtil.getSession();
     Transaction tx = s.beginTransaction();
     IdCard idCard =(IdCard) s.get(IdCard.class, id);
     System.out.println(idCard.getPerson().getName());
     tx.commit();
    }

static void query(int id){
     Session s = HibernateUtil.getSession();
     Transaction tx = s.beginTransaction();
     IdCard idCard =(IdCard) s.get(IdCard.class, id);
     System.out.println(idCard.getPerson().getName());
     tx.commit();
    }

 

对从类进行查询只会查询出从类的信息，主类被懒加载。从查询语句上看，以上代码执行了两次查询。

Hibernate: select idcard0_.id as id2_0_, idcard0_.usefulLife as usefulLife2_0_ from t_idcard idcard0_ where idcard0_.id=?
Hibernate: select person0_.id as id1_1_, person0_.name as name1_1_, idcard1_.id as id2_0_, idcard1_.usefulLife as usefulLife2_0_ from t_person person0_ left outer join t_idcard idcard1_ on person0_.id=idcard1_.id where person0_.id=?
jianchen

 

把后面的打印语句删掉，就只会执行上面的一次查询。

Hibernate: select idcard0_.id as id2_0_, idcard0_.usefulLife as usefulLife2_0_ from t_idcard idcard0_ where idcard0_.id=?

 

使用lazy属性：
修改映射文件：

<hibernate-mapping package="hibernatetest01">
    <class name="IdCard" table="t_idcard">
        <id name="id" >
            <generator class="foreign">
                <param name="property">person</param>
            </generator>
        </id>
        <property name="usefulLife"></property>
       <one-to-one name="person"  constrained="true" lazy="false"/> //加上约束，保证了person必须有idcard与之对应，未加时，保存person可以没有idcard
    </class>
</hibernate-mapping>

<hibernate-mapping package="hibernatetest01">
    <class name="IdCard" table="t_idcard">
        <id name="id" >
            <generator class="foreign">    
                <param name="property">person</param>
            </generator>
        </id>
        <property name="usefulLife"></property>
       <one-to-one name="person"  constrained="true" lazy="false"/> //加上约束，保证了person必须有idcard与之对应，未加时，保存person可以没有idcard
    </class>
</hibernate-mapping>

 

分析：
lazy=”false“表明对person不进行懒加载。这样就会导致在查询从类时也会把主类进行加载。
lazy在hibernate 3.0之前只有两个值：true ，false
3.0后lazy有三个值：true，false，proxy。其中proxy是默认的方式，采用代理实现懒加载。

在以上基础上进行进一步的修改：
 <one-to-one name="person"  constrained="true" lazy="true" fetch="select" />
虽然配置为懒加载，但由于设置了抓取为select ，就是说在抓取idcard时，也会抓取person。这也会导致在加载从对象时即时的加载主对象。

由此可见，hibernate中的属性配置是互相制约和影响的。hibernate的难点也在此。

基于外键的一对一关联

可以描述为多对一，加uniquer=“ture”，就变为一对一关联了
对象模型不需要改变，对映射文件进行修改。
person.hbm.xml文件：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="hibernatetest01">
    <class name="Person" table="t_person">
        <id name="id" >
            <generator class="native"></generator>
        </id>
        <property name="name"></property>
       <one-to-one name="idCard" property-ref="person"/>   //person通过idcard的属性person来查找属于自己的idcard
    </class>
</hibernate-mapping>

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="hibernatetest01">
    <class name="Person" table="t_person">
        <id name="id" >
            <generator class="native"></generator>
        </id>
        <property name="name"></property>
       <one-to-one name="idCard" property-ref="person"/>   //person通过idcard的属性person来查找属于自己的idcard
    </class>
</hibernate-mapping>

 

注：  property-ref: (可选) 指定关联类的一个属性，这个属性将会和本外键相对应。如果没有指定，会使用对方关联类的主键。

idcard.hbm.xml映射文件：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="hibernatetest01">
    <class name="IdCard" table="t_idcard">
        <id name="id" >
            <generator class="native"></generator>
        </id>
        <property name="usefulLife"></property>
       <many-to-one name="person"  column="person_id" unique="true"/>
    </class>
</hibernate-mapping>

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="hibernatetest01">
    <class name="IdCard" table="t_idcard">
        <id name="id" >
            <generator class="native"></generator>
        </id>
        <property name="usefulLife"></property>
       <many-to-one name="person"  column="person_id" unique="true"/>
    </class>
</hibernate-mapping>

 

表结构为：


从这个表结构上看，person和idcard是一对多的关系，因为idcard表有一个外键指向person表。但是在idcard的映射文件中，把外键强制设定为唯一，这样就构成了一对一映射。
  <many-to-one name="person"  column="person_id" unique="true"/>