hibernate的关系配置

一:一对多
实体类:

package cn.cat.model;
public class User {
    private int id;
    private String name;
    Private Set<Order> orders = new HashSet<Order>();

    //setter & getter ......
}

public class Order{
    private int id;
    private String orderName;
    private int user_id;

    //setter & getter.....
}

hbm.xml配置

<property name="name" type="string" column="name">
<set name="orders" cascade="true" lazy="false" fetch="join">
    <!--指定关联的外键-->
    <key column="user_id"/>
    <one-to-many class="cc. cn.cat.model.Order"/>
</set>

二:多对一
  实体是Employee和Department，它们之间是多对一的关系。
Department类:

public class Department {
	 private int id;
	 private String name;

	public Department() {
	}
	public Department(String name) {
		this.name = name;
	}
	// getters and setters are omitted
}
Employee类:

public class Employee {  
    private int id;  
    private String name;  
    private Department department;  
  
    public Employee() {  
    }  
    public Employee(String name) {  
        this.name = name;  
    }  
    // getters and setters are omitted 

Department.hbm.xml:

<hibernate-mapping  
    package="com.john.myhibernate.domain">  
  
    <class name="Department">  
        <id name="id">  
            <generator class="native"/>  
        </id>  
        <property name="name" length="20" not-null="true"/>  
    </class>  
</hibernate-mapping>

Employee.hbm.xml

<hibernate-mapping package="com.john.myhibernate.domain">

<class name="Employee">
	<id name="id">
		<generator class="native"/>
	</id>
	<property name="name" length="20" not-null="true"/>
	<many-to-one name="department" column="department_id" class="Department" fetch="select"/>
</class>
</hibernate-mapping>

many-to-one没有inverse属性，因为关系的维护是many的一方，不可能放弃对关系的维护。
many-to-one的lazy属性有三个取值：false, proxy, no-proxy。
1.测试cascade属性:

	public void testSaveCascade() {
		Session s = null;
		Transaction tx = null;
		
		Department depart = new Department();
		depart.setName("FCI");
		
		Employee em1 = new Employee("John");
		em1.setDepartment(depart);
		
		Employee em2 = new Employee("Lucy");
		em2.setDepartment(depart);
		
		try {
			s = HibernateUtil.getSession();
			tx = s.beginTransaction();
			s.save(em1);
			s.save(em2);
			tx.commit();
		} catch (HibernateException e) {
			tx.rollback();
			e.printStackTrace();
		} finally {
			if (s != null)
				s.close();
		}
	}

结果是报org.hibernate.TransientObjectException异常，因为没有保存Department实例。

可以加cascade属性，解决问题：

<many-to-one name="department" column="department_id" class="Department" fetch="select" cascade="save-update"/>

2.测试fetch属性:

	Session s = null;
	
	s = HibernateUtil.getSession();
	Employee em = (Employee) s.get(Employee.class, 2);
	System.out.println(em.getName());
	System.out.println(em.getDepartment());

查询语句如下：
Hibernate: select employee0_.id as id1_0_, employee0_.name as name1_0_, employee0_.department as department1_0_, employee0_.skill as skill1_0_, employee0_.sell as sell1_0_, employee0_.type as type1_0_ from Employee employee0_ where employee0_.id=?

Hibernate: select department0_.id as id0_0_, department0_.name as name0_0_ from Department department0_ where department0_.id=?
因为fetch设置为select，所以对每个实体，都分别用一个SELECT语句

如果把fetch设置为join，也就是连表查询，只使用一个SELECT语句。如下：
Hibernate: select employee0_.id as id1_1_, employee0_.name as name1_1_, employee0_.department as department1_1_, employee0_.skill as skill1_1_, employee0_.sell as sell1_1_, employee0_.type as type1_1_, department1_.id as id0_0_, department1_.name as name0_0_ from Employee employee0_ left outer join Department department1_ on employee0_.department=department1_.id where employee0_.id=?
3.测试lazy属性:
当fetch为select时，设置lazy为proxy或者no-proxy。

<many-to-one name="department" column="department_id" class="Department" fetch="select" cascade="save-update" lazy="no-proxy"/>

测试代码:

	Session s = null;
	
	s = HibernateUtil.getSession();
	Employee em = (Employee) s.get(Employee.class, 2);
	s.close();
	System.out.println(em.getName());
	System.out.println(em.getDepartment());

结果是报org.hibernate.LazyInitializationException异常。
因为fetch为select，而且lazy为proxy或者no-proxy，所以开始仅仅查询Employee，当需要用SELECT语句查询Department时，Session已经关闭。

解决办法：
1. 设置lazy为false，hibernate会第一时间把Employee和Department查询出来。
   如果fetch为select，使用两个SELECT查询语句。
   如果fetch为join，使用一个SELECT连表查询语句。
2. 设置fetch为join，这时不管lazy的取值，hibernate会进行连表查询，把两个实体都查询出来。
三:多对多
多对多（many-to-many）：在操作和性能方面都不太理想，所以多对多的映射使用较少，实际使用中最好转换成一对多的对象模型；hibernate会为我们创建中间关联表，转换成两个一对多。
E-R图

Teacher类:

package com.reiyen.hibernate.domain;

import java.util.Set;

public class Teacher {

	private int id;
	private String name;
	private Set<Student> students;
       //setter和getter方法

}

Student类:

package com.reiyen.hibernate.domain;

import java.util.Set;

public class Student {

	private int id;
	private String name;
	private Set<Teacher> teachers;
        //setter和getter方法

}

Teacher.hbm.xml:

<?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="com.reiyen.hibernate.domain">
	<class name="Teacher">
		<id name="id">
			<generator class="native" />
		</id>
		<property name="name" />
		<!-- 通过table项告诉hibernate中间表的名称 -->
		<set name="students" table="teacher_student">
			<!-- 通过key属性告诉hibernate在中间表里面查询teacher_id值相应的teacher记录 -->
			<key column="teacher_id" />
			<!-- 通过column项告诉hibernate对student表中查找student_id值相就的studnet记录 -->
			<many-to-many class="Student" column="student_id" />
		</set>
	</class>
</hibernate-mapping>

Student.hbm.xml:

<?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="com.reiyen.hibernate.domain">
	<class name="Student" >
		<id name="id" >
			<generator class="native" />
		</id>
		<property name="name" />
		<set name="teachers" table="teacher_student">
		<key column="student_id" />
		<many-to-many class="Teacher" column="teacher_id"/>
		</set>
	</class>
</hibernate-mapping>

一定要注意映射文件中<many-to-many class="Teacher" column="teacher_id"/>中class的值，它必须与你另一个关联映射文件中的class属性的name值一致，其实就是与你的实体类的类名一致,如：<many-to-many class="Teacher" column="teacher_id"/>中class的值就不能写成"teacher"。如果写成这样的话，就会抛出如下异常：An association from the table teacher_student refers to an unmapped class: com.reiyen .hibernate.domain.teacher
测试代码:

public class Many2Many {
	public static void main(String[] args) {
		add();
		//query(1);
	}

	static void query(int id) {
		Session s = null;
		Transaction tx = null;
		try {
			s = HibernateUtil.getSession();
			tx = s.beginTransaction();
			Teacher t = (Teacher) s.get(Teacher.class, id);
			System.out.println("students:" + t.getStudents().size());
			tx.commit();
		} finally {
			if (s != null)
				s.close();
		}
	}

	static void add() {
		Session s = null;
		Transaction tx = null;
		try {
			Set<Teacher> ts = new HashSet<Teacher>();

			Teacher t1 = new Teacher();
			t1.setName("t1 name");
			ts.add(t1);

			Teacher t2 = new Teacher();
			t2.setName("t2 name");
			ts.add(t2);

			Set<Student> ss = new HashSet<Student>();
			Student s1 = new Student();
			s1.setName("s1");
			ss.add(s1);

			Student s2 = new Student();
			s2.setName("s2");
			ss.add(s2);

			t1.setStudents(ss);  //1
			t2.setStudents(ss);  //1
//			
//			s1.setTeachers(ts);  //2
//			s2.setTeachers(ts);  //2

			s = HibernateUtil.getSession();
			tx = s.beginTransaction();
			s.save(t1);
			s.save(t2);
			s.save(s1);
			s.save(s2);
			tx.commit();
		} finally {
			if (s != null)
				s.close();
		}
	}
}

运行此程序后：控制台打印的sql语句如下所示：

Hibernate: insert into Teacher (name) values (?)
Hibernate: insert into Teacher (name) values (?)
Hibernate: insert into Student (name) values (?)
Hibernate: insert into Student (name) values (?)
Hibernate: insert into teacher_student (teacher_id, student_id) values (?, ?)
Hibernate: insert into teacher_student (teacher_id, student_id) values (?, ?)
Hibernate: insert into teacher_student (teacher_id, student_id) values (?, ?)
Hibernate: insert into teacher_student (teacher_id, student_id) values (?, ?)

一共在中间表里面插入了4条记录。

中间表结构如下所示：

DROP TABLE IF EXISTS `test`.`teacher_student`;
CREATE TABLE  `test`.`teacher_student` (
  `teacher_id` int(11) NOT NULL,
  `student_id` int(11) NOT NULL,
  PRIMARY KEY (`student_id`,`teacher_id`),
  KEY `FK2E2EF2DE6C8A2663` (`teacher_id`),
  KEY `FK2E2EF2DE5BEEDBC3` (`student_id`),
  CONSTRAINT `FK2E2EF2DE5BEEDBC3` FOREIGN KEY (`student_id`) REFERENCES `student` (`id`),
  CONSTRAINT `FK2E2EF2DE6C8A2663` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

表中插入的记录如下所示：

mysql> select * from teacher_student;
+------------+------------+
| teacher_id | student_id |
+------------+------------+
|          1 |          1 |
|          1 |          2 |
|          2 |          1 |
|          2 |          2 |
+------------+------------+
4 rows in set (0.00 sec)



程序中注释为1的语句非常重要，它是建立Teacher与Student关联的语句，如果没有这两条语句，虽然程序照样会执行，但是在中间表teacher_student没有任何记录，也就是Teacher与Student之间未关联。

当然你也可以通过程序中注释为2的语句来建立Teacher与Student之间的关联关系，同样会产生与注释为1的语句的效果。但是你不能在程序中同时出现以上四句程序，否则会抛出异常（ PRIMARY KEY (`student_id`,`teacher_id`),所以会出现主键冲突的异常 ）,：

Hibernate: insert into teacher_student (teacher_id, student_id) values (?, ?)
Hibernate: insert into teacher_student (teacher_id, student_id) values (?, ?)
Hibernate: insert into teacher_student (teacher_id, student_id) values (?, ?)
Hibernate: insert into teacher_student (teacher_id, student_id) values (?, ?)
Hibernate: insert into teacher_student (student_id, teacher_id) values (?, ?)
Hibernate: insert into teacher_student (student_id, teacher_id) values (?, ?)
Hibernate: insert into teacher_student (student_id, teacher_id) values (?, ?)
Hibernate: insert into teacher_student (student_id, teacher_id) values (?, ?)
Exception in thread "main" org.hibernate.exception.ConstraintViolationException: Could not execute JDBC batch update

解决上面产生异常的办法是设置inverse属性。即在Tearcher一端或Student一端设置inverse="true"，即让他们之中的某一方放弃维护关联关系。此时，虽然上面四句程序在测试程序中同时出现了（其实就是在对象模型上相互设置了他们的关联关系），但程序照样能运行正常，因为了在数据库模型上，只会有两句程序生效，也就是没有设置inverse="true"的那一端会去维护关联关系。有关inverse的说细信息，可以参看我的文章hibernate级联（cascade和inverse）.



执行测试程序中的查询测试，控制台打印的信息如下所示：

Hibernate: select teacher0_.id as id5_0_, teacher0_.name as name5_0_ from Teacher teacher0_ where teacher0_.id=?
Hibernate: select students0_.teacher_id as teacher1_1_, students0_.student_id as student2_1_, student1_.id as id7_0_, student1_.name as name7_0_ from teacher_student students0_ left outer join Student student1_ on students0_.student_id=student1_.id where students0_.teacher_id=?
students:2

从打印出的sql语句可以看出，多对多关系进行查询时，效率是比较低的。
四:一对一
一对一（one-to-one）实例（Person-IdCard）

一对一的关系在数据库中表示为主外关系．例如．人和身份证的关系．每个人都对应一个身份证号．我们应该两个表．一个是关于人信息的表（Person）．别外一个是身份证相关信息的表(id_card)．id_card表的主键对应该Person表的主键id，也是Person表的外键．有人才能有身份证．所以此例中Person是主表，id_card表为从表。

hibernate的一对一关系有两种形式，一种是共享主键方式，另一种是唯一外键方式.
<一>、共享主键方式实现一对一
1. 实体类设计如下：
Person类：

package com.reiyen.hibernate.domain;

public class Person {

	private int id;
	private String name;
	private IdCard idCard;

       //setter和getter方法
}

Idcard类:

package com.reiyen.hibernate.domain;

public class IdCard {

	private int id;
	private Date authorizeDate;
	private Person person;
       
       //setter和getter方法
}

Person.hbm.xml:

<?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="com.reiyen.hibernate.domain">
	<class name="Person" >
		<id name="id" >
			<generator class="native" />
		</id>
		<property name="name" />
		<one-to-one name="idCard" />
	</class>
</hibernate-mapping>

IdCard.hbm.xml:

<?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="com.reiyen.hibernate.domain">
	<class name="IdCard" table="id_card">
		<id name="id">
			<!-- id_card的主键来源person，也就是共享idCard的主键 -->
			<generator class="foreign">
				<param name="property">person</param>
			</generator>
		</id>
		<property name="authorizeDate" column="authorize_date" />
		<!-- one-to-one标签的含义，指示hibernate怎么加载它的关联对象，默认根据主键加载， 
			constrained="true"， 表明当前主键上存在一个约束，id_card的主键作为外键参照了person -->
		<one-to-one name="person" constrained="true"></one-to-one>
	</class>
</hibernate-mapping>

3.测试代码:

public class One2One {
	public static void main(String[] args) {
		Person person = add();
		System.out.println("peron name:" + person.getName());
	}
	
	static Person add(){
		Session session = null;
		Transaction tran = null;
		try {
			session = HibernateUtil.getSession();
			IdCard idCard = new IdCard();
			idCard.setAuthorizeDate(new Date());

			Person p = new Person();
			p.setName("person1");
			 p.setIdCard(idCard);   // 1
			idCard.setPerson(p);   // 2
			tran = session.beginTransaction();
			session.save(p);
			session.save(idCard);  
			tran.commit();
			return p;
		} finally {
			if (session != null)
				session.close();
		}

控制台打印信息如下所示：

Hibernate: insert into Person (name) values (?)
Hibernate: insert into id_card (authorize_date, id) values (?, ?)
person name : person1



数据库表id_card的创建语句如下所示：（重点注意红色字体部分）

DROP TABLE IF EXISTS `test`.`id_card`;
CREATE TABLE  `test`.`id_card` (
  `id` int(11) NOT NULL,
  `authorize_date` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `FK627C1FB4284AAF67` (`id`),
  CONSTRAINT `FK627C1FB4284AAF67` FOREIGN KEY (`id`) REFERENCES `person` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

数据库中记录如下所示：
mysql> select * from person;
+----+---------+
| id | name    |
+----+---------+
|  1 | person1 |
+----+---------+
1 row in set (0.00 sec)

mysql> select * from id_card;
+----+---------------------+
| id | authorize_date      |
+----+---------------------+
|  1 | 2010-03-23 01:07:25 |
+----+---------------------+
1 row in set (0.00 sec)
4.查询测试:

public class One2One {
	public static void main(String[] args) {
		query(1);
	}
	
	static void query(Integer id){
		Session session = null;
		try {
			session = HibernateUtil.getSession();
			Person person = (Person)session.get(Person.class, id);//4
                       System.out.println(person.getIdCard().getAuthorizeDate());//3
                        //IdCard idCard = (IdCard)session.get(IdCard.class, id); //1
			//System.out.println(idCard.getPerson().getName());      //2
		} finally {
			if (session != null)
				session.close();
		}
	}
}

执行此测试类时，控制台打印信息如下所示：

Hibernate: select person0_.id as id3_1_, person0_.name as name3_1_, idcard1_.id as id4_0_, idcard1_.authorize_date as authorize2_4_0_ from Person person0_ left outer join id_card idcard1_ on person0_.id=idcard1_.id where person0_.id=?
2010-03-23 21:46:07.0

从打印的SQL语句可以看出，一对一关系查询主对象时，然后得到主对象中的从属对象，通过left join一次就把查询结果查询出来了，因为从对象从属于主对象。而一对多，多对一关系时，从打印的SQL语句可知，它经过了两次查询才将查询结果查询出来。这是它们的区别。如果一对一关系中先查询从属对象，然后得到从属中的主对象时（即把上面测试类中的注释1, 2都去掉再运行），控制台打印信息如下：

Hibernate: select idcard0_.id as id4_0_, idcard0_.authorize_date as authorize2_4_0_ from id_card idcard0_ where idcard0_.id=?
Hibernate: select person0_.id as id3_1_, person0_.name as name3_1_, idcard1_.id as id4_0_, idcard1_.authorize_date as authorize2_4_0_ from Person person0_ left outer join id_card idcard1_ on person0_.id=idcard1_.id where person0_.id=?
person1

从打印的SQL可以看出,这种先查从对象，然后得到从属对象中的主对象，要经过两次查询才能得到查询结果。

如果只把测试程序中注释1去掉运行，则只会执行上面两次查询中的第一次查询。

6.one-to-one（元素）懒加载分析：

必须同时满足下面的三个条件时才能实现懒散加载：1).lazy!=false (lazy缺省方式就!=false)2).constrained=true 3).fetch=select(fetch缺省方式即为select)
(因为主表不能有constrained=true，所以主表没有懒加载功能)。能够懒加载的对象都是被改写过的代理对象，当相关联的session没有关闭时，访问这些懒加载对象（代理对象）的属性（getId和getClass除外）时，hibernate会初始化这些代理，或用Hibernate.initialize(proxy)来初始化代理对象；当相关联的session关闭后，再访问懒加载的对象将会出现异常。

测试：(1).注释掉查询测试程序中标记为3的语句，运行程序，控制台打印信息如下：

Hibernate: select person0_.id as id4_1_, person0_.name as name4_1_, idcard1_.id as id5_0_, idcard1_.authorize_date as authorize2_5_0_ from Person person0_ left outer join id_card idcard1_ on person0_.id=idcard1_.id where person0_.id=?

说明查询主对象Person时没有懒加载特性，因此它通过left outer join id_card表，同时把它的从对象IdCard也查询出来了。那为什么一对一中查询主对象时，不能实现懒加载呢？？大家可以看看person表的结构，你从表结构中根本不能决断出Person对象有没有相应的IdCard对象，所以它无法给setIdCard赋值（hibernate不能想当然的认为你有值，给你new一个代理对象给它），所以它一定要去查询相关联的对象表，看是否有与此Person对应的IdCard记录。而如果查询的是从对象IdCard时，因为idcard中的id是一个person表的一个外键，所以它必定有一个相对应的Person对象（因为有constrained=true），所以它可以先返回给你一个代理对象，当你真正需要Person对象的数据时，它再去查询数据库。

(2).注释掉查询测试程序中标记为3,4的语句，同进将标记为1的语句前的注释去掉再运行程序，控制台打印信息如下：

Hibernate: select idcard0_.id as id5_0_, idcard0_.authorize_date as authorize2_5_0_ from id_card idcard0_ where idcard0_.id=?

从打印信息可以看出，查询从对象IdCard时实现了懒加载功能，因为它只查询了IdCard对象，而关联的Person对象它没有进行查询。

(3).如果在(2)基础上将标记为2的语句前的注释也去掉再运行程序，控制台打印信息如下：

Hibernate: select idcard0_.id as id5_0_, idcard0_.authorize_date as authorize2_5_0_ from id_card idcard0_ where idcard0_.id=?
Hibernate: select person0_.id as id4_1_, person0_.name as name4_1_, idcard1_.id as id5_0_, idcard1_.authorize_date as authorize2_5_0_ from Person person0_ left outer join id_card idcard1_ on person0_.id=idcard1_.id where person0_.id=?
person1

它就进行了两次查询，将IdCard关联的Person对象也进行了查询。因为访问这些懒加载对象（代理对象）的属性（getId和getClass除外）时，hibernate会初始化这些代理.

(4).如果修改IdCard.hbm.xml映射文件,增加fetch="join"，如下所示：
xml代码:

<one-to-one name="person" constrained="true" fetch="join"/>

再按(2)进行测试，此时控制台打印信息如下：

Hibernate: select idcard0_.id as id5_1_, idcard0_.authorize_date as authorize2_5_1_, person1_.id as id4_0_, person1_.name as name4_0_ from id_card idcard0_ inner join Person person1_ on idcard0_.id=person1_.id where idcard0_.id=?
此时查询从对象IdCard时也不再懒加载了，通过inner join一次性将主从对象都查询出来。

(5).如果修改IdCard.hbm.xml映射文件,增加lazy="false"，如下所示：
xml代码:

<one-to-one name="person" constrained="true" lazy="false" />

再按(2)进行测试，此时控制台打印信息如下：

Hibernate: select idcard0_.id as id5_0_, idcard0_.authorize_date as authorize2_5_0_ from id_card idcard0_ where idcard0_.id=?
Hibernate: select person0_.id as id4_1_, person0_.name as name4_1_, idcard1_.id as id5_0_, idcard1_.authorize_date as authorize2_5_0_ from Person person0_ left outer join id_card idcard1_ on person0_.id=idcard1_.id where person0_.id=?
虽然也不实现懒加载功能，一次性将主从对象都查询出来，但此时是经过两次查询才得到结果。

如果修改IdCard.hbm.xml映射文件,增加lazy="proxy"，如下所示,与缺省时一样的效果，因为缺省时，lazy是=proxy
xml代码:

<one-to-one name="person" constrained="true" lazy="proxy" />

如果修改IdCard.hbm.xml映射文件,如下所示,则lazy(懒加载失效)，此时效果如测试（4）。
xml代码:

<one-to-one name="person" constrained="true" lazy="proxy" fetch="join"/>

<二>、唯一外键方式实现一对一

基于外键的one-to-one可以描述为多对一。

hibernate 一对一唯一外键关联映射（双向关联 Person<---->IdCard ）
一对一唯一外键 双向 关联，需要在另一端（person ），添加 <one-to-one> 标签，指示 hibernate 如何加载
其关联对象，默认根据主键加载idcard ，外键关联映射中，因为两个实体采用的是 idcard 的外键维护的关系， 所以不能指定主键加载 idcard ，而要根据 idcard 的外键加载，所以采用如下映射方式：
<one-to-one name="idcard" property-ref="person"/>

IdCard.hbm.xml的映射文件如下：

<?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="com.itcast.hibernate.domain">
	<class name="IdCard" table="id_card">
		<id name="id">
			<generator class="native" />
		</id>
		<property name="authorizeDate" column="authorize_date" />
<!-- 指定多的一端的unique=true，这样就限制了多的一端的多重性为一 
			通过这种手段映射一对一唯一外键关联 -->
		<many-to-one name="person" column="person_id" unique="true" />
	</class>
</hibernate-mapping>

Person.hbm.xml的映射文件如下：

<?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="com.itcast.hibernate.domain">
	<class name="Person" >
		<id name="id" >
			<generator class="native" />
		</id>
		<property name="name" />
		<!-- 没有下面的one-to-one标签也行,但那样就变成了单向关联(IdCard ----》 Person) ，也就是当知道IdCard后，能找到它属于的对应的人，但知道某人后，却无法找到相对应的IdCard-->
		<one-to-one name="idCard" property-ref="person"/>
	</class>
</hibernate-mapping>

实体类不用修改，还是用上面的测试类进行测试即可。

保存测试类运行后，相对共享主键方式的one-to-one，id_card表的结构发生了变化，表结构如下所示：

DROP TABLE IF EXISTS `test`.`id_card`;
CREATE TABLE  `test`.`id_card` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `authorize_date` datetime DEFAULT NULL,
  `person_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `person_id` (`person_id`) ,
  KEY `FK627C1FB45B253C91` (`person_id`),
  CONSTRAINT `FK627C1FB45B253C91` FOREIGN KEY (`person_id`) REFERENCES `person` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;

数据库表中记录如下：

mysql> select * from person;
+----+---------+
| id | name    |
+----+---------+
|  1 | person1 |
+----+---------+
1 row in set (0.00 sec)

mysql> select * from id_card;
+----+---------------------+-----------+
| id | authorize_date      | person_id |
+----+---------------------+-----------+
|  1 | 2010-03-23 22:40:38 |         1 |
+----+---------------------+-----------+
1 row in set (0.00 sec)



如果Person.hbm.xml映射文件中没有<one-to-one/>这一项的话，运行测试：

Person person = (Person)session.get(Person.class, id);
System.out.println(person.getIdCard().getAuthorizeDate());

会抛出如下异常：

java.lang.NullPointerException

因为这种关系成了IdCard--->Person的单向关联了。知道了Person，找不到对应的IdCard.

当运行如下测试时：

Person person = (Person)session.get(Person.class, id);
System.out.println(person.getIdCard());

控制台会打印出Person相对应的IdCard为null.



但如果得到了IdCard，却能找到相应的Person.测试如下：

IdCard idCard = (IdCard)session.get(IdCard.class, id);
System.out.println(idCard.getPerson().getName());

能得到正常的结果，person name为person1.

总结： 在缺省情况下，hibernate只有在一对一关联中，查询主对象时，是进行关联查询一次得到查询结果，其它（多对多、多对一、一对多、一对一查询从对象）的查询都是分两次查询得到查询结果。