hibernate中一对一的关联有两种方式：
      主键关联
      唯一外键关联

    下面以一个人只能有一个身份证，一张身份证只能属于一个人为例，分别说明这两种的关联方式。

   1）主键关联方式
    主键关联不需要额外的表字段；如果两行是通过这种一对一关系相关联的，那么这两行就共享同样的主关键字值。
   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>
 <class name="org.example.bean.Person" table="TPERSON">
  <id name="id" column="PERSON_ID">
   <generator class="increment"/>
  </id>
  <property name="age"/>
  <property name="firstname"/>
  <property name="lastname"/>

  <one-to-one name="card" class="org.example.bean.PersonCard"
   cascade="all" />
 </class>
</hibernate-mapping>

   对应的类：Person.java

public class Person implements Serializable {

 private long id;

 private long age;

 private String firstname;

 private String lastname;

 private PersonCard card;
}


   PersonCard.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>
 <class name="org.example.bean.PersonCard" table="TPERSONCARD">
  <id name="id" column="ID">
   <generator class="increment"/>
  </id>
  <property name="cardID"/>
  <property name="beginDate"/>
  <property name="endDate"/>

  <one-to-one name="person" class="org.example.bean.Person" constrained="true"/>
 </class>
</hibernate-mapping>

   对应的类：PersonCard.java
public class PersonCard {

 private long id;

 private String cardID;

 private String beginDate;

 private String endDate;

 private Person person;
}


   2）唯一外键关联
   唯一外键关联主要是一个外键和一个惟一关键字对应。
   下面是修改后的配置文件和对应的类。
   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>
 <class name="org.example.bean.Person" table="TPERSON">
  <id name="id" column="PERSON_ID">
   <generator class="increment"/>
  </id>
  <property name="age"/>
  <property name="firstname"/>
  <property name="lastname"/>
  <!-- one-to-one
   one person to one personcard
   -->
  <one-to-one name="card" class="org.example.bean.PersonCard"
   cascade="all-delete-orphan" property-ref="person" />
 </class>
</hibernate-mapping>

   对应的类：Person.java
public class Person implements Serializable {

 private long id;

 private long age;

 private String firstname;

 private String lastname;

 private PersonCard card;
}

   PersonCard.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>
 <class name="org.example.bean.PersonCard" table="TPERSONCARD">
  <id name="id" column="ID">
   <generator class="increment"/>
  </id>
  <property name="cardID"/>
  <property name="beginDate"/>
  <property name="endDate"/>
  <!--
   one personCard to one person
   -->
  <many-to-one name="person" class="org.example.bean.Person"
   unique="true" not-null="true" >
   <column name="PERSON_ID" />
  </many-to-one>
 </class>
</hibernate-mapping>

   对应的类：PersonCard.java
public class PersonCard {

 private long id;

 private String cardID;

 private String beginDate;

 private String endDate;

 private Person person;
}