public class User {private int userId;private String userName;private IDCard idCard;set...get...
}

public class IDCard {private String cardNum;private String place;private User user;set...get...
}

public class IDCard {//必须有此属性作为主键private int userId;private String cardNum;private String place;private User user;set...get...
}

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE hibernate-mapping PUBLIC"-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.juaner.hibernate.idcard"><class name="User" table="t_usercard"><id name="userId" column="u_id"><generator class="native"/></id><property name="userName" column="u_name"/><!--一对一映射--><one-to-one name="idCard" class="IDCard"/></class></hibernate-mapping>

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE hibernate-mapping PUBLIC"-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.juaner.hibernate.idcard"><class name="IDCard" table="t_idcard" ><id name="cardNum"><!--允许赋值--><generator class="assigned"/></id><property name="place"/><!--一对一映射，有外键而且唯一--><many-to-one name="user" class="User" column="user_id" cascade="save-update" unique="true" /></class></hibernate-mapping>

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE hibernate-mapping PUBLIC"-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.juaner.hibernate.idcard"><class name="IDCard" table="t_idcard" ><id name="userId"><!--主键生成方式是foreign，即把别的表的主键作为当前表的主键property指定引用的对象，根据对象可以找到对象的类及映射文件，从而找到表的主键--><generator class="foreign"><param name="property">user</param></generator></id><property name="cardNum"/><property name="place"/><!--基于主键的映射constraint=true指定在主键上添加外键约束--><one-to-one name="user"  class="User" constrained="true" cascade="save-update"  /></class></hibernate-mapping>