mybatis一对一映射

drop table students;
drop table cards;
create table cards(
    id    int(5)    primary key,
    num varchar(20)
);
create table students(
    id    int(5)    primary key,
    name varchar(10),
    cid int(5),
    constraint cid_fk foreign key(cid) references cards(id)
);
insert into cards(id,num) values(1,‘111‘);
insert into students(id,name,cid) values(1,‘哈哈‘,1);

public class Card {
    private Integer id;
    private String num;
    private Student student;
    public Card(){}
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getNum() {
        return num;
    }
    public void setNum(String num) {
        this.num = num;
    }
    public Student getStudent() {
        return student;
    }
    public void setStudent(Student student) {
        this.student = student;
    }
}
public class Student {
    private Integer id;
    private String name;
    private Card card;
    public Student(){}
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Card getCard() {
        return card;
    }
    public void setCard(Card card) {
        this.card = card;
    }
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cardNamespace">
    <resultMap type="loaderman.one2one.Card" id="cardMap">
        <id property="id" column="id" />
        <result property="num" column="num" />
    </resultMap>
</mapper>

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="studentNamespace">
    <resultMap type="loaderman.one2one.Student" id="studentMap">
        <id property="id" column="id" />
        <result property="name" column="name"/>
        <association property="card" resultMap="cardNamespace.cardMap"/>
    </resultMap>
    <select id="findById" parameterType="int" resultMap="studentMap">
        select s.id,s.name,c.id,c.num
        from students s inner join cards c
        on s.cid = c.id 
        and s.id = #{id}
    </select>
</mapper>

public class StudentCardDao {
    /**
     * 查询1号【学生】
     */
    public Student findById(int id) throws Exception{
        SqlSession sqlSession = null;
        try{
            sqlSession = MybatisUtil.getSqlSession();
            return sqlSession.selectOne("studentNamespace.findById",id);
        }catch(Exception e){
            e.printStackTrace();
            throw e;
        }finally{
            MybatisUtil.closeSqlSession();
        }
    }
    public static void main(String[] args) throws Exception{
        StudentCardDao dao = new StudentCardDao();
        Student student = dao.findById(1);
        System.out.println(student.getId()+":"+student.getName());
        System.out.println(student.getCard().getId()+":"+student.getCard().getNum());
    }
}