MyBatis ResultMap Assocation 返回属性为null的问题

Model:

public class Employee {
    private Integer id;
    private String lastName;
    private String email;
    private String gender;
    private Department dept;
}

public class Department {
    private Integer id;
    private String departmentName;
}

　　

不使用Assocation的方式，可以正常返回数据，没有关联的属性email和gender也可以绑定上：

    <resultMap id="myEmpPlus" type="com.roy.simple.model.Employee">
        <id column="id" property="id"/>
        <result column="last_name" property="lastName"/>
        <result column="d_id" property="dept.id"/>
        <result column="dept_name" property="dept.departmentName"/>
    </resultMap>

    <select id="getEmpAndDeptById" resultMap="myEmpPlus">
        select e.*, d.id d_id, d.dept_name from tbl_employee e
        inner join tbl_dept d on e.dept_id = d.id
        where e.id = #{id}
    </select>

输出：

Employee{id=3, lastName='roy', email='adfa@asdf', gender='1', dept=Department{id=1, departmentName='开发部'}}

使用Assocation，如果不显示指定列的关联，gender和emai为null

    <resultMap id="myEmpPlus2" type="com.roy.simple.model.Employee">
        <id column="id" property="id"/>
        <result column="last_name" property="lastName"/>
        <!--<result column="gender" property="gender"/>-->
        <!--<result column="email" property="email"/>-->
        <!--可以指定联合的javabean的对象
            property：指定哪个属性是联合的对象
            javaType:指定对象的类型【不能省略】
        -->
        <association property="dept"javaType="com.roy.simple.model.Department">
            <id column="d_id" property="id"/>
            <result column="dept_name" property="departmentName"/>
        </association>
    </resultMap>

输出：

Employee{id=3, lastName='roy', email='null', gender='null', dept=Department{id=1, departmentName='开发部'}}

 在学习过程中碰到了这个问题，暂时还没有找到是什么原因引起的。望各位高手帮忙看看