Mapping返回值map

mapping.xml  --------<select id="get" resultMap="ReturnMap" >        SELECT             a.id,            a.userid as "UserId",            s.name as "user.name"        FROM user a        LEFT JOIN log_user s ON        a.userid=s.id    </select>    <resultMap id="ReturnMap" type="User">        <result property="id" column="id" />        <result property="UserId" column="userid" />        <result property="user.name" column="user.name" />    </resultMap>    ---dao层        @MapKey("user.name")        public Map<String,User> get();