出现的原因是数据库查询出空数据并填充了实体

查询结果

SpringBoot是用map接收

    <select id="check" resultType="java.util.Map">
        SELECT
        p.address_name AS `name`
        FROM
        pm_rules_items r
        LEFT JOIN pm_mountain_address p ON p.mountain_id = r.mountains_id
        <where>
            <if test="id!=null">
                r.rules_id &lt;> #{id}
            </if>
            <if test="items!=null">
                AND r.mountains_id IN
                <foreach collection="items" item="item" open="(" close=")" separator=",">
                    #{item.mountainsId}
                </foreach>
            </if>

        </where>

    </select>

dao层接口

    List<Map<String, String>> check(@Param("id") Integer id, @Param("items") List<RulesItemParam> items);

最后确实组装进集合，集合的长度为1但是内容为空,提示All elements are null

解决方案，处理前去除null元素，不然判断该集合是不为空

resultMap.removeAll(Collections.singleton(null));

处理查询数据All elements are null