相信你也遇到过这种场景，判断二级目录属于哪个一级目录，一个员工属于哪个上级员工领导…
当Mybatis遇上目录树，有哪些解决方法？
一般来说，有xml直接实现和java代码递归赋值实现。
方式一：xml直接实现
这里列出category数据表数据
表结构如下
type表示分类类型，也就是目录级别，1表示一级目录，3表示三级目录
大家就不要关注数据类型规范了，比如这里id应该bigint，type明明可以tinyint之类的，我抓我看到的例子直接讲解。
目录为甜点/蛋糕的id为1，而蛋糕和点心的father_id为1，目录为饼干/膨化的id为2，饼干、薯片、虾条的father_id就是2，一级目录id对应二级子目录的father_id，这就是所属对应关系，可以理解为父子关系。
实体类是mybatis-generator插件自动生成的
public class Category { private Integer id; private String name; private Integer type; private Integer fatherId; private String logo; private String slogan; private String catImage; private String bgColor; //=====篇幅原因，省掉Getter和Setter方法======......}
一般我们看到的商城，鼠标放到一级分类目录就会展示出二级分类目录。我们的需求是当鼠标移动到一级分类，我们需要提供二级分类和三级分类。
这里贴出需要返回给前端的聚合模型view object数据
/** * 二级分类VO */public class CategoryVO { private Integer id; private String name; private String type; private Integer fatherId; // 三级分类vo list private List<SubCategoryVO> subCatList;//=====篇幅原因，省掉Getter和Setter方法======......}
public class SubCategoryVO { private Integer subId; private String subName; private String subType; private Integer subFatherId; //=====篇幅原因，省掉Getter和Setter方法======......}
这就涉及到自连接查询子目录的技巧了，我们试试查找father_id是1的子分类数据，也就是查询甜点/蛋糕分类下面的二级和三级分类，执行如下语句
SELECTf.id AS id,f.`name` AS `name`,f.type AS type,f.father_id AS fatherId,c.id AS subId,c.`name` AS subName,c.type AS subType,c.father_id AS subFatherId FROMcategory fLEFT JOIN category c ON f.id = c.father_id WHEREf.father_id = 1
结果如下
可以看到二级分类为蛋糕、点心时，有哪些对应的三级分类可以提供给前端，便于展示。
我这里分为CategoryVO、SubCategoryVO ，而不是把所有属性放在一个VO，是为了便于理解。如果不用List集合，而把所有属性放在一个VO，前端收到的数据形式和你此时在数据库查询出来的一样，有多条蛋糕记录，底下对应着不同具体食品，这让前端不好处理也不符合逻辑，正常逻辑应该是只有一个蛋糕分类，然后这个分类里面有数组去装着蛋糕对应子分类才对。
这里其实只用一个CategoryVO里面也可以处理，在后面第二种方式用java代码处理多级目录时，你会看到我只用了一个CategoryVO就能处理。
注意，二级分类的实体类CategoryVO有个
private List<SubCategoryVO> subCatList;
这个subCatList是为了存放三级分类的vo list，在xml中三级分类用了collection对应这个list
<?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="com.me.mapper.CategoryMapperCustom" > <resultMap id="myCategoryVO" type="com.me.pojo.vo.CategoryVO"> <id column="id" property="id"/> <result column="name" property="name"/> <result column="type" property="type"/> <!-- column一定要在sql语句中找到，property一定要在对应实体类中找到 因为sql用as写了别名，所以column才能用fatherId，如果不用别名，还是得写father_id --> <result column="fatherId" property="fatherId"/> <!-- collection 标签：用于定义关联的list集合类型的封装规则 property：对应三级分类的list属性名 ofType：集合的类型，三级分类的vo --> <collection property="subCatList" ofType="com.me.pojo.vo.SubCategoryVO"> <id column="subId" property="subId"/> <result column="subName" property="subName"/> <result column="subType" property="subType"/> <result column="subFatherId" property="subFatherId"/> </collection> </resultMap> <select id="getSubCatList" resultMap="myCategoryVO" parameterType="int"> SELECT f.id as id, f.`name` as `name`, f.type as type, f.father_id as fatherId, c.id as subId, c.`name` as subName, c.type as subType, c.father_id as subFatherId FROM category f LEFT JOIN category c on f.id = c.father_id WHERE f.father_id = #{rootCatId} </select></mapper>
首先让前端展示在首页的一级分类，前端调用一级分类接口，我们只需要查询type为1的数据返回给前端，鼠标移动到一级分类，就调用获取子分类的接口，前端传入对应一级分类的id给后端，后端将这个id作为father_id去查询子分类。最后我们可以调用getSubCatList来得到所有目录。
@Transactional(propagation = Propagation.SUPPORTS) @Override public List<CategoryVO> getSubCatList(Integer rootCatId) { return categoryMapperCustom.getSubCatList(rootCatId); }
最后数据就是这样，如下
{"status": 200,"msg": "OK","data": [{"id": 11,"name": "蛋糕","type": "2", <==================type=2表示二级目录"fatherId": 1,"subCatList": [{"subId": 37,"subName": "蒸蛋糕","subType": "3", <================subType=3表示3级目录"subFatherId": 11}, {"subId": 38,"subName": "软面包","subType": "3","subFatherId": 11}, {"subId": 39,"subName": "脱水蛋糕","subType": "3","subFatherId": 11}, {"subId": 40,"subName": "马卡龙","subType": "3","subFatherId": 11}, {"subId": 41,"subName": "甜甜圈","subType": "3","subFatherId": 11}, {"subId": 42,"subName": "三明治","subType": "3","subFatherId": 11}, {"subId": 43,"subName": "铜锣烧","subType": "3","subFatherId": 11}]}, {"id": 12,"name": "点心","type": "2","fatherId": 1,"subCatList": [{"subId": 44,"subName": "肉松饼","subType": "3","subFatherId": 12}, {"subId": 45,"subName": "华夫饼","subType": "3","subFatherId": 12}, {"subId": 46,"subName": "沙琪玛","subType": "3","subFatherId": 12}, {"subId": 47,"subName": "鸡蛋卷","subType": "3","subFatherId": 12}, {"subId": 48,"subName": "蛋饼","subType": "3","subFatherId": 12}, {"subId": 49,"subName": "凤梨酥","subType": "3","subFatherId": 12}, {"subId": 50,"subName": "手撕面包","subType": "3","subFatherId": 12}]}]}
方式二：java代码递归处理二级三级目录
此刻我换一个数据库例子，但是还是和上面一个处理一级二级三级分类的例子一样数据表如下
表结构如下
和上一个例子大同小异，type依然表示目录级别
此刻需要返回给前端的VO如下，此刻我只写了一个CategoryVO，没有写子VO，可以对比前一种方式看看，道理都是一样的。
public class CategoryVO { private Integer id; private String name; private Integer type; private Integer parentId; private Integer orderNum; private Date createTime; private Date updateTime; private List<CategoryVO> childCategory = new ArrayList<>(); //=====篇幅原因，省掉Getter和Setter方法======......}
@Override public List<CategoryVO> listCategoryForCustomer(Integer parentId) { ArrayList<CategoryVO> categoryVOList = new ArrayList<>(); recursivelyFindCategories(categoryVOList, parentId); return categoryVOList; }// 以该parentId对应的目录为根节点，查询下面所有子目录信息，categoryVOList是要返回给前端展示的聚合模型数据 private void recursivelyFindCategories(List<CategoryVO> categoryVOList, Integer parentId) { // 递归获取所有子类别，并组合成为一个"目录树" List<Category> list= categoryMapper.selectCategoriesByParentId(parentId); // 通过父id查询子分类 if (!CollectionUtils.isEmpty(list)) { for (int i = 0; i < list.size(); ++i) { Category category = list.get(i); CategoryVO categoryVO = new CategoryVO(); BeanUtils.copyProperties(category, categoryVO); categoryVOList.add(categoryVO); // 这里当前目录id作为下一次的父id，查询有没有对应的子目录，getChildCategory()方法是返回定义的List<CategoryVO> childCategory recursivelyFindCategories(categoryVO.getChildCategory(), categoryVO.getId()); } } }
XML文件如下：
...... <resultMap id="BaseResultMap" type="com.me.mall.model.pojo.Category"> <id column="id" jdbcType="INTEGER" property="id" /> <result column="name" jdbcType="VARCHAR" property="name" /> <result column="type" jdbcType="INTEGER" property="type" /> <result column="parent_id" jdbcType="INTEGER" property="parentId" /> <result column="order_num" jdbcType="INTEGER" property="orderNum" /> <result column="create_time" jdbcType="TIMESTAMP" property="createTime" /> <result column="update_time" jdbcType="TIMESTAMP" property="updateTime" /> </resultMap> <sql id="Base_Column_List"> id, `name`, `type`, parent_id, order_num, create_time, update_time </sql> <select id="selectCategoriesByParentId" parameterType="int" resultMap="BaseResultMap"> select <include refid="Base_Column_List"/> from category where parent_id = #{parentId} </select> ......
我们手动查询模拟一下递归的过程，首先查询parent_id为3的二级分类
select *from categorywhere parent_id = 3
结果递归查询的时候，又会发现parent_id=4时还有数据，即还有三级分类，我们手动查询试试
select *from categorywhere parent_id = 4
示例数据如下：
{ "status": 10000, "msg": "SUCCESS", "data": [ { "id": 4, "name": "橘子橙子", "type": 2, <=================代表二级目录 "parentId": 3, "orderNum": 1, "createTime": "2019-12-17T17:17:00.000+0000", "updateTime": "2019-12-28T08:25:10.000+0000", "childCategory": [ <===============代表还有三级目录 { "id": 19, "name": "果冻橙", "type": 3, "parentId": 4, "orderNum": 1, "createTime": "2019-12-17T17:17:00.000+0000", "updateTime": "2020-02-10T16:37:02.000+0000", "childCategory": [] } ] }, { "id": 11, "name": "草莓", "type": 2, "parentId": 3, "orderNum": 2, "createTime": "2019-12-17T17:17:00.000+0000", "updateTime": "2019-12-28T07:44:42.000+0000", "childCategory": [] }, { "id": 12, "name": "奇异果", "type": 2, "parentId": 3, "orderNum": 3, "createTime": "2019-12-17T17:17:00.000+0000", "updateTime": "2019-12-28T08:25:12.000+0000", "childCategory": [] }, { "id": 14, "name": "车厘子", "type": 2, "parentId": 3, "orderNum": 4, "createTime": "2019-12-17T17:17:00.000+0000", "updateTime": "2019-12-28T08:25:12.000+0000", "childCategory": [] }, { "id": 28, "name": "其他水果", "type": 2, "parentId": 3, "orderNum": 4, "createTime": "2019-12-17T17:17:00.000+0000", "updateTime": "2019-12-28T08:25:12.000+0000", "childCategory": [] } ]}
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