项目中我们可能经常有这样的需求，需要返回二级或三级的菜单，返回一个树形结构，面试中也可能经常被问到。
最近的项目中就用到了，这里整理分享一下。
应用场景：
地区树，国家，省，市，区县共4级。
表结构：
建表语句：
SET NAMES utf8;SET FOREIGN_KEY_CHECKS = 0;返回JSON结果示例：
-- ----------------------------
-- Table structure for `base_area`
-- ----------------------------
DROP TABLE IF EXISTS `base_area`;
CREATE TABLE `base_area` (
`ID` bigint(20) NOT NULL AUTO_INCREMENT,
`AREA_NAME` varchar(255) DEFAULT NULL COMMENT '地区名称',
`AREA_CODE` varchar(255) DEFAULT NULL COMMENT '地区编码',
`PARENT_ID` bigint(20) DEFAULT NULL,
`PLAT_MARK` bigint(20) DEFAULT NULL COMMENT '区域标识，也就是平台标识',
`LEVEL` tinyint(4) DEFAULT '1' COMMENT '层',
`STATUS` tinyint(4) DEFAULT '1' COMMENT '是否可用、是否显示',
`EXPAND` tinyint(4) DEFAULT '0' COMMENT '是否展开子节点，非0为展开。',
PRIMARY KEY (`ID`),
KEY `index2` (`PLAT_MARK`)
) ENGINE=InnoDB AUTO_INCREMENT=3514 DEFAULT CHARSET=utf8;
SET FOREIGN_KEY_CHECKS = 1;
数据比较多，这里只展示一部分，有需要完整表数据的小伙伴，稍后提供下载地址。
{"result":[{"createTimeString":"","updateTimeString":"","level":3,"platMark":100001001000000,"parentId":1,"areaCode":"110000","expand":0,"areaName":"北京市","id":2,"childrenList":[{"createTimeString":"","updateTimeString":"","level":4,"platMark":100001001001000,"parentId":2,"areaCode":"110101","expand":0,"areaName":"东城区","id":4,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100001001002000,"parentId":2,"areaCode":"110102","expand":0,"areaName":"西城区","id":5,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100001001003000,"parentId":2,"areaCode":"110105","expand":0,"areaName":"朝阳区","id":6,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100001001004000,"parentId":2,"areaCode":"110106","expand":0,"areaName":"丰台区","id":7,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100001001005000,"parentId":2,"areaCode":"110107","expand":0,"areaName":"石景山区","id":8,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100001001006000,"parentId":2,"areaCode":"110108","expand":0,"areaName":"海淀区","id":9,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100001001007000,"parentId":2,"areaCode":"110109","expand":0,"areaName":"门头沟区","id":10,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100001001008000,"parentId":2,"areaCode":"110111","expand":0,"areaName":"房山区","id":11,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100001001009000,"parentId":2,"areaCode":"110112","expand":0,"areaName":"通州区","id":12,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100001001010000,"parentId":2,"areaCode":"110113","expand":0,"areaName":"顺义区","id":13,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100001001011000,"parentId":2,"areaCode":"110114","expand":0,"areaName":"昌平区","id":14,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100001001012000,"parentId":2,"areaCode":"110115","expand":0,"areaName":"大兴区","id":15,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100001001013000,"parentId":2,"areaCode":"110116","expand":0,"areaName":"怀柔区","id":16,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100001001014000,"parentId":2,"areaCode":"110117","expand":0,"areaName":"平谷区","id":17,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100001001015000,"parentId":2,"areaCode":"110228","expand":0,"areaName":"密云县","id":18,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100001001016000,"parentId":2,"areaCode":"110229","expand":0,"areaName":"延庆县","id":19,"childrenList":[]}]},{"createTimeString":"","updateTimeString":"","level":3,"platMark":100002001000000,"parentId":1,"areaCode":"120000","expand":0,"areaName":"天津市","id":20,"childrenList":[{"createTimeString":"","updateTimeString":"","level":4,"platMark":100002001001000,"parentId":20,"areaCode":"120101","expand":0,"areaName":"和平区","id":22,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100002001002000,"parentId":20,"areaCode":"120102","expand":0,"areaName":"河东区","id":23,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100002001003000,"parentId":20,"areaCode":"120103","expand":0,"areaName":"河西区","id":24,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100002001004000,"parentId":20,"areaCode":"120104","expand":0,"areaName":"南开区","id":25,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100002001005000,"parentId":20,"areaCode":"120105","expand":0,"areaName":"河北区","id":26,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100002001006000,"parentId":20,"areaCode":"120106","expand":0,"areaName":"红桥区","id":27,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100002001007000,"parentId":20,"areaCode":"120110","expand":0,"areaName":"东丽区","id":28,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100002001008000,"parentId":20,"areaCode":"120111","expand":0,"areaName":"西青区","id":29,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100002001009000,"parentId":20,"areaCode":"120112","expand":0,"areaName":"津南区","id":30,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100002001010000,"parentId":20,"areaCode":"120113","expand":0,"areaName":"北辰区","id":31,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100002001011000,"parentId":20,"areaCode":"120114","expand":0,"areaName":"武清区","id":32,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100002001012000,"parentId":20,"areaCode":"120115","expand":0,"areaName":"宝坻区","id":33,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100002001013000,"parentId":20,"areaCode":"120116","expand":0,"areaName":"滨海新区","id":34,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100002001014000,"parentId":20,"areaCode":"120221","expand":0,"areaName":"宁河县","id":35,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100002001015000,"parentId":20,"areaCode":"120223","expand":0,"areaName":"静海县","id":36,"childrenList":[]},{"createTimeString":"","updateTimeString":"","level":4,"platMark":100002001016000,"parentId":20,"areaCode":"120225","expand":0,"areaName":"蓟县","id":37,"childrenList":[]}]}]}
实体类：
/** * 表名：base_area * 备注：BaseArea */@SuppressWarnings("serial")public class BaseArea {//date formats//columns START//idprivate Long id;//地区名称private String areaName;//地区编码private String areaCode;//parentIdprivate Long parentId;//区域标识，也就是平台标识private Long platMark;//层private Byte level;//是否可用、是否显示private Integer status;//是否展开子节点，非0为展开。private Byte expand;//columns END//extend columns START//子节点列表List<BaseArea> childrenList;//extend columns ENDpublic List<BaseArea> getChildrenList() {return childrenList;}public void setChildrenList(List<BaseArea> childrenList) {this.childrenList = childrenList;}public BaseArea(){}public BaseArea(Long id){this.id = id;}/** * id * @return */public Long getId() {return this.id;}/** * id * @param value */public void setId(Long value) {this.id = value;}/** * 地区名称 * @return */public String getAreaName() {return this.areaName;}/** * 地区名称 * @param value */public void setAreaName(String value) {this.areaName = value;}/** * 地区编码 * @return */public String getAreaCode() {return this.areaCode;}/** * 地区编码 * @param value */public void setAreaCode(String value) {this.areaCode = value;}/** * parentId * @return */public Long getParentId() {return this.parentId;}/** * parentId * @param value */public void setParentId(Long value) {this.parentId = value;}/** * 区域标识，也就是平台标识 * @return */public Long getPlatMark() {return this.platMark;}/** * 区域标识，也就是平台标识 * @param value */public void setPlatMark(Long value) {this.platMark = value;}/** * 层 * @return */public Byte getLevel() {return this.level;}/** * 层 * @param value */public void setLevel(Byte value) {this.level = value;}/** * 是否可用、是否显示 * @return */public Integer getStatus() {return this.status;}/** * 是否可用、是否显示 * @param value */public void setStatus(Integer value) {this.status = value;}/** * 是否展开子节点，非0为展开。 * @return */public Byte getExpand() {return this.expand;}/** * 是否展开子节点，非0为展开。 * @param value */public void setExpand(Byte value) {this.expand = value;}}
Mapperxml 映射方法
<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE mapperPUBLIC "-//ibatis.apache.org//DTD Mapper 3.0//EN""http://ibatis.apache.org/dtd/ibatis-3-mapper.dtd"><!-- 不使用namespace的话sql搜索定位会比较方便 --><!-- BaseArea --><mapper namespace="BaseArea"><resultMap id="baseAreaResult" type="com.wanyu.smarthome.model.BaseArea"> <id property="id" column="ID"/> <result property="areaName" column="AREA_NAME"/> <result property="areaCode" column="AREA_CODE"/> <result property="parentId" column="PARENT_ID"/> <result property="platMark" column="PLAT_MARK"/> <result property="level" column="LEVEL"/> <result property="status" column="STATUS"/> <result property="expand" column="EXPAND"/> <collection property="childrenList" javaType="java.util.ArrayList" column="id" ofType="com.wanyu.smarthome.model.BaseArea" select="childrenSelect"></collection></resultMap><resultMap id="childrenResult" type="com.wanyu.smarthome.model.BaseArea"> <id property="id" column="ID"/> <result property="areaName" column="AREA_NAME"/> <result property="areaCode" column="AREA_CODE"/> <result property="parentId" column="PARENT_ID"/> <result property="platMark" column="PLAT_MARK"/> <result property="level" column="LEVEL"/> <result property="status" column="STATUS"/> <result property="expand" column="EXPAND"/> <collection property="childrenList" javaType="java.util.ArrayList" column="id" ofType="com.wanyu.smarthome.model.BaseArea" select="childrenSelect"></collection></resultMap><!-- 用于select查询公用抽取的列 --><sql id="commonColumns"> <![CDATA[ ID, AREA_NAME, AREA_CODE, PARENT_ID, PLAT_MARK, LEVEL, STATUS, EXPAND ]]></sql><sql id="commonAliasColumns"> <![CDATA[ x.ID, x.AREA_NAME, x.AREA_CODE, x.PARENT_ID, x.PLAT_MARK, x.LEVEL, x.STATUS, x.EXPAND ]]></sql><!-- useGeneratedKeys="true" keyProperty="xxx" for sqlserver and mysql --><insert id="insert" useGeneratedKeys="true" keyProperty="id" parameterType="com.wanyu.smarthome.model.BaseArea"> <![CDATA[ INSERT INTO base_area ( ID, AREA_NAME, AREA_CODE, PARENT_ID, PLAT_MARK, LEVEL, STATUS, EXPAND ) VALUES ( #{id}, #{areaName}, #{areaCode}, #{parentId}, #{platMark}, #{level}, #{status}, #{expand} ) ]]></insert> <update id="update" parameterType="com.wanyu.smarthome.model.BaseArea"> <![CDATA[ UPDATE base_area SET AREA_NAME = #{areaName}, AREA_CODE = #{areaCode}, PARENT_ID = #{parentId}, PLAT_MARK = #{platMark}, LEVEL = #{level}, STATUS = #{status}, EXPAND = #{expand} WHERE ID = #{id} ]]></update><delete id="delete" parameterType="map"> <![CDATA[ delete from base_area ]]><include refid="dynamicWhere"/> </delete> <delete id="batchDelete" parameterType="list"> delete from base_area where ID IN <foreach collection="list" item="ids" open="(" separator="," close=")"> #{ids} </foreach> </delete> <delete id="batchDeleteByLocalId" parameterType="list"> delete from base_area where LOCAL_ID in <foreach collection="list" item="ids" open="(" separator="," close=")"> #{ids}</foreach> </delete> <select id="getById" resultMap="baseAreaResult">SELECT <include refid="commonColumns" /> <![CDATA[ FROM base_area WHERE ID = #{id} ]]></select><select id="getByIds" resultMap="baseAreaResult">SELECT <include refid="commonColumns" />FROM base_area WHERE ID IN <foreach collection="list" item="ids" open="(" separator="," close=")"> #{ids} </foreach> </select><sql id="dynamicWhere"><!-- ognl访问静态方法的表达式 为@class@method(args),以下为调用rapid中的Ognl.isNotEmpty()方法,还有其它方法如isNotBlank()可以使用，具体请查看Ognl类 --><where> <if test="@Ognl@isNotEmpty(id)">AND ID = #{id}</if> <if test="@Ognl@isNotEmpty(areaName)">AND AREA_NAME = #{areaName}</if> <if test="@Ognl@isNotEmpty(areaCode)">AND AREA_CODE = #{areaCode}</if> <if test="@Ognl@isNotEmpty(parentId)">AND PARENT_ID = #{parentId}</if> <if test="@Ognl@isNotEmpty(platMark)">AND PLAT_MARK = #{platMark}</if> <if test="@Ognl@isNotEmpty(level)">AND LEVEL = #{level}</if> <if test="@Ognl@isNotEmpty(status)">AND STATUS = #{status}</if> <if test="@Ognl@isNotEmpty(expand)">AND EXPAND = #{expand}</if></where></sql> <select id="count" resultType="long"> SELECT count(*) FROM base_area <include refid="dynamicWhere"/> </select> <!-- 分页查询已经使用Dialect进行分页,也可以不使用Dialect直接编写分页 因为分页查询将传 offset,pageSize,lastRows 三个参数,不同的数据库可以根于此三个参数属性应用不同的分页实现 --> <select id="pageSelect" resultMap="baseAreaResult"> SELECT <include refid="commonColumns" /> FROM base_area <include refid="dynamicWhere"/><if test="@Ognl@isNotEmpty(sortColumns)">ORDER BY ${sortColumns}</if> </select> <select id="childrenSelect" resultMap="childrenResult"> SELECT <include refid="commonColumns" /> FROM base_area WHERE PARENT_ID = #{id}ORDER BY ID ASC </select></mapper>
Mapperxml 解析
1、主查询语句为： pageSelect
2、结果映射 resultMap id="baseAreaResult"
这里的关键在于：
<id property="id" column="ID"/><collection property="childrenList" javaType="java.util.ArrayList" column="id" ofType="com.wanyu.smarthome.model.BaseArea" select="childrenSelect"></collection>
collection 标签中定义属性名称为 childrenList，对应实体类中的：childrenList
属性类型为：java.util.ArrayList
column="id" 将 id 列的值做为参数传递给子查询
ofType 定义List 中保存的数据类型
select 定义子查询
3、注意子查询 childrenSelect 对应的结果映射 childrenResult ，又包含了 collection 标签，形成了循环递归调用
小结
不知道我有没有描述清楚，有不明白的地方请留言。
强烈不推荐这种循环递归调用的写法，因为性能非常差。
最好是有几级就写几级，也就是写几个 resultMap。
本粟中是演示同一个表相同实体类的实现方式，同样也可以不同的表不同的实体类，只要把子节点类型修改一下就可以了。
为了性能，这种查询最好配合缓存使用。
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