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;
-- ----------------------------
-- 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
//id
private Long id;
//地区名称
private String areaName;
//地区编码
private String areaCode;
//parentId
private 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 END
public 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 mapper
PUBLIC "-//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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文章出处：[无知人生，记录点滴](http://blog.csdn.NET/testcs_dn)
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