How to implement menu tree recursively in Java

pom file

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <!--  可选修改：之前的一些案例按照此版本搭建      -->
        <version>2.1.3.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.example</groupId>
    <artifactId>demo</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>demo</name>
    <description>Demo project for Spring Boot</description>
    <properties>
        <java.version>1.8</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-thymeleaf</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <!--  spring boot 整合mybatis的核心依赖-->
        <dependency>
            <groupId>org.mybatis.spring.boot</groupId>
            <artifactId>mybatis-spring-boot-starter</artifactId>
            <version>2.1.1</version>
        </dependency>
        <!--        数据库驱动-->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <scope>runtime</scope>
        </dependency>
        <!--pageHelper分页插件-->
        <dependency>
            <groupId>com.github.pagehelper</groupId>
            <artifactId>pagehelper-spring-boot-starter</artifactId>
            <version>1.3.0</version>
        </dependency>
        <!-- 引入lombok，简化pojo类-->
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>
        <!--        引入Mybatis plus 依赖  增强mybatis-->
        <dependency>
            <groupId>com.baomidou</groupId>
            <artifactId>mybatis-plus-boot-starter</artifactId>
            <version>3.4.2</version>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

application.yaml file

spring:
  datasource:   
    url: jdbc:mysql://localhost:3306/springboot?useUnicode=true&characterEncoding=utf-8&serverTimezone=CTT
    username: root
    password: 2020
    driver-class-name: com.mysql.cj.jdbc.Driver
pagehelper:
  helperDialect: mysql
  reasonable: true # 修改默认值
# mybatis-plus配置
mybatis-plus:
  configuration:
    log-impl: org.apache.ibatis.logging.stdout.StdOutImpl
  typeAliasesPackage: com.qcby.entity
  mapperLocations: classpath:mapper/*.xml
  # 全局配置id自增  =>
  global-config:
    db-config:
      id-type: auto

Database table design As follows

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;
-- ----------------------------
-- Table structure for menu
-- ----------------------------
DROP TABLE IF EXISTS `menu`;
CREATE TABLE `menu`  (
  `id` bigint(20) NOT NULL AUTO_INCREMENT COMMENT '主键id',
  `name` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_bin DEFAULT NULL COMMENT '名称',
  `pid` bigint(20) DEFAULT NULL COMMENT '父级id',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 10 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_bin ROW_FORMAT = Compact;
-- ----------------------------
-- Records of menu
-- ----------------------------
INSERT INTO `menu` VALUES (1, '主菜单1', 0);
INSERT INTO `menu` VALUES (2, '主菜单2', 0);
INSERT INTO `menu` VALUES (3, '主菜单3', 0);
INSERT INTO `menu` VALUES (4, '菜单1.1', 1);
INSERT INTO `menu` VALUES (5, '菜单1.2', 1);
INSERT INTO `menu` VALUES (6, '菜单1.1.1', 4);
INSERT INTO `menu` VALUES (7, '菜单2.1', 2);
INSERT INTO `menu` VALUES (8, '菜单2.2', 2);
INSERT INTO `menu` VALUES (9, '菜单1.1.2', 4);
SET FOREIGN_KEY_CHECKS = 1;

Menu class

package com.qcby.entity;
import lombok.Data;
import java.util.List;
@Data//lombok实现简化 get、set、tostring方法
public class Menu {
    // 菜单id
    private String  id;
    //菜单名称
    private String name;
    // 父菜单id
    private String pid;
    // 子菜单
    private List<Menu> menuChildren;
}

xml file

<?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.qcby.mapper.MenuMapper">
    <select id="selectByPid" resultType="com.qcby.entity.Menu">
        SELECT * FROM menu WHERE  pid=#{pid}
    </select>
    <select id="selectAll" resultType="com.qcby.entity.Menu">
        SELECT * FROM menu
    </select>
    <select id="selectAllNotBase" resultType="com.qcby.entity.Menu">
        SELECT * FROM menu where pid!= 0
    </select>
</mapper>

Mapper layer

package com.qcby.mapper;
import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import com.qcby.entity.Menu;
import org.apache.ibatis.annotations.Mapper;
import java.util.List;
@Mapper
public interface MenuMapper extends BaseMapper<Menu> {
    List<Menu> selectByPid(Integer pid);
    List<Menu> selectAll();
    List<Menu> selectAllNotBase();
}

service layer

package com.qcby.service;
import com.baomidou.mybatisplus.extension.service.IService;
import com.qcby.entity.Menu;
import java.util.List;
public interface MenuService extends IService<Menu> {
    List<Menu> selectByPid(Integer pid);
    List<Menu> selectAll();
    List<Menu> selectAllNotBase();
}

serviceImpl

package com.qcby.service.Impl;
import com.baomidou.mybatisplus.extension.service.impl.ServiceImpl;
import com.qcby.entity.Menu;
import com.qcby.mapper.MenuMapper;
import com.qcby.service.MenuService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import java.util.List;
@Service
public class MenuServiceImpl extends ServiceImpl<MenuMapper, Menu> implements MenuService {
    @Autowired
    private MenuMapper menuMapper;
    @Override
    public List<Menu> selectByPid(Integer pid) {
        return menuMapper.selectByPid(pid);
    }
    @Override
    public List<Menu> selectAll() {
        return menuMapper.selectAll();
    }
    @Override
    public List<Menu> selectAllNotBase() {
        return menuMapper.selectAllNotBase();
    }
}

controller layer

package com.qcby.controller;
import com.baomidou.mybatisplus.core.toolkit.StringUtils;
import com.qcby.entity.Menu;
import com.qcby.mapper.MenuMapper;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import java.util.ArrayList;
import java.util.List;
@RestController
@RequestMapping("menu")
public class MenuController {
    @Autowired
    private MenuMapper menuMapper;
    @RequestMapping("/getMenuTree")
    public List<Menu> getMenuTree(){
        List<Menu> menusBase = menuMapper.selectByPid(0);
        List<Menu> menuLNotBase = menuMapper.selectAllNotBase();
        for (Menu menu : menusBase) {
            List<Menu> menus = iterateMenus(menuLNotBase, menu.getId());
            menu.setMenuChildren(menus);
        }
        return  menusBase;
    }
    /**
     *多级菜单查询方法
     * @param menuVoList 不包含最高层次菜单的菜单集合
     * @param pid 父类id
     * @return
     */
    public List<Menu> iterateMenus(List<Menu> menuVoList,String pid){
        List<Menu> result = new ArrayList<Menu>();
        for (Menu menu : menuVoList) {
            //获取菜单的id
            String menuid = menu.getId();
            //获取菜单的父id
            String parentid = menu.getPid();
            if(StringUtils.isNotBlank(parentid)){
                if(parentid.equals(pid)){
                    //递归查询当前子菜单的子菜单
                    List<Menu> iterateMenu = iterateMenus(menuVoList,menuid);
                    menu.setMenuChildren(iterateMenu);
                    result.add(menu);
                }
            }
        }
        return result;
    }
}

Result display

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