java后端树形菜单结构

课程分类的建表语句

CREATE TABLE `edu_subject` (
  `id` char(19) NOT NULL COMMENT '课程类别ID',
  `title` varchar(10) NOT NULL COMMENT '类别名称',
  `parent_id` char(19) NOT NULL DEFAULT '0' COMMENT '父ID',
  `sort` int(10) unsigned NOT NULL DEFAULT '0' COMMENT '排序字段',
  `gmt_create` datetime NOT NULL COMMENT '创建时间',
  `gmt_modified` datetime NOT NULL COMMENT '更新时间',
  PRIMARY KEY (`id`),
  KEY `idx_parent_id` (`parent_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 ROW_FORMAT=COMPACT COMMENT='课程科目';

#
# Data for table "edu_subject"
#

INSERT INTO `edu_subject` VALUES ('1178214681118568449','后端开发','0',1,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681139539969','Java','1178214681118568449',1,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681181483010','前端开发','0',3,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681210843137','JavaScript','1178214681181483010',4,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681231814658','云计算','0',5,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681252786178','Docker','1178214681231814658',5,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681294729217','Linux','1178214681231814658',6,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681324089345','系统/运维','0',7,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681353449473','Linux','1178214681324089345',7,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681382809602','Windows','1178214681324089345',8,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681399586817','数据库','0',9,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681428946945','MySQL','1178214681399586817',9,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681454112770','MongoDB','1178214681399586817',10,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681483472898','大数据','0',11,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681504444418','Hadoop','1178214681483472898',11,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681529610242','Spark','1178214681483472898',12,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681554776066','人工智能','0',13,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681584136193','Python','1178214681554776066',13,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681613496321','编程语言','0',14,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178214681626079234','Java','1178214681613496321',14,'2019-09-29 15:47:25','2019-09-29 15:47:25'),('1178585108407984130','Python','1178214681118568449',2,'2019-09-30 16:19:22','2019-09-30 16:19:22'),('1178585108454121473','HTML/CSS','1178214681181483010',3,'2019-09-30 16:19:22','2019-09-30 16:19:22');

课程对应的实体

package com.atguigu.eduservice.entity;

import com.baomidou.mybatisplus.annotation.IdType;
import java.util.Date;
import com.baomidou.mybatisplus.annotation.TableId;
import java.io.Serializable;
import java.util.List;

import io.swagger.annotations.ApiModel;
import io.swagger.annotations.ApiModelProperty;
import lombok.Data;
import lombok.EqualsAndHashCode;
import lombok.experimental.Accessors;

/**
 * <p>
 * 课程科目
 * </p>
 *
 * @author wangjie
 * @since 2020-07-08
 */
@Data
@EqualsAndHashCode(callSuper = false)
@Accessors(chain = true)
@ApiModel(value="EduSubject对象", description="课程科目")
public class EduSubject implements Serializable {

    private static final long serialVersionUID = 1L;

    @ApiModelProperty(value = "课程类别ID")
    @TableId(value = "id", type = IdType.ID_WORKER_STR)
    private String id;

    @ApiModelProperty(value = "类别名称")
    private String title;

    @ApiModelProperty(value = "父ID")
    private String parentId;

    @ApiModelProperty(value = "排序字段")
    private Integer sort;

    @ApiModelProperty(value = "创建时间")
    private Date gmtCreate;

    @ApiModelProperty(value = "更新时间")
    private Date gmtModified;

    @ApiModelProperty(value = "子菜单")
    private List<EduSubject> children;


}

　　获取树形结构的主要代码

package com.atguigu.eduservice.service.impl;

import com.atguigu.eduservice.entity.EduSubject;
import com.atguigu.eduservice.mapper.EduSubjectMapper;
import com.atguigu.eduservice.service.EduSubjectService;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.baomidou.mybatisplus.extension.service.impl.ServiceImpl;
import org.springframework.stereotype.Service;

import java.util.*;

/**
 * <p>
 * 课程科目 服务实现类
 * </p>
 *
 * @author wangjie
 * @since 2020-07-08
 */
@Service
public class EduSubjectServiceImpl extends ServiceImpl<EduSubjectMapper, EduSubject> implements EduSubjectService {

    @Override
    public List<EduSubject> findAllSubjectList() {
        return findTree();
    }

    /*
     * 排序,根据order排序
     */
    public Comparator<EduSubject> order() {
        Comparator<EduSubject> comparator = new Comparator<EduSubject>() {
            @Override
            public int compare(EduSubject o1, EduSubject o2) {
                if (o1.getSort() != o2.getSort()) {
                    return o1.getSort() - o2.getSort();
                }
                return 0;
            }
        };
        return comparator;
    }

    /**
     * 生成树
     *
     * @return
     */
    public List<EduSubject> findTree() {
        //最终的树形结构
        List<EduSubject> data = new ArrayList<>();
        try {//查询所有菜单
            QueryWrapper<EduSubject>  subjectWrapper = new QueryWrapper<>();
            subjectWrapper.select(EduSubject.class,info->!info.getColumn().equals("children"));
            List<EduSubject> allMenu = baseMapper.selectList(subjectWrapper);
            //根节点
            List<EduSubject> rootMenu = new ArrayList<EduSubject>();
            for (EduSubject nav : allMenu) {
                if (nav.getParentId().equals("0")) {//父节点是0的，为根节点。
                    rootMenu.add(nav);
                }
            }
            /* 根据Menu类的order排序 */
            Collections.sort(rootMenu, order());
            //为根菜单设置子菜单，getClild是递归调用的
            for (EduSubject nav : rootMenu) {
                /* 获取根节点下的所有子节点 使用getChild方法*/
                List<EduSubject> childList = getChild(nav.getId(), allMenu);
                nav.setChildren(childList);//给根节点设置子节点
            }
            data.addAll(rootMenu);
            return data;
        } catch (Exception e) {
            e.printStackTrace();
            return data;
        }
    }

    /**
     * 获取子节点
     *
     * @param id      父节点id
     * @param allMenu 所有菜单列表
     * @return 每个根节点下，所有子菜单列表
     */
    public List<EduSubject> getChild(String id, List<EduSubject> allMenu) {
        //子菜单
        List<EduSubject> childList = new ArrayList<EduSubject>();
        for (EduSubject nav : allMenu) {
            // 遍历所有节点，将所有菜单的父id与传过来的根节点的id比较
            //相等说明：为该根节点的子节点。
            if (nav.getParentId().equals(id)) {
                childList.add(nav);
            }
        }
        //递归
        for (EduSubject nav : childList) {
            nav.setChildren(getChild(nav.getId(), allMenu));
        }
        Collections.sort(childList, order());//排序
        //如果节点下没有子节点，返回一个空List（递归退出）
        if (childList.size() == 0) {
            return new ArrayList<EduSubject>();
        }
        return childList;
    }
}