实现步骤：

1. 分析数据库

如图所示，聪明的你可能发现了！有的数据的属性pid有多个相同的值并且对应了id的值。由此发现它们是有某种关系的，我也不打哑谜了，数据库设计呢，就是把所有一级菜单（最大的菜单）的某一个值设置为一致，这样后来我们只要查询这个值就可以获取所有一级菜单。

这些都是父级内容

pid为10就是id为10下的子菜单！

pid：“所以我们现在是什么关系？”

id：“I'm your father🫰”

2. 构建数据源

2.1 编写实体类

package com.ycxw.entity;

/**
 * 会议系统实体类
 * 
 * @author 云村小威
 *
 * @2023年7月11日 下午9:41:18
 */
public class Permission {
	private long id;
	private String name;
	private String description;
	private String url;
	private long pid;
	private int ismenu;
	private long displayno;

	public Permission() {
		// TODO Auto-generated constructor stub
	}

	public long getId() {
		return id;
	}

	public void setId(long id) {
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getDescription() {
		return description;
	}

	public void setDescription(String description) {
		this.description = description;
	}

	public String getUrl() {
		return url;
	}

	public void setUrl(String url) {
		this.url = url;
	}

	public long getPid() {
		return pid;
	}

	public void setPid(long pid) {
		this.pid = pid;
	}

	public int getIsmenu() {
		return ismenu;
	}

	public void setIsmenu(int ismenu) {
		this.ismenu = ismenu;
	}

	public long getDisplayno() {
		return displayno;
	}

	public void setDisplayno(long displayno) {
		this.displayno = displayno;
	}

	public Permission(long id, String name, String description, String url, long pid, int ismenu, long displayno) {
		super();
		this.id = id;
		this.name = name;
		this.description = description;
		this.url = url;
		this.pid = pid;
		this.ismenu = ismenu;
		this.displayno = displayno;
	}

	@Override
	public String toString() {
		return "Permission [id=" + id + ", name=" + name + ", description=" + description + ", url=" + url + ", pid="
				+ pid + ", ismenu=" + ismenu + ", displayno=" + displayno + "]";
	}

}

2.2 编写节点实体类

package com.zking.util;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;

/**
 * 
 * @author 云村小威
 *
 * @2023年7月12日 上午11:13:05
 */
public class TreeVo<T> {
	/**
	 * 节点ID
	 */
	private String id;
	/**
	 * 显示节点文本
	 */
	private String text;
	/**
	 * 节点状态，open closed
	 */
	private Map<String, Object> state;
	/**
	 * 节点是否被选中 true false
	 */
	private boolean checked = false;
	/**
	 * 节点属性
	 */
	private Map<String, Object> attributes;

	/**
	 * 节点的子节点
	 */
	private List<TreeVo<T>> children = new ArrayList<TreeVo<T>>();

	/**
	 * 父ID
	 */
	private String parentId;
	/**
	 * 是否有父节点
	 */
	private boolean hasParent = false;
	/**
	 * 是否有子节点
	 */
	private boolean hasChildren = false;

	public String getId() {
		return id;
	}

	public void setId(String id) {
		this.id = id;
	}

	public String getText() {
		return text;
	}

	public void setText(String text) {
		this.text = text;
	}

	public Map<String, Object> getState() {
		return state;
	}

	public void setState(Map<String, Object> state) {
		this.state = state;
	}

	public boolean isChecked() {
		return checked;
	}

	public void setChecked(boolean checked) {
		this.checked = checked;
	}

	public Map<String, Object> getAttributes() {
		return attributes;
	}

	public void setAttributes(Map<String, Object> attributes) {
		this.attributes = attributes;
	}

	public List<TreeVo<T>> getChildren() {
		return children;
	}

	public void setChildren(List<TreeVo<T>> children) {
		this.children = children;
	}

	public boolean isHasParent() {
		return hasParent;
	}

	public void setHasParent(boolean isParent) {
		this.hasParent = isParent;
	}

	public boolean isHasChildren() {
		return hasChildren;
	}

	public void setChildren(boolean isChildren) {
		this.hasChildren = isChildren;
	}

	public String getParentId() {
		return parentId;
	}

	public void setParentId(String parentId) {
		this.parentId = parentId;
	}

	public TreeVo(String id, String text, Map<String, Object> state, boolean checked, Map<String, Object> attributes,
                  List<TreeVo<T>> children, boolean isParent, boolean isChildren, String parentID) {
		super();
		this.id = id;
		this.text = text;
		this.state = state;
		this.checked = checked;
		this.attributes = attributes;
		this.children = children;
		this.hasParent = isParent;
		this.hasChildren = isChildren;
		this.parentId = parentID;
	}

	public TreeVo() {
		super();
	}

}

2.3 构建BuildTree节点结构方法类

package com.zking.util;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * @author 云村小威
 *
 * @2023年7月12日 上午11:15:30
 */
public class BuildTree {

	
	/**
	 * 指定idparam为顶级节点
	 * 
	 * @param nodes
	 * @param idParam
	 * @param <T>
	 * @return
	 */
	public static <T> List<TreeVo<T>> buildList(List<TreeVo<T>> nodes, String idParam) {
		if (nodes == null) {
			return null;
		}
		List<TreeVo<T>> topNodes = new ArrayList<TreeVo<T>>();
		//遍历传过来的节点数据
		for (TreeVo<T> children : nodes) {
			//获取父级id
			String pid = children.getParentId();
			//如果跟传过来的id相同
			if (pid == null || idParam.equals(pid)) {
				//就判定这个父级菜单
				topNodes.add(children);
				//继续执行下操作
				continue;
			}
			
			for (TreeVo<T> parent : nodes) {
				//获取id
				String id = parent.getId();
				//判断父级id是否等于pid
				if (id != null && id.equals(pid)) {
					//等于就判定它是谁的儿子
					parent.getChildren().add(children);
					//设置是否有父节点和子节点
					children.setHasParent(true);
					parent.setChildren(true);

					continue;
				}
			}

		}
		return topNodes;
	}

}

2.4 编写dao类

后台方法构建好后，只需将数据源进行Json解析后，进行后台获取输出到页面即可：

package com.ycxw.dao;

import java.util.ArrayList;
import java.util.List;

import com.ycxw.entity.Permission;
import com.zking.util.BaseDao;
import com.zking.util.BuildTree;
import com.zking.util.PageBean;
import com.zking.util.TreeVo;

/**
 * @author 云村小威
 *
 * @2023年7月12日 上午11:11:54
 */
public class PermissionDao extends BaseDao<Permission>{

	/**
	 * 获取数据库数据
	 * @param p
	 * @param pageBean
	 * @return
	 * @throws Exception
	 */
	public List<Permission> list(Permission p, PageBean pageBean) throws Exception {
		String sql = "select * from t_oa_permission where 1=1";
		return super.executeQuery(sql, Permission.class, pageBean);
	}
	
	/**
	 * 将数据库查询的平级数据，转换成有父子关系的数据
	 * @param permission
	 * @param pageBean
	 * @return
	 * @throws Exception
	 */
	public List<TreeVo<Permission>> menus(Permission permission, PageBean pageBean) throws Exception{
		List<TreeVo<Permission>> list = new ArrayList<TreeVo<Permission>>();
		//获取的数据库集合
		List<Permission> per = this.list(permission, pageBean);
		for (Permission p : per) {
			TreeVo<Permission> vo = new TreeVo<>();
			//设置节点类内容并添加到节点集合
			vo.setId(p.getId()+"");//设置节点id 
			vo.setText(p.getName()); //设置节点文本
			vo.setParentId(p.getPid()+"");//设置父节点
			list.add(vo);
		}
		//返回节点构建好的数据
		return BuildTree.buildList(list, "-1");
	}

}

2.5 编写数据Acntion控制类

对传入节点进行Json解析，并输出到页面

package demo.web;

import java.util.List;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.ycxw.dao.PermissionDao;
import com.ycxw.entity.Permission;
import com.zking.framework.ActionSupport;
import com.zking.framework.ModelDriver;
import com.zking.util.ResponseUtil;
import com.zking.util.TreeVo;
/**
 * 
 * @author 云村小威
 *
 * @2023年7月12日 上午11:31:20
 */
public class PermissionAction extends ActionSupport implements ModelDriver<Permission> {
	private Permission permission = new Permission();
	private PermissionDao dao = new PermissionDao();

	public void menus(HttpServletRequest req, HttpServletResponse resp) {
		try {
			List<TreeVo<Permission>> menus = dao.menus(null, null);
			//调用封装好的解析json类
			ResponseUtil.writeJson(resp, menus);
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

	@Override
	public Permission getModel() {
		return permission;
	}

}

将数据解析成json方法

public static void writeJson(HttpServletResponse response, Object o) throws Exception {
		ObjectMapper om = new ObjectMapper();
		// om.writeValueAsString(o)代表了json串
		write(response, om.writeValueAsString(o));
	}

3. 前台准备

3.1 配置mvc.xml文件

<action path="/permission" type="demo.web.PermissionAction"></action>

3.2 页面编写

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>主界面</title>
<link rel="stylesheet"
	href="${pageContext.request.contextPath}/static/js/layui/css/layui.css">
<script
	src="${pageContext.request.contextPath}/static/js/layui/layui.js"></script>
</head>
<body>
<div class="layui-layout layui-layout-admin">
  <div class="layui-header">
    <div class="layui-logo layui-hide-xs layui-bg-black">layout demo</div>
    <!-- 头部区域（可配合layui 已有的水平导航） -->
    <ul class="layui-nav layui-layout-left">
      <!-- 移动端显示 -->
      <li class="layui-nav-item layui-show-xs-inline-block layui-hide-sm" lay-header-event="menuLeft">
        <i class="layui-icon layui-icon-spread-left"></i>
      </li>
      <!-- Top导航栏 -->
      <li class="layui-nav-item layui-hide-xs"><a href="">nav 1</a></li>
      <li class="layui-nav-item layui-hide-xs"><a href="">nav 2</a></li>
      <li class="layui-nav-item layui-hide-xs"><a href="">nav 3</a></li>
      <li class="layui-nav-item">
        <a href="javascript:;">nav groups</a>
        <dl class="layui-nav-child">
          <dd><a href="">menu 11</a></dd>
          <dd><a href="">menu 22</a></dd>
          <dd><a href="">menu 33</a></dd>
        </dl>
      </li>
    </ul>
    <!-- 个人头像及账号操作 -->
    <ul class="layui-nav layui-layout-right">
      <li class="layui-nav-item layui-hide layui-show-md-inline-block">
        <a href="javascript:;">
          <img src="//tva1.sinaimg.cn/crop.0.0.118.118.180/5db11ff4gw1e77d3nqrv8j203b03cweg.jpg" class="layui-nav-img">
          tester
        </a>
        <dl class="layui-nav-child">
          <dd><a href="">Your Profile</a></dd>
          <dd><a href="">Settings</a></dd>
          <dd><a href="login.jsp">Sign out</a></dd>
        </dl>
      </li>
      <li class="layui-nav-item" lay-header-event="menuRight" lay-unselect>
        <a href="javascript:;">
          <i class="layui-icon layui-icon-more-vertical"></i>
        </a>
      </li>
    </ul>
  </div>
  
  <div class="layui-side layui-bg-black">
    <div class="layui-side-scroll">
      <!-- 左侧导航区域（可配合layui已有的垂直导航） -->
      <ul id="menu" class="layui-nav layui-nav-tree" lay-filter="menu">
       <li class="layui-nav-item layui-nav-itemed">
          <a class="" href="javascript:;">menu group 1</a>
          <dl class="layui-nav-child">
            <dd><a href="javascript:;">menu 1</a></dd>
            <dd><a href="javascript:;">menu 2</a></dd>
            <dd><a href="javascript:;">menu 3</a></dd>
            <dd><a href="">the links</a></dd>
          </dl>
        </li>
        <li class="layui-nav-item">
          <a href="javascript:;">menu group 2</a>
          <dl class="layui-nav-child">
            <dd><a href="javascript:;">list 1</a></dd>
            <dd><a href="javascript:;">list 2</a></dd>
            <dd><a href="">超链接</a></dd>
          </dl>
        </li>
        <li class="layui-nav-item"><a href="javascript:;">click menu item</a></li>
        <li class="layui-nav-item"><a href="">the links</a></li>
      </ul>
    </div>
  </div>
  
  <div class="layui-body">
    <!-- 内容主体区域 -->
    <div style="padding: 15px;">内容主体区域。记得修改 layui.css 和 js 的路径</div>
  </div>
  
  <div class="layui-footer">
    <!-- 底部固定区域 -->
            底部固定区域
  </div>
</div>
<script>
layui.use(['element', 'layer', 'util'], function(){
  var element = layui.element
  ,layer = layui.layer
  ,util = layui.util
  ,$ = layui.$;
  
   $.post("${pageContext.request.contextPath}/permission.action?methodName=menus",
	function(data){
	//将json串转换成数组对象
	var per = $.parseJSON(data);
	//拼接左侧导航栏
	var htmlStr = "";
	$.each(per,function(i,n){
		htmlStr+="<li class=\"layui-nav-item layui-nav-itemed\">";
		htmlStr+="<a class='' href=\"javascript:;\">"+n.text+"</a>";
		//判断如果子级是否有值
		if(n.hasChildren){
			var children = n.children;
			//遍历子级内容
			htmlStr+="<dl class=\"layui-nav-child\">";
			$.each(children,function(i,n){
				htmlStr+="<dd><a href=\"javascript:;\">"+n.text+"</a></dd>";
			})
			htmlStr+="</dl>";
		}
		htmlStr+="</li>";
	})
	//替换原来标签内容
	$("#menu").html(htmlStr);
	//因为要修改之前元素的内容，防止数据出不来需要进行渲染结构传入目标容器的选择器和数据。
	element.render("menu");
}) 
  
});
</script>
</body>
</html>

注意：

防止数据出不来需要进行渲染结构传入目标容器的选择器和数据。