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。

element.render("menu");

LayUI 实现树形二级导航栏