DROP TABLE IF EXISTS `t_category`;
CREATE TABLE `t_category` (
  `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '標(biāo)識目錄的唯一id',
  `parent_id` int(11) DEFAULT NULL COMMENT '存儲父級目錄的id，0代表一級目錄',
  `name` varchar(255) COLLATE utf8_bin DEFAULT NULL COMMENT '目錄的名稱',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;

2.java后端代碼

① 對應(yīng)數(shù)據(jù)庫的實體類

public class Category {
    private int id;
    private int parentId;
    private String name;
}

② 返回給前端的實體類 //注意寫一個有參的構(gòu)造方法，方便我們把Category的數(shù)據(jù)注入進來

public class CategoryTwo extends Category{
    private List<CategoryTwo> categorieTwos;
 
    public CategoryTwo(){}
 
    //這個有參構(gòu)造方法方便我們?yōu)閷傩再x值即把categoy的成員變量的值賦值到自身中
    public CategoryTwo(Category category){
        super(category.getId(),category.getParentId(),category.getName());
    }
}

③ 具體實現(xiàn)

//多級目錄
    public void findCategoryAll(){
 
        //查詢出所有的目錄數(shù)據(jù)
        List<Category> categorys = categoryMapper.findCategoryAll(null);//這個大家自己完成
 
 
        //存放所有一級目錄
        List<CategoryTwo> categoryTows = new ArrayList();
 
        //找出一級目錄，即 parentId = 0 的目錄
        for(Category category :categorys){
            if(category.getParentId() == 0){
                CategoryTwo categoryTwo = new CategoryTwo(category);
                categoryTows.add(categoryTwo);//存入一級目錄集合中
            }
        }
 
        //利用遞歸，找出所有一級目錄的List<CategoryTwo>,這里我把具體實現(xiàn)包裝到 findSubCategory（）方法中了
        categoryTows = findSubCategory(categorys,categoryTows);
 
        //最后就完成了，把數(shù)據(jù)按需求返回給前端就可以了。
        System.out.println(categoryTows);
    }
 
    //找出List<CategoryTwo>中所有目錄的子級目錄
    public List<CategoryTwo> findSubCategory(List<Category> categorys,List<CategoryTwo> categoryTwos){
 
        for (CategoryTwo categoryTwo:categoryTwos) {
            //找出自身的子級目錄，存放到List<CategoryTwo>中去
            List<CategoryTwo> categoryTwos2 = new ArrayList<CategoryTwo>();
 
            for (Category category:categorys){
                //如果自身的id = 與某個目錄的parentId 相等，則這個目錄是自身的子級目錄
                if(categoryTwo.getId() == category.getParentId()){
                    categoryTwos2.add(new CategoryTwo(category));
                }
            }
 
            //所有子級目錄找到后，再回調(diào)findSubCategory()方法 讓子級目錄找到自己的子級
            List<CategoryTwo> categoryTwos3 = findSubCategory(categorys,categoryTwos2);
 
            //categoryTwos3真正的List<categoryTwo>，把他添加到自身
            categoryTwo.setCategorieTwos(categoryTwos3);
        }
 
        return categoryTwos;//返回注入好的對象
    }

核心就是那個回調(diào)的方法，只要把這個搞清楚了，那就很容易了

附另一個實現(xiàn)方法

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
 
 
public class MenuTest {
 
    /**
     * @param args
     */
    public static void main(String[] args) {
        MenuTest menuTest = new MenuTest();
        List menuList = initMenu();//初始化菜單
        Map menuMap = menuTest.rebuildMenus(menuList);//重構(gòu)菜單
        menuTest.showMenu(menuMap);//打印菜單
 
    }
     
    /**
     * 重構(gòu)菜單，構(gòu)造成需要的結(jié)構(gòu)
     * @param menuList
     * @return
     */
    private Map rebuildMenus(List menuList) {
        List rootMenuList=new ArrayList();//這個List用來存放根菜單
        for (int j = 0; j < menuList.size(); j++) {
            Map menu = (Map) menuList.get(j);
            String parentId = menu.get("parentId").toString();
            if (null==parentId || "".equals(parentId)) {
                rootMenuList.add(menu);
            }
        }
        menuList.removeAll(rootMenuList);//將根菜單從menuList中除去
        HashMap menuMap = new HashMap();
        for (int j = 0; j < rootMenuList.size(); j++) {//遍歷根菜單，每一個跟菜單都遍歷到最后一個子菜單（葉子節(jié)點）
            Map menu = (Map) rootMenuList.get(j);
            menu = this.pase(menu, menuList);
            menuMap.put(menu.get("id"), menu);
        }
        return menuMap;
    }
 
    /**
     * 遞歸構(gòu)造菜單Map
     * @param menu
     * @param menuList
     * @return
     */
    private Map pase(Map menu, List menuList) {
        for (int j = 0; j < menuList.size(); j++) {
            Map menu2 = (Map) menuList.get(j);
            if (menu2.get("parentId").toString().equals(menu.get("id").toString())) {
                if(menu.get("children") == null){
                    menu.put("children", new ArrayList());
                }
                ((List)menu.get("children")).add(menu2);
                pase(menu2, menuList);
            }
        }
        return menu;
    }
    /**
     * 打印菜單
     * @param menuMap
     */
    private void showMenu(Map menuMap){
        Iterator it = menuMap.keySet().iterator();
        while(it.hasNext()){
            Map menu = (Map) menuMap.get(it.next());
            System.out.println(menu.get("id")+"--"+menu.get("name"));
            this.buildChildMenu(menu);
        }
    }
    /**
     * 構(gòu)造子菜單
     * @param menu
     */
    private void buildChildMenu(Map menu){
        List childList = (List) menu.get("children");
        if(null!=childList){
            for(int i=0;i<childList.size();i++){
                Map childMap = (Map) childList.get(i);
                System.out.println(childMap.get("id")+"--"+childMap.get("name"));
                this.buildChildMenu(childMap);
            }
        }
    }
    /**
     * 初始化菜單，實際項目中這部分?jǐn)?shù)據(jù)是從數(shù)據(jù)庫中獲取
     * @return
     */
    public static List initMenu(){
        List menuList = new ArrayList();
         
        Map menuMap = new HashMap();
        menuMap.put("id", "0000");
        menuMap.put("name", "根菜單");
        menuMap.put("parentId","");//根菜單的父菜單為空
         
        Map menuMap1 = new HashMap();
        menuMap1.put("id", "1000");
        menuMap1.put("name", "一級菜單A");
        menuMap1.put("parentId","0000");
         
        Map menuMap2 = new HashMap();
        menuMap2.put("id", "2000");
        menuMap2.put("name", "一級菜單B");
        menuMap2.put("parentId","0000");
         
        Map menuMap3 = new HashMap();
        menuMap3.put("id", "1100");
        menuMap3.put("name", "二級菜單A");
        menuMap3.put("parentId","1000");
         
        Map menuMap4 = new HashMap();
        menuMap4.put("id", "1110");
        menuMap4.put("name", "三級菜單A");
        menuMap4.put("parentId","1100");
         
        menuList.add(menuMap);
        menuList.add(menuMap1);
        menuList.add(menuMap2);
        menuList.add(menuMap3);
        menuList.add(menuMap4);
         
        return menuList;
    }
     
     
 
}