Hutool是一个小而全的Java工具类库，通过静态方法封装，降低相关API的学习成本，提高工作效率，使Java拥有函数式语言般的优雅

引入Hutool依赖

<dependency>
    <groupId>cn.hutool</groupId>
    <artifactId>hutool-all</artifactId>
    <version>5.8.0.M2</version>
</dependency>

第一种方式

/**
 * @author songzixian
 */
public class TestHutoolTree {
    public static void main(String[] args) {

        // 构建node列表(数据源)
        List<TreeNode<String>> nodeList = CollUtil.newArrayList();
        nodeList.add(new TreeNode<>("1", "0", "系统管理", 5));
        nodeList.add(new TreeNode<>("11", "1", "用户管理", 222222));
        nodeList.add(new TreeNode<>("111", "11", "用户添加", 0));
        nodeList.add(new TreeNode<>("2", "0", "店铺管理", 1));
        nodeList.add(new TreeNode<>("21", "2", "商品管理", 44));
        nodeList.add(new TreeNode<>("221", "2", "商品管理2", 2));
        // 0表示最顶层的id是0
        List<Tree<String>> treeList = TreeUtil.build(nodeList, "0");
        System.out.println(JSONUtil.toJsonStr(treeList));

    }
}

输出三级Json

第二种方式

准备测试表和数据

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;
 
-- ----------------------------
-- Table structure for system_classification
-- ----------------------------
DROP TABLE IF EXISTS `system_classification`;
CREATE TABLE `system_classification`  (
  `id` bigint(20) NOT NULL AUTO_INCREMENT COMMENT '分类id',
  `name` varchar(30) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT '' COMMENT '分类名称',
  `parent_id` bigint(20) NULL DEFAULT 0 COMMENT '父分类id',
  `sort` int(11) NOT NULL DEFAULT 0 COMMENT '显示顺序',
  `level` int(11) NULL DEFAULT NULL COMMENT '层级',
  `status` tinyint(4) NOT NULL DEFAULT 0 COMMENT '分类状态（0正常 1停用）',
  `deth` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NULL DEFAULT NULL COMMENT '纵深',
  `creator` varchar(64) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NULL DEFAULT '' COMMENT '创建者',
  `create_time` datetime(0) NOT NULL DEFAULT CURRENT_TIMESTAMP(0) COMMENT '创建时间',
  `updater` varchar(64) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NULL DEFAULT '' COMMENT '更新者',
  `update_time` datetime(0) NOT NULL DEFAULT CURRENT_TIMESTAMP(0) ON UPDATE CURRENT_TIMESTAMP(0) COMMENT '更新时间',
  `deleted` bit(1) NOT NULL DEFAULT b'0' COMMENT '是否删除',
  `tenant_id` bigint(20) NOT NULL DEFAULT 0 COMMENT '租户编号',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 8 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_unicode_ci COMMENT = '部门表' ROW_FORMAT = Dynamic;
 
-- ----------------------------
-- Records of system_classification
-- ----------------------------
INSERT INTO `system_classification` VALUES (1, '根节点', 0, 0, 1, 0, '1', 'admin', '2021-01-05 17:03:47', '1', '2022-03-22 10:18:56', b'0', 1);
INSERT INTO `system_classification` VALUES (2, '王二', 1, 1024, 2, 0, '1', '1', '2022-03-18 11:35:43', '1', '2022-03-22 10:19:00', b'0', 1);
INSERT INTO `system_classification` VALUES (3, '王1二', 1, 1024, 2, 0, '1', '1', '2022-03-18 11:38:05', '1', '2022-03-22 10:19:02', b'0', 1);
INSERT INTO `system_classification` VALUES (4, '一级目', 0, 1, 1, 0, NULL, '1', '2022-03-22 10:09:25', '1', '2022-03-22 10:09:25', b'0', 1);
INSERT INTO `system_classification` VALUES (5, '一级目2', 0, 1, 1, 0, NULL, '1', '2022-03-22 10:09:32', '1', '2022-03-22 10:09:32', b'0', 1);
INSERT INTO `system_classification` VALUES (6, '二级目2', 5, 1, 2, 0, '5', '1', '2022-03-22 10:09:40', '1', '2022-03-22 10:14:17', b'0', 1);
INSERT INTO `system_classification` VALUES (7, 'san级目2', 6, 1, 3, 0, '5,6', '1', '2022-03-22 10:19:28', '1', '2022-03-22 10:19:28', b'0', 1);
 
SET FOREIGN_KEY_CHECKS = 1;

创建测试数据对应实体类

public class ClassificationDO {
 
    /**
     * 分级ID
     */
    @TableId
    private Long id;
    /**
     * 分级名称
     */
    private String name;
    /**
     * 父分级ID
     *
     * 关联 {@link #id}
     */
    private Long parentId;
    /**
     * 显示顺序
     */
    private Integer sort;
 
    /**
     * 层级
     */
    private Integer level;
    /**
     * 纵深
     *
     */
    private String deth;
 
    /**
     * 分级状态
     *
     * 枚举 {@link CommonStatusEnum}
     */
    private Integer status;
    /**
     * 多租户编号
     */
    private Long tenantId;
 
}

四、实现细节
根据pid与id的逻辑关系构建tree;

​public static void main(String[] args) {
    //配置
    TreeNodeConfig treeNodeConfig = new TreeNodeConfig();
    // 自定义属性名 都要默认值的
    treeNodeConfig.setWeightKey("order");
    treeNodeConfig.setIdKey("rid");
    // 最大递归深度
    treeNodeConfig.setDeep(3);
    //数据源
    List<Student> nodeList = list();
    //转换器
    List<Tree<String>> treeNodes = TreeUtil.build(nodeList, "0", treeNodeConfig,
        (treeNode, tree) -> {
            tree.setId(treeNode.getId());
            tree.setParentId(treeNode.getParentId());
            tree.setWeight(treeNode.getWeight());
            tree.setName(treeNode.getName());
            // 扩展属性 ...
            tree.putExtra("extraField", 666);
            tree.putExtra("other", new Object());
    System.out.println(JSON.toJSONString(treeNodes));
}

输出3级树形Json格式