一、前言

    在后台代码的开发中，经常碰到有父子关联的表设计，前端要求在后台数据以树形封装进行返回，因此如何建树，或者如何更优雅的建树成了后台开发工作中绕不过去的一个坎，根据工作所需，这里做下总结，以堪后由。

（推荐方式二）

二、方法

1.方式一：独立对象建树

1.1 封装独立对象TreeNode

package cn.keyidea.common.bean;

import io.swagger.v3.oas.annotations.media.Schema;
import lombok.Data;

import java.util.List;

/**
 * 树节点实体
 */
@Data
@Schema(name = "TreeNode", description = "树节点实体")
public class TreeNode {

    @Schema(description = "节点id", requiredMode = Schema.RequiredMode.REQUIRED)
    private Integer id;

    @Schema(description = "节点标题", requiredMode = Schema.RequiredMode.REQUIRED)
    private String title;

    @Schema(description = "子节点", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
    private List<TreeNode> children;

    @Schema(description = "父节点id", requiredMode = Schema.RequiredMode.REQUIRED)
    private Integer parentId;

    /**
     * 节点是否初始为选中状态（如果开启复选框的话），默认 false
     */
    @Schema(description = "节点是否初始为选中状态（如果开启复选框的话），默认 false", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
    private Boolean checked;

    @Schema(description = "URL地址", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
    private String url;

    @Schema(description = "类型", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
    private Integer type;

    @Schema(description = "code码", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
    private String code;

    @Schema(description = "图标", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
    private String icon;

    @Schema(description = "路由地址", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
    private String component;
}

1.2 建树工具类

package cn.keyidea.common.util;

import cn.keyidea.common.bean.TreeNode;

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

/**
 * Tree工具类
 */
public class TreeUtil {
    /**
     * 两层循环实现建树
     *
     * @param treeNodes 传入的树节点列表
     * @return
     */
    public static List<TreeNode> bulidTree(List<TreeNode> treeNodes) {

        List<TreeNode> trees = new ArrayList<TreeNode>();

        for (TreeNode treeNode : treeNodes) {

            if (0 == treeNode.getParentId()) {
                trees.add(treeNode);
            }

            for (TreeNode it : treeNodes) {
                if (null != it.getParentId()) {
                    if (it.getParentId().equals(treeNode.getId())) {
                        if (treeNode.getChildren() == null) {
                            treeNode.setChildren(new ArrayList<TreeNode>());
                        }
                        treeNode.getChildren().add(it);
                    }
                }
            }
        }
        return trees;
    }
}

2.方式二：原生对象建树（推荐）

2.1 以Entity实体作为自身节点

由于菜单一般设计成带有父子项，因此这里以菜单对象做示例。

package cn.keyidea.sys.entity;

import cn.keyidea.common.annotation.EnumValue;
import cn.keyidea.common.bean.BaseModel;
import cn.keyidea.common.valid.GroupAdd;
import cn.keyidea.common.valid.GroupUpdate;
import com.baomidou.mybatisplus.annotation.FieldFill;
import com.baomidou.mybatisplus.annotation.TableField;
import com.baomidou.mybatisplus.annotation.TableLogic;
import com.baomidou.mybatisplus.annotation.TableName;
import com.fasterxml.jackson.annotation.JsonIgnore;
import com.fasterxml.jackson.annotation.JsonProperty;
import io.swagger.v3.oas.annotations.media.Schema;
import jakarta.persistence.Column;
import jakarta.persistence.Entity;
import jakarta.persistence.Table;
import jakarta.persistence.Transient;
import jakarta.validation.constraints.NotBlank;
import jakarta.validation.constraints.NotNull;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.EqualsAndHashCode;
import lombok.NoArgsConstructor;
import lombok.experimental.SuperBuilder;
import org.hibernate.annotations.Comment;

import java.util.List;
import java.util.Objects;
import java.util.stream.Collectors;

/**
 * 系统菜单
 *
 * @author qyd
 * @date 2022-10-17
 */
@SuperBuilder
@AllArgsConstructor
@NoArgsConstructor
@Data
@EqualsAndHashCode(callSuper = true)
@TableName("sys_menu")
@Schema(name = "SysMenu", description = "系统菜单")
@Entity
@Table(name = "sys_menu")
@Comment("系统菜单表")
public class SysMenu extends BaseModel {

    @NotNull(message = "父菜单不能为NULL", groups = {GroupAdd.class, GroupUpdate.class})
    @Schema(description = "父菜单ID，一级菜单为0", requiredMode = Schema.RequiredMode.REQUIRED, example = "0")
    @Column(name = "parent_id", columnDefinition = "int(10) not null COMMENT '父菜单ID，一级菜单为0'")
    private Integer parentId;

    // name ==> label
    @NotBlank(message = "菜单名称不能为空", groups = {GroupAdd.class, GroupUpdate.class})
    @Schema(description = "菜单名称", requiredMode = Schema.RequiredMode.REQUIRED)
    @Column(name = "label", columnDefinition = "varchar(100) not null COMMENT '菜单名称'")
    private String label;

    @NotNull(message = "菜单类型不能为NULL", groups = {GroupAdd.class, GroupUpdate.class})
    @EnumValue(intValues = {0, 1, 2, 3}, message = "菜单类型不符合条件")
    @Schema(description = "菜单类型：0-系统，1-目录，2-菜单，3-按钮", requiredMode = Schema.RequiredMode.REQUIRED)
    @Column(name = "type", columnDefinition = "int(2) not null COMMENT '菜单类型：0-系统，1-目录，2-菜单，3-按钮'")
    private Integer type;

    @Schema(description = "菜单图标url", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
    @Column(name = "icon", columnDefinition = "varchar(100) COMMENT '菜单图标url'")
    private String icon;

    // url ==> key
    // 数据库存储url，返回给前端使用key（key为MySQL关键字无法作为存储字段）
    @JsonProperty("key")
    @Schema(description = "菜单URL", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
    @Column(name = "url", columnDefinition = "varchar(100) COMMENT '菜单URL'")
    private String url;

    @Schema(description = "菜单路由地址", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
    @Column(name = "component", columnDefinition = "varchar(100) COMMENT '菜单路由地址'")
    private String component;

    @Schema(description = "权限标识，如：user:list", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
    @Column(name = "perms", columnDefinition = "varchar(100) COMMENT '权限标识，如：user:list'")
    private String perms;

    @NotNull(message = "排序号不能为NULL", groups = {GroupAdd.class, GroupUpdate.class})
    @Schema(description = "排序号", requiredMode = Schema.RequiredMode.REQUIRED)
    @Column(name = "order_num", columnDefinition = "int(10) not null COMMENT '排序号'")
    private Integer orderNum;

    @TableLogic
    @JsonIgnore
    @Column(name = "del_flag", columnDefinition = "int(2) not null COMMENT '删除状态：0-正常，1-删除'")
    @TableField(value = "del_flag", fill = FieldFill.INSERT)
    private Integer delFlag;

    // ---------------------- 以下非数据库字段 ----------------------

    @Schema(description = "子孩子", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
    @TableField(exist = false)
    @Transient
    private List<SysMenu> children;

    /**
     * 建树
     *
     * @param list     待建树集合
     * @param parentId 父节点，一般为0作为父节点
     * @return 返回树
     */
    public static List<SysMenu> streamToTree(List<SysMenu> list, Integer parentId) {
        return list.stream()
                .filter(parent -> Objects.equals(parent.getParentId(), parentId))
                .peek(child -> child.setChildren(streamToTree(list, child.getId())))
                .collect(Collectors.toList());
    }
}

2.2 涉及BaseModel类

package cn.keyidea.common.bean;

import cn.keyidea.common.valid.GroupChangePwd;
import cn.keyidea.common.valid.GroupCustomWithId;
import cn.keyidea.common.valid.GroupUpdate;
import com.alibaba.fastjson.annotation.JSONField;
import com.baomidou.mybatisplus.annotation.FieldFill;
import com.baomidou.mybatisplus.annotation.TableField;
import com.fasterxml.jackson.annotation.JsonFormat;
import io.swagger.v3.oas.annotations.media.Schema;
import jakarta.persistence.*;
import jakarta.validation.constraints.NotNull;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.experimental.SuperBuilder;
import org.springframework.data.jpa.domain.support.AuditingEntityListener;

import java.io.Serial;
import java.io.Serializable;
import java.util.Date;


/**
 * 实体父类
 * <p>
 * 特殊说明：
 * 继承此基类的实体类，如果使用lombok的@Data注解时，主要同时添加@EqualsAndHashCode(callSuper = true)注解
 * 1)注解@EqualsAndHashCode(callSuper = true)，就是用自己的属性和从父类继承的属性来生成hashcode；
 * 2)注解@EqualsAndHashCode(callSuper = false)，就是只用自己的属性来生成hashcode；
 * 3)@Data相当于@Getter @Setter @RequiredArgsConstructor @ToString @EqualsAndHashCode这5个注解的合集, 和@EqualsAndHashCode默认是false。
 * <p>
 * MappedSuperclass注解说明：
 * 1.@MappedSuperclass注解仅仅能标准在类上；这个注解标识在父类上面的，用来标识父类
 * 2.标注为@MappedSuperclass的类将不是一个完整的实体类，他将不会映射到数据库表，可是他的属性都将映射到其子类的数据库字段中
 * 3.标注为@MappedSuperclass的类不能再标注@Entity或@Table注解
 * <p>
 * EntityListeners注解及相关说明：
 * EntityListeners：该注解用于指定Entity或者superclass上的回调监听类
 * AuditingEntityListener：这是一个JPA Entity Listener，用于捕获监听信息，当Entity发生持久化和更新操作时
 *
 * @author qyd
 * @date 2022-10-17
 */
@SuperBuilder   // @SuperBuilder支持对基类成员属性的构造；如果子类继承了BaseModel，也需要使用该注解
@AllArgsConstructor // 全参构造函数
@NoArgsConstructor  // 空参构造函数
@Data
@MappedSuperclass
@EntityListeners(AuditingEntityListener.class)
@Schema(name = "BaseModel", description = "实体父类")
public class BaseModel implements Serializable {

    @Serial
    private static final long serialVersionUID = 1L;

    @NotNull(message = "ID不能为NUll", groups = {GroupUpdate.class, GroupChangePwd.class, GroupCustomWithId.class})
    @Schema(description = "主键ID", requiredMode = Schema.RequiredMode.REQUIRED, example = "1")
    @Id
    @Column(name = "id", columnDefinition = "int(10) COMMENT '自增长ID'")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;

    @Schema(description = "创建时间", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
    @JsonFormat(pattern = "yyyy-MM-dd HH:mm:ss", timezone = "GMT+8")
    @Column(name = "create_time", updatable = false, columnDefinition = "datetime not null COMMENT '创建时间'")
    @TableField(value = "create_time", fill = FieldFill.INSERT)
    @JSONField(format = "yyyy-MM-dd HH:mm:ss")
    private Date createTime;

    @Schema(description = "创建人ID", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
    @Column(name = "create_by", updatable = false, columnDefinition = "int(10) not null COMMENT '创建人ID'")
    @TableField(value = "create_by", fill = FieldFill.INSERT)
    private Integer createBy;

    @Schema(description = "更新时间", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
    @JsonFormat(pattern = "yyyy-MM-dd HH:mm:ss", timezone = "GMT+8")
    @Column(name = "update_time", columnDefinition = "datetime COMMENT '更新时间'")
    @TableField(value = "update_time", fill = FieldFill.UPDATE)
    @JSONField(format = "yyyy-MM-dd HH:mm:ss")
    private Date updateTime;

    @Schema(description = "更新人ID", requiredMode = Schema.RequiredMode.NOT_REQUIRED)
    @Column(name = "update_by", columnDefinition = "int(10) COMMENT '更新人ID'")
    @TableField(value = "update_by", fill = FieldFill.UPDATE)
    private Integer updateBy;

}

3.方式一 vs 方式二

对于方式二来说，新增的字段主要就是在原生对象中新增private List<T> children;， 然后新增静态方法streamToTree，并且能完美支持原生所有字段，所差的是，如果有10个类都需要建树的话，那么每个类都需要写同样的方法。不过对于方式一来说，如果真存在10个类，并且每个类字段都存在差异性，那么方式一中的TreeNode的冗余字段将变的非常之多。

END