实体类

@Data
@TableName("pms_category")
public class CategoryEntity implements Serializable {
    private static final long serialVersionUID = 1L;
    /**
     * 分类id
     */
    @TableId
    private Long catId;
    /**
     * 分类名称
     */
    private String name;
    /**
     * 父分类id
     */
    private Long parentCid;
    /**
     * 层级
     */
    private Integer catLevel;
    /**
     * 是否显示[0-不显示，1显示]
     */
    @TableLogic(value = "1", delval = "0")
    private Integer showStatus;
    /**
     * 排序
     */
    private Integer sort;
    /**
     * 图标地址
     */
    private String icon;
    /**
     * 计量单位
     */
    private String productUnit;
    /**
     * 商品数量
     */
    private Integer productCount;

    @JsonInclude(JsonInclude.Include.NON_EMPTY)
    @TableField(exist = false)
    private List<CategoryEntity> children;
}

实现

 @Override
    public List<CategoryEntity> listWithTree() {

        //1、查询出所有分类
        List<CategoryEntity> entities = super.baseMapper.selectList(null);

        //2、组装成父子的树形结构

        //2.1)、找到所有一级分类
        List<CategoryEntity> levelMenus = entities.stream()
                .filter(e -> e.getParentCid() == 0)
                .peek((menu) -> menu.setChildren(getChildrens(menu, entities)))
                .sorted((menu, menu2) -> {
                    return (menu.getSort() == null ? 0 : menu.getSort()) - (menu2.getSort() == null ? 0 : menu2.getSort());
                })
                .collect(Collectors.toList());

        return levelMenus;
    }

    //递归查找所有菜单的子菜单
    private List<CategoryEntity> getChildrens(CategoryEntity root, List<CategoryEntity> all) {

        return all.stream().filter(categoryEntity -> {
            return categoryEntity.getParentCid().equals(root.getCatId());
        }).peek(categoryEntity -> {
            //1、找到子菜单(递归)
            categoryEntity.setChildren(getChildrens(categoryEntity, all));
        }).sorted((menu, menu2) -> {
            //2、菜单的排序
            return (menu.getSort() == null ? 0 : menu.getSort()) - (menu2.getSort() == null ? 0 : menu2.getSort());
        }).collect(Collectors.toList());

    }