用stream流查询三级分类，以树形结构返回

首先在实体类中添加一个可以递归存储的列表

@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]
	 */
	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;

	@TableField(exist = false)
	private List<CategoryEntity> children;

}

然后在service中编写方法

public List<CategoryEntity> listWithTree() {
        List<CategoryEntity> entities = baseMapper.selectList(null);

        List<CategoryEntity> level1Menus = entities.stream()
                .filter((categoryEntity) -> {
                    return categoryEntity.getParentCid() == 0;
                })
                .map(menu -> {
                    menu.setChildren(getChildren(menu,entities));
                    return menu;
                })
                .sorted((m1, m2) -> {
                    return (m1.getSort()==null?0:m1.getSort()) - (m2.getSort()==null?0:m2.getSort());
                })
                .collect(Collectors.toList());

        return level1Menus;
    }

    private List<CategoryEntity> getChildren(CategoryEntity root,List<CategoryEntity> all){
        List<CategoryEntity> children = all.stream()
                .filter(categoryEntity -> {
                    return root.getCatId() == categoryEntity.getParentCid();
                })
                .peek(categoryEntity -> categoryEntity.setChildren(getChildren(categoryEntity, all)))
                .sorted((m1, m2) -> {
                    return (m1.getSort()==null?0:m1.getSort()) - (m2.getSort()==null?0:m2.getSort());
                })
                .collect(Collectors.toList());

        return children;
    };