前言

以前在java中构建菜单、区划等都是使用递归或是数据库。 数据库如pg、mysql等原生不支持tree结构，递归不是很好理解，同时代码太长。所以考虑使用java1.8以后支持的lambda操作一波

---

PS:数据库我使用了mybatis-plus，所以查数据的写法简化了

代码

		//查找所有菜单，正序排序
		List<MenuEntity> list = this.list(Wrappers.lambdaQuery(MenuEntity.class)
				                                  .eq(MenuEntity::getHide, 		BoolStateEnum.FALSE.getCode())
				                                  .orderByAsc(MenuEntity::getSort));
		//找寻下级菜单，利用.map来保证引用地址相同
		list.stream()
				.map(ele -> {
					ele.setChildren(this.fromTree(ele, list));
					return ele;
				})
				.collect(Collectors.toList());

		//过滤出最上级的菜单
		return list.stream()
				       .filter(ele -> ele.getParentId() == null)
				       .collect(Collectors.toList());

/**
	 * 找寻指定菜单的下级菜单
	 *
	 * @param menuEntity 当前菜单
	 * @param list       所有的菜单list
	 * @return 下级菜单
	 */
	private List<MenuEntity> fromTree(MenuEntity menuEntity, List<MenuEntity> list) {
		return list.stream()
				       .filter(ele -> menuEntity.getId().equals(ele.getParentId()))
				       .collect(Collectors.toList());
	}

public class MenuEntity {
	@ApiModelProperty(value = "ID")
	@TableId
	private Long id;

	@ApiModelProperty(value = "创建时间")
	private LocalDateTime createTime;

	@ApiModelProperty(value = "创建人ID")
	private String createUserId;

	@ApiModelProperty(value = "修改时间")
	private LocalDateTime updateTime;

	@ApiModelProperty(value = "修改人ID")
	private String updateUserId;
	@ApiModelProperty(value = "节点名")
	private String name;
	@ApiModelProperty(value = "所有上级节点id列表 为空时表示最顶层")
	private Long parentId;

	@ApiModelProperty(value = "菜单类型 0=节点 1=菜单 2=按钮")
	private int menuType;

	@ApiModelProperty(value = "图标")
	private String icon;
	@ApiModelProperty(value = "url")
	private String url;
	@ApiModelProperty(value = "hash路由")
	private String hash;

	@ApiModelProperty(value = "是否隐藏 0=不隐藏 1=隐藏")
	private int hide;
	@ApiModelProperty(value = "排序，从小到大正向排序")
	private int sort;

	@ApiModelProperty(value = "下级菜单集合")
	@TableField(exist = false)
	private List<MenuEntity> children;


}