Java怎样构造树形结构数据并查询？

因为项目需要，页面上需要树形结构的数据进行展示（类似下图这样），因此需要后端返回相应格式的数据。 不说废话，直接开干！！！

我这里用的是springboot+mybatis-plus+mysql，示例的接口是查询一级权限以及二级权限、三级权限整个权限树...

下面是导入的maven依赖

 		<dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
<!--数据库连接-->
        <dependency>
            <groupId>com.alibaba</groupId>
            <artifactId>druid-spring-boot-starter</artifactId>
            <version>1.1.21</version>
        </dependency>
 
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
        </dependency>
        <!--mybatis增强工具-->
        <dependency>
            <groupId>com.baomidou</groupId>
            <artifactId>mybatis-plus-boot-starter</artifactId>
            <version>3.4.0</version>
        </dependency>
  		<dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>cn.hutool</groupId>
            <artifactId>hutool-all</artifactId>
            <version>5.0.6</version>
        </dependency>

下面是实体类Permission

@Data
public class Permission implements Serializable {
    @TableId
    private String permissionId;
    @NotNull(message = "权限名称不能为空")
    private String permissionName;
    /**
     * 权限标识
     */
    @NotNull(message = "权限标识不能为空")
    private String permissionCode;
    /**
     * 父菜单ID，如果是-1就表示是一级权限菜单。
     */
    @NotBlank(message = "父菜单ID不能为空")
    private String parentId;

    /**
     * 前端URL访问接口路径
     */
    private String path;

    /**
     * 排序值
     */
    private Integer sort;
    /**
     * 创建时间
     */

    private LocalDateTime createTime;

    /**
     * 更新时间
     */

    private LocalDateTime updateTime;

    /**
     * 0--正常 1--删除
     */
    private String delFlag;

    public Permission() {
        this.permissionId = IdUtil.simpleUUID();
    }

树形结点类

@Data
public class TreeNode {
    protected String id;
    protected String parentId;
    protected List<TreeNode> children = new ArrayList<TreeNode>();
    protected boolean hasChildren;

   public void addTreeNode(TreeNode node){
       children.add(node);
   }
}

树形结点详细信息类

@Data
@EqualsAndHashCode(callSuper = true)
public class PermissionTree extends TreeNode implements Serializable {
    private String permissionName;
    private String permissionCode;
    private String path;
    private Integer sort;
    private String label;
    private boolean hasChildren;
    public PermissionTree() {
    }
}

构建树形结点工具类（关键），在这里我用@UtilityClass注解就表示这个类中的方法都是静态方法：

@UtilityClass
public class TreeUtil {


    public <T extends TreeNode> List<T> build(List<T> treeNodes, String root) {
        List<T> trees = new ArrayList<>();
        for (T treeNode : treeNodes) {

            if (root.equals(treeNode.getParentId())) {
                trees.add(treeNode);
            }

            for (T node : treeNodes) {
                if (node.getParentId().equals(treeNode.getId())) {
                    treeNode.addTreeNode(node);
                    treeNode.setHasChildren(true);

                }
            }
        }
        return trees;
    }


    /**
     * 通过permission创建树形节点
     *
     * @param permissionList
     * @param root
     * @return
     */
    public List<PermissionTree> buildTree(List<Permission> permissionList, String root) {
        System.out.println(Arrays.toString(permissionList.toArray()));
        List<PermissionTree> treeNodeList = new ArrayList<>();
        PermissionTree treeNode = null;
        for (Permission permission : permissionList) {
            treeNode = new PermissionTree();
            treeNode.setId(permission.getPermissionId());
            treeNode.setPermissionName(permission.getPermissionName());
            treeNode.setPath(permission.getPath());
            treeNode.setSort(permission.getSort());
            treeNode.setParentId(permission.getParentId());
            treeNode.setLabel(permission.getPermissionName());
            treeNode.setHasChildren(false);
            treeNodeList.add(treeNode);
        }
        return TreeUtil.build(treeNodeList, root);
    }
  }

响应消息主体类

/**
 * 响应信息主体
 *
 * @param <T>
 */
@ToString
@NoArgsConstructor
@AllArgsConstructor
@Accessors(chain = true)
public class R<T> implements Serializable {

	private static final long serialVersionUID = 1L;

	private int code;

	private String msg;

	private T data;

	public int getCode() {
		return code;
	}

	public void setCode(int code) {
		this.code = code;
	}

	public String getMsg() {
		return msg;
	}

	public void setMsg(String msg) {
		this.msg = msg;
	}

	public T getData() {
		return data;
	}

	public void setData(T data) {
		this.data = data;
	}

	public static <T> R<T> ok() {
		return restResult(null, CommonConstants.SUCCESS, CommonConstants.MSG_SUCCESS);
	}

	public static <T> R<T> ok(T data) {
		return restResult(data, CommonConstants.SUCCESS, CommonConstants.MSG_SUCCESS);
	}

	public static <T> R<T> ok(T data, String msg) {
		return restResult(data, CommonConstants.SUCCESS, msg);
	}

	public static <T> R<T> failed() {
		return restResult(null, CommonConstants.FAIL, null);
	}

	public static <T> R<T> failed(String msg) {
		return restResult(null, CommonConstants.FAIL, msg);
	}

	public static <T> R<T> failed(T data) {
		return restResult(data, CommonConstants.FAIL, null);
	}

	public static <T> R<T> failed(T data, String msg) {
		return restResult(data, CommonConstants.FAIL, msg);
	}

	private static <T> R<T> restResult(T data, int code, String msg) {
		R<T> apiResult = new R<>();
		apiResult.setCode(code);
		apiResult.setData(data);
		apiResult.setMsg(msg);
		return apiResult;
	}

}

数据查询接口mapper类

@Mapper
public interface PermissionMapper extends BaseMapper<Permission>{
    
}

数据逻辑处理业务接口

public interface PermissionService extends IService<Permission> {
  

    /**
     * 构建权限树
     *
     * @param lazy
     * @param parentId
     * @return
     */
    List<PermissionTree> treePermission(boolean lazy, String parentId);
}

数据逻辑处理业务接口实现类

@Service
public class PermissionServiceImpl extends ServiceImpl<PermissionMapper, Permission> implements PermissionService {
/**
     * 构建权限树：1、不是懒加载情况，查询全部
     * 2、是懒加载，根据parentId查询
     *
     * @param lazy
     * @param parentId
     * @return
     */

    @Override
    public List<PermissionTree> treePermission(boolean lazy, String parentId) {

        if (!lazy) {
            return TreeUtil.buildTree(
                    baseMapper.selectList(Wrappers.<Permission>lambdaQuery().orderByAsc(Permission::getSort)),
                    CommonConstants.PERMISSION_ROOT_ID);
        }
        String parent = parentId == null ? CommonConstants.PERMISSION_ROOT_ID : parentId;
        return TreeUtil.buildTree(
                baseMapper.selectList(Wrappers.<Permission>lambdaQuery().eq(Permission::getParentId, parent).orderByAsc(Permission::getSort)), parent
        );
    }
}

查询权限树请求接口类

@RestController
@RequestMapping("/permission")
public class PermissionController {
    @Autowire
    private PermissionService permissionService;

    /**
     * 查询权限列表，并以树状结构显示
     *
     * @param lazy 懒加载: false时, parentId这个参数失效, 加载所有的权限; true时, 根据parentId加载
     * @param parentId
     * @return
     */
    @RequestMapping(value = "/getTree", method = RequestMethod.GET)
    public R getTree(boolean lazy, String parentId) {
        return R.ok(permissionService.treePermission(lazy, parentId));
    }
 }

表中测试数据如下（注意它的parent_id）

测试一：不是懒加载，查询整个权限树。 结果如下。

{
    "code": 0,
    "msg": "SUCCESS",
    "data": [
        {
            "id": "1",
            "parentId": "-1",
            "children": [
                {
                    "id": "2",
                    "parentId": "1",
                    "children": [
                        {
                            "id": "3",
                            "parentId": "2",
                            "children": [],
                            "hasChildren": false,
                            "permissionName": "update",
                            "permissionCode": null,
                            "path": null,
                            "sort": 3,
                            "label": "update",
                            "owned": false
                        },
                        {
                            "id": "4",
                            "parentId": "2",
                            "children": [],
                            "hasChildren": false,
                            "permissionName": "insert_role",
                            "permissionCode": null,
                            "path": null,
                            "sort": 4,
                            "label": "insert_role",
                            "owned": false
                        }
                    ],
                    "hasChildren": true,
                    "permissionName": "delete",
                    "permissionCode": null,
                    "path": null,
                    "sort": 2,
                    "label": "delete",
                    "owned": false
                }
            ],
            "hasChildren": true,
            "permissionName": "add",
            "permissionCode": null,
            "path": null,
            "sort": 1,
            "label": "add",
            "owned": false
        },
        {
            "id": "5",
            "parentId": "-1",
            "children": [],
            "hasChildren": false,
            "permissionName": "role:saveRole",
            "permissionCode": null,
            "path": "/role/saveRole",
            "sort": 5,
            "label": "role:saveRole",
            "owned": false
        }
    ]
}

测试二：是懒加载，根据parent_id查询当前分支。 结果如下。

{
    "code": 0,
    "msg": "SUCCESS",
    "data": [
        {
            "id": "3",
            "parentId": "2",
            "children": [],
            "hasChildren": false,
            "permissionName": "update",
            "permissionCode": null,
            "path": null,
            "sort": 3,
            "label": "update",
            "owned": false
        },
        {
            "id": "4",
            "parentId": "2",
            "children": [],
            "hasChildren": false,
            "permissionName": "insert_role",
            "permissionCode": null,
            "path": null,
            "sort": 4,
            "label": "insert_role",
            "owned": false
        }
    ]
}