java转树形结构工具类详解

import com.alibaba.fastjson.JSON;
import lombok.Data;
import lombok.ToString;
import org.springframework.util.ReflectionUtils;
import org.springframework.util.StringUtils;

import java.lang.reflect.Field;
import java.util.*;

/**
 * @author : liyk
 * @version 1.0
 * @date : 2020/6/9 
 */
public class TreeUtil {


  /**
   * 将 List 转为树形结构
   *
   * @param origList     : 要转换的 List
   * @param idFieldName    : id字段名
   * @param parentIdFieldName : parentId 字段名
   * @param childrenFieldName : children 字段名
   * @param <T>        : 拥有父子结构的 Entity
   * @return : 树形结果
   * @throws Exception .
   */
  public static <T> List<T> convert(List<T> origList, String idFieldName,
                   String parentIdFieldName, String childrenFieldName) throws Exception {
    // 用于保存当前 id 索引的实体类
    Map<String, T> idMaps = new HashMap<>();
    // 暂存区, 用于保存没有找到父 id 的控件
    List<T> tempList = new ArrayList<>();
    List<T> result = new ArrayList<>();
    for (T entity : origList) {
      // 获取 id, parentId, children
      String id = Objects.toString(getFieldValue(entity, idFieldName), "");
      String parentId = Objects.toString(getFieldValue(entity, parentIdFieldName), "");
      if (StringUtils.isEmpty(id)) {
        throw new Exception("存在id为空的资料");
      }
      idMaps.put(id, entity);
      if (StringUtils.isEmpty(parentId)) {
        // 如果父 id 为空, 则实体类为第一层
        result.add(entity);
      } else {
        // 根据父 id 获取实体类
        T parentEntity = idMaps.get(parentId);
        if (parentEntity == null) {
          // 没找到先放入暂存区
          tempList.add(entity);
        } else {
          // 父组件判断是否存在 children, 不存在新增, 存在则直接假如
          setChildrenValue(childrenFieldName, entity, parentEntity);
        }
      }
    }
    // 处理暂存区, 暂存区的一定不为根节点, 所以它只要父节点存在, 那么此轮查询一定能找到父节点(上一轮已经将全部节点放入 idMaps)
    for (T entity : tempList) {
      // 获取 parentId
      String parentId = Objects.toString(getFieldValue(entity, parentIdFieldName), "");
      // 根据父id获取实体类
      T parentEntity = idMaps.get(parentId);
      if (parentEntity == null) {
        throw new Exception("存在孤立的子节点");
      } else {
        // 父组件判断是否存在children, 不存在新增, 存在则直接假如
        setChildrenValue(childrenFieldName, entity, parentEntity);
      }
    }
    return result;
  }

  private static <T> void setChildrenValue(String childrenFieldName, T entity, T parentEntity) throws Exception {
    Object children = getFieldValue(parentEntity, childrenFieldName);
    List<T> childrenList;
    if (children == null) {
      childrenList = new ArrayList<>();
      childrenList.add(entity);
      setFieldValue(parentEntity, childrenFieldName, childrenList);
    } else {
      List<T> childrenReal = (List<T>) children;
      childrenReal.add(entity);
    }
  }

  private static <T> Object getFieldValue(T entity, String fieldName) throws Exception {
    Field field = ReflectionUtils.findField(entity.getClass(), fieldName);
    if (field == null) {
      throw new Exception(String.format("字段名称[%s]不存在", fieldName));
    }
    boolean accessible = field.isAccessible();
    field.setAccessible(true);
    Object result = ReflectionUtils.getField(field, entity);
    field.setAccessible(accessible);
    return result;
  }

  private static <T> void setFieldValue(T entity, String fieldName, Object value) throws Exception {
    Field field = ReflectionUtils.findField(entity.getClass(), fieldName);
    if (field == null) {
      throw new Exception(String.format("字段名称[%s]不存在", fieldName));
    }
    boolean accessible = field.isAccessible();
    field.setAccessible(true);
    ReflectionUtils.setField(field, entity, value);
    field.setAccessible(accessible);
  }

  public static void main(String[] args) throws Exception {
    List<Demo> list = new ArrayList<>();
    for (int i = 0; i < 5; i++) {
      Demo demo = new Demo(i, "一级节点" + i);
      list.add(demo);
    }
    for (int i = 5; i < 15; i++) {
      Demo demo = new Demo(i, i % 5, "二级节点" + i);
      list.add(demo);
    }
    for (int i = 15; i < 100; i++) {
      Demo demo = new Demo(i, i % 10 + 5, "三级节点" + i);
      list.add(demo);
    }
    Demo demo = new Demo(100, 102, "非法节点");
    list.add(demo);
    List<Demo> convert = TreeUtil.convert(list, "id", "pid", "children");
    String s = JSON.toJSONString(convert);
    System.out.println(s);
  }

}

@Data
@ToString
class Demo {
  private Integer id;
  private Integer pid;
  private String name;
  private List<Demo> children;

  public Demo(Integer id, Integer pid, String name) {
    this.id = id;
    this.pid = pid;
    this.name = name;
  }

  public Demo(Integer id, String name) {
    this.id = id;
    this.name = name;
  }
}