平常我们工作中基本最多两级嵌套，但是有时候难免会遇到三级嵌套的业务场景，笔者最近就碰到了，使用一般的嵌套发现赋值为空，这可难倒了菜逼的我，后来在stackoverflow的帮助下终于搜到了解决办法，完美解决了问题 ，总结一下，方便有需要的同学，下面直接上栗子：

首先上实体类：三级嵌套如下 （电站 -----> 电桩 ---->电枪）

电站实体类 (实体为JPA写法，不影响mybatis的使用)

package com.weima.cecapp.entities;

import lombok.Data;
import lombok.NoArgsConstructor;
import org.hibernate.annotations.BatchSize;
import org.hibernate.annotations.GenericGenerator;

import javax.persistence.*;
import java.util.Collections;
import java.util.List;
import java.util.Set;
@NoArgsConstructor
@Data
@Entity
@Table(name = "station_info")
public class StationInfo {
  /**
   * Auto-generated primary key.
   */
  @Id
  @GeneratedValue(generator = "uuid")
  @GenericGenerator(name = "uuid", strategy = "org.hibernate.id.UUIDGenerator")
  @Column(unique = true, nullable = false, updatable = false)
  private String id;

  @Column(name = "station_id")
  private String stationId;

  @Column(name = "operator_id")
  private String operatorId;

  @Column(name = "equipment_owner_id")
  private String equipmentOwnerId;

  @Column
  private String stationName;

  @Column
  private String countryCode;

  @Column
  private String areaCode;

  @Column
  private String address;

  @Column
  private String stationTel;

  @Column
  private String serviceTel;

  @Column
  private Integer stationType;

  @Column
  private Integer stationStatus;

  @Column
  private Integer parkNums;

  @Column
  private Double stationLng;

  @Column
  private Double stationLat;

  @Column
  private String siteGuide;

  @Column
  private Integer construction;

  @OneToMany(fetch = FetchType.LAZY, cascade = {CascadeType.ALL}, orphanRemoval = true, mappedBy = "ownerStationInfo")
  private List pictures;

  @Column
  private String matchCars;

  @Column
  private String parkInfo;

  @Column
  private String busineHours;

  @Column(name = "busine_hours_in_milliseconds")
  private Long busineHoursInMilliseconds;

  @Column
  private String electricityFee;

  @Column
  private String serviceFee;

  @Column
  private String parkFee;

  @Column
  private String payment;

  @Column
  private Integer supportOrder;

  @Column
  private String remark;

  @OneToMany(fetch = FetchType.LAZY, cascade = {CascadeType.ALL}, orphanRemoval = true, mappedBy = "ownerStationInfo")
  @BatchSize(size = 20)
  private List equipmentInfos;

}

电站图片实体

package com.weima.cecapp.entities;

import lombok.Data;
import lombok.EqualsAndHashCode;
import lombok.ToString;
import org.hibernate.annotations.GenericGenerator;

import javax.persistence.*;

@Data
@Entity
@EqualsAndHashCode(of = {"url"})
@ToString(exclude = {"ownerStationInfo"})
@Table(name = "station_picture")
public class StationPicture {
  /**
   * Auto-generated primary key.
   */
  @Id
  @GeneratedValue(generator = "uuid")
  @GenericGenerator(name = "uuid", strategy = "org.hibernate.id.UUIDGenerator")
  @Column(unique = true, nullable = false, insertable = true, updatable = false)
  private String id;

  @ManyToOne(fetch = FetchType.EAGER, optional = false)
  @JoinColumn(nullable = false, updatable = false)
  private StationInfo ownerStationInfo;

  @Column
  private String url;
}

电桩实体类

package com.weima.cecapp.entities;

import lombok.Data;
import lombok.EqualsAndHashCode;
import lombok.ToString;
import org.hibernate.annotations.BatchSize;
import org.hibernate.annotations.GenericGenerator;

import javax.persistence.*;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Collections;
import java.util.List;

@Data
@Entity
@EqualsAndHashCode(of = {"equipmentId"})
@ToString(exclude = {"ownerStationInfo"})
@Table(name = "equipment_info")
public class EquipmentInfo {
  /**
   * Auto-generated primary key.
   */
  @Id
  @GeneratedValue(generator = "uuid")
  @GenericGenerator(name = "uuid", strategy = "org.hibernate.id.UUIDGenerator")
  @Column(unique = true, nullable = false, updatable = false)
  private String id;

  @Column(name = "equipment_id")
  private String equipmentId;

  @ManyToOne(fetch = FetchType.EAGER, optional = false)
  @JoinColumn(nullable = false, updatable = false)
  private StationInfo ownerStationInfo;

  @Column(name = "manufacturer_id")
  private String manufacturerId;

  @Column
  private String manufacturerName;

  @Column
  private String equipmentModel;

  @Column
  private String productionDate;

  public String getProductionDate() {
    String format = null;
    if (this.productionDate != null) {
      SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
      try {
        format = sdf.format(sdf.parse(this.productionDate));
      } catch (ParseException e) {
        e.printStackTrace();
      }
      return format;
    }
    return format;
  }

  @Column
  private String equipmentType;

  @OneToMany(fetch = FetchType.LAZY, orphanRemoval = true, mappedBy = "ownerEquipmentInfo", cascade = {CascadeType.PERSIST})
  @BatchSize(size = 20)
  private List connectorInfos;

  @Column
  private Double equipmentLng;

  @Column
  private Double equipmentLat;

  @Column
  private Double power;

  @Column
  private String equipmentName;

  @Column(name = "equipment_no")
  //cpo's custom equipmentId mostly for Evstation
  private String equipmentNo;
}

电枪实体类

package com.weima.cecapp.entities;

import lombok.Data;
import lombok.EqualsAndHashCode;
import lombok.ToString;
import org.hibernate.annotations.GenericGenerator;

import javax.persistence.*;

@Data
@Entity
@EqualsAndHashCode(of = {"connectorId"})
@ToString(exclude = {"ownerEquipmentInfo"})
@Table(name = "connector_info")
public class ConnectorInfo {
  /**
   * Auto-generated primary key.
   */
  @Id
  @GeneratedValue(generator = "uuid")
  @GenericGenerator(name = "uuid", strategy = "org.hibernate.id.UUIDGenerator")
  @Column(unique = true, nullable = false, updatable = false)
  private String id;

  @ManyToOne(fetch = FetchType.EAGER, optional = false)
  @JoinColumn(nullable = false, updatable = false)
  private EquipmentInfo ownerEquipmentInfo;

  @Column(name = "connector_id")
  private String connectorId;

  @Column
  private String connectorName;

  @Column
  private Integer connectorType;

  @Column
  private Integer voltageUpperLimits;

  @Column
  private Integer voltageLowerLimits;

  @Column
  private Integer current;

  @Column
  private Double power;

  @Column
  private String parkNo;

  @Column
  private Integer nationalStandard;

  @Column(name = "connector_no")
  //cpo's custom connectorId mostly for Evstation
  private String connectorNo;
}

mapper 文件的resultMap映射及Sql语句的书写，要特别注意映射关系

<?xml version="1.0" encoding="UTF-8"?>



  
    
    
    
    
    
    
    
    
    
    
    
    
    
      
    
      
      
      
      
      
      
      
      
      
      
      
    
  

  
  
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
  


-- 要特别注意的是 t_f_ 的前缀，这个关乎到映射不映射上的根本。

    select
       a.id                   as    id,
       a.station_id               as    station_id,
       a.operator_id              as    operator_id,
       a.equipment_owner_id           as    equipment_owner_id,
       a.station_name              as    station_name,
       a.country_code              as    country_code,
       a.area_code               as    area_code,
       a.address                as    address,
       a.station_tel              as    station_tel,
       a.service_tel              as    service_tel,
       a.station_type              as    station_type,
       a.station_status             as    station_status,
       a.park_nums               as    park_nums,
       a.station_lng              as    station_lng,
       a.station_lat              as    station_lat,
       a.site_guide               as    site_guide,
       a. construction             as    construction,
       a.match_cars               as    match_cars,
       a.park_info               as    park_info,
       a.busine_hours              as    busine_hours,
       a.busine_hours_in_milliseconds      as    busine_hours_in_milliseconds,
       a. electricity_fee            as    electricity_fee,
       a. service_fee              as    service_fee,
       a. park_fee               as    park_fee,
       a. payment                as    payment,
       a. support_order             as    support_order,
       a.remark                 as    remark
       ,
       e.id                   as    t_id               ,
       e.equipment_id              as    t_equipment_id          ,
       e.equipment_lat             as    t_equipment_lat         ,
       e.equipment_lng             as    t_equipment_lng         ,
       e.equipment_model            as    t_equipment_model        ,
       e.equipment_name             as    t_equipment_name         ,
       e.equipment_type             as    t_equipment_type         ,
       e.manufacturer_id            as    t_manufacturer_id        ,
       e.manufacturer_name           as    t_manufacturer_name       ,
       e.power                 as    t_power             ,
       e.production_date            as    t_production_date        ,
       e.owner_station_info_id         as    t_owner_station_info_id     ,
       e.equipment_no              as    t_equipment_no
       ,

       c.id                   as    t_f_id             ,
       c.power                 as    t_f_power           ,
       c.current                as    t_f_current          ,
       c.park_no                as    t_f_park_no          ,
       c.connector_id              as    t_f_connector_id        ,
       c.connector_name             as    t_f_connector_name       ,
       c.connector_type             as    t_f_connector_type       ,
       c.connector_no              as    t_f_connector_no        ,
       c.national_standard           as    t_f_national_standard     ,
       c.voltage_lower_limits          as    t_f_voltage_lower_limits    ,
       c.voltage_upper_limits          as    t_f_voltage_upper_limits    ,
       c.owner_equipment_info_id        as    t_f_owner_equipment_info_id

       from station_info a join equipment_info e on a.id = e.owner_station_info_id

         join connector_info c on e.id = c.owner_equipment_info_id

         where a.operator_id='MA59J8YL8'