Hibernate异常行为：集合返回null而非空列表问题排查

数据库结构示意：MonitorUser关联多个MonitoredUser；MonitoredUser关联Take集合（takeList）和IsMonitoredBy集合（isMonitoredByList），同时IsMonitoredBy关联MonitoredUser和Device。

执行findMonitorUserById(userId)方法时，Hibernate应返回指定ID的MonitorUser及其关联的MonitoredUsers列表。按Hibernate默认逻辑，集合属性即便没有数据也应是空列表而非null，但调试发现MonitoredUser的takeList符合预期（空列表），但isMonitoredByList却为null。请问这是代码错误还是配置问题？

调试时观察到的monitorUser对象状态：monitoredUserList中的每个MonitoredUser实例，takeList为[]，isMonitoredByList为null。

Controller.java #

@RestController
public class Controller {

@Autowired
IUserService userService;

    @PostMapping(value = "/addMonitoredUser", consumes = MediaType.APPLICATION_JSON_VALUE, produces = MediaType.APPLICATION_JSON_VALUE)
    @Transactional(rollbackOn = SaveException.class)
    public ResponseEntity<MonitorUserDTO> addMonitoredUser(@RequestBody MonitoredUserDTO monitoredUserDTO) throws DrugNotFoundException, SaveException, UserNotFoundException {

        MonitorUser monitorUser = userService.findMonitorUserById(monitoredUserDTO.getMonitorUser().getId());
        MonitorUserDTO monitorUserDTO = setMonitorDTO(monitorUser);

        return ResponseEntity
                .ok()
                .body(monitorUserDTO);
    }

}

UserService.java #

@Service
public class UserService implements IUserService {
    @Autowired
    UserRepository userRepository;

    @Override
    @Transactional
    public MonitorUser findMonitorUserById(int idUser) throws UserNotFoundException {
        MonitorUser res = userRepository.findMonitorUserById(idUser);

        if(res == null)
            throw new UserNotFoundException("UserService findMonitorUserById(" +idUser +"): not found");
        return res;
    }
}

UserRepository.java #

@Repository
public interface UserRepository extends JpaRepository<User, Integer> {

    MonitorUser findMonitorUserById(int idMonitor);

}

实体类代码 #

MonitorUser.java #

@Entity
@DiscriminatorValue("monitor")
public class MonitorUser extends User{
    public MonitorUser() {
    }

    String username;
    String password;

    @OneToMany(mappedBy = "monitorUser", targetEntity = Monitor.class, cascade = CascadeType.ALL , fetch = FetchType.LAZY )
    List<Monitor> monitorList;

    @OneToMany(mappedBy = "monitorUser", targetEntity = MonitoredUser.class, cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    List<MonitoredUser> monitoredUserList;

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public List<Monitor> getMonitorList() {
        return monitorList;
    }

    public void setMonitorList(List<Monitor> monitorList) {
        this.monitorList = monitorList;
    }

    public List<MonitoredUser> getMonitoredUserList() {
        return monitoredUserList;
    }

    public void setMonitoredUserList(List<MonitoredUser> monitoredUserList) {
        this.monitoredUserList = monitoredUserList;
    }
}

MonitoredUser.java #

@Entity
@DiscriminatorValue("monitored")
public class MonitoredUser extends User{
    public MonitoredUser() {
    }

    float weight;
    int height; //in cm
    @Enumerated(EnumType.STRING)
    Gender sex;
    boolean smoker;
    String phoneNumber;
    int nyhaclass;
    @OneToMany(mappedBy = "monitoredUser", targetEntity = Take.class, cascade = CascadeType.ALL , fetch = FetchType.LAZY )
    List<Take> takeList;
    @OneToMany(mappedBy = "monitoredUser", targetEntity = IsMonitoredBy.class, cascade = CascadeType.ALL , fetch = FetchType.LAZY )
    List<IsMonitoredBy> isMonitoredByList;
    boolean hospitalized;
    @ManyToOne
    MonitorUser monitorUser;

    //and all the getter and setter

}

IsMonitoredBy.java #

@Entity

public class IsMonitoredBy {

    public IsMonitoredBy() {
    }

    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Id
    int id;
    long lastSyncTimeStamp;

    @ManyToOne
    MonitoredUser monitoredUser;
    @ManyToOne
    Device device;

    long timestampStart;
    long timestampEnd;

//all the getter an setter
}

内容的提问来源于stack exchange，提问作者Luca Lotifi