从 Hibernate 反序列化 JSON

Question

我在反序列化 Spring Boot 中的类时遇到问题。当我的控制器尝试反序列化它时，它会崩溃。这是课程：

@Entity
@Table(name="trash_cans")
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property="id")
public class TrashCan {
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    @Column(name="TRASH_CAN_ID")
    long id;

    @Column(name="TRASH_CAN_NAME")
    String name;

    @ManyToOne
    @JoinColumn(name="PLACE_ID")
    private Place place;

    @OneToMany(mappedBy="trashCan", targetEntity=TrashMeter.class, fetch=FetchType.LAZY)
    private List<TrashCan> trashMeterList;

    @OneToMany(mappedBy="trashCan", targetEntity=TrashSensor.class, fetch=FetchType.LAZY)
    private List<TrashSensor> trashSensorList;

    public TrashCan() {     
    }

    public TrashCan(long id, String name) {
        super();
        this.id = id;
        this.name = name;
    }

    [getters and setters]
}

这取决于这个：

@Entity
@Table(name="trash_sensor")
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property="id")
public class TrashSensor {

    @Id
    @Column(name="id")
    @GeneratedValue(strategy=GenerationType.AUTO)
    private long id;

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

    @Column(name="description")
    private String description;

    @ManyToOne
    @JoinColumn(name="TRASH_CAN_ID")
    private TrashCan trashCan;

    @OneToMany(mappedBy = "trashSensor", targetEntity = Measurement.class, fetch = FetchType.LAZY)
    private List<Measurement> measurementList;

    public TrashSensor() {
        super();
    }

而垃圾传感器依赖于这个类：

@Entity
@Table(name="measurement")
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property="id")
public class Measurement {

    @Id
    @Column(name="id")
    @GeneratedValue(strategy=GenerationType.AUTO)
    private long id;

    @Column(name="value")
    private float value;

    @Column(name="last_measure")
    private LocalDateTime dateTime;

    @ManyToOne
    @JoinColumn(name="trash_sensor_id")
    private TrashCan trashSensor;

    public Measurement() {
    }

}

我的控制者：

@RequestMapping(value="/trashCan", method=RequestMethod.GET)
    public ResponseEntity<Iterable<TrashCan>> getPlaces(){
        Iterable<TrashCan> trashCanIterable = trashCansRepository.findAll();

        return new ResponseEntity<>(trashCanIterable, HttpStatus.OK);
    }

当我调用 web 服务时，我得到这个错误：

无法写入HTTP消息：org.springframework.http.converter.HttpMessageNotWritableException：无法写入内容：无法反序列化（通过引用链：java.util.ArrayList[0]-br.com.simplepass.cleanerway.domain .TrashCan["trashSensorList"]-org.hibernate.collection.internal.PersistentBag[0]-br.com.simplepass.cleanerway.domain.TrashSensor["measurementList"]);嵌套异常是 com.fasterxml.jackson.databind.JsonMappingException: 无法反序列化（通过引用链：java.util.ArrayList[0]-br.com.simplepass.cleanerway.domain.TrashCan["trashSensorList"]-org.hibernate .collection.internal.PersistentBag[0]-br.com.simplepass.cleanerway.domain.TrashSensor["measurementList"])

我无法解释这个错误=/。非常感谢您对此问题的任何帮助。

Answer 1

由于您的 json 正在进入循环，因此您收到此错误，为避免这种情况，请使用 @JsonIgnore 注释：

@OneToMany(mappedBy = "trashSensor", targetEntity = Measurement.class, fetch = FetchType.LAZY)
@JsonIgnore
private List<Measurement> measurementList;

Answer 2

当您使用实体之间的关系时会发生这种情况。想象一下，您的 TrashCan 中有指向 Trash 的链接。你的垃圾有链接到它的包装 - 垃圾桶。因此，您尝试序列化 TrashCan 实体的同时也序列化了 Trash。然后当你序列化垃圾时，垃圾桶会在里面再次序列化。等等。这是一个循环。您可以在每个可能导致循环的实体上使用@JsonIgnore。

@JsonIgnore
@ManyToOne
@JoinColumn(name="PLACE_ID")
private Place place;

@JsonIgnore
@OneToMany(mappedBy="trashCan", targetEntity=TrashMeter.class, fetch=FetchType.LAZY)
private List<TrashCan> trashMeterList;

@JsonIgnore
@OneToMany(mappedBy="trashCan", targetEntity=TrashSensor.class, fetch=FetchType.LAZY)
private List<TrashSensor> trashSensorList;

但这是一个糟糕的方法。强烈建议您使用 DTO（数据传输对象）模式进行序列化/反序列化。它还为您提供了更大的灵活性。你可以阅读它here

Answer 3

如果您需要 trashMeterList 和 trashSensorList 作为响应，请遵循此答案。

由于休眠延迟加载和反序列化时没有会话，您会收到此异常。

要修复，只需更改您的控制器：

    @RequestMapping(value="/trashCan", method=RequestMethod.GET)
    public ResponseEntity<Iterable<TrashCan>> getPlaces(){
        Iterable<TrashCan> trashCanIterable = trashCansRepository.findAll();
        List<TrashCan> responseList = new ArrayList<TrashCan>(trashCanIterable.size())
        while(trashCanIterable.hasNext()){
            TrashCan trashCan = trashCanIterable.next();
            for(TrashMeter trashMeter : trashCan.trashMeterList){

            }
            for(TrashSensor trashSensor : trashCan.trashSensorList){

            }
            responseList.add(trashCan);
        }
        return new ResponseEntity<>(responseList, HttpStatus.OK);
    }