JPA：多对多关系 - JsonMappingException：无限递归

Question

我在与 JPA 的多对多关系方面遇到问题。 我的代码如下所示：

传感器类：

@Entity
@Table(name = "sensor")
@Data
@NoArgsConstructor
@AllArgsConstructor
public class Sensor {
    @Id
    private long chipId;
    @OneToMany(mappedBy = "sensor")
    @JsonBackReference
    private Set<Link> userLinks;
    private String firmwareVersion;
    private long creationTimestamp;
    private String notes;
    private long lastMeasurementTimestamp;
    private long lastEditTimestamp;
    private double gpsLatitude;
    private double gpsLongitude;
    private double gpsAltitude;
    private String country;
    private String city;
    private boolean indoor;
    private boolean published;
}

用户类：

@Entity
@Table(name = "user")
@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @JsonManagedReference
    private int id;
    private String firstName;
    private String lastName;
    private String email;
    private String password;
    @OneToMany(mappedBy = "user")
    private Set<Link> sensorLinks;
    private int role;
    private int status;
    private long creationTimestamp;
    private long lastEditTimestamp;
}

还有Link类（关系类）：

@Entity
@Table(name = "link")
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Link {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;

    @ManyToOne
    @JoinColumn(name = "user_id")
    @MapsId("user_id")
    private User user;

    @ManyToOne
    @JoinColumn(name = "sensor_id")
    @MapsId("sensor_id")
    private Sensor sensor;

    private boolean owner;
    private String name;
    private int color;
    private long creationTimestamp;
}

控制器：

...

@RequestMapping(method = RequestMethod.GET, path = "/user/{email}", produces = MediaType.APPLICATION_JSON_VALUE)
@ApiOperation(value = "Returns details for one specific user")
public User getUserByEmail(@PathVariable("email") String email) {
    return userRepository.findByEmail(email).orElse(null);
}

...

用户存储库：

public interface UserRepository extends JpaRepository<User, Integer> {

    Optional<User> findByEmail(String email);

    @Modifying
    @Query("UPDATE User u SET u.firstName = ?2, u.lastName = ?3, u.password = ?4, u.role = ?5, u.status = ?6 WHERE u.id = ?1")
    Integer updateUser(int id, String firstName, String lastName, String password, int role, int status);
}

我想实现，用户端点显示该特定用户的所有链接传感器。 我得到的只是一条错误消息：

JSON 映射问题： com.chillibits.particulatematterapi.model.db.main.User["sensorLinks"]; 嵌套异常是 com.fasterxml.jackson.databind.JsonMappingException：无限 递归（StackOverflowError）（通过引用链： com.chillibits.particulatematterapi.model.db.main.User["sensorLinks"])

我该如何解决这个问题？

提前致谢

马克

------------------------------------ 编辑 ---------- --------------------------

根据 Abinash Ghosh 的回答，我添加了以下 DTO：

UserDto:

@Data
@NoArgsConstructor
@AllArgsConstructor
public class UserDto {
    private int id;
    private String firstName;
    private String lastName;
    private Set<LinkDto> sensorLinks;
    private int role;
    private int status;
    private long creationTimestamp;
    private long lastEditTimestamp;
}

链接地址：

@Data
@NoArgsConstructor
@AllArgsConstructor
public class LinkDto {
    private Integer id;
    private SensorDto sensor;
    private boolean owner;
    private String name;
    private int color;
    private long creationTimestamp;
}

还有映射器（我意识到有点不同，但应该是一样的）：

public UserDto getUserByEmail(@PathVariable("email") String email) {
    User user = userRepository.findByEmail(email).orElse(null);
    return convertToDto(user);
}

private UserDto convertToDto(User user) {
    return mapper.map(user, UserDto.class);
}

这会导致以下异常：

2020-04-13 14:22:24.383  WARN 8176 --- [nio-8080-exec-2] o.h.e.loading.internal.LoadContexts      : HHH000100: Fail-safe cleanup (collections) : org.hibernate.engine.loading.internal.CollectionLoadContext@68ab57c7<rs=HikariProxyResultSet@2017009664 wrapping Result set representing update count of -1>

1) Error mapping com.chillibits.particulatematterapi.model.db.main.User to com.chillibits.particulatematterapi.model.io.UserDto
1 error] with root cause
java.lang.StackOverflowError: null
at com.mysql.cj.NativeSession.execSQL(NativeSession.java:1109) ~[mysql-connector-java-8.0.19.jar:8.0.19]
...

Answer 1

它正在工作！ 这篇文章有帮助：https://stackoverflow.com/a/57111004/6296634

在这种情况下，您似乎不应该使用 Lombok @Data。

Answer 2

当User为响应序列化时，User的字段的所有getter方法都会被调用。 所以，User 关系字段sensorLinks 的getter 也被调用来设置值。这是递归发生的。这就是无限递归的原因。

最好不要使用实体作为响应。为User 创建一个DTO 类，然后将User 实体值映射到DTO，然后发送响应。 不要在 DTO 中再次使用任何 Enity 类，否则会导致同样的问题

要将一个模型动态映射到另一个模型，您可以使用ModleMapper

public class UserDTO {
   //Fields you want to show in response & don't use enity class 
   private Set<LinkDTO> sensorLinks;
}
public class LinkDTO{
   //Fields you want to show in response &don't use enity class 
}


public User getUserByEmail(@PathVariable("email") String email) {
    User user = userRepository.findByEmail(email).orElse(null);
    UserDTO userDto = merge(user,UserDTO.class)
    return userDto;
}

public static <T> void merge(T source, T target) {
    ModelMapper modelMapper = new ModelMapper();
    modelMapper.getConfiguration().setMatchingStrategy(MatchingStrategies.STRICT);
    modelMapper.map(source, target);
  }