Spring data JPA- Hibernate多对多关系在链接实体表中插入null

@Entity
@Table(name = "entity1")
public class Entity1 {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    // other fields and getters/setters
}

@Entity
@Table(name = "entity2")
public class Entity2 {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    // other fields and getters/setters
}

@Embeddable
public class EntityLinkKey implements Serializable {
    @Column(name = "entity1_id")
    private Long entity1Id;

    @Column(name = "entity2_id")
    private Long entity2Id;

    // getters/setters and equals/hashCode methods
}

@Entity
@Table(name = "entity_link")
public class EntityLink {
    @EmbeddedId
    private EntityLinkKey id;

    // other fields and getters/setters
}

@Repository
public interface EntityLinkRepository extends JpaRepository<EntityLink, EntityLinkKey> {
}

@Service
public class EntityLinkService {
    @Autowired
    private EntityLinkRepository entityLinkRepository;

    public void createLink(Long entity1Id, Long entity2Id) {
        EntityLinkKey linkKey = new EntityLinkKey();
        linkKey.setEntity1Id(entity1Id);
        linkKey.setEntity2Id(entity2Id);

        EntityLink entityLink = new EntityLink();
        entityLink.setId(linkKey);

        entityLinkRepository.save(entityLink);
    }
}