我正在使用SpringJPA将实体保存到数据库中。

客户.java

@Entity
@Table(name="customer")
@Data
public class Customer {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String name;

    @OneToMany(mappedBy = "customer", cascade = CascadeType.ALL)
    private Set<Order> orders = new HashSet<>();

    public void add(Order order) {
        if(order != null) {
            if(orders == null) {
                this.orders = new HashSet<>();
            }
            this.orders.add(order);
            order.setCustomer(this);
        }
    }
}

订单.java

@Entity
@Table(name="order")
@Data
public class Order {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String orderTrackingNumber;
    private String status;

    @ManyToOne
    @JoinColumn(name = "customer_id")
    private Customer customer;
}

现在，我正在尝试将客户沿着订单保存在数据库中，如下所示

订单服务.java

@Transactional
public PurchaseResponse placeOrder(Purchase purchase) {

    Order order = purchase.getOrder();

    String orderTrackingNumber = UUID.randomUUID().toString();
    order.setOrderTrackingNumber(orderTrackingNumber);

    Customer customer = purchase.getCustomer();
    customer.add(order);

    this.customerRepository.save(customer);

    return new PurchaseResponse(orderTrackingNumber);
}

当this.customerRepository.save(customer);语句执行时，我得到以下异常

java.lang.StackOverflowError: null
at com.ecomhunt.entities.Order.hashCode(Order.java:15) ~[classes/:na]
at java.base/java.util.AbstractSet.hashCode(AbstractSet.java:124) ~[na:na]
at com.ecomhunt.entities.Customer.hashCode(Customer.java:11) ~[classes/:na]
at com.ecomhunt.entities.Order.hashCode(Order.java:15) ~[classes/:na]
at java.base/java.util.AbstractSet.hashCode(AbstractSet.java:124) ~[na:na]
at com.ecomhunt.entities.Customer.hashCode(Customer.java:11) ~[classes/:na]
at com.ecomhunt.entities.Order.hashCode(Order.java:15) ~[classes/:na]
at java.base/java.util.AbstractSet.hashCode(AbstractSet.java:124) ~[na:na]
at com.ecomhunt.entities.Customer.hashCode(Customer.java:11) ~[classes/:na]
......

如果我在Customer#add（order）中注解order.setCustomer(this);行，则错误消失，但在数据库中，customer_id未保存在order表中。
我不知道我做错了什么。