我无法将输入的日期值从窗体传递到控制器。所有值都被正确地传递和存储在数据库中，但日期为空。创建一个javadate对象并手动设置它是可行的，所以我认为问题出在前端格式化上。
以下是带有thymeleaf语法的html代码：

<form th:action="@{/register}" method="post" th:object="${user}" id="userInfo">
    ...
        <h4>Enter Payment Information</h4>
                <div th:object="${card}">
                    <div class="col-12">
                        <label for="cardType" id="cardType"><strong>Card
                            type:</strong></label> <select class="form-select" th:field="*{cardType}">
                        <option th:value="None">Select one:</option>
                        <option th:value="Visa" th:text="Visa"></option>
                        <option th:value="Mastercard" th:text="Mastercard"></option>
                        <option th:value="Discover" th:text="Discover"></option>
                        <option th:value="AmericanExpress" th:text="AmericanExpress"></option>
                    </select>
                    </div>
                    <div class="col-12">
                        <label for="card-number" id="cardNumberLabel"><strong>Card
                            number:</strong></label> <input type="text" th:field="*{cardNumber}" class="form-control"
                                                            id="card-number" name="card-number"
                                                            pattern="^\d{16}$"
                                                            title="Card number must be 16 digits long."
                                                            value="">
                    </div>
                    <div class="col-12 mb-2">
                        <label for="exp" id="expLabel"><strong>Expiration
                            date:</strong></label> <input type="date" th:value="*{expirationDate}" class="form-control" id="exp"
                                                          name="exp"
                                                          value="">
                    </div>
                </div>
    ...
    </form>

以下是控制器（为了简洁起见，我省略了一些细节，但保留了相关部分）：

@GetMapping(value={"/register", "register.html"})
public String displayForm(Model model) {
    // process form data and add user to db
    User user = new User();
    PaymentCard card = new PaymentCard();
    model.addAttribute("user", user);
    model.addAttribute("card", card);
    return "register";
}

@PostMapping(value={"/register", "register.html"})
public String createAccount(@ModelAttribute("user") User user, @ModelAttribute("card") PaymentCard card) {
    User tempUser = new User();
    PaymentCard tempCard = new PaymentCard();
    tempCard.setUser(tempUser);
    tempCard.setPaymentCardId(card.getPaymentCardId());
    tempCard.setCardType(card.getCardType());
    tempCard.setCardNumber(card.getCardNumber());
    //tempCard.setExpirationDate(card.getExpirationDate()); <--- this returns null so I get a NPE
    Date tempDate = new Date(2020, 11, 12); <--- dummy date that works and is saved to db correctly
    tempCard.setExpirationDate(tempDate);
    cardRepo.save(tempCard);

    return "confirmregistration";
}

支付卡pojo的开始：

@Entity
@Table(name = "payment_card", catalog = "bookstore")
public class PaymentCard implements java.io.Serializable {

@Id
@Column(name = "PaymentCardID", unique = true, nullable = false)
@GeneratedValue(strategy=GenerationType.IDENTITY)
private int paymentCardId;

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "UserID")
private User user;

@Column(name = "cardNumber", nullable = false)
private String cardNumber; // int and long aren't big enough to hold 16 digits

@Column(name = "cardType", nullable = false, length = 20)
private String cardType;

@Temporal(TemporalType.DATE)
@Column(name = "expirationDate", nullable = false, length = 10)
private Date expirationDate;
...
}