当我提交表单时，我得到了这个错误-“org.hibernate.propertyvalueexception:notnull属性引用了一个null或暂时值”，请帮助。我在stackoverflow中提到了类似的问题，但我无法解决
//下面的用户类

@Entity
@Table(name = "users")
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(nullable=false, unique = true, length = 45)
    private String email;

    @Column(nullable=false, length = 64)
    private String password;

    @Column(nullable=false, length = 20)
    private String firstName;

    @Column(nullable=false, length = 20)
    private String lastName;

    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }

    public String getPassword() {
        return password;
    }
    public void setPassword(String password) {
        this.password = password;
    }

    public String getFirstName() {
        return firstName;
    }
    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }
    public String getLastName() {
        return lastName;
    }
    public void setLastName(String lastName) {
        this.lastName = lastName;
    }   
}