为什么验证器中的@Id UUID在持久运行时不为空？

Question

我有一个实体

@Getter
@Setter
@EqualsAndHashCode(callSuper = false)
@MappedSuperclass
@ValidUuidPrimaryEntity
public class UuidPrimaryEntity implements Serializable {
    private static final long serialVersionUID = 6602362437085355133L;

    @Id
    @Type(type = "pg-uuid")
    @Column(name = "gid")
    @GeneratedValue(generator = "hibernate-uuid")
    @GenericGenerator(name = "hibernate-uuid", strategy = "uuid2")
    private UUID id;
}

当我的实体通过JPA存储库来验证时

public class UuidPrimaryEntityValidator implements ConstraintValidator<ValidUuidPrimaryEntity, UuidPrimaryEntity > {

    @Override
    public void initialize(ValidUuidPrimaryEntity constraintAnnotation) {

    }

    @Override
    public boolean isValid(UuidPrimaryEntity value, ConstraintValidatorContext context) {
        return true;
    }
}

id字段（value.getId（））不为null。所以我无法检查实体是否是新的

ext.hibernateVersion = '5.0.2.Final'
ext.hibernateValidatorVersion = '5.2.1.Final'