Spring 验证：要求可选字段的精确长度

Question

在 Spring MVC 中，我尝试使用注释来执行表单输入的验证。我有一些可选字段，但提供输入时，它必须满足某些条件。例如，我有一个字段可以留空，但是当填写它时，答案必须恰好是五个字符长。

我尝试过类似的事情

@Size(min=5, max=5)
private String value;

但如果用户不填写该字段，则会给用户带来错误。

如何为可选字段提供一些“规则”？

Answer 1

1. 第一个选项是将值设置为空。

如果你看一下 Hibernate 使用的 SizeValidatorForCharSequence：

/**
 * Check that the length of a character sequence is between min and max.
 *
 * @author Emmanuel Bernard
 * @author Gavin King
 * @author Hardy Ferentschik
 */
public class SizeValidatorForCharSequence implements ConstraintValidator<Size, CharSequence> {

    private static final Log log = LoggerFactory.make();

    private int min;
    private int max;

    public void initialize(Size parameters) {
        min = parameters.min();
        max = parameters.max();
        validateParameters();
    }

    /**
     * Checks the length of the specified character sequence (e.g. string).
     *
     * @param charSequence The character sequence to validate.
     * @param constraintValidatorContext context in which the constraint is evaluated.
     *
     * @return Returns <code>true</code> if the string is <code>null</code> or the length of <code>charSequence</code> between the specified
     *         <code>min</code> and <code>max</code> values (inclusive), <code>false</code> otherwise.
     */
    public boolean isValid(CharSequence charSequence, ConstraintValidatorContext constraintValidatorContext) {
        if ( charSequence == null ) {
            return true;
        }
        int length = charSequence.length();
        return length >= min && length <= max;
    }

    private void validateParameters() {
        if ( min < 0 ) {
            throw log.getMinCannotBeNegativeException();
        }
        if ( max < 0 ) {
            throw log.getMaxCannotBeNegativeException();
        }
        if ( max < min ) {
            throw log.getLengthCannotBeNegativeException();
        }
    }
}

仅当大小的长度与最小、最大约束不匹配时，大小才无效，并且如果值为 null，则没有问题。

2. 另一种选择是创建您自己的注释和验证器：

   @Target({ METHOD, FIELD, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER })
   @Retention(RUNTIME)
   @Documented
   @Constraint(validatedBy = { StringEmptyOrSizeValidator.class })
   public @interface StringEmptyOrSize {
   
       String message() default "{javax.validation.constraints.Size.message}";
   
       Class<?>[] groups() default { };
   
       Class<? extends Payload>[] payload() default { };
   
       /**
        * @return size the element must be higher or equal to
        */
       int min() default 0;
   
       /**
        * @return size the element must be lower or equal to
        */
       int max() default Integer.MAX_VALUE;
   
       /**
        * Defines several {@link Size} annotations on the same element.
        *
        * @see Size
        */
       @Target({ METHOD, FIELD, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER })
       @Retention(RUNTIME)
       @Documented
       @interface List {
   
           Size[] value();
       }
   }
   

带验证器

public class StringEmptyOrSizeValidator implements ConstraintValidator<Size, CharSequence> {

    ....
    public boolean isValid(CharSequence charSequence, ConstraintValidatorContext constraintValidatorContext) {
        if ( charSequence == null || charSequence.isEmpty()) {
            return true;
        }
        int length = charSequence.length();
        return length >= min && length <= max;
    }

    ....
}