Formatting (string-) array as part of bean-validation message

Question

I created my first custom validation annotation with validator class as inner class (which i find quite well-arranged). It looks like this:

@Target( { ElementType.TYPE })
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = {OneOfGroupNotNull.Validator.class})
@Documented
public @interface OneOfGroupNotNull {

    // custom annotation properties
    String[] fields();

    // required by JSR-303
    String message() default "One of group must be not null. {fields}";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};

    public class Validator implements ConstraintValidator<OneOfGroupNotNull, Object> {

        private String[] fields;

        @Override
        public boolean isValid(Object bean, ConstraintValidatorContext cvc) {

            int countNotNull = 0;

            for (String field : fields) {
                try {
                    String property = BeanUtils.getProperty(bean, field);
                    if (property != null) {
                        countNotNull++;
                    }
                } catch (Exception ex) {
                    throw new RuntimeException("Validation for field " + field + " of type " + bean.getClass()+ " raised exception.", ex);
                }
            }

            return countNotNull == 1;
        }

        @Override
        public void initialize(OneOfGroupNotNull a) {
            fields = a.fields();
        }
    }
}

A bean class that is annotated with this validator may look like this:

@OneOfGroupNotNull(fields = {"a", "b", "c"})
public interface MyBean {
    String getA();
    Rarity getB();
    Boolean getC();
}

The problem is that I cannot find a way to format the string array "fields". It just takes the to string method which results in something like: One of group must be not null. [Ljava.lang.String;@157d954

Answer 1

If you changed type of fields from String[] to String then message with field names will be shown correctly. To get field names in constraint just split() it by comma.

Another option is to generating custom message inside constraint, like this:

cvc.disableDefaultConstraintViolation();
cvc.buildConstraintViolationWithTemplate("error message")
    .addNode("field name with error")
    .addConstraintViolation();

Answer 2

Which implementation of Bean Validation are you using? If you are using Hibernate Validator 4.3 this should actually work. See also https://hibernate.onjira.com/browse/HV-506 .

As a workaround, why not use a List ? There the default toString is more sensible.