Question

I want to validate the trimmed string length, like:

@Size(min = 5, max = 20, message = "Please enter a valid username (5-20 characters)")
String userName;

Given user name foo (6 chars), it will be trimmed to foo (3 chars) in the setter method, however, validate against the raw input seems a bit of useless.

Answer 1

Instead of altering the validation procedure, you could write your own constraint which wraps the expected validation logic.

I have found such an implementation of a @TrimmedSize annotation at the auto-trader-spring-spike project:

package com.autotrader.frameworks;

import javax.validation.Constraint;
import javax.validation.Payload;
import java.lang.annotation.Documented;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import static java.lang.annotation.ElementType.*;

@Documented
@Constraint(validatedBy = TrimmedSizeValidator.class)
@Target({METHOD, FIELD, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
public @interface TrimmedSize {
    String message() default "size too small";

    Class<?>[] groups() default {};

    int min() default 0;

    Class<? extends Payload>[] payload() default {};
}

And the validator:

package com.autotrader.frameworks;


import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;

public class TrimmedSizeValidator implements ConstraintValidator<TrimmedSize, String> {

    private int length;

    public void initialize(TrimmedSize trimmedSize) {
        length = trimmedSize.min();
    }

    public boolean isValid(String value, ConstraintValidatorContext constraintValidatorContext) {
        if (value == null || value.length() == 0) {
            return false;
        }
        return value.trim().length() >= length;
    }
}

It uses an Apache License 2.0, but even then you could write this code from scratch once you’ve seen it.