Bean validation provides constraint annotations to check DTO parameters. However, what if we want to check a parameter type not supported by a standard constraint annotation?

Let’s say, we want to check a LocalDateTime is in the future. But, our parameter type is in String, a type not supported by the annotation @Future.

Instead of creating a new annotation, we will add configure so that it will support it.

First, let’s create our custom validator:

package lin.louis;

import java.time.LocalDateTime;
import java.time.format.DateTimeParseException;

import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;
import javax.validation.constraints.Future;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;


public class FutureValidatorForString implements ConstraintValidator<Future, String> {
    private static final Logger LOGGER = LoggerFactory.getLogger(FutureValidatorForString.class);

    @Override
    public boolean isValid(String value, ConstraintValidatorContext context) {
        if (value == null || "".equals(value.trim())) {
            return true;
        }
        
        try {
            LocalDateTime localDateTime = LocalDateTime.parse(value);
            return LocalDateTime.now().isBefore(localDateTime);
        } catch (DateTimeParseException e) {
            LOGGER.debug("The timestamp is not valid.", e);
            return true;
        }
    }
}

Then provide this implementation in META-INF/services/javax.validation.ConstraintValidator:

lin.louis.FutureValidatorForString

Now, you can use the annotation for String type:

public class Apple {
    @Future
    private String expirationDate;
}

Source: