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Kiwi is a utility library. We really like Google's Guava, and also use Apache Commons. But if they don't have something we need, and we think it is useful, this is where we put it.

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package org.kiwiproject.validation;

import static java.util.Objects.isNull;
import static java.util.Objects.requireNonNull;

import lombok.extern.slf4j.Slf4j;
import org.apache.commons.lang3.StringUtils;

import javax.annotation.Nonnull;
import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;
import java.lang.reflect.Method;
import java.util.Arrays;
import java.util.Collection;
import java.util.Map;
import java.util.Optional;

/**
 * Check that a validated value is required.
 */
@Slf4j
public class RequiredValidator implements ConstraintValidator {

    private boolean allowBlank;
    private boolean allowEmpty;

    @Override
    public void initialize(Required constraintAnnotation) {
        allowBlank = constraintAnnotation.allowBlank();
        allowEmpty = constraintAnnotation.allowEmpty();
    }

    /**
     * Perform the validation on the given value. See {@link Required} for the validation logic.
     *
     * @param value   the value to validate
     * @param context context in which the constraint is evaluated
     * @return true if valid, false otherwise
     */
    @Override
    public boolean isValid(Object value, ConstraintValidatorContext context) {
        //noinspection RedundantIfStatement
        if (isNull(value) || notAllowingBlanksButIsBlank(value) || notAllowingEmptyButIsEmpty(value)) {
            return false;
        }

        return true;
    }

    private boolean notAllowingBlanksButIsBlank(Object value) {
        return !allowBlank && isBlank(value);
    }

    private static boolean isBlank(Object value) {
        return value instanceof CharSequence && StringUtils.isBlank((CharSequence) value);
    }

    private boolean notAllowingEmptyButIsEmpty(Object value) {
        return !allowEmpty && isEmpty(value);
    }

    private static boolean isEmpty(@Nonnull Object value) {
        requireNonNull(value);

        if (value instanceof CharSequence) {
            return StringUtils.isEmpty((CharSequence) value);
        } else if (value instanceof Collection) {
            return ((Collection) value).isEmpty();
        } else if (value instanceof Map) {
            return ((Map) value).isEmpty();
        }

        try {
            var isEmptyMethod = findIsEmptyMethod(value).orElse(null);

            if (isNull(isEmptyMethod)) {
                return false;
            }

            return (boolean) isEmptyMethod.invoke(value);
        } catch (Exception e) {
            LOG.trace("Error invoking isEmpty method; assuming not empty", e);
        }

        // Nothing we checked above matched; assume not empty
        return false;
    }

    /**
     * Find public isEmpty() method that returns a primitive {@code boolean}.
     */
    private static Optional findIsEmptyMethod(Object value) {
        return Arrays.stream(value.getClass().getMethods())
                .filter(method -> method.getName().equals("isEmpty"))
                .filter(method -> method.getParameterCount() == 0)
                .filter(method -> method.getReturnType().equals(Boolean.TYPE))
                .findFirst();
    }
}