Spring JPA GenericSpecification for GROUP BY and WHERE IN

gistfile1.txt

package com.company.jpa;

import org.springframework.data.jpa.domain.Specification;

import javax.persistence.criteria.Expression;
import java.util.Collection;
import java.util.List;
import java.util.stream.Collectors;

public class GenericSpecifications<T> {

    public Specification<T> groupBy(Specification<T> specification, List<String> columnNames) {
        return (Specification<T>) (root, query, criteriaBuilder) -> {
            List<Expression<?>> columnNamesExpression = columnNames.stream().map(x -> root.get(x)).collect(Collectors.toList());
            query.groupBy(columnNamesExpression);
            return specification.toPredicate(root, query, criteriaBuilder);
        };
    }

    public Specification<T> whereIn(String columnName, Collection<String> value) {
        return (Specification<T>) (root, query, criteriaBuilder) -> root.get(columnName).in(value);
    }

}