EntityManagerを使った汎用的なSELECT文

package sql;

import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.stream.Collectors;

public class Sql {
	
	private EntityManager em;

	public <T> T select(Class<? extends Object> clazz, Map<String, List<Object>> conditions) {
		
		String tableName = camelToSnake(clazz.getSimpleName());
		String select = "SELECT entity FROM " + tableName + " entity ";
		
		String state = select + conditions.entrySet().stream()
				.map(Entry::getKey)
				.map(column -> "WHERE " + camelToSnake(column) + " IN :" + column)
				.collect(Collectors.joining(" "));
		
		TypedQuery query = em.createNativeQuery(state, clazz);
		conditions.entrySet().forEach(map -> query.setParameter(map.getKey(), map.getValue()));
		return query.getResultList;
	}
	
	public String camelToSnake(final String camel) {
		final StringBuilder sb = new StringBuilder(camel.length() + camel.length());
		for (int i = 0; i < camel.length(); i++) {
			final char c = camel.charAt(i);
			if (Character.isUpperCase(c)) {
				sb.append(sb.length() != 0 ? '_' : "").append(Character.toLowerCase(c));
			} else {
				sb.append(Character.toLowerCase(c));
			}
		}
		return sb.toString();
	}
}