使用InputSource将XML解析为字符串会产生空文档

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.Document;
import org.xml.sax.InputSource;

public class XMLParser {
    public static String parseXMLToString(InputSource inputSource) {
        try {
            DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
            DocumentBuilder builder = factory.newDocumentBuilder();
            Document document = builder.parse(inputSource);
            document.getDocumentElement().normalize();
            return document.getDocumentElement().getTextContent();
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        }
    }
}