How to convet XML file to string properly?

I have an XML signature verification code below:

public static boolean isXmlDigitalSignatureValid(String xmlString) throws Exception {
    boolean validFlag = false;
    String pubicKeyFilePath = "src/main/resources/keys/publickey.key";
    Document doc = convertStringToDocument(xmlString);
    NodeList nl = doc.getElementsByTagNameNS(XMLSignature.XMLNS, "Signature");
    if (nl.getLength() == 0) {
        throw new Exception("No XML Digital Signature Found, document is discarded");
    }
    PublicKey publicKey = new CryptoUtil().getStoredPublicKey(pubicKeyFilePath);
    DOMValidateContext valContext = new DOMValidateContext(publicKey, nl.item(0));
    XMLSignatureFactory fac = XMLSignatureFactory.getInstance("DOM");
    XMLSignature signature = fac.unmarshalXMLSignature(valContext);
    validFlag = signature.validate(valContext);
    return validFlag;
}

But there is a problem. Im sure that doc file has signature part cause I'm printing it. But

 NodeList nl = doc.getElementsByTagNameNS(XMLSignature.XMLNS, "Signature"); 

cant find the signature part so

n1.item(0)

is null and I have the exception I mentioned in the header.

I figure it out what going on. My XMLReader method wasn't working properly. Yes it was returning string exactly but missing some point in bytes. So I change it a little.

First I read and convert to Doc file

public static Document getXmlDocument(String xmlFilePath) {
    Document doc = null;
    DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
    dbf.setNamespaceAware(true);
    try {
        doc = dbf.newDocumentBuilder().parse(new FileInputStream(xmlFilePath));
    } catch (ParserConfigurationException ex) {
        ex.printStackTrace();
    } catch (FileNotFoundException ex) {
        ex.printStackTrace();
    } catch (SAXException ex) {
        ex.printStackTrace();
    } catch (IOException ex) {
        ex.printStackTrace();
    }
    return doc;
}

Then I convert doc to string.

public static Document convertStringToDocument(String xmlStr) {
    DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
    factory.setNamespaceAware(true);
    DocumentBuilder builder;
    try
    {
        builder = factory.newDocumentBuilder();
        Document doc = builder.parse( new InputSource( new StringReader( xmlStr ) ) );
        return doc;
    } catch (Exception e) {
        e.printStackTrace();
    }
    return null;
}

And my final xmlReader Function is :

public static String xmlReader(String path){
    return convertDocumentToString(XmlDigitalSignatureGenerator.getXmlDocument(path));
}

This is the only way that I convert an XML file to string properly