打印时Java Document为null，但作为Xml正确保存

Question

I'm converting a String to an XML, this way:

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        DocumentBuilder builder = null;
        try {
            builder = factory.newDocumentBuilder();
        } catch (ParserConfigurationException e) {
            e.printStackTrace();
        }
        Document doc = null;
        try {
            doc = builder.parse(new InputSource(new StringReader(xmlSource)));
        } catch (SAXException | IOException e) {
            e.printStackTrace();
        }

I've tried printing a node or something to see how to retrieve node informations but it keeps printing null whenever I try doc.getNodeName() or doc.getElementById(elementId) , or whatever.
After this conversion I do this:

// Write the parsed document to an xml file
        TransformerFactory transformerFactory = TransformerFactory.newInstance();
        Transformer transformer= null;
        try {
            transformer = transformerFactory.newTransformer();
        } catch (TransformerConfigurationException e) {
            e.printStackTrace();
        }
        DOMSource source = new DOMSource(doc);
        StreamResult result =  new StreamResult(new File("TestFile.xml"));
        try {
            transformer.transform(source, result);
        } catch (TransformerException e) {
            e.printStackTrace();
        }

My TestFile.xml is a correct XML file. So I don't understand why my Document is null. I need to retrieve informations from the document but this way I'm not able to do anything. If the XML file created and saved to disk is correct, how can he Document return null with every method? What am I missing?

Answer 1

Please check my solution: App.java class

  DocumentBuilderFactory docFactory = DocumentBuilderFactory.newInstance();
                DocumentBuilder docBuilder = docFactory.newDocumentBuilder();

                // root elements
                Document doc = docBuilder.newDocument();