使用Java将元素添加到现有XML文件

Question

我有一个像这样的空白XML文件：

<root>
<!--Some Comment goes here>
</root>

现在我想拥有一个GUI，其中包含1个textfield和1个textarea，它需要一些字符串（标题和内容），点击“保存”按钮后，它会将它们添加到元素下的XML文件中。我在一个单独的GUI中创建了root并保存到'sample.xml'中，单击一个按钮后，会出现内容添加窗口，我打开相同的XML文件，并希望将项目添加到该文件中。但每次我这样做，我都会收到'Null Pointer Exception'错误。以下是我的代码。请告诉我哪里出错了：

主GUI中的代码：

JButton btnNewButton = new JButton("Add New");
        btnNewButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {
                //File f = new File("sample.xml");
                DocumentBuilderFactory dbfac = DocumentBuilderFactory.newInstance();
                DocumentBuilder docBuilder;
                try {
                    Writer output = new BufferedWriter(new FileWriter("sample.xml"));
                    docBuilder = dbfac.newDocumentBuilder();
                    Document doc = docBuilder.newDocument();

                    Element root = doc.createElement("root");
                    doc.appendChild(root);

                    Comment comment = doc.createComment("Just a thought");
                    root.appendChild(comment);

                    TransformerFactory transfac = TransformerFactory.newInstance();
                    Transformer trans = transfac.newTransformer();
                    trans.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
                    trans.setOutputProperty(OutputKeys.INDENT, "yes");

                    StringWriter sw = new StringWriter();
                    StreamResult result = new StreamResult(sw);
                    DOMSource source = new DOMSource(doc);
                    trans.transform(source, result);
                    String xmlString = sw.toString();

                    PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("sample.xml", true)));
                    out.println(xmlString);
                    out.close();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (ParserConfigurationException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (TransformerException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

辅助GUI中的代码添加元素：

JButton btnSave = new JButton("Save");
        btnSave.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {

                DocumentBuilderFactory dbfac = DocumentBuilderFactory.newInstance();
                dbfac.setIgnoringComments(true);
                DocumentBuilder docBuilder;
                try {
                    docBuilder = dbfac.newDocumentBuilder();
                    Document doc = docBuilder.parse("sample.xml");

                    NodeList rt = doc.getElementsByTagName("root");

                    Element child = doc.createElement("content");
                    child.setAttribute("title", textField.getText().toString());
                    rt.item(0).appendChild(child);

                    Text text = doc.createTextNode(textArea.getText().toString());
                    child.appendChild(text);

                    TransformerFactory transfac = TransformerFactory.newInstance();
                    Transformer trans = transfac.newTransformer();
                    trans.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
                    trans.setOutputProperty(OutputKeys.INDENT, "yes");

                    StringWriter sw = new StringWriter();
                    StreamResult result = new StreamResult(sw);
                    DOMSource source = new DOMSource(doc);
                    trans.transform(source, result);
                    String xmlString = sw.toString();

                    PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("sample.xml", true)));
                    out.println(xmlString);
                    out.close();


                    System.out.println(xmlString);
                } catch (ParserConfigurationException e) {
                    e.printStackTrace();
                } catch (TransformerConfigurationException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (TransformerException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (FileNotFoundException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (SAXException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }



            }
        });