使用Java向现有XML文件添加元素

我有一个现有的空白XML文件，如下所示：

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

在辅助GUI中添加元素的代码：

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();
                }



            }
        });

---

我不知道这是一个打字错误还是一个错误，但XML注释是这样的：

。它必须以

-->

结尾。哦，对不起。这是一个输入错误。但是，它并不能解决问题。您说您得到了一个NullPointerException，所以在代码中显示stacktrace。并指出异常出现在哪一行。

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();
                }



            }
        });