如何向根节点添加属性

Question

我有一个XML作为String

<color>
    <name>black</name>
<color>

我想向根节点添加一个属性，然后再次将xml保存为String。

<color id="1">
    <name>black</name>
<color>

但是我不能。 这是我的代码

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

String xml = "<color><name>black</name></color>";
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
DocumentBuilder builder = factory.newDocumentBuilder();
InputSource is = new InputSource(new StringReader(xml));
Document document = builder.parse(is);
Element element = (Element) document.getFirstChild();
element.setAttribute("id", "1");
String result = document.toString();
System.out.println(result);

输出为[#document：null]。 帮我解决我的问题

Answer 1

您不能使用document.toString（）获取输出。 您需要将文档对象转换回xml字符串。 请检查以下代码。 这将有助于解决您的问题。

import java.io.IOException;
import java.io.StringReader;
import java.io.StringWriter;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerException;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;

import org.xml.sax.InputSource;
import org.xml.sax.SAXException;
import org.w3c.dom.Document;
import org.w3c.dom.Element;

public class test1 {

    public static void main(String[] args) throws SAXException, IOException,
            ParserConfigurationException {
        final String xmlStr = "<color><name>black</name></color>";
        Document doc = convertStringToDocument(xmlStr);

        String str = convertDocumentToString(doc);
        System.out.println(str);
    }

    private static Document convertStringToDocument(String xmlStr)
            throws SAXException, IOException, ParserConfigurationException {
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        DocumentBuilder builder = factory.newDocumentBuilder();
        InputSource is = new InputSource(new StringReader(xmlStr));
        Document document = builder.parse(is);
        Element element = (Element) document.getFirstChild();
        element.setAttribute("id", "1");
        String result = document.toString();
        System.out.println(result);
        return document;

    }

    private static String convertDocumentToString(Document doc) {
        TransformerFactory tf = TransformerFactory.newInstance();
        Transformer transformer;
        try {
            transformer = tf.newTransformer();
            StringWriter writer = new StringWriter();
            transformer.transform(new DOMSource(doc), new StreamResult(writer));
            String output = writer.getBuffer().toString();
            return output;
        } catch (TransformerException e) {
            e.printStackTrace();
        }

        return null;
    }

}