如何向根节点添加属性

Question

I have an xml as String

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

I want to add an attribute to root node and save a xml as String again.

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

But I can't. Here is my code

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);

The output is [#document: null]. Help me please resolve my problem

Answer 1

You can not use document.toString() to get output. You need to convert document object back to xml string. Please check following code. It will help to solve your problem.

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;
    }

}