这是一个简单的java访问xml文件的例子：

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

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 javax.xml.parsers.DocumentBuilder;

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

public class XMLMain {
    // read from xml file
    public Document readXML(String filename) throws ParserConfigurationException, SAXException, IOException {
        FileInputStream inputFile = new FileInputStream(filename);
        DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
        DocumentBuilder dBuilder = dbFactory.newDocumentBuilder();
        Document doc = dBuilder.parse(inputFile);
        return doc;
    }

    // write to xml file
    public void writeXML(Document doc, String filename) throws FileNotFoundException, TransformerException {
        FileOutputStream output = new FileOutputStream(filename);
        TransformerFactory transformerFactory = TransformerFactory.newInstance();
        Transformer transformer = transformerFactory.newTransformer();

        DOMSource source = new DOMSource(doc);
        StreamResult result = new StreamResult(output);
        transformer.transform(source, result);
    }
    
    // process doc
    public void processXML(Document doc) {
        Element root = doc.getDocumentElement();
        System.out.println("root=" + root.getNodeName());
    }
    
    public static void main(String[] args) {
        XMLMain m = new XMLMain();
        try {
            Document doc = m.readXML("input.xml");
            m.processXML(doc);
            m.writeXML(doc, "output.xml");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

注意这里目前JDK(1.8.x)在处理root节点之外的comment节点时会有一个换行的bug:
比如源输入文件：

<?xml version="1.0" standalone="yes" ?>
<!-- comment line -->
<root_node>
   <sub_node>
      <attr1>Tom</attr1>
      <attr2>123</attr2>
   </sub_node>
</root_node>

运行完之后会变成：

<?xml version="1.0" encoding="UTF-8"?><!-- comment line --><root_node>
   <sub_node>
      <attr1>Tom</attr1>
      <attr2>123</attr2>
   </sub_node>

在comment节点前后的换行符弄丢了；但是如果这个comment节点在root节点之内，则不会有这个问题。