如何读写XML文件？

我必须读取和写入XML http://en.wikipedia.org/wiki/XML文件。使用 Java 读写 XML 文件的最简单方法是什么？

下面是一个快速 DOM 示例，展示了如何使用 dtd 读取和写入简单的 xml 文件：

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE roles SYSTEM "roles.dtd">
<roles>
    <role1>User</role1>
    <role2>Author</role2>
    <role3>Admin</role3>
    <role4/>
</roles>

和 dtd：

<?xml version="1.0" encoding="UTF-8"?>
<!ELEMENT roles (role1,role2,role3,role4)>
<!ELEMENT role1 (#PCDATA)>
<!ELEMENT role2 (#PCDATA)>
<!ELEMENT role3 (#PCDATA)>
<!ELEMENT role4 (#PCDATA)>

首先导入这些：

import javax.xml.parsers.*;
import javax.xml.transform.*;
import javax.xml.transform.dom.*;
import javax.xml.transform.stream.*;
import org.xml.sax.*;
import org.w3c.dom.*;

以下是您需要的一些变量：

private String role1 = null;
private String role2 = null;
private String role3 = null;
private String role4 = null;
private ArrayList<String> rolev;

这是一个阅读器（String xml 是您的 xml 文件的名称）：

public boolean readXML(String xml) {
        rolev = new ArrayList<String>();
        Document dom;
        // Make an  instance of the DocumentBuilderFactory
        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        try {
            // use the factory to take an instance of the document builder
            DocumentBuilder db = dbf.newDocumentBuilder();
            // parse using the builder to get the DOM mapping of the    
            // XML file
            dom = db.parse(xml);

            Element doc = dom.getDocumentElement();

            role1 = getTextValue(role1, doc, "role1");
            if (role1 != null) {
                if (!role1.isEmpty())
                    rolev.add(role1);
            }
            role2 = getTextValue(role2, doc, "role2");
            if (role2 != null) {
                if (!role2.isEmpty())
                    rolev.add(role2);
            }
            role3 = getTextValue(role3, doc, "role3");
            if (role3 != null) {
                if (!role3.isEmpty())
                    rolev.add(role3);
            }
            role4 = getTextValue(role4, doc, "role4");
            if ( role4 != null) {
                if (!role4.isEmpty())
                    rolev.add(role4);
            }
            return true;

        } catch (ParserConfigurationException pce) {
            System.out.println(pce.getMessage());
        } catch (SAXException se) {
            System.out.println(se.getMessage());
        } catch (IOException ioe) {
            System.err.println(ioe.getMessage());
        }

        return false;
    }

这里有一位作家：

public void saveToXML(String xml) {
    Document dom;
    Element e = null;

    // instance of a DocumentBuilderFactory
    DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
    try {
        // use factory to get an instance of document builder
        DocumentBuilder db = dbf.newDocumentBuilder();
        // create instance of DOM
        dom = db.newDocument();

        // create the root element
        Element rootEle = dom.createElement("roles");

        // create data elements and place them under root
        e = dom.createElement("role1");
        e.appendChild(dom.createTextNode(role1));
        rootEle.appendChild(e);

        e = dom.createElement("role2");
        e.appendChild(dom.createTextNode(role2));
        rootEle.appendChild(e);

        e = dom.createElement("role3");
        e.appendChild(dom.createTextNode(role3));
        rootEle.appendChild(e);

        e = dom.createElement("role4");
        e.appendChild(dom.createTextNode(role4));
        rootEle.appendChild(e);

        dom.appendChild(rootEle);

        try {
            Transformer tr = TransformerFactory.newInstance().newTransformer();
            tr.setOutputProperty(OutputKeys.INDENT, "yes");
            tr.setOutputProperty(OutputKeys.METHOD, "xml");
            tr.setOutputProperty(OutputKeys.ENCODING, "UTF-8");
            tr.setOutputProperty(OutputKeys.DOCTYPE_SYSTEM, "roles.dtd");
            tr.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "4");

            // send DOM to file
            tr.transform(new DOMSource(dom), 
                                 new StreamResult(new FileOutputStream(xml)));

        } catch (TransformerException te) {
            System.out.println(te.getMessage());
        } catch (IOException ioe) {
            System.out.println(ioe.getMessage());
        }
    } catch (ParserConfigurationException pce) {
        System.out.println("UsersXML: Error trying to instantiate DocumentBuilder " + pce);
    }
}

getTextValue 在这里：

private String getTextValue(String def, Element doc, String tag) {
    String value = def;
    NodeList nl;
    nl = doc.getElementsByTagName(tag);
    if (nl.getLength() > 0 && nl.item(0).hasChildNodes()) {
        value = nl.item(0).getFirstChild().getNodeValue();
    }
    return value;
}

添加一些访问器和修改器，就完成了！