JAVA学习笔记 -- 读写XML

XML是一种可扩展标记语言

以下是一个完整的XML文件（也是下文介绍读写XML的样本）：

<?
xml version="1.0" encoding="UTF-8"?
>
<poem author="William Carlos Williams" title="The Great Figure">
	<line>Among the rain</line>
	<line>and ligths</line>
	<line>I saw the figure 5</line>
	<line>in gold</line>
	<line>on a red</line>
	<line>fire truck</line>
	<line>moving</line>
	<line>tense</line>
	<line>unheeded</line>
	<line>to gong clangs</line>
	<line>siren howls</line>
	<line>and wheels rumbling</line>
	<line>through the dark city</line>
</poem>

一、写XML

本文介绍两种方式：使用DOM开发包来写XML文件和用String对象的方式

将Poem类作为数据源，提供须要转换成XML的内容：

class Poem {
	private static String title = "The Great Figure";
	private static String author = "William Carlos Williams";
	private static ArrayList<String> lines = new ArrayList<String>();
	static {
		lines.add("Among the rain");
		lines.add("and ligths");
		lines.add("I saw the figure 5");
		lines.add("in gold");
		lines.add("on a red");
		lines.add("fire truck");
		lines.add("moving");
		lines.add("tense");
		lines.add("unheeded");
		lines.add("to gong clangs");
		lines.add("siren howls");
		lines.add("and wheels rumbling");
		lines.add("through the dark city");
	}
	public static String getTitle() {
		return title;
	}
	public static String getAuthor() {
		return author;
	}
	public static ArrayList<String> getLines() {
		return lines;
	}
}

1、用DOM写XML文件

流程：

（1）创建一个空的Document对象（最顶层的DOM对象，包括了创建XML所须要的其它一切）。

（2）创建元素和属性，把元素和属性加到Document对象中。

（3）把Document对象的内容转换成String对象。

（4）把String对象写到目标文件中去。

import java.util.ArrayList;
import java.io.*;
import javax.xml.parsers.*;
import javax.xml.transform.*;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;
import org.w3c.dom.*;

public class XmlTest {
	public static void main(String[] args) {
		Document doc = createXMLContent1(); // 创建空文档
		createElements(doc); // 创建XML
		String xmlContent = createXMLString(doc);// 创建字符串以表示XML
		writeXMLToFile1(xmlContent);
	}

	/*********** 用DOM写XML文件 ***********/
	private static Document createXMLContent1() {
		Document doc = null;
		try {
			// 使应用程序可以从XML文档获取生成 DOM 对象树的解析器
			DocumentBuilderFactory dbfac = DocumentBuilderFactory.newInstance();
			DocumentBuilder docBuilder = dbfac.newDocumentBuilder();
			doc = docBuilder.newDocument();
			// 作为 XML 声明 的一部分指定此文档是否是单独的的属性。
未指定时，此属性为 false。
			doc.setXmlStandalone(true);
		} catch (ParserConfigurationException pce) {
			System.out.println("Couldn't create a DocumentBuilder");
			System.exit(1);
		}
		return doc;
	}
	private static void createElements(Document doc) {
		// 创建根元素
		Element poem = doc.createElement("poem");
		poem.setAttribute("title", Poem.getTitle());
		poem.setAttribute("author", Poem.getAuthor());
		// 把根元素加到文档里去
		doc.appendChild(poem);
		// 创建子元素
		for (String lineIn : Poem.getLines()) {
			Element line = doc.createElement("line");
			Text lineText = doc.createTextNode(lineIn);
			line.appendChild(lineText);
			// 把每一个元素加到根元素里去
			poem.appendChild(line);
		}
	}
	private static String createXMLString(Document doc) {
		// 将DOM转换成字符串
		Transformer transformer = null;
		StringWriter stringWriter = new StringWriter();
		try {
			TransformerFactory transformerFactory = TransformerFactory.newInstance();
			transformer = transformerFactory.newTransformer();
			// 是否应输出 XML 声明
			transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
			// 是否以XML格式自己主动换行
			transformer.setOutputProperty(OutputKeys.INDENT, "yes");
			// 创建字符串以包括XML
			stringWriter = new StringWriter();
			StreamResult result = new StreamResult(stringWriter);// 充当转换结果的持有者
			DOMSource source = new DOMSource(doc);
			transformer.transform(source, result);
		} catch (TransformerConfigurationException e) {
			System.out.println("Couldn't create a Transformer");
			System.exit(1);
		} catch (TransformerException e) {
			System.out.println("Couldn't transforme DOM to a String");
			System.exit(1);
		}
		return stringWriter.toString();
	}
	private static void writeXMLToFile1(String xmlContent) {
		String fileName = "E:\test\domoutput.xml";
		try {
			File domOutput = new File(fileName);
			FileOutputStream domOutputStream = new FileOutputStream(domOutput);
			domOutputStream.write(xmlContent.getBytes());
			domOutputStream.close();
			System.out.println(fileName + " was successfully written");
		} catch (FileNotFoundException e) {
			System.out.println("Couldn't find a file called" + fileName);
			System.exit(1);
		} catch (IOException e) {
			System.out.println("Couldn't write a file called" + fileName);
			System.exit(1);
		}
	}

2、用String写XML文件

这样的方法就比較简单。就是直接用字符串把整个XML文件描写叙述出来，然后保存文件。

二、读取XML文件

两种方式：用DOM读取XML文件和用SAX方式。一般DOM处理内容比較小的XML文件。而SAX能够处理随意大小的XML文件。

1、用DOM读取XML文件

public class XmlTest {
	public static void main(String[] args) {
		String fileName = "E:\test\domoutput.xml";
		writeFileContentsToConsole(fileName);
	}
	/*********** 用DOM读取XML文件 ***********/
	private static void writeFileContentsToConsole(String fileName) {
		Document doc = createDocument(fileName);
		Element root = doc.getDocumentElement();// 获取根元素
		StringBuilder sb = new StringBuilder();
		sb.append("The root element is named:"" + root.getNodeName() + """);
		sb.append("and has the following attributes: ");
		NamedNodeMap attributes = root.getAttributes();
		for (int i = 0; i < attributes.getLength(); i++) {
			Node thisAttribute = attributes.item(i);
			sb.append(thisAttribute.getNodeName());
			sb.append("("" + thisAttribute.getNodeValue() + "")");
			if (i < attributes.getLength() - 1) {
				sb.append(",");
			}
		}
		System.out.println(sb);// 根元素的描写叙述信息
		NodeList nodes = doc.getElementsByTagName("line");
		for (int i = 0; i < nodes.getLength(); i++) {
			Element element = (Element) nodes.item(i);
			System.out.println("Found an element named ""
					+ element.getTagName() + """
					+ "With the following content: ""
					+ element.getTextContent() + """);
		}
	}
	private static Document createDocument(String fileName) {// 从文件创建DOM的Document对象
		Document doc = null;
		try {
			File xmlFile = new File(fileName);
			DocumentBuilderFactory dbfac = DocumentBuilderFactory.newInstance();
			DocumentBuilder docBuilder = dbfac.newDocumentBuilder();
			doc = docBuilder.parse(xmlFile);// 解析xml文件载入为dom文档
			doc.setXmlStandalone(true);
		} catch (IOException e) {
			e.printStackTrace();
		} catch (SAXException e) {
			e.printStackTrace();
		} catch (ParserConfigurationException e) {
			e.printStackTrace();
		}
		return doc;
	}
}/*  
 * Output:
 * The root element is named:"poem"and has the following attributes: author("William Carlos Williams"),title("The Great Figure")
 * Found an element named "line"With the following content: "Among the rain"
 * Found an element named "line"With the following content: "and ligths"
 * ... ... 
 */// :~ 

2、用SAX读取XML文件

SAX是使用ContentHandler接口来公开解析事件，并且SAX包提供了一个默认实现类DefaultHandler，它的默认行为就是什么都不做。以下就通过XMLToConsoleHandler类来覆盖当中的一些方法，来捕获XML文件的内容。

import org.w3c.dom.CharacterData;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;
public class XmlTest {
	public static void main(String[] args) {
		String fileName = "E:\test\domoutput.xml";
		getFileContents(fileName);
	}
	private static void getFileContents(String fileName) {
		try {
			XMLToConsoleHandler handler = new XMLToConsoleHandler();
			SAXParserFactory factory = SAXParserFactory.newInstance();
			SAXParser saxParser = factory.newSAXParser();
			saxParser.parse(fileName, handler);
		} catch (IOException e) {
			e.printStackTrace();
		} catch (ParserConfigurationException e) {
			e.printStackTrace();
		} catch (SAXException e) {
			e.printStackTrace();
		}
	}
}
/***********  用SAX读取XML文件   ***********/   
class XMLToConsoleHandler extends DefaultHandler {
	public void characters(char[] content, int start, int length)
			throws SAXException { // 处理元素的真正内容
			System.out.println("Found content: " + new String(content, start, length));
	}
	public void endElement(String arg0, String localName, String qName)throws SAXException {
		System.out.println("Found the end of an element named "" + qName + """);
	}
	public void startElement(String uri, String localName, String qName,
			Attributes attributes) throws SAXException {
		StringBuilder sb = new StringBuilder();
		sb.append("Found the start of an element named "" + qName + """);
		if (attributes != null && attributes.getLength() > 0) {
			sb.append(" with attributes named ");
			for (int i = 0; i < attributes.getLength(); i++) {
				String attributeName = attributes.getLocalName(i);
				String attributeValue = attributes.getValue(i);
				sb.append(""" + attributeName + """);
				sb.append(" (value = ");
				sb.append(""" + attributeValue + """);
				sb.append(")");
				if (i < attributes.getLength() - 1) {
					sb.append(",");
				}
			}
		}
		System.out.println(sb.toString());
	}
}