xml的dom解析思想如下图

dom解析思想：得到文档节点通过节点往下搜寻，它就是一棵树。

下面是dom4j的入门操作。

（1）解析xml文档

    public Document parse(URL url) throws DocumentException {
        SAXReader reader = new SAXReader();
        Document document = reader.read(url);
        return document;
    }

（2）遍历xml的元素，使用迭代器

 public void bar(Document document) throws DocumentException {

        Element root = document.getRootElement();

        // iterate through child elements of root
        for ( Iterator i = root.elementIterator(); i.hasNext(); ) {
            Element element = (Element) i.next();
            // do something
        }

        // iterate through child elements of root with element name "foo"
        for ( Iterator i = root.elementIterator( "foo" ); i.hasNext(); ) {
            Element foo = (Element) i.next();
            // do something
        }

        // iterate through attributes of root 
        for ( Iterator i = root.attributeIterator(); i.hasNext(); ) {
            Attribute attribute = (Attribute) i.next();
            // do something
        }

（3）快速遍历

 public void treeWalk(Document document) {
        treeWalk( document.getRootElement() );
    }

    public void treeWalk(Element element) {
        for ( int i = 0,size = element.nodeCount(); i < size; i++ ) {
            Node node = element.node(i);
            if ( node instanceof Element ) {
                treeWalk( (Element) node );
            }
            else {
                // do something....
            }
        }

（4）创建xml文件

 public Document createDocument() {
        Document document = DocumentHelper.createDocument();
        Element root = document.addElement( "root" );

        Element author1 = root.addElement( "author" )
            .addAttribute( "name","James" )
            .addAttribute( "location","UK" )
            .addText( "James Strachan" );
        
        Element author2 = root.addElement( "author" )
            .addAttribute( "name","Bob" )
            .addAttribute( "location","US" )
            .addText( "Bob McWhirter" );

        return document;
    }

 public void write(Document document) throws IOException {

        // lets write to a file
        XMLWriter writer = new XMLWriter(
            new FileWriter( "output.xml" )
        );
        writer.write( document );
        writer.close();


        // Pretty print the document to System.out
        OutputFormat format = OutputFormat.createPrettyPrint();
        writer = new XMLWriter( System.out,format );
        writer.write( document );

        // Compact format to System.out
        format = OutputFormat.createCompactFormat();
        writer = new XMLWriter( System.out,format );
        writer.write( document );
    }

（6）修改后的dom树更新到内存中去

	XMLWriter writer = new XMLWriter(new FileWriter("output.xml"),OutputFormat.createPrettyPrint());
        writer.write( document );
	writer.close();    

例子一：解析xml文件并且输出书名作者售价

以book.xml为例

<?xml version="1.0" encoding="UTF-8"?>

<书架>
  <书>
    <书名>Java就业培训教程</书名>
    <作者>小凌</作者>
    <售价>39.00元</售价>
    <特价>20</特价>
  </书>
  <书>
    <书名>JavaScript网页开发</书名>
    <作者>张孝祥</作者>
    <售价>28.00元</售价>
  </书>
</书架>

java代码

	/**
	 * 解析xml文件
	 * @throws DocumentException
	 */
	public void ParsingXml() throws DocumentException {
		SAXReader reader = new SAXReader();
		Document doc = reader.read("book.xml");
		Element root = doc.getRootElement();
		//遍历xml的元素
		for (Iterator i = root.elementIterator(); i.hasNext();) {
			Element Root = (Element) i.next();
			String titile = Root.element("书名").getText();
			String author = Root.element("作者").getText();
			String price = Root.element("售价").getText();
			System.out.println(titile+":"+author+":"+price);
		}
	}

	public void add() throws DocumentException,IOException{
		
		SAXReader reader = new SAXReader();
		Document doc = reader.read("book.xml");
		Element root = doc.getRootElement();   //得到根节点
		
		 Element price = DocumentHelper.createElement( "特价" );
		 price.setText("20");
		 
		 Element book = root.element("书");
		 book.add(price);
		 

		 XMLWriter writer = new XMLWriter(new FileOutputStream("book.xml"),OutputFormat.createPrettyPrint());
	        writer.write( doc );
	        writer.close();  
	}

	public void del() throws DocumentException,IOException {
		SAXReader reader = new SAXReader();
		Document doc = reader.read("output.xml");
		Element root = doc.getRootElement();
		
		Element e = root.element("书").element("特价");
		e.getParent().remove(e);
		
		//更新内存
		 XMLWriter writer = new XMLWriter(new FileOutputStream("book.xml"),OutputFormat.createPrettyPrint());
	        writer.write( doc );
	        writer.close(); 
	}

	public void updata() throws DocumentException,IOException{
			SAXReader reader = new SAXReader();
			Document doc = reader.read("book.xml");
			Element root = doc.getRootElement();
			
			root.element("书").element("作者").setText("小凌");
			
			//更新内存
			 XMLWriter writer = new XMLWriter(new FileOutputStream("book.xml"),OutputFormat.createPrettyPrint());
		        writer.write( doc );
		        writer.close(); 
	}