XML parsing basics-XML/RSS Tutorial-php.cn

Java Web Development Practical Classic

In project development, the main function of HTML is to display data, and to standardize the data storage structure, you need to useXML. XML has its own syntax, and all markup elements can be defined arbitrarily by the user.

1. Understanding XML

XML (eXtended Markup Language, extensible markup language) provides a cross-platform , cross-network, cross-program language data description method, using XML to easily implement common functions such as data exchange, system configuration, and content management.
XML is similar to HTML, both are markup languages. The biggest difference is that the elements in HTML are fixed and mainly focused on display, while the tags in XML language are customized by users and mainly focus on data storage.

XML parsing basics

Comparison of XML and HTML

In fact, all XML files are divided into two parts: the leading area and the data areaGroupbecome.
Leading area: Specifies some attributes of the XML page , there are the following 3 attributes:

version: Indicates the XML version used, now is 1.0.

encoding: The text encoding used in the page. If there is Chinese, the encoding must be specified.

standalone: Whether this XML file is run independently. If it needs to be displayed, it can be controlled using CSS or XSL.

The order in which attributes appear is fixed, version, encoding, standalone. Once the order is incorrect, XML will cause an error.
 <?xml version="1.0" encoding="GB2312" standalone="no"?>
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Data area: All data areas must have a root element. Multiple sub-elements can be stored under one root element, but each element must be completed. Each mark is case-sensitive. .

The CDATA tag is provided in the XML language to identify the file data. When the XML parser processes the CDATA tag, it will not parse any symbols or tags in the data, but will The original data is passed to the application intact.
CDATA syntax format:

<![CDATA[]  不解析内容  ]>

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2. XML parsing

In the XML file, since it is more about the content of the descriptive information, use this application after getting an XML document. The definition name of the element is used to extract the corresponding content. This operation is called XML parsing.
In XML parsingW3C defines two parsing methods: SAX and DOM. The program operations of these two parsing methods are as follows:

XML parsing basics

XML parsing operation

It can be seen that the application does not directly operate on the XML document. Instead, the XML document is first analyzed by the XML parser, and then the application passes the DOM provided by the XML parser. Interface or SAX interface operates on the analysis structure, thus indirectly achieving access to XML documents.

2.1. DOM parsing operation

In the application, based on DOM (Document Object Model, documentObject Model) XML parser converts an XML document into a collection of object models (usually called a DOM tree). It is through the operation of this object model that the application Cheng Xun realizes the Operations on XML document data. Through the DOM interface, the application can access any part of the data in the XML document at any time. Therefore, this mechanism that utilizes the DOM interface is also called a random access mechanism.
Since the DOM parser converts the entire XML document into a DOM tree and stores it in memory, when the document is large or the structure is complex, the memory requirements are relatively high, and the traversal of trees with complex structures is also difficult. A time-consuming operation.
DOM operation will turn all XML files into DOM trees in memory.

There are the following 4 core operation interfaces in DOM parsing:

Document: This interface represents the entire XML document, represents the root of the entire DOM tree, and provides The entrance to access and operate the data in the document. All element contents in the XML file can be accessed through the Document node.

XML parsing basics

Common methods of Document interface

Node：DOM操作的核心接口中有很大一部分是从Node接口继承过来的。例如Document、Element、Attri等接口。在DOM树中，每一个Node接口代表了DOM树中的一个节点。

XML parsing basics

Node接口的常用方法

NodeList：此接口表示一个节点的集合，一般用于表示有顺序关系的一组节点。例如，一个节点的子节点，当文档改变时会直接影响到NodeList集合。

XML parsing basics

NodeList接口的常用方法

NameNodeMap：此接口表示一组节点和其唯一名称对应的一一对应关系，主要用于属性节点的表示。

出以上4个核心接口外，如果一个程序需要进行DOM解析读操作，则需要按如下步骤进行：
（1）建立DocumentBuilderFactory：DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
（2）建立DocumentBuilder：DocumentBuilder builder = factory.newDocumentBuilder();
（3）建立Document：Document doc= builder.parse("要读取的文件路径");
（4）建立NodeList：NodeList nl = doc.getElementsByTagName("读取节点");
（5）进行XML信息读取。

// xml_demo.xml

<?xml version="1.0" encoding="GBK"?>
<addresslist>
    <linkman>
        <name>小明</name>
        <email>asaasa@163.com</email>
    </linkman>
    <linkman>
        <name>小张</name>
        <email>xiaoli@163.com</email>
    </linkman>
</addresslist>

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DOM完成XML的读取。

package com.demo;

import java.io.IOException;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;

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

public class XmlDomDemo {
    public static void main(String[] args) {
        // （1）建立DocumentBuilderFactory，以用于取得DocumentBuilder
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        // （2）通过DocumentBuilderFactory，取得DocumentBuilder
        DocumentBuilder builder = null;
        try {
            builder = factory.newDocumentBuilder();
        } catch (ParserConfigurationException e) {
            e.printStackTrace();
        }
        // （3）定义Document接口对象，通过DocumentBuilder类进行DOM树是转换操作
        Document doc = null;
        try {
            // 读取指定路径的XML文件，读取到内存中
            doc = builder.parse("xml_demo.xml");
        } catch (SAXException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        // （4）查找linkman节点
        NodeList nl = doc.getElementsByTagName("linkman");
        // （5）输出NodeList中第一个子节点中文本节点的内容
        for (int i = 0; i < nl.getLength(); i++) {
            // 取出每一个元素    
            Element element = (Element) nl.item(i);
            System.out.println("姓名：" + element.getElementsByTagName("name").item(0).getFirstChild().getNodeValue());
            System.out.println("邮箱：" + element.getElementsByTagName("email").item(0).getFirstChild().getNodeValue());
        }
    }
}

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DOM完成XML的文件输出。
此时就需要使用DOM操作中提供的各个接口（如Element接口）并手工设置各个节点的关系，同时在创建Document对象时就必须使用newDocument()方法建立一个新的DOM树。
如果现在需要将XML文件保存在硬盘上，则需要使用TransformerFactory、Transformer、DOMSource、StreamResult 4个类完成。
TransformerFactory类：取得一个Transformer类的实例对象。
DOMSource类：接收Document对象。
StreamResult 类：指定要使用的输出流对象（可以向文件输出，也可以向指定的输出流输出）。
Transformer类：通过该类完成内容的输出。

XML parsing basics

StreamResult类的构造方法

package com.demo;

import java.io.File;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.transform.OutputKeys;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerConfigurationException;
import javax.xml.transform.TransformerException;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;

import org.w3c.dom.Document;
import org.w3c.dom.Element;

public class XmlDemoWrite {

    public static void main(String[] args) {
        // （1）建立DocumentBuilderFactory，以用于取得DocumentBuilder
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        // （2）通过DocumentBuilderFactory，取得DocumentBuilder
        DocumentBuilder builder = null;
        try {
            builder = factory.newDocumentBuilder();
        } catch (ParserConfigurationException e) {
            e.printStackTrace();
        }
        // （3）定义Document接口对象，通过DocumentBuilder类进行DOM树是转换操作
        Document doc = null;
        // 创建一个新的文档
        doc = builder.newDocument();
        // （4）建立各个操作节点
        Element addresslist = doc.createElement("addresslist");
        Element linkman = doc.createElement("linkman");
        Element name = doc.createElement("name");
        Element email = doc.createElement("email");
        // （5）设置节点的文本内容，即为每一个节点添加文本节点
        name.appendChild(doc.createTextNode("小明"));
        email.appendChild(doc.createTextNode("xiaoming@163.com"));
        // （6）设置节点关系
        linkman.appendChild(name);
        linkman.appendChild(email);
        addresslist.appendChild(linkman);
        doc.appendChild(addresslist);
        // （7）输出文档到文件中
        TransformerFactory tf = TransformerFactory.newInstance();
        Transformer t = null;
        try {
            t = tf.newTransformer();
        } catch (TransformerConfigurationException e) {
            e.printStackTrace();
        }
        // 设置编码
        t.setOutputProperty(OutputKeys.ENCODING, "GBK");
        // 输出文档
        DOMSource source = new DOMSource(doc);
        // 指定输出位置
        StreamResult result = new StreamResult(new File("xml_wirte.xml"));
        try {
            // 输出
            t.transform(source, result);
            System.out.println("yes");
        } catch (TransformerException e) {
            e.printStackTrace();
        }
    }

}

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生成文档：

//xml_write.xml

<?xml version="1.0" encoding="GBK" standalone="no"?>
<addresslist>
    <linkman>
        <name>小明</name>
        <email>xiaoming@163.com</email>
    </linkman>
</addresslist>

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2.2、SAX解析操作

SAX（Simple APIs for XML，操作XML的简单接口）与DOM操作不同的是，SAX采用的是一种顺序的模式进行访问，是一种快速读取XML数据的方式。
当使用SAX 解析器进行操作时会触发一系列的事件。

XML parsing basics

SAX主要事件

当扫描到文档（Document）开始与结束、元素（Element）开始与结束时都会调用相关的处理方法，并由这些操作方法做出相应的操作，直到整个文档扫描结束。
如果在开发中想要使用SAX解析，则首先应该编写一个SAX解析器，再直接定义一个类，并使该类继承自DefaultHandler类，同时覆写上述的表中的方法即可。

package com.sax.demo;

import org.xml.sax.Attributes;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;

public class XmlSax extends DefaultHandler {
    @Override
    public void startDocument() throws SAXException {
        System.out.println("<?xml version=\"1.0\" encoding=\"GBK\"?>");
    }
    @Override
    public void endDocument() throws SAXException {
        System.out.println("\n 文档读取结束。。。");
    }
    @Override
    public void startElement(String url, String localName, String name,
            Attributes attributes) throws SAXException {
        System.out.print("<");
        System.out.print(name);
        if (attributes != null) {
            for (int x = 0; x < attributes.getLength(); x++) {
                System.out.print("" + attributes.getQName(x) + "=\"" + attributes.getValue(x) + "\"");
            }
        }
        System.out.print(">");
    }
    @Override
    public void characters(char[] ch, int start, int length) throws SAXException {
        System.out.print(new String(ch, start, length));
    }
    @Override
    public void endElement(String url, String localName, String name)
            throws SAXException {
        System.out.print("</");
        System.out.print(name);
        System.out.print(">");
    }
}

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建荔湾SAX解析器后，还需要建立SAXParserFactory和SAXParser对象，之后通过SAXPaeser的parse()方法指定要解析的XML文件和指定的SAX解析器。

建立要读取的文件：sax_demo.xml

<?xml version="1.0" encoding="GBK"?>
<addresslist>
    <linkman id="xm">
        <name>小明</name>
        <email>xiaoming@163.com</email>
    </linkman>
    <linkman id="xl">
        <name>小李</name>
        <email>xiaoli@163.com</email>
    </linkman>
</addresslist>

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使用SAX解析器

package com.sax.demo;

import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;

public class SaxTest {

    public static void main(String[] args) throws Exception {
        //（1）建立SAX解析工厂
        SAXParserFactory factory = SAXParserFactory.newInstance();
        //（2）构造解析器
        SAXParser parser = factory.newSAXParser();
        //（3）解析XML使用handler
        parser.parse("sax_demo.xml", new XmlSax());
    }

}

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通过上面的程序可以发现，使用SAX解析比使用DOM解析更加容易。

XML parsing basics

DOM解析与SAX解析的区别

有两者的特点可以发现两者的区别：
DOM解析适合于对文件进行修改和随机存取的操作，但是不适合于大型文件的操作。
SAX采用部分读取的方式，所以可以处理大型文件，而且只需要从文件中读取特定内容。SAX解析可以由用户自己建立自己的对象模型。

2.3、XML解析的好帮手：jdom

jdom是使用Java编写的，用于读、写、操作XML的一套组件。

jdom = dom 修改文件的有点 + SAX读取快速的优点

XML parsing basics

jdom的主要操作类

使用jdom生成XML文件

package com.jdom.demo;

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

import org.jdom.Attribute;
import org.jdom.Document;
import org.jdom.Element;
import org.jdom.output.XMLOutputter;


public class WriteXml {
    public static void main(String[] args) {
        // 定义节点
        Element addresslist = new Element("addresslist");
        Element linkman = new Element("linkman");
        Element name = new Element("name");
        Element email = new Element("email");
        // 定义属性
        Attribute id = new Attribute("id", "xm");
        // 声明一个Document对象
        Document doc = new Document(addresslist);
        // 设置元素的内容
        name.setText("小明");
        email.setText("xiaoming@163.com");
        name.setAttribute(id);
        // 设置子节点
        linkman.addContent(name);
        linkman.addContent(email);
        // 将linkman加入根子节点
        addresslist.addContent(linkman);
        // 用来输出XML文件
        XMLOutputter out = new XMLOutputter();
        // 设置输出编码
        out.setFormat(out.getFormat().setEncoding("GBK"));

        // 输出XML文件
        try {
            out.output(doc, new FileOutputStream("jdom_write.xml"));
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

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// jdom_write.xml

<?xml version="1.0" encoding="GBK"?>
<addresslist>
    <linkman>
        <name id="xm">小明</name>
        <email>xiaoming@163.com</email>
    </linkman>
</addresslist>

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使用jdom读取XML文件

package com.jdom.demo;

import java.io.IOException;
import java.util.List;

import org.jdom.Document;
import org.jdom.Element;
import org.jdom.JDOMException;
import org.jdom.input.SAXBuilder;

public class ReadXml {

    public static void main(String[] args) throws JDOMException, IOException {
        //建立SAX解析
        SAXBuilder builder = new SAXBuilder();
        //找到Document
        Document readDoc = builder.build("jdom_write.xml");
        //读取根元素
        Element stu = readDoc.getRootElement();
        //得到全部linkman子元素
        List list = stu.getChildren("linkman");
        for (int i = 0; i < list.size(); i++) {
            Element e = (Element) list.get(i);
            String name = e.getChildText("name");
            String id = e.getChild("name").getAttribute("id").getValue();
            String email = e.getChildText("email");
            System.out.println("----联系人----");
            System.out.println("姓名：" + name + "编号：" + id);
            System.out.println("Email：" + email);
        }
    }

}

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jdom是一种常见的操作组件
在实际的开发中使用非常广泛。

2.4、解析工具：dom4j

dom4j也是一组XML操作组件包，主要用来读写XML文件，由于dom4j性能优异、功能强大，且具有易用性，所以现在已经被广泛地应用开来。如，Hibernate、Spring框架中都使用了dom4j进行XML的解析操作。
开发时需要引入的jar包：dom4j-1.6.1.jar、lib/jaxen-1.1-beta-6.jar

dom4j中的所用操作接口都在org.dom4j包中定义。其他包根据需要把选择使用。

XML parsing basics

dom4j的主要接口

用dom4j生成XML文件：

package com.dom4j.demo;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.UnsupportedEncodingException;

import org.dom4j.Document;
import org.dom4j.DocumentHelper;
import org.dom4j.Element;
import org.dom4j.io.OutputFormat;
import org.dom4j.io.XMLWriter;

public class Dom4jWrite {
    public static void main(String[] args) {
        // 创建文档
        Document doc = DocumentHelper.createDocument();
        // 定义个节点
        Element addresslist = doc.addElement("addresslist");
        Element linkman = addresslist.addElement("linkman");
        Element name = linkman.addElement("name");
        Element email = linkman.addElement("email");
        // 设置节点内容
        name.setText("小明");
        email.setText("xiaoming@163.com");
        // 设置输出格式
        OutputFormat format = OutputFormat.createPrettyPrint();
        // 指定输出编码
        format.setEncoding("GBK");
        try {
            XMLWriter writer = new XMLWriter(new FileOutputStream(new File(
                    "dom4j_demo.xml")), format);
            writer.write(doc);
            writer.close();
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}

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// dom4j_demo.xml

<?xml version="1.0" encoding="GBK"?>

<addresslist>
  <linkman>
    <name>小明</name>
    <email>xiaoming@163.com</email>
  </linkman>
</addresslist>

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用dom4j读取XML文件：

package com.dom4j.demo;

import java.io.File;
import java.util.Iterator;

import org.dom4j.Document;
import org.dom4j.DocumentException;
import org.dom4j.Element;
import org.dom4j.io.SAXReader;

public class Dom4jRead {

    public static void main(String[] args) {
        //读取文件
        File file = new File("dom4j_demo.xml");
        //建立SAX解析读取
        SAXReader reader = new SAXReader();
        Document doc = null;
        try {
            //读取文档
            doc = reader.read(file);
        } catch (DocumentException e) {
            e.printStackTrace();
        }
        //取得根元素
        Element root = doc.getRootElement();
        //取得全部的子节点
        Iterator iter = root.elementIterator();
        while (iter.hasNext()) {
            //取得每一个linkman
            Element linkman = (Element) iter.next();
            System.out.println("姓名："+linkman.elementText("name"));
            System.out.println("邮件地址："+linkman.elementText("email"));
        }
    }

}

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