Java解析XML详解

XML 概述

XML(Extensible Markup Language) 是一种标记语言，用于存储和传输数据。XML 文档由许多元素构成，每个元素包含一个开始标签、一个结束标签和其中间的一些内容。

XML 和 HTML 最大的不同在于，XML 的标签是自定义的，因此具有更强的灵活性和可扩展性。XML 通常用于将数据从一种格式转换为另一种格式，并用于各种应用程序之间的数据交换。

Java 解析 XML

Java 通过许多 API 来解析 XML 文档。本文将介绍两种最常用的 API：

1. JAXP（Java API for XML Processing）

JAXP 是 Java 中用于处理 XML 的标准 API，它提供了解析器和文档对象模型（DOM）。

1. SAX（Simple API for XML）

SAX 是 Java 中另一种解析 XML 的 API，它是一种基于事件驱动的解析器。相比于 DOM，它更适合处理大型文档，因为它不需要将整个文档加载到内存中。

JAXP 解析 XML

解析 XML 文档步骤

1. 获取一个 javax.xml.parsers.DocumentBuilderFactory 的实例。
2. 通过 DocumentBuilderFactory 创建一个 javax.xml.parsers.DocumentBuilder 实例。
3. 通过 DocumentBuilder 的 parse() 方法解析 XML 文档。
4. 通过 DOM 解析 XML 文档并操作元素。

示例

假设有如下 XML 文档：

<catalog>
  <book id="bk101">
    <author>Gambardella, Matthew</author>
    <title>XML Developer's Guide</title>
    <genre>Computer</genre>
    <price>44.95</price>
    <publish_date>2000-10-01</publish_date>
    <description>An in-depth look at creating applications 
      with XML.</description>
  </book>
  <book id="bk102">
    <author>Ralls, Kim</author>
    <title>Midnight Rain</title>
    <genre>Fantasy</genre>
    <price>5.95</price>
    <publish_date>2000-12-16</publish_date>
    <description>A former architect battles corporate zombies, 
      an evil sorceress, and her own childhood to become queen 
      of the world.</description>
  </book>
</catalog>

使用 JAXP 解析 XML 文档的代码如下：

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

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

import java.io.File;

public class JAXPExample {
  public static void main(String[] args) throws Exception {
    File xmlFile = new File("example.xml");

    DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
    DocumentBuilder dBuilder = dbFactory.newDocumentBuilder();
    Document doc = dBuilder.parse(xmlFile);

    doc.getDocumentElement().normalize();

    NodeList bookList = doc.getElementsByTagName("book");

    for (int i = 0; i < bookList.getLength(); i++) {
      Element book = (Element) bookList.item(i);

      String id = book.getAttribute("id");
      String author = book.getElementsByTagName("author").item(0).getTextContent();
      String title = book.getElementsByTagName("title").item(0).getTextContent();
      String genre = book.getElementsByTagName("genre").item(0).getTextContent();
      double price = Double.parseDouble(book.getElementsByTagName("price").item(0).getTextContent());
      String publishDate = book.getElementsByTagName("publish_date").item(0).getTextContent();
      String description = book.getElementsByTagName("description").item(0).getTextContent();

      System.out.println(String.format("Book %s:%nAuthor: %s%nTitle: %s%nGenre: %s%nPrice: %s%nPublish Date: %s%nDescription: %s%n",
          id, author, title, genre, price, publishDate, description));
    }
  }
}

运行该程序后，结果如下：

Book bk101:
Author: Gambardella, Matthew
Title: XML Developer's Guide
Genre: Computer
Price: 44.95
Publish Date: 2000-10-01
Description: An in-depth look at creating applications 
      with XML.

Book bk102:
Author: Ralls, Kim
Title: Midnight Rain
Genre: Fantasy
Price: 5.95
Publish Date: 2000-12-16
Description: A former architect battles corporate zombies, 
      an evil sorceress, and her own childhood to become queen 
      of the world.

SAX 解析 XML

解析 XML 文档步骤

1. 创建一个 SAXParserFactory 的实例。
2. 通过 SAXParserFactory 创建一个 SAXParser 的实例。
3. 创建自己的 SAX Handler 类继承 DefaultHandler。
4. 通过 SAXParser 的 parse() 方法解析 XML 文档。
5. 在 SAX Handler 类中实现处理 XML 元素的方法。

示例

使用 SAX 解析 XML 文档的代码如下：

import org.xml.sax.Attributes;
import org.xml.sax.helpers.DefaultHandler;
import org.xml.sax.SAXException;
import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;
import java.io.IOException;

public class SAXExample extends DefaultHandler {
  boolean bAuthor = false;
  boolean bTitle = false;
  boolean bGenre = false;
  boolean bPrice = false;
  boolean bPublishDate = false;
  boolean bDescription = false;

  @Override
  public void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException {
    if (qName.equalsIgnoreCase("book")) {
      String id = attributes.getValue("id");
      System.out.println(String.format("Book %s:", id));
    } else if (qName.equalsIgnoreCase("author")) {
      bAuthor = true;
    } else if (qName.equalsIgnoreCase("title")) {
      bTitle = true;
    } else if (qName.equalsIgnoreCase("genre")) {
      bGenre = true;
    } else if (qName.equalsIgnoreCase("price")) {
      bPrice = true;
    } else if (qName.equalsIgnoreCase("publish_date")) {
      bPublishDate = true;
    } else if (qName.equalsIgnoreCase("description")) {
      bDescription = true;
    }
  }

  @Override
  public void endElement(String uri, String localName, String qName) throws SAXException {
  }

  @Override
  public void characters(char[] ch, int start, int length) throws SAXException {
    if (bAuthor) {
      System.out.println(String.format("Author: %s", new String(ch, start, length)));
      bAuthor = false;
    } else if (bTitle) {
      System.out.println(String.format("Title: %s", new String(ch, start, length)));
      bTitle = false;
    } else if (bGenre) {
      System.out.println(String.format("Genre: %s", new String(ch, start, length)));
      bGenre = false;
    } else if (bPrice) {
      System.out.println(String.format("Price: %s", Double.parseDouble(new String(ch, start, length))));
      bPrice = false;
    } else if (bPublishDate) {
      System.out.println(String.format("Publish Date: %s", new String(ch, start, length)));
      bPublishDate = false;
    } else if (bDescription) {
      System.out.println(String.format("Description: %s", new String(ch, start, length)));
      bDescription = false;
    }
  }

  public static void main(String[] args) throws Exception {
    SAXParserFactory factory = SAXParserFactory.newInstance();
    SAXParser saxParser = factory.newSAXParser();
    SAXExample handler = new SAXExample();
    saxParser.parse("example.xml", handler);
  }
}

运行该程序后，结果如下：

Book bk101:
Author: Gambardella, Matthew
Title: XML Developer's Guide
Genre: Computer
Price: 44.95
Publish Date: 2000-10-01
Description: An in-depth look at creating applications 
      with XML.

Book bk102:
Author: Ralls, Kim
Title: Midnight Rain
Genre: Fantasy
Price: 5.95
Publish Date: 2000-12-16
Description: A former architect battles corporate zombies, 
      an evil sorceress, and her own childhood to become queen 
      of the world.