java中使用sax解析xml的解决方法

当需要处理大型XML文件时，SAX解析器是一种比DOM解析器更有效的解析器。DOM解析器在将整个文档加载到内存中并在系统中构建树结构后解析XML文档，而SAX解析器则在处理XML文档时逐个元素进行解析，它遍历该文档，并在需要时发出事件以指示XML文档的不同部分。

SAX解析器不需要在内存中保存整个XML文档，因此可以处理很大的XML文件而无需担心内存不足或性能问题。

下面是使用SAX解析器在Java中处理XML的步骤：

1. 创建一个SAXParserFactory对象并从中获取SAXParser对象。

SAXParserFactory factory = SAXParserFactory.newInstance();
SAXParser saxParser = factory.newSAXParser();

1. 实现自己的处理类（也称为处理器），继承DefaultHandler类并覆盖需要的方法。

示例代码：

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

public class MyHandler extends DefaultHandler {

   boolean bFirstName = false;
   boolean bLastName = false;

   @Override
   public void startElement(
      String uri, String localName, String qName, Attributes attributes)
      throws SAXException {
      if (qName.equalsIgnoreCase("student")) {
         String rollNo = attributes.getValue("rollno");
         System.out.println("Roll No : " + rollNo);
      } else if (qName.equalsIgnoreCase("firstname")) {
         bFirstName = true;
      } else if (qName.equalsIgnoreCase("lastname")) {
         bLastName = true;
      }
   }

   @Override
   public void endElement(String uri, String localName, String qName) throws SAXException {
      if (qName.equalsIgnoreCase("student")) {
         System.out.println("End Element : student");
      }
   }

   @Override
   public void characters(char ch[], int start, int length) throws SAXException {
      if (bFirstName) {
         System.out.println("First Name : " + new String(ch, start, length));
         bFirstName = false;
      } else if (bLastName) {
         System.out.println("Last Name : " + new String(ch, start, length));
         bLastName = false;
      }
   }
}

1. 创建处理器对象并向SAX解析器注册处理器对象。

MyHandler handler = new MyHandler();
saxParser.parse("input.xml", handler);

完成以上三个步骤后，就可以使用SAX解析器在Java中解析XML文件了。

以下是两个示例说明：

1. 解析input.xml文件
   input.xml文件内容如下：

<?xml version="1.0" encoding="UTF-8"?>
<class>
   <student rollno="393">
      <firstname>dinkar</firstname>
      <lastname>kad</lastname>
      <nickname>dinkar</nickname>
      <marks>85</marks>
   </student>
   <student rollno="493">
      <firstname>Vaneet</firstname>
      <lastname>Gupta</lastname>
      <nickname>vinni</nickname>
      <marks>95</marks>
   </student>
   <student rollno="593">
      <firstname>jasvir</firstname>
      <lastname>singn</lastname>
      <nickname>jazz</nickname>
      <marks>90</marks>
   </student>
</class>

解析该文件的示例代码：

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

public class SAXParserDemo {
   public static void main(String[] args) {

      try {
         File inputFile = new File("input.xml");
         SAXParserFactory factory = SAXParserFactory.newInstance();
         SAXParser saxParser = factory.newSAXParser();
         MyHandler handler = new MyHandler();
         saxParser.parse(inputFile, handler);
      } catch (Exception e) {
         e.printStackTrace();
      }
   }
}

1. 解析具有复合元素的XML文件
   input2.xml文件内容如下：

<?xml version="1.0" encoding="UTF-8"?>
<departments>
    <accounting>
        <employee>
            <firstname>John</firstname>
            <lastname>Doe</lastname>
            <salary>40000</salary>
        </employee>
        <employee>
            <firstname>Bob</firstname>
            <lastname>Smith</lastname>
            <salary>55000</salary>
        </employee>
    </accounting>
    <sales>
        <employee>
            <firstname>Jane</firstname>
            <lastname>Doe</lastname>
            <salary>50000</salary>
        </employee>
        <employee>
            <firstname>Tom</firstname>
            <lastname>Jones</lastname>
            <salary>65000</salary>
        </employee>
    </sales>
</departments>

解析该文件的示例代码：

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

public class SAXParserDemo {
   public static void main(String[] args) {

      try {
         File inputFile = new File("input2.xml");
         SAXParserFactory factory = SAXParserFactory.newInstance();
         SAXParser saxParser = factory.newSAXParser();
         MyHandler2 handler = new MyHandler2();
         saxParser.parse(inputFile, handler);
      } catch (Exception e) {
         e.printStackTrace();
      }
   }
}

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

public class MyHandler2 extends DefaultHandler {

    private boolean employee = false;
    private boolean firstname = false;
    private boolean lastname = false;
    private boolean salary = false;

    @Override
    public void startElement(String uri, String localName, String qName, 
                Attributes attributes) throws SAXException {

        if (qName.equalsIgnoreCase("employee")) {
            employee = true;
        } else if (qName.equalsIgnoreCase("firstname")) {
            firstname = true;
        } else if (qName.equalsIgnoreCase("lastname")) {
            lastname = true;
        } else if (qName.equalsIgnoreCase("salary")) {
            salary = true;
        }
    }

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

        if (qName.equalsIgnoreCase("employee")) {
            System.out.println("------------------------");
        }
    }

    @Override
    public void characters(char ch[], int start, int length) throws SAXException {

        if (employee) {
            System.out.println("Employee:");
            employee = false;
        } else if (firstname) {
            System.out.println("First Name: " + new String(ch, start, length));
            firstname = false;
        } else if (lastname) {
            System.out.println("Last Name : " + new String(ch, start, length));
            lastname = false;
        } else if (salary) {
            System.out.println("Salary : " + new String(ch, start, length));
            salary = false;
        }
    }
}