Java Xml 简明教程

Java SAX Parser - Modify XML Document

Java SAX解析器是一种Java API，可用于解析和修改XML文档。要使用SAX解析器修改任何XML文档，我们必须将数据存储在任何数据结构中，因为我们无法返回，因为它只有向前只读访问权限。在本章中，我们将详细了解如何读取和修改现有XML文档。

Modify XML Using Java SAX Parser

  1. 步骤 1: 实施一个 Handler 类

  2. 步骤 2: 创建一个 SAXParser 对象

  3. 步骤 3: 读取 XML

  4. 步骤 4: 为 Handler 类创建对象

  5. 步骤 5: 解析 XML 文档

  6. *步骤6：*将更新的内容写入文件

有关前五个步骤，请参阅本部分的 Parse XML Document 章。

Step 6: Writing the Updated Content into File

可以使用 FileWriter 对象的write()方法将更新的内容写入文件，如下所示：

FileWriter filewriter = new FileWriter("newfile.xml");
filewriter.write("content_here");

Modifying XML File

SAX解析器不会像DOM那样提供对元素的随机访问。我们可以在实现DefaultHandler的Handler类中实现回调方法，以创建并向已存在的XML文档添加新元素。

Example

在此示例中，我们将了解如何通过将结果元素添加到学生元素来修改studentData.xml文件。以下是需要通过在</marks>标记的末尾附加<Result>Pass<Result/>来修改的 sudentData.xml 文件。

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

在以下 ModifyXML.java 程序中，我们创建了SAXParser对象并解析了输入文件。我们用必需的回调方法实现了Handler类，并利用writeFile()方法将更新的内容写入文件。我们使用了一个字符串数组来存储内容并将其写入文件。

import java.io.*;
import org.xml.sax.*;
import javax.xml.parsers.*;
import org.xml.sax.helpers.DefaultHandler;

//Implementing a Handler class
class UserDemoHandler extends DefaultHandler {

   static String displayText[] = new String[1000];
   static int numberLines = 0;
   static String indentation = "";

   public void startDocument() {
      displayText[numberLines] = indentation;
	  displayText[numberLines] += "<?xml version = \"1.0\" encoding = \""+
	         "UTF-8" + "\"?>";
	  numberLines++;
   }

   public void processingInstruction(String target, String data) {
      displayText[numberLines] = indentation;
	  displayText[numberLines] += "<?";
	  displayText[numberLines] += target;

	  if (data != null && data.length() > 0) {
	     displayText[numberLines] += ' ';
	     displayText[numberLines] += data;
	  }
	   displayText[numberLines] += "?>";
	   numberLines++;
   }

   public void startElement(String uri, String localName, String qualifiedName,
		   Attributes attributes) {
      displayText[numberLines] = indentation;
	  indentation += "    ";
      displayText[numberLines] += '<';
	  displayText[numberLines] += qualifiedName;

      if (attributes != null) {

         int numberAttributes = attributes.getLength();
	     for (int loopIndex = 0; loopIndex < numberAttributes; loopIndex++) {
            displayText[numberLines] += ' ';
	        displayText[numberLines] += attributes.getQName(loopIndex);
	        displayText[numberLines] += "=\"";
	        displayText[numberLines] += attributes.getValue(loopIndex);
	        displayText[numberLines] += '"';
	      }
	   }
	      displayText[numberLines] += '>';
	      numberLines++;
   }

   public void characters(char characters[], int start, int length) {
      String characterData = (new String(characters, start, length)).trim();
	  if(characterData.indexOf("\n") < 0 && characterData.length() > 0) {
         displayText[numberLines] = indentation;
	     displayText[numberLines] += characterData;
	     numberLines++;
	  }
   }

   public void endElement(String uri, String localName, String qualifiedName) {

      indentation = indentation.substring(0, indentation.length() - 4) ;
	  displayText[numberLines] = indentation;
	  displayText[numberLines] += "</";
	  displayText[numberLines] += qualifiedName;
	  displayText[numberLines] += '>';
	  numberLines++;
	  if (qualifiedName.equals("marks")) {
         startElement("", "Result", "Result", null);
	     characters("Pass".toCharArray(), 0, "Pass".length());
	     endElement("", "Result", "Result");
	  }
   }

   public void writeFile(FileWriter filewriter) throws IOException {

      for(int loopIndex = 0; loopIndex < numberLines; loopIndex++) {
         filewriter.write(displayText[loopIndex].toCharArray());
	     filewriter.write('\n');
	     System.out.println(displayText[loopIndex].toString());
	  }
      filewriter.close();
   }
}

public class ModifyXML extends DefaultHandler {
   public static void main(String args[]) {
      try {

    	 //Creating SAXParser object
         SAXParserFactory factory = SAXParserFactory.newInstance();
         SAXParser saxParser = factory.newSAXParser();

         //Reading the XML
         File inputFile = new File("src/input.txt");

         //Creating object for Handler class
         UserDemoHandler userHandler = new UserDemoHandler();

         //Parsing the XML
         saxParser.parse(inputFile, userHandler);

         //Writing the updated content into file
         FileWriter filewriter = new FileWriter("src/newfile.xml");
         userHandler.writeFile(filewriter);

      }
      catch (Exception e) {
         e.printStackTrace(System.err);
      }
   }
}

输出窗口显示在向每个学生元素中添加结果元素后更新的XML文档内容。

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