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Java实现学习笔记(一)


  首先,先给出一个比较基本的处理xml文件的程序。你不必细看,直接跳过即可。需要时可以返回来看。
  
  Echo01.java
  
  import java.io.*;
  
  import org.xml.sax.*;
  
  import org.xml.sax.helpers.DefaultHandler;
  
  import javax.xml.parsers.SAXParserFactory;
  
  import javax.xml.parsers.ParserConfigurationException;
  
  import javax.xml.parsers.SAXParser;
  
  public class Echo01 extends DefaultHandler
  
  {
  
  StringBuffer textBuffer;
  
  public static void main(String argv[])
  
  {
  
    if (argv.length != 1) {
  
      System.err.println("Usage: cmd filename");
  
      System.exit(1);
  
    }
  
    // Use an instance of ourselves as the SAX event handler
  
    DefaultHandler handler = new Echo01();
  
    // Use the default (non-validating) parser
  
       SAXParserFactory factory = SAXParserFactory.newInstance();
  
    try {
  
      // Set up output stream
  
      out = new OutputStreamWriter(System.out, "UTF-8");
  
      // Parse the input
  
      SAXParser saxParser = factory.newSAXParser();
  
      saxParser.parse( new File(argv[0]), handler);
  
    } catch (Throwable t) {
  
      t.printStackTrace();
  
    }
  
    System.exit(0);
  
  }
  
  static private Writer out;
  
  //===========================================================
  
  // SAX DocumentHandler methods
  
  //===========================================================
  
  public void startDocument()
  
  throws SAXException
  
  {
  
    emit("");
  
    nl();
  
  }
  
  public void endDocument()
  
  throws SAXException
  
  {
  
    try {
  
      nl();
  
      out.flush();
  
    } catch (IOException e) {
  
      throw new SAXException("I/O error", e);
  
    }
  
  }
  
  public void startElement(String namespaceURI,
  
               String sName, // simple name
  
               String qName, // qualified name
  
               Attributes attrs)
  
  throws SAXException
  
  {
  
    echoText();
  
       String eName = sName; // element name
  
    if ("".equals(eName)) eName = qName; // not namespaceAware
  
    emit("<"+eName);
  
    if (attrs != null) {
  
      for (int i = 0; i < attrs.getLength(); i++) {
  
        String aName = attrs.getLocalName(i); // Attr name
  
        if ("".equals(aName)) aName = attrs.getQName(i);
  
        emit(" ");
  
        emit(aName+"=\""+attrs.getValue(i)+"\"");
  
             }
    }
  
    emit(">");
  
  }
  
  public void endElement(String namespaceURI,
  
              String sName, // simple name
  
              String qName // qualified name
  
             )
  
  throws SAXException
  
  {
  
    echoText();
  
    String eName = sName; // element name
  
    if ("".equals(eName)) eName = qName; // not namespaceAware
  
    emit("");
  
  }
  
  public void characters(char buf[], int offset, int len)
  
  throws SAXException
  
  {
  
           String s = new String(buf, offset, len);
  
    if (textBuffer == null) {
  
      textBuffer = new StringBuffer(s);
  
    } else {
  
      textBuffer.append(s);
  
    }
  
  }
  
  //===========================================================
  
  // Utility Methods ...
  
  //===========================================================
  
  // Display text accumulated in the character buffer
  
  private void echoText()
  
  throws SAXException
  
  {
  
    if (textBuffer == null) return;
  
              String s = ""+textBuffer;
  
       emit(s);
  
       textBuffer = null;
  
  }
  
  // Wrap I/O exceptions in SAX exceptions, to
  
  // suit handler signature requirements
  
  private void emit(String s)
  
  throws SAXException
  
  {
  
    try {
  
      out.write(s);
  
      out.flush();
  
    } catch (IOException e) {
  
      throw new SAXException("I/O error", e);
  
    }
  
  }
  
  // Start a new line
  
  private void nl()
  
  throws SAXException
  
  {
  
   String lineEnd = System.getProperty("line.separator");
  
    try {
  
      out.write(lineEnd);
  
    } catch (IOException e) {
  
      throw new SAXException("I/O error", e);
  
    }
  
  }
  
  }
  
  从程序中可以看出,解析一个XML文件的核心语句是下面一部分:
  
   // Use an instance of ourselves as the SAX event handler
  
    DefaultHandler handler = new Echo01();
  
     // Use the default (non-validating) parser
  
       SAXParserFactory factory = SAXParserFactory.newInstance();
  
    try {
  
      // Set up output stream
  
      out = new OutputStreamWriter(System.out, "UTF-8");
  
       // Parse the input
  
      SAXParser saxParser = factory.newSAXParser();
  
      saxParser.parse( new File(argv[0]), handler);
  
     } catch (Throwable t) {
  
      t.printStackTrace();
  
    }
  
  先是创建一个SAXParserFactory工厂类的实例,然后通过SAXParser saxParser = factory.newSAXParser(); 这个工厂类的方法创建了一个saxParser。将xml文件(new File(argv[0]))和一个Sax Event Handler(handler)(在这个程序里面,这个Handler其实是本身这个类,这个类继承了org.xml.sax.helpers.DefaultHandler 这个类,并且在前面初始化了它:DefaultHandler handler = new Echo01(); )传递给它,让它进行解析。
  
  关于xml文件的解析过程中的处理全部在Handler里面实现。一般Parser接受的是DefaultHandler或者HandlerBase这两个类。 这个例子里面的类是继承DefaultHandler这个虚类的。看下图:
  
 

  而DefaultHandler是实现了EntityResolver, DTDHandler, ContentHandler, ErrorHandler四个接口的虚类。分别定义了如下的方法:
  
 

  不同的方法,在不同的时候被Parser调用,(这个不同的时候就是Event-based)
  
  详细介绍:(暂略)
  
  DefualtHandler的UML图如下:
  
 
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