jdom解析xml

A typical Java application is a domain-specific XML editor:

• nobody wants to write the markup by hand
• general-purpose XML editors are too clunky

We generalize the business card language to allow collections of business cards:

<cards>
  <card>
    <name>John Doe</name>
    <title>CEO, Widget Inc.</title>
    <email>john.doe@widget.com</email>
    <phone>(202) 456-1414</phone>
    <logo url="widget.gif" />
  </card>
  <card>
    <name>Michael Schwartzbach</name>
    <title>Associate Professor</title>
    <email>mis@brics.dk</email>
    <phone>+45 8610 8790</phone>
    <logo url="http://www.brics.dk/~mis/portrait.gif" />
  </card>
  <card>
    <name>Anders Møller</name>
    <title>Research Assistant Professor</title>
    <email>amoeller@brics.dk</email>
    <phone>+45 8942 3475</phone>
    <logo url="http://www.brics.dk/~amoeller/am.jpg"/>
  </card>
</cards>

We then write a Java program to edit such collections.

First, we need a high-level representation of a business card:

class Card {
  public String name, title, email, phone, logo;

  public Card(String name, String title, String email, String phone, String logo) {
    this.name = name;
    this.title = title;
    this.email = email;
    this.phone = phone;
    this.logo = logo;
  }
}

An XML document must then be translated into a vector of such objects:

Vector doc2vector(Document d) {
  Vector v = new Vector();
  Iterator i = d.getRootElement().getChildren().iterator();
  while (i.hasNext()) {
    Element e = (Element)i.next();
    String phone = e.getChildText("phone");
    if (phone==null) phone="";
    Element logo = e.getChild("logo");
    String url;
    if (logo==null) url = ""; else url = logo.getAttributeValue("url");
    Card c = new Card(e.getChildText("name"),  // exploit schema,
                      e.getChildText("title"), // assume validity
                      e.getChildText("email"),
                      phone,
                      url);
    v.add(c);
  }
  return v;
}

And back into an XML document:

Document vector2doc() {
  Element cards = new Element("cards");
  for (int i=0; i<cardvector.size(); i++) {
    Card c = (Card)cardvector.elementAt(i);
    if (c!=null) {
      Element card = new Element("card");
      Element name = new Element("name");
      name.addContent(c.name);
      card.addContent(name);
      Element title = new Element("title");
      title.addContent(c.title);
      card.addContent(title);
      Element email = new Element("email");
      email.addContent(c.email);
      card.addContent(email);
      if (!c.phone.equals("")) {
        Element phone = new Element("phone");
        phone.addContent(c.phone);
        card.addContent(phone);
      }
      if (!c.logo.equals("")) {
        Element logo = new Element("logo");
        logo.setAttribute("url",c.logo);
        card.addContent(logo);
      }
      cards.addContent(card);
    }
  }
  return new Document(cards);
}

A little logic and some GUI then completes the editor:

Compile with: javac -classpath xerces.jar:jdom.jar BCedit.java

This example contains some general observations:

• XML documents are parsed via JDOM into domain-specific data structures
• if the input is known to validate according to some schema, then many runtime errors can be assumed never to occur
• how do we ensure that the output of vector2doc is valid according to the schema? (well-formedness is for free) 
  - that's a current research challenge!

• import java.awt.*;
  import java.awt.event.*;
  import java.io.*; 
  import java.util.*;
  import org.jdom.*; 
  import org.jdom.input.*; 
  import org.jdom.output.*; 
  
  class Card {
    public String name, title, email, phone, logo;
  
    public Card(String name, String title, String email, String phone, String logo) {
      this.name = name;
      this.title = title;
      this.email = email;
      this.phone = phone;
      this.logo = logo;
    }
  }
  
  public class BCedit extends Frame implements ActionListener {
    Button ok = new Button("ok");
    Button delete = new Button("delete");
    Button clear = new Button("clear");
    Button save = new Button("save");
    Button quit = new Button("quit");
    TextField name = new TextField(20);
    TextField title = new TextField(20);
    TextField email = new TextField(20);
    TextField phone = new TextField(20);
    TextField logo = new TextField(20);
    Panel cardpanel = new Panel(new GridLayout(0,1));
    String cardfile;
    Vector cardvector;
    int current = -1;
  
    public static void main(String[] args) { new BCedit(args[0]); }
  
    Vector doc2vector(Document d) {
      Vector v = new Vector();
      Iterator i = d.getRootElement().getChildren().iterator();
      while (i.hasNext()) {
        Element e = (Element)i.next();
        String phone = e.getChildText("phone");
        if (phone==null) phone="";
        Element logo = e.getChild("logo");
        String url;
        if (logo==null) url=""; else url=logo.getAttributeValue("url");
        Card c = new Card(e.getChildText("name"),
                          e.getChildText("title"),
                          e.getChildText("email"),
                          phone,
                          url);
        v.add(c);
      }
      return v;
    }
  
    Document vector2doc() {
      Element cards = new Element("cards");
      for (int i=0; i<cardvector.size(); i++) {
        Card c = (Card)cardvector.elementAt(i);
        if (c!=null) {
          Element card = new Element("card");
          Element name = new Element("name");
          name.addContent(c.name);
          card.addContent(name);
          Element title = new Element("title");
          title.addContent(c.title);
          card.addContent(title);
          Element email = new Element("email");
          email.addContent(c.email);
          card.addContent(email);
          if (!c.phone.equals("")) {
            Element phone = new Element("phone");
            phone.addContent(c.phone);
            card.addContent(phone);
          }
          if (!c.logo.equals("")) {
            Element logo = new Element("logo");
            logo.setAttribute("url",c.logo);
            card.addContent(logo);
          }
          cards.addContent(card);
        }
      }
      return new Document(cards);
    }
  
    void addCards() {
      cardpanel.removeAll();
      for (int i=0; i<cardvector.size(); i++) {
        Card c = (Card)cardvector.elementAt(i);
        if (c!=null) {
          Button b = new Button(c.name);
          b.setActionCommand(String.valueOf(i));
          b.addActionListener(this);
          cardpanel.add(b);
        }
      }
      this.pack();
    }
  
    public BCedit(String cardfile) {
      super("BCedit");
      this.cardfile=cardfile;
      try {
        cardvector = doc2vector(new SAXBuilder().build(new File(cardfile)));
      } catch (Exception e) {e.printStackTrace();}
      this.setLayout(new BorderLayout());
      ScrollPane s = new ScrollPane();
      s.setSize(200,0);
      s.add(cardpanel);
      this.add(s,BorderLayout.WEST);
      Panel l = new Panel(new GridLayout(5,1));
      l.add(new Label("Name"));                  
      l.add(new Label("Title"));                  
      l.add(new Label("Email"));                  
      l.add(new Label("Phone"));                  
      l.add(new Label("Logo"));                  
      this.add(l,BorderLayout.CENTER);
      Panel f = new Panel(new GridLayout(5,1));
      f.add(name);    
      f.add(title);    
      f.add(email);    
      f.add(phone);    
      f.add(logo);    
      this.add(f,BorderLayout.EAST);
      Panel p = new Panel();
      ok.addActionListener(this);
      p.add(ok);
      delete.addActionListener(this);
      p.add(delete);
      clear.addActionListener(this);
      p.add(clear);
      save.addActionListener(this);
      p.add(save);
      quit.addActionListener(this);
      p.add(quit);
      this.add(p,BorderLayout.SOUTH);
      addCards();
      this.show();
    }
  
    public void actionPerformed(ActionEvent event) {
       Card c;
       String command = event.getActionCommand();
       if (command.equals("ok")) {
         c = new Card(name.getText(),
                      title.getText(),
                      email.getText(),
                      phone.getText(),
                      logo.getText());
         if (current==-1) {
            cardvector.add(c);
         } else {
            cardvector.setElementAt(c,current);
         }
         addCards();
       } else if (command.equals("delete")) {
          if (current!=-1) {
            cardvector.setElementAt(null,current);
            addCards();
          }
       } else if (command.equals("clear")) {
          current = -1;
          name.setText("");
          title.setText("");
          email.setText("");
          phone.setText("");
          logo.setText("");
       } else if (command.equals("save")) {
          try {
            new XMLOutputter().output(vector2doc(),new FileOutputStream(cardfile));
          } catch (Exception e) {e.printStackTrace();}
       } else if (command.equals("quit")) {
          System.exit(0);
       } else {
          current = Integer.parseInt(command);
          c = (Card)cardvector.elementAt(current);
          name.setText(c.name);
          title.setText(c.title);
          email.setText(c.email);
          phone.setText(c.phone);
          logo.setText(c.logo);
       }
    }
  }