jdom解析xml

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

<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) 

}

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);
     }
  }
}