Effective XML

Effective XML

Elliotte Rusty Harold

Tuesday, December 13, 2005

elharo@metalab.unc.edu

http://www.cafeconleche.org/

Speaker’s Qualifications

Part I: XML Overview

XML succeeded, and in ways that weren't expected - at least not by many. Originally it was conceived as a document-oriented technology for robust quality publishing of documents over networks. the original workplan had three pillars - XML syntax, XML link, and XML stylesheets. Schemas were not high on the agenda and XML was not seen as an infrastructure for middleware or glueware. It was expected that at some stage it would be necessary to manage data but there was little activity in this area in 1997. When developing Chemical Markup Language (which must be one of the first published XML applications), I found the lack of datatypes very frustrating!

Well, XML is now a basic infrastructure of much modern information. I doubt that anyone now designs a protocol, or operating system without including XML. Although this list sometimes complains that XML isn't as clean as we would like, it works, and it works pretty well.

--Peter Murray-Rust on the xml-dev mailing list, Thursday, February 7, 2002

What is XML?

XML is a Meta Markup Language

Rule 1: Focus on Meaning, Not Appearance

A Song Description in HTML

<dt>Hot Cop
<dd> by Jacques Morali, Henri Belolo, and Victor Willis
<ul>
<li>Jacques Morali
<li>PolyGram Records
<li>6:20
<li>1978
<li>Village People
</ul>
View Document in Browser

A Song Description in XML

<?xml version="1.0"?>
<SONG>
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <LENGTH>6:20</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>
</SONG>
View Document in Browser

Elements

Rule 2: Subdivide Elements Into the Smallest Units of Information

Cascading Style Sheets

CSS Stylesheet for Songs

SONG     {display: block}
TITLE    {display: block; 
          font-family: Helvetica, sans-serif;
          font-size: 20pt; font-weight: bold;}
COMPOSER {display: block;
          font-family: Times, "Times New Roman", serif;
          font-size: 14pt;
          font-style: italic;}
ARTIST   {display: block;
          font-family: Times, "Times New Roman", serif;
          font-size: 14pt; font-weight: bold;
          font-style: italic;}
PUBLISHER {display: block;
           font-family: Times, "Times New Roman", serif;
           font-size: 14pt;}
LENGTH    {display: block;
           font-family: Times, "Times New Roman", serif;
           font-size: 14pt;}
YEAR      {display: block;
           font-family: Times, "Times New Roman", serif;
           font-size: 14pt;}

Attaching style sheets to documents

<?xml version="1.0"?>
<?xml-stylesheet type="text/css" href="song1.css"?>
<SONG>
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <LENGTH>6:20</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>
</SONG>
View Document in Browser

song.xsl

<?xml version="1.0"?>
<xsl:stylesheet version="1.0"
     xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

  <xsl:template match="SONG">
    <html>
      <body>
       <h1>
        <xsl:value-of select="TITLE"/> 
        by the 
        <xsl:value-of select="ARTIST"/> 
       </h1>
       <ul>
         <xsl:apply-templates select="COMPOSER"/>
         <li>Publisher: <xsl:value-of select="PUBLISHER"/></li>
         <li>Year: <xsl:value-of select="YEAR"/></li>
         <li>Producer: <xsl:value-of select="PRODUCER"/></li>
       </ul>
      </body>
    </html>
  </xsl:template>

  <xsl:template match="COMPOSER">
    <li>Composer: <xsl:value-of select="."/></li>
  </xsl:template>

</xsl:stylesheet>

Applying an XSLT Style Sheet

Output

<html>
   <body>
      <h1>Hot Cop 
         by the 
         Village People
      </h1>
      <ul>
         <li>Composer: Jacques Morali</li>
         <li>Composer: Henri Belolo</li>
         <li>Composer: Victor Willis</li>
         <li>Publisher: PolyGram Records</li>
         <li>Year: 1978</li>
         <li>Producer: Jacques Morali</li>
      </ul>
   </body>
</html>
View in browser

CSS or XSL?

Editing and Saving XML Documents

Rule 3: Prefer UTF-8

Well-formedness

Well-formedness Rules

Open and close all tags

Empty-element tags end with />

There is a unique root element

Elements may not overlap

Attribute values are quoted

< and & are only used to start tags and entities

Only the five predefined entity references are used

Numeric Character References

Rule 4: Never compromise on well-formedness

Validity

To be valid an XML document must be

  1. Well-formed

  2. Must have a DOCTYPE declaration specifying a Document Type Definition (DTD)

  3. Must comply with the constraints specified in the DTD

A DTD for Songs

<!ELEMENT SONG (TITLE, COMPOSER+, PRODUCER*, PUBLISHER*, 
                 LENGTH?, YEAR?, ARTIST+)>

<!ELEMENT TITLE (#PCDATA)>

<!ELEMENT COMPOSER (#PCDATA)>
<!ELEMENT PRODUCER (#PCDATA)>
<!ELEMENT PUBLISHER (#PCDATA)>
<!ELEMENT LENGTH (#PCDATA)>
<!-- This should be a four digit year like "1999",
     not a two-digit year like "99" -->
<!ELEMENT YEAR (#PCDATA)>

<!ELEMENT ARTIST (#PCDATA)>

A Valid Song Document

<?xml version="1.0"?>
<!DOCTYPE SONG SYSTEM "song.dtd">
<SONG>
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <LENGTH>6:20</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>
</SONG>

Checking Validity

To check validity you pass the document through a validating parser which should report any errors it finds. For example, 

$ xmllint --valid --noout invalidhotcop.xml
invalidhotcop.xml:10: element SONG: validity error : Element 
SONG content does not follow the DTD, expecting 
(TITLE , COMPOSER+ , PRODUCER* , PUBLISHER* , LENGTH? , 
YEAR? , ARTIST+), got (TITLE PRODUCER PUBLISHER LENGTH 
YEAR ARTIST )
</SONG>
       ^

A valid document:

$ xmllint --valid --noout validhotcop.xml
$

Well-formedness vs. validity

DTDs and Validity

What is a DTD?

Internal DTD Subsets

<?xml version="1.0"?>
<!DOCTYPE SONG [
  <!ELEMENT SONG (TITLE, COMPOSER+, PRODUCER*, PUBLISHER*, 
                  LENGTH?, YEAR?, ARTIST+)>

  <!ELEMENT TITLE (#PCDATA)>

  <!ELEMENT COMPOSER (#PCDATA)>
  <!ELEMENT PRODUCER (#PCDATA)>
  <!ELEMENT PUBLISHER (#PCDATA)>
  <!ELEMENT LENGTH (#PCDATA)>
  <!-- This should be a four digit year like "1999",
     not a two-digit year like "99" -->
  <!ELEMENT YEAR (#PCDATA)>

  <!ELEMENT ARTIST (#PCDATA)>
]>
<SONG>
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <LENGTH>6:20</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>
</SONG>

The importance of validation

Comments in DTDs

<!-- e.g. "1999 New York Women Composers", 
     not "Copyright 1999 New York Women Composers" -->
<!ELEMENT copyright (#PCDATA)>

Rule 5: Do compromise on validity

A More Complex Example

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<?xml-stylesheet type="text/css" href="song.css"?>
<!DOCTYPE SONG SYSTEM "intermediate_song.dtd">
<SONG CATEGORY="DISCO" ALBUM="The Best of the Village People">
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <LYRICS_AVAILABLE/>
  <SHEET_MUSIC_AVAILABLE />
  <!-- The publisher is actually Polygram but I needed 
       an example of a general entity reference. -->
  <PUBLISHER URL="http://www.amrecords.com/">
    A &amp; M Records
  </PUBLISHER>
  <LENGTH>6:20</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST URL="http://www.officialvillagepeople.com/">Village People</ARTIST>
  <!-- an empty element -->
  <PHOTO SRC="hotcop.jpg" ALT="Victor Willis in Cop Outfit" 
         WIDTH="100" HEIGHT="200" />
  <DESCRIPTION>
    <!-- mixed content -->
    <PERSON>Victor Willis</PERSON>'s theme song is 
    one of the lesser known of the <ARTIST>Village People</ARTIST>'s
    <IRONIC>hits</IRONIC>, and <OPINION>deservedly so</OPINION>. 
    It never charted, <!-- need to verify this? --> 
    but they didn't have enough genuine 
    hits to fill a Best Of album, <OPINION>so some lesser numbers had 
    to be <METAPHOR>pulled out of the recycle bin</METAPHOR></OPINION>.
  </DESCRIPTION>
</SONG>
<!-- You can tell what album I was 
     listening to when I wrote this example -->

The XML Declaration

Rule 6: Include an XML Declaration

Empty-element Tags

  <LYRICS_AVAILABLE />
  <SHEET_MUSIC_AVAILABLE />

Rule 7: Don't assign meaning to Empty-element Tags

Attributes

<SONG CATEGORY="DISCO" ALBUM="The Best of the Village People">
...
  <ARTIST URL="http://www.officialvillagepeople.com/">Village People</ARTIST>
  <PHOTO SRC="hotcop.jpg" ALT="Victor Willis in Cop Outfit" 
         WIDTH="100" HEIGHT="200" />

Rule 8: Use Attributes for Metadata; Elements for Content

Rule 8.5: When not to use attributes

Comments

<!-- You can tell what album I was listening to when I wrote this example -->

Rule 9: Don't Rely on Comments

Processing Instructions

Rule 10: Use processing instructions for prcess specific content

Mixed Content

  <DESCRIPTION>
    <PERSON>Victor Willis</PERSON>'s theme song is 
    one of the lesser known of the 
    <ARTIST>Village People</ARTIST>'s <IRONIC>hits</IRONIC>, 
    and <OPINION>deservedly so</OPINION>. It never 
    charted, <!-- need to verify this? --> 
    but the group didn't have enough genuine 
    hits to fill a Best Of album, <OPINION>so some lesser 
    numbers had to be <METAPHOR>pulled out of the recycle 
    bin</METAPHOR></OPINION>.
  </DESCRIPTION>

Rule 11: Provide for Mixed Content

A More Complex DTD

<!ELEMENT SONG (TITLE, PHOTO?, COMPOSER+, PRODUCER*, 
                LYRICS_AVAILABLE?, SHEET_MUSIC_AVAILABLE?,
                PUBLISHER*, LENGTH?, YEAR?, ARTIST+, 
                PHOTO?, DESCRIPTION)>

<!ATTLIST SONG CATEGORY (DISCO | POP | ROCK) #REQUIRED
               ALBUM CDATA #IMPLIED>
<!ATTLIST ARTIST URL CDATA #IMPLIED>
<!ATTLIST PUBLISHER URL CDATA #IMPLIED>

<!ELEMENT LYRICS_AVAILABLE EMPTY>
<!ELEMENT PHOTO EMPTY>
<!ELEMENT SHEET_MUSIC_AVAILABLE EMPTY>

<!ATTLIST PHOTO SRC    CDATA #REQUIRED
                ALT    CDATA #REQUIRED
                WIDTH  CDATA #REQUIRED
                HEIGHT CDATA #REQUIRED
>

<!ELEMENT ARTIST    (#PCDATA)>
<!ELEMENT METAPHOR  (#PCDATA)>
<!ELEMENT IRONIC    (#PCDATA)>
<!ELEMENT PERSON    (#PCDATA)>
<!ELEMENT TITLE     (#PCDATA)>
<!ELEMENT COMPOSER  (#PCDATA)>
<!ELEMENT PRODUCER  (#PCDATA)>
<!ELEMENT PUBLISHER (#PCDATA)>
<!ATTLIST PUBLISHER xlink:type CDATA #IMPLIED
                    xlink:href CDATA #IMPLIED
>

<!ELEMENT LENGTH (#PCDATA)>
<!-- This should be a four digit year like "1999",
     not a two-digit year like "99" -->
<!ELEMENT YEAR (#PCDATA)>

<!ELEMENT OPINION (#PCDATA | METAPHOR)*>
<!ELEMENT DESCRIPTION (#PCDATA | PERSON | IRONIC | OPINION 
                               | METAPHOR | ARTIST | TITLE)*>

Namespaces

<SONG xmlns="http://www.cafeconleche.org/namespace/song"
      xmlns:xlink="http://www.w3.org/1999/xlink">
  <TITLE>Hot Cop</TITLE>
  <PHOTO 
    xlink:type="simple" xlink:show="onLoad" xlink:href="hotcop.jpg"
    ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>Jacques Morali</COMPOSER>
  <PUBLISHER xlink:type="simple" xlink:href="http://www.amrecords.com/">
    A &amp; M Records
  </PUBLISHER>
  <ARTIST>Village People</ARTIST>
</SONG>

Raison d'etre

  1. To distinguish between elements and attributes from different vocabularies with the same names.

  2. To group all related elements and attributes together so that a parser can easily recognize them.

The Need for Namespaces

Namespaces disambiguate elements

Namespace Syntax

Namespace URIs

Binding Prefixes to Namespace URIs

Binding Prefixes to Namespace URIs Example

<xhtml:html xmlns:xhtml="http://www.w3.org/1999/xhtml"
            xmlns:xlink="http://www.w3.org/1999/xlink">
  <xhtml:head><xhtml:title>Three Namespaces</xhtml:title></xhtml:head>
  <xhtml:body>
    <xhtml:h1 align="center">An Ellipse and a Rectangle</xhtml:h1>
    <svg:svg xmlns:svg="http://www.w3.org/2000/svg" 
             width="12cm" height="10cm">
      <svg:ellipse rx="110" ry="130" />
      <svg:rect x="4cm" y="1cm" width="3cm" height="6cm" />
    </svg:svg>
    <xhtml:p xlink:type="simple" 
      xlink:href="ellipses.html">
      More about ellipses
    </xhtml:p>
    <xhtml:p xlink:type="simple" xlink:href="rectangles.html">
      More about rectangles
    </xhtml:p>
    <xhtml:hr/>
    <xhtml:p>Last Modified February 13, 2000</xhtml:p>    
  </xhtml:body>
</xhtml:html>

The Default Namespace

Unprefixed attributes are never in any namespace

Rule 13: URIs matter; not prefixes

Namespace URIs do not necessarily point to a document, page, or schema

Namespaces and DTDs

Rule 12: Prefer Numeric Character References

What is XML used for?

Domain-Specific Markup Languages

Self-Describing Data

An XML Fragment

<PERSON ID="p1100" SEX="M">
  <NAME>
    <GIVEN>Judson</GIVEN>
    <SURNAME>McDaniel</SURNAME>
  </NAME>
  <BIRTH>
    <DATE>21 Feb 1834</DATE>
  </BIRTH>
  <DEATH>
    <DATE>9 Dec 1905</DATE>
  </DEATH>
</PERSON>

Interchange of Data Among Applications

Can assemble data from multiple sources

XML Applications

Example XML Applications

Mathematical Markup Language

<?xml version="1.0"?>
<!DOCTYPE html 
     PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
            "../xhtml1/transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Fiat Lux</title>
</head>
<body>

<p>
And God said,
</p>

<math xmlns="http://www.w3.org/1998/Math/MathML">
  <mrow>
    <msub>
      <mi>&delta;</mi>
      <mi>&alpha;</mi>
    </msub>
    <msup>
      <mi>F</mi>
      <mi>&alpha;&beta;</mi>
    </msup>
    <mo>=</mo>
    <mfrac>
      <mrow>
        <mn>4</mn>
        <mi>&pi;</mi>
      </mrow>
      <mi>c</mi>
    </mfrac>
    <msup>
      <mi>J</mi>
      <mrow>
        <mi>&beta;</mi>
      </mrow>
    </msup>
  </mrow>
</math>

<p>
and there was light.
</p>
</body>
</html>
View in Browser

RSS

<?xml version="1.0"?>
<rss version="0.92">
  <channel>
    <title>Cafe con Leche XML News and Resources</title>
    <link>http://www.cafeconleche.org/</link>
    <description>Cafe con Leche is the preeminent independent source of XML information on the net. Cafe con Leche is neither beholden to specific companies nor to advertisers. At Cafe con Leche you'll find many resources to help you develop your XML skills here including daily news summaries, examples, book reviews, mailing lists and more.</description>
    <language>en-us</language>
    <copyright>Copyright 2005 Elliotte Rusty Harold</copyright>
    <webMaster>elharo@metalab.unc.edu</webMaster>
    <image>
      <title>Cafe con Leche</title>
      <url>http://www.cafeconleche.org/cup.gif</url>
      <link>http://www.cafeconleche.org/</link>
      <width>89</width>
      <height>67</height>
      <description>Cafe con Leche is the preeminent independent source of XML information on the net. Cafe con Leche is neither beholden to specific companies nor to advertisers. At Cafe con Leche you'll find many resources to help you develop your XML skills here including daily news summaries, examples, book reviews, mailing lists and more.</description>
    </image>
    <item>
      <title>IBM developerWorks has published my latest article, Encode your XML documents in UTF-8.
          </title>
      <description>IBM developerWorks has published my latest article, Encode your XML documents in UTF-8. In this article inspired by Google's Sitemaps service, I explain why I think it's time to stop bothering with other encodings and just choose UTF-8 once and for all.</description>
      <link>http://www.cafeconleche.org/#news2005September2</link>
    </item>
    <item>
      <title>The Apache XML Project has released version 2.7.0 of Xerces-C, an open source schema validating XML parser written in reasonably cross-platform C++.
          </title>
      <description>The Apache XML Project has released version 2.7.0 of Xerces-C, an open source schema validating XML parser written in reasonably cross-platform C++. Version 2.7.0 includes a number of small improvements: More...</description>
      <link>http://www.cafeconleche.org/#news2005September2</link>
    </item>
  </channel>
</rss>
Today's News on Cafe con Leche

Books

DocBook
OpenOffice
TEI
Over 100 different corpora including:

Vector Graphics

An SVG document

SOAP

WSDL

Database interchange and export

Narrative vs. Record-Like

XML is not:

XML for XML

XSL: The Extensible Stylesheet Language

Schemas

W3C XML Schema Language Example

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <xsd:element name="SONG" type="SongType"/>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE" type="xsd:string" 
                   minOccurs="1" maxOccurs="1"/>
      <xsd:element name="COMPOSER"  type="xsd:string" 
                   minOccurs="1" maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
                   minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
                   minOccurs="0" maxOccurs="1"/>
    
      <xsd:element name="LENGTH" type="xsd:duration" 
                   minOccurs="0" maxOccurs="1"/>
      <xsd:element name="YEAR"   type="xsd:gYear" 
                   minOccurs="1" maxOccurs="1"/>
  
      <xsd:element name="ARTIST" type="xsd:string" 
                   minOccurs="1" maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

</xsd:schema>

XLinks

<footnote xlink:type="simple" xlink:href="footnote7.xml">7</footnote>

File Formats, in-house applications, and other behind the scenes uses

When should you not use XML?

Processing XML

SAX

Processing XML with Java is easy

XML API Styles

Parser APIs

XML Infoset

The Infoset is the unfortunate standard to which those in retreat from the radical and most useful implications of well-formedness have rallied. At its core the Infoset insists that there is 'more' to XML than the straightforward syntax of well-formedness. By imposing its canonical semantics the Infoset obviates the infinite other semantic outcomes which might be elaborated in particular unique circumstances from an instance of well-formed XML 1.0 syntax. The question we should be asking is not whether the Infoset has chosen the correct canonical semantics, but whether the syntactic possibilities of XML 1.0 should be curtailed in this way at all.
--Walter Perry on the xml-dev mailing list

Markup and Character Data

Markup and Character Data Example

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<?xml-stylesheet type="text/css" href="song.css"?>
<!DOCTYPE SONG SYSTEM "song.dtd">
<SONG xmlns="http://www.cafeconleche.org/namespace/song"
      xmlns:xlink="http://www.w3.org/1999/xlink">
  <TITLE>Hot Cop</TITLE>
  <PHOTO 
    xlink:type="simple" xlink:show="onLoad" xlink:href="hotcop.jpg"
    ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <!-- The publisher is actually Polygram but I needed 
       an example of a general entity reference. -->
  <PUBLISHER xlink:type="simple" xlink:href="http://www.amrecords.com/">
    A &amp; M Records
  </PUBLISHER>
  <LENGTH>6:20</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>
</SONG>
<!-- You can tell what album I was 
     listening to when I wrote this example -->

Entities

Parsed Character Data

CDATA sections

How Parsers Handle Namespaces

The Five Layers of XML Processing

Semantics
Structure
Syntax
Lexical
Binary

Writing XML Documents with Java

I have learned to be even more skeptical than before about the slew of APIs doing the rounds in the XML development community. An XML instance is just a documents, guys; you need to understand the document structure and document interchange choreography of your systems. Don't let some API get in the way of your understanding of XML systems at the document level. If you do, you run the risk becoming a slave to the APIs and hitting a wall when the APIs fail you.

--Sean McGrath
Read the rest in ITworld.com - XML IN PRACTICE - APIs Considered Harmful

You don't always need a new API

Unicode

Readers and Writers

A Java program that writes Fibonacci numbers into a text file

import java.math.BigInteger;
import java.io.*;


public class FibonacciText {

  public static void main(String[] args) {

    try {
      OutputStream fout = new FileOutputStream("fibonacci.txt");
      Writer out = new OutputStreamWriter(fout, "8859_1");

      BigInteger low  = BigInteger.ONE;
      BigInteger high = BigInteger.ONE;

      for (int i = 1; i <= 25; i++) {
        out.write(low.toString() + "\r\n");
        BigInteger temp = high;
        high = high.add(low);
        low = temp;
      }
      out.write(high.toString() + "\r\n");

      out.close();
    }
    catch (IOException e) {
      System.err.println(e);
    }

  }

}

fibonacci.txt

1
1
2
3
5
8
13
21
34
55
89
144
233
377
610
987
1597
2584
4181
6765
10946
17711
28657
46368
75025
121393
317811

A Java program that writes Fibonacci numbers into an XML file

import java.math.BigInteger;
import java.io.*;


public class FibonacciXML {

  public static void main(String[] args) {
   
    try {
      OutputStream  fout = new FileOutputStream("fibonacci.xml");
      Writer out = new OutputStreamWriter(fout, "UTF-8");      
      
      BigInteger low  = BigInteger.ONE;
      BigInteger high = BigInteger.ONE;      
      
      out.write("<?xml version=\"1.0\"?>\r\n");  
      out.write("<Fibonacci_Numbers>\r\n");  
      for (int i = 1; i <= 25; i++) {
        out.write("  <fibonacci index=\"" + i + "\">");
        out.write(low.toString());
        out.write("</fibonacci>\r\n");
        BigInteger temp = high;
        high = high.add(low);
        low = temp;
      }
      out.write("</Fibonacci_Numbers>");  
 
      out.close();
    }
    catch (IOException ex) {
      System.err.println(ex);
    }

  }

}

fibonacci.xml

<?xml version="1.0"?>
<Fibonacci_Numbers>
  <fibonacci index="1">1</fibonacci>
  <fibonacci index="2">1</fibonacci>
  <fibonacci index="3">2</fibonacci>
  <fibonacci index="4">3</fibonacci>
  <fibonacci index="5">5</fibonacci>
  <fibonacci index="6">8</fibonacci>
  <fibonacci index="7">13</fibonacci>
  <fibonacci index="8">21</fibonacci>
  <fibonacci index="9">34</fibonacci>
  <fibonacci index="10">55</fibonacci>
  <fibonacci index="11">89</fibonacci>
  <fibonacci index="12">144</fibonacci>
  <fibonacci index="13">233</fibonacci>
  <fibonacci index="14">377</fibonacci>
  <fibonacci index="15">610</fibonacci>
  <fibonacci index="16">987</fibonacci>
  <fibonacci index="17">1597</fibonacci>
  <fibonacci index="18">2584</fibonacci>
  <fibonacci index="19">4181</fibonacci>
  <fibonacci index="20">6765</fibonacci>
  <fibonacci index="21">10946</fibonacci>
  <fibonacci index="22">17711</fibonacci>
  <fibonacci index="23">28657</fibonacci>
  <fibonacci index="24">46368</fibonacci>
  <fibonacci index="25">75025</fibonacci>
</Fibonacci_Numbers>

Single quoted attribute values are a little cleaner

import java.math.BigInteger;
import java.io.*;


public class FibonacciApos {

  public static void main(String[] args) {
   
    try {
      OutputStream  fout = new FileOutputStream("fibonacci_apos.xml");
      Writer out = new OutputStreamWriter(fout);      
      
      BigInteger low  = BigInteger.ONE;
      BigInteger high = BigInteger.ONE;      
      
      out.write("<?xml version='1.0'?>\r\n");  
      out.write("<Fibonacci_Numbers>\r\n");  
      for (int i = 1; i <= 25; i++) {
        out.write("  <fibonacci index='" + i + "'>");
        out.write(low.toString());
        out.write("</fibonacci>\r\n");
        BigInteger temp = high;
        high = high.add(low);
        low = temp;
      }
      out.write("</Fibonacci_Numbers>");  
 
      out.close();
    }
    catch (IOException e) {
      System.err.println(e);
    }

  }

}

Suppose you want to include a DTD

import java.math.BigInteger;
import java.io.*;


public class FibonacciDTD {

  public static void main(String[] args) {
   
    try {
      OutputStream fout = new FileOutputStream("valid_fibonacci.xml");
      Writer out = new OutputStreamWriter(fout, "UTF-8");      
      
      BigInteger low  = BigInteger.ONE;
      BigInteger high = BigInteger.ONE;      
      
      out.write("<?xml version=\"1.0\"?>\r\n");  
      out.write("<!DOCTYPE Fibonacci_Numbers [\r\n");
      out.write("  <!ELEMENT Fibonacci_Numbers (fibonacci*)>\r\n");      
      out.write("  <!ELEMENT fibonacci (#PCDATA)>\r\n");      
      out.write("  <!ATTLIST fibonacci index CDATA #IMPLIED>\r\n");      
      out.write("]>\r\n");  
      out.write("<Fibonacci_Numbers>\r\n");  
      for (int i = 1; i <= 25; i++) {
        out.write("  <fibonacci index=\"" + i + "\">");
        out.write(low.toString());
        out.write("</fibonacci>\r\n");
        
        BigInteger temp = high;
        high = high.add(low);
        low = temp;
      }
      out.write("</Fibonacci_Numbers>");  
 
      out.close();
    }
    catch (IOException e) {
      System.err.println(e);
    }

  }

}

valid_fibonacci.xml

<?xml version="1.0"?>
<!DOCTYPE Fibonacci_Numbers [
  <!ELEMENT Fibonacci_Numbers (fibonacci*)>
  <!ELEMENT fibonacci (#PCDATA)>
  <!ATTLIST fibonacci index CDATA #IMPLIED>
]>
<Fibonacci_Numbers>
  <fibonacci index="0">0</fibonacci>
  <fibonacci index="1">1</fibonacci>
  <fibonacci index="2">1</fibonacci>
  <fibonacci index="3">2</fibonacci>
  <fibonacci index="4">3</fibonacci>
  <fibonacci index="5">5</fibonacci>
  <fibonacci index="6">8</fibonacci>
  <fibonacci index="7">13</fibonacci>
  <fibonacci index="8">21</fibonacci>
  <fibonacci index="9">34</fibonacci>
  <fibonacci index="10">55</fibonacci>
  <fibonacci index="11">89</fibonacci>
  <fibonacci index="12">144</fibonacci>
  <fibonacci index="13">233</fibonacci>
  <fibonacci index="14">377</fibonacci>
  <fibonacci index="15">610</fibonacci>
  <fibonacci index="16">987</fibonacci>
  <fibonacci index="17">1597</fibonacci>
  <fibonacci index="18">2584</fibonacci>
  <fibonacci index="19">4181</fibonacci>
  <fibonacci index="20">6765</fibonacci>
  <fibonacci index="21">10946</fibonacci>
  <fibonacci index="22">17711</fibonacci>
  <fibonacci index="23">28657</fibonacci>
  <fibonacci index="24">46368</fibonacci>
</Fibonacci_Numbers>

To Learn More

Reading XML Documents with SAX

Actually, SAX2 has ** MUCH ** better infoset support than DOM does. Yes, I've done the detailed analysis.

--David Brownell on the xml-dev mailing list

Reading XML Documents

SAX

SAX Parsers for Java

Parser URL Validating Namespaces SAX1 SAX2 License
Yuval Oren's Piccolo http://piccolo.sourceforge.net/ X X X LGPL
Apache XML Project's Xerces Java http://xml.apache.org/xerces2-j/index.html X X X X Apache Software License, Version 1.1
IBM's XML for Java http://www.alphaworks.ibm.com/formula/xml X X X X Apache Software License, Version 1.1
Ælfred http://www.gnu.org/software/classpathx/jaxp/jaxp.html X X X X GPL with library exception
Sun's Crimson http://xml.apache.org/crimson/ X X X X Apache
Oracle's XML Parser for Java http://technet.oracle.com/ X X X Xfree beer
Caucho Resin http://www.caucho.com/products/resin-xml/index.xtp ? X X Xpayware
Saxon's AElfred http://saxon.sourceforge.net/aelfred.html   X X XBSD-ish license

The Horrors of the CLASSPATH

The SAX2 Process

  1. Use the factory method XMLReaderFactory.createXMLReader() to retrieve a parser-specific implementation of the XMLReader interface

  2. Your code registers a ContentHandler with the parser

  3. An InputSource feeds the document into the parser

  4. As the document is read, the parser calls back to the methods of the ContentHandler to tell it what it's seeing in the document.

Making an XMLReader

Parsing a Document with XMLReader

import org.xml.sax.*;
import org.xml.sax.helpers.*;
import java.io.*;


public class SAX2Checker {

  public static void main(String[] args) {

    XMLReader parser;
    try {
      parser = XMLReaderFactory.createXMLReader();
    }
    catch (SAXException ex) {
      try {
        parser = XMLReaderFactory.createXMLReader(
         "org.apache.xerces.parsers.SAXParser");
      }
      catch (SAXException ex2) {
        System.out.println("Could not locate a parser."
         + "Please set the the org.xml.sax.driver property.");
        return;
      }
    }

    if (args.length == 0) {
      System.out.println("Usage: java SAX2Checker URL1 URL2...");
    }

    // start parsing...
    for (int i = 0; i < args.length; i++) {

      // command line should offer URIs or file names
      try {
        parser.parse(args[i]);
        // If there are no well-formedness errors
        // then no exception is thrown
        System.out.println(args[i] + " is well formed.");
      }
      catch (SAXParseException e) { // well-formedness error
        System.out.println(args[i] + " is not well formed.");
        System.out.println(e.getMessage()
         + " at line " + e.getLineNumber()
         + ", column " + e.getColumnNumber());
      }
      catch (SAXException e) { // some other kind of error
        System.out.println(e.getMessage());
      }
      catch (IOException e) {
        System.out.println("Could not check " + args[i]
         + " because of the IOException " + e);
      }

    }

  }

}

Sample Output from SAX2Checker

C:\>java SAX2Checker http://www.cafeconleche.org/
http://www.cafeconleche.org/ is not well formed.
The element type "dt" must be terminated by the 
matching end-tag "</dt>". 
at line 186, column 5

JAXP Brain Damage

The ContentHandler interface

package org.xml.sax;


public interface ContentHandler {

    public void setDocumentLocator(Locator locator);
    
    public void startDocument() throws SAXException;
    
    public void endDocument() throws SAXException;
    
    public void startPrefixMapping(String prefix, String uri) 
     throws SAXException;

    public void endPrefixMapping(String prefix) throws SAXException;

    public void startElement(String namespaceURI, String localName,
     String qualifiedName, Attributes atts) throws SAXException;

    public void endElement(String namespaceURI, String localName,
     String qualifiedName) throws SAXException;

    public void characters(char[] text, int start, int length) 
     throws SAXException;

    public void ignorableWhitespace(char[] text, int start, int length)
     throws SAXException;

    public void processingInstruction(String target, String data)
     throws SAXException;

    public void skippedEntity(String name) throws SAXException;
     
}

A Sample Application

Full list

Goal: Return a list of all the URLs in this list as java.net.URL objects

Design Decisions

SAX Design

User Interface Class

import org.xml.sax.*;
import org.xml.sax.helpers.XMLReaderFactory;
import java.util.*;
import java.io.*;


public class WeblogsSAX {
     
  public static List listChannels() 
   throws IOException, SAXException {
    return listChannels(
     "http://static.userland.com/weblogMonitor/logs.xml"); 
  }
  
  public static List listChannels(String uri) 
   throws IOException, SAXException {
    
    XMLReader parser;
    try {
      parser = XMLReaderFactory.createXMLReader();
    }
    catch (SAXException ex) {
      parser = XMLReaderFactory.createXMLReader(
       "org.apache.xerces.parsers.SAXParser"
      );
    }
    Vector urls = new Vector(1000);
    ContentHandler handler = new URIGrabber(urls);
    parser.setContentHandler(handler);
    parser.parse(uri);
    return urls;
    
  }
  
  public static void main(String[] args) {
   
    try {
      List urls;
      if (args.length > 0) urls = listChannels(args[0]);
      else urls = listChannels();
      Iterator iterator = urls.iterator();
      while (iterator.hasNext()) {
        System.out.println(iterator.next()); 
      }
    }
    catch (IOException e) {
      System.err.println(e); 
    }
    catch (SAXParseException e) {
      System.err.println(e); 
      System.err.println("at line " + e.getLineNumber() 
       + ", column " + e.getColumnNumber()); 
    }
    catch (SAXException e) {
      System.err.println(e); 
    }
    catch (/* Unexpected */ Exception e) {
      e.printStackTrace(); 
    }
    
  }
  
}

ContentHandler Class

import org.xml.sax.*;
import java.net.*;
import java.util.Vector;

             // conflicts with java.net.ContentHandler
class URIGrabber implements org.xml.sax.ContentHandler {

  private Vector urls;

  URIGrabber(Vector urls) {
    this.urls = urls;
  }

  // do nothing methods
  public void setDocumentLocator(Locator locator) {}
  public void startDocument() throws SAXException {}
  public void endDocument() throws SAXException {}
  public void startPrefixMapping(String prefix, String uri)
   throws SAXException {}
  public void endPrefixMapping(String prefix) throws SAXException {}
  public void skippedEntity(String name) throws SAXException {}
  public void ignorableWhitespace(char[] text, int start, int length)
   throws SAXException {}
  public void processingInstruction(String target, String data)
   throws SAXException {}


  // Remember, there's no guarantee all the text of the
  // url element will be returned in a single call to characters
  private StringBuffer urlBuffer;
  private boolean collecting = false;

  public void startElement(String namespaceURI, String localName,
   String qualifiedName, Attributes atts) throws SAXException {

    if (qualifiedName.equals("url")) {
      collecting = true;
      urlBuffer = new StringBuffer();
    }

  }

  public void characters(char[] text, int start, int length)
   throws SAXException {

    if (collecting) {
      urlBuffer.append(text, start, length);
    }

  }

  public void endElement(String namespaceURI, String localName,
   String qualifiedName) throws SAXException {

    if (qualifiedName.equals("url")) {
      collecting = false;
      String url = urlBuffer.toString();
      try {
        urls.addElement(new URL(url));
      }
      catch (MalformedURLException e) {
        // skip this url
      }
    }

  }

}

Weblogs Output

% java Weblogs shortlogs.xml
http://www.mozillazine.org
http://www.salonherringwiredfool.com/
http://www.slashdot.org/

Event Based API Caveats

To Learn More


DOM, The Document Object Model

The DOM (like XML) is not a triumph of elegance; it's a triumph of "if we do not hang together, we shall hang separately." At least the Browser Wars were not followed by API Wars. Better a common API that we all love to hate than a bazillion contending APIs that carve the Web up into contending enclaves of True Believers.

--Mike Champion on the xml-dev mailing list, Thursday, September 27, 2001

Trees

Document Object Model

DOM Evolution

DOM Implementations for Java

Eight Modules:

DOM Trees

org.w3c.dom

The DOM Process

  1. Library specific code creates a parser

  2. The parser parses the document and returns a DOM org.w3c.dom.Document object.

  3. The entire document is stored in memory.

  4. DOM methods and interfaces are used to extract data from this object

Parsing documents with a DOM Parser Example 

import org.apache.xerces.parsers.*;
import org.w3c.dom.*;
import org.xml.sax.*;
import java.io.*;

public class DOMParserMaker {

  public static void main(String[] args) {
     
    // This is simpler but less flexible than the SAX approach.
    // Perhaps a good creational design pattern is needed here?   
  
    DOMParser parser = new DOMParser();
    
    for (int i = 0; i < args.length; i++) {
      try {
        // Read the entire document into memory
        parser.parse(args[i]); 
       
        Document d = parser.getDocument();
        // work with the document...
      }
      catch (SAXException e) {
        System.err.println(e); 
      }
      catch (IOException e) {
        System.err.println(e); 
      }
      
    }
   
  }

}

The JAXP Process

  1. javax.xml.parsers.DocumentBuilderFactory.newInstance() creates a DocumentBuilderFactory

  2. Configure the factory

  3. The factory's newBuilder() method creates a DocumentBuilder

  4. Configure the builder

  5. The builder parses the document and returns a DOM org.w3c.dom.Document object.

  6. The entire document is stored in memory.

  7. DOM methods and interfaces are used to extract data from this object

Parsing documents with a JAXP DocumentBuilder 

import javax.xml.parsers.*;
import org.w3c.dom.*;
import org.xml.sax.*;
import java.io.*;

public class JAXPParserMaker {

  public static void main(String[] args) {
     
    try {       
      DocumentBuilderFactory builderFactory 
       = DocumentBuilderFactory.newInstance();
      builderFactory.setNamespaceAware(true);
      DocumentBuilder parser 
       = builderFactory.newDocumentBuilder();
    
      for (int i = 0; i < args.length; i++) {
        try {
          // Read the entire document into memory
          Document d = parser.parse(args[i]); 
          // work with the document...
        }
        catch (SAXException e) {
        System.err.println(e); 
        }
        catch (IOException e) {
          System.err.println(e); 
        }
      
      } // end for
      
    }
    catch (ParserConfigurationException e) {
      System.err.println("You need to install a JAXP aware parser.");
    }
   
  }

}

The Node Interface 

package org.w3c.dom;

public interface Node {

  // NodeType
  public static final short ELEMENT_NODE                = 1;
  public static final short ATTRIBUTE_NODE              = 2;
  public static final short TEXT_NODE                   = 3;
  public static final short CDATA_SECTION_NODE          = 4;
  public static final short ENTITY_REFERENCE_NODE       = 5;
  public static final short ENTITY_NODE                 = 6;
  public static final short PROCESSING_INSTRUCTION_NODE = 7;
  public static final short COMMENT_NODE                = 8;
  public static final short DOCUMENT_NODE               = 9;
  public static final short DOCUMENT_TYPE_NODE          = 10;
  public static final short DOCUMENT_FRAGMENT_NODE      = 11;
  public static final short NOTATION_NODE               = 12;

  public String       getNodeName();
  public String       getNodeValue() throws DOMException;
  public void         setNodeValue(String nodeValue) throws DOMException;
  public short        getNodeType();
  public Node         getParentNode();
  public NodeList     getChildNodes();
  public Node         getFirstChild();
  public Node         getLastChild();
  public Node         getPreviousSibling();
  public Node         getNextSibling();
  public NamedNodeMap getAttributes();
  public Document     getOwnerDocument();
  public Node         insertBefore(Node newChild, Node refChild) throws DOMException;
  public Node         replaceChild(Node newChild, Node oldChild) throws DOMException;
  public Node         removeChild(Node oldChild) throws DOMException;
  public Node         appendChild(Node newChild) throws DOMException;
  public boolean      hasChildNodes();
  public Node         cloneNode(boolean deep);
  public void         normalize();
  public boolean      supports(String feature, String version);
  public String       getNamespaceURI();
  public String       getPrefix();
  public void         setPrefix(String prefix) throws DOMException;
  public String       getLocalName();
  
}

The NodeList Interface 

package org.w3c.dom;

public interface NodeList {
  public Node item(int index);
  public int  getLength();
}

Now we're really ready to read a document

Node Reporter

import javax.xml.parsers.*;
import org.w3c.dom.*;
import org.xml.sax.*;
import java.io.*;


public class NodeReporter {

  public static void main(String[] args) {
     
    try {       
      DocumentBuilderFactory builderFactory 
       = DocumentBuilderFactory.newInstance();
      DocumentBuilder parser 
       = builderFactory.newDocumentBuilder();
      NodeReporter iterator = new NodeReporter();
        
      for (int i = 0; i < args.length; i++) {
        try {
          // Read the entire document into memory
          Document doc = parser.parse(args[i]); 
          iterator.followNode(doc);
        }
        catch (SAXException ex) {
          System.err.println(args[i] + " is not well-formed."); 
        }
        catch (IOException ex) {
          System.err.println(ex); 
        }
      }
    }
    catch (ParserConfigurationException ex) {
      System.err.println("You need to install a JAXP aware parser.");
    }
  
  } // end main

  // note use of recursion
  public void followNode(Node node) {
    
    processNode(node);
    if (node.hasChildNodes()) {
      NodeList children = node.getChildNodes();
      for (int i = 0; i < children.getLength(); i++) {
        followNode(children.item(i));
      } 
    }
    
  }

  public void processNode(Node node) {
    
    String name = node.getNodeName();
    String type = getTypeName(node.getNodeType());
    System.out.println("Type " + type + ": " + name);
    
  }
  
  public static String getTypeName(int type) {
    
    switch (type) {
      case Node.ELEMENT_NODE: 
        return "Element";
      case Node.ATTRIBUTE_NODE: 
        return "Attribute";
      case Node.TEXT_NODE: 
        return "Text";
      case Node.CDATA_SECTION_NODE: 
        return "CDATA Section";
      case Node.ENTITY_REFERENCE_NODE: 
        return "Entity Reference";
      case Node.ENTITY_NODE: 
        return "Entity";
      case Node.PROCESSING_INSTRUCTION_NODE: 
        return "Processing Instruction";
      case Node.COMMENT_NODE : 
        return "Comment";
      case Node.DOCUMENT_NODE: 
        return "Document";
      case Node.DOCUMENT_TYPE_NODE: 
        return "Document Type Declaration";
      case Node.DOCUMENT_FRAGMENT_NODE: 
        return "Document Fragment";
      case Node.NOTATION_NODE: 
        return "Notation";
      default: 
        return "Unknown Type"; 
    }
    
  }

}

Node Reporter Output

% java NodeReporter hotcop.xml
Type Document: #document
Type Processing Instruction: xml-stylesheet
Type Document Type Declaration: SONG
Type Element: SONG
Type Text: #text
Type Element: TITLE
Type Text: #text
Type Text: #text
Type Element: PHOTO
Type Text: #text
Type Element: COMPOSER
Type Text: #text
Type Text: #text
Type Element: COMPOSER
Type Text: #text
Type Text: #text
Type Element: COMPOSER
Type Text: #text
Type Text: #text
Type Element: PRODUCER
Type Text: #text
Type Text: #text
Type Comment: #comment
Type Text: #text
Type Element: PUBLISHER
Type Text: #text
Type Text: #text
Type Element: LENGTH
Type Text: #text
Type Text: #text
Type Element: YEAR
Type Text: #text
Type Text: #text
Type Element: ARTIST
Type Text: #text
Type Text: #text
Type Comment: #comment

Attributes are missing from this output. They are not nodes. They are properties of nodes.

Node Values as returned by getNodeValue()

Node TypeNode Value
element nodenull
attribute nodeattribute value
text nodetext of the node
CDATA section nodetext of the section
entity reference nodenull
entity nodenull
processing instruction nodecontent of the processing instruction, not including the target
comment nodetext of the comment
document nodenull
document type declaration nodenull
document fragment nodenull
notation nodenull

The Document Node

The Document Interface 

package org.w3c.dom;

  public interface Document extends Node {
  
    public DocumentType      getDoctype();
    public DOMImplementation getImplementation();
    public Element           getDocumentElement();

    public NodeList        getElementsByTagName(String tagname);
    public NodeList        getElementsByTagNameNS(String namespaceURI, String localName);
    public Element         getElementById(String elementId);

    // Factory methods    
    public Element           createElement(String tagName) throws DOMException;
    public Element           createElementNS(String namespaceURI, String qualifiedName) throws DOMException;
    public DocumentFragment  createDocumentFragment();
    public Text              createTextNode(String data);
    public Comment           createComment(String data);
    public CDATASection      createCDATASection(String data) throws DOMException;
    public ProcessingInstruction createProcessingInstruction(String target, String data)
     throws DOMException;
    public Attr            createAttribute(String name) throws DOMException;
    public Attr            createAttributeNS(String namespaceURI, String qualifiedName) throws DOMException;
    public EntityReference createEntityReference(String name) throws DOMException;

    public Node            importNode(Node importedNode, boolean deep) throws DOMException;
    
}

A Sample Application

Full list

DOM Design

Weblogs with DOM

import org.w3c.dom.*;
import org.xml.sax.SAXException;
import java.io.IOException;
import java.util.*;
import java.net.*;


public class WeblogsDOM {

  public static String DEFAULT_URL
   = "http://static.userland.com/weblogMonitor/logs.xml";

  public static List listChannels() throws DOMException {
    return listChannels(DEFAULT_URL);
  }

  public static List listChannels(String uri) throws DOMException {

    if (uri == null) {
      throw new NullPointerException("URL must be non-null");
    }

    org.apache.xerces.parsers.DOMParser parser
     = new org.apache.xerces.parsers.DOMParser();

    Vector urls = null;

    try {
      // Read the entire document into memory
      parser.parse(uri);
      Document doc = parser.getDocument();
      NodeList logs = doc.getElementsByTagName("url");

      urls = new Vector(logs.getLength());

      for (int i = 0; i < logs.getLength(); i++) {
        try {
          Node element = logs.item(i);
          Node text = element.getFirstChild();
          String content = text.getNodeValue();
          URL u = new URL(content);
          urls.addElement(u);
        }
        catch (MalformedURLException e) {
          // bad input data from one third party; just ignore it
        }
      }
    }
    catch (SAXException e) {
      System.err.println(e);
    }
    catch (IOException e) {
      System.err.println(e);
    }

    return urls;

  }

  public static void main(String[] args) {

    try {
      List urls;
      if (args.length > 0) {
        try {
          URL url = new URL(args[0]);
          urls = listChannels(args[0]);
        }
        catch (MalformedURLException e) {
          System.err.println("Usage: java WeblogsDOM url");
          return;
        }
      }
      else {
        urls = listChannels();
      }
      Iterator iterator = urls.iterator();
      while (iterator.hasNext()) {
        System.out.println(iterator.next());
      }
    }
    catch (/* Unexpected */ Exception e) {
      e.printStackTrace();
    }

  } // end main

}

Weblogs Output

% java WeblogsDOM
http://2020Hindsight.editthispage.com/
http://www.sff.net/people/mitchw/weblog/weblog.htp
http://nate.weblogs.com/
http://plugins.launchpoint.net
http://404.psistorm.net
http://home.att.net/~geek9000
http://daubnet.tzo.com/weblog
several hundred more...

Element Nodes

The Element Interface

package org.w3c.dom;

public interface Element extends Node {

  public String   getTagName();

  public NodeList getElementsByTagName(String name);
  public NodeList getElementsByTagNameNS(String namespaceURI, 
   String localName);

  public String   getAttribute(String name);
  public String   getAttributeNS(String namespaceURI, 
   String localName);
  public void     setAttribute(String name, String value) 
   throws DOMException;
  public void     setAttributeNS(String namespaceURI, 
   String qualifiedName, String value) throws DOMException;
  public void     removeAttribute(String name) throws DOMException;
  public void     removeAttributeNS(String namespaceURI, 
   String localName) throws DOMException;
  public Attr     getAttributeNode(String name);
  public Attr     getAttributeNodeNS(String namespaceURI, String localName);
  public Attr     setAttributeNode(Attr newAttr) throws DOMException;
  public Attr     setAttributeNodeNS(Attr newAttr) throws DOMException;
  public Attr     removeAttributeNode(Attr oldAttr) throws DOMException;

}

IDTagger

import org.apache.xerces.parsers.DOMParser;
import org.w3c.dom.*;
import org.xml.sax.*;
import java.io.IOException;
import org.apache.xml.serialize.*;


public class IDTagger {

  int id = 1;

  public void processNode(Node node) {
    
    if (node.getNodeType() == Node.ELEMENT_NODE) {
      
      Element element = (Element) node;
      String currentID = element.getAttribute("ID");
      if (currentID == null || currentID.equals("")) {
        element.setAttribute("ID", "_" + id);
        id = id + 1; 
      }
    }
    
  }

  // note use of recursion
  public void followNode(Node node) {
    
    processNode(node);
    if (node.hasChildNodes()) {
      NodeList children = node.getChildNodes();
      for (int i = 0; i < children.getLength(); i++) {
        followNode(children.item(i));
      } 
    }
    
  }

  public static void main(String[] args) {
     
    DOMParser parser  = new DOMParser();
    IDTagger iterator = new IDTagger();
    
    for (int i = 0; i < args.length; i++) {
      try {
        // Read the entire document into memory
        parser.parse(args[i]); 
       
        Document document = parser.getDocument();
        iterator.followNode(document);
        
        // now we serialize the document...
        OutputFormat format = new OutputFormat(document);
        XMLSerializer serializer 
         = new XMLSerializer(System.out, format);
        serializer.serialize(document);       
        
      }
      catch (SAXException e) {
        System.err.println(e); 
      }
      catch (IOException e) {
        System.err.println(e); 
      }
      
    }
  
  } // end main

}

Output from IDTagger

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE SONG SYSTEM "song.dtd">
<?xml-stylesheet type="text/css" href="song.css"?><!-- This should be a four digit year like "1999",
     not a two-digit year like "99" --><SONG xmlns="http://www.cafeconleche.org/namespace/song" ID="_1" xmlns:xlink="http://www.w3.org/1999/xlink">   <TITLE ID="_2">Hot Cop</TITLE>   <PHOTO ALT="Victor Willis in Cop Outfit" HEIGHT="200" ID="_3" WIDTH="100" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="hotcop.jpg" xlink:show="onLoad" xlink:type="simple"/>   <COMPOSER ID="_4">Jacques Morali</COMPOSER>   <COMPOSER ID="_5">Henri Belolo</COMPOSER>   <COMPOSER ID="_6">Victor Willis</COMPOSER>   <PRODUCER ID="_7">Jacques Morali</PRODUCER>   <!-- The publisher is actually Polygram but I needed 
       an example of a general entity reference. -->   <PUBLISHER ID="_8" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.amrecords.com/" xlink:type="simple">     A &amp; M Records   </PUBLISHER>   <LENGTH ID="_9">6:20</LENGTH>   <YEAR ID="_10">1978</YEAR>   <ARTIST ID="_11">Village People</ARTIST> </SONG><!-- You can tell what album I was 
     listening to when I wrote this example -->
View Output in Browser

Namespaces in DOM

Writing XML Documents with DOM

The DOMImplementation interface

package org.w3c.dom;

public interface DOMImplementation {

  public boolean hasFeature(String feature, String version) 
  
  public DocumentType createDocumentType(String qualifiedName, 
   String publicID, String systemID, String internalSubset)
                                          
  public Document createDocument(String namespaceURI, 
   String qualifiedName, DocumentType doctype)
   throws DOMException

}

org.apache.xerces.dom.DOMImplementationImpl

package org.apache.xerces.dom;

public class DOMImplementationImpl implements DOMImplementation {

  public boolean hasFeature(String feature, String version) 
  
  public static DOMImplementation getDOMImplementation()
  
  public DocumentType createDocumentType(String qualifiedName, 
   String publicID, String systemID, String internalSubset)
                                          
  public Document createDocument(String namespaceURI, 
   String qualifiedName, DocumentType doctype)
   throws DOMException

}

A Xerces/DOM program that writes Fibonacci numbers into an XML document

import java.math.BigInteger;
import java.io.*;
import org.w3c.dom.*;
import org.apache.xerces.dom.*;


public class FibonacciDOM {

  public static void main(String[] args) {

    try {

      DOMImplementation impl 
       = DOMImplementationImpl.getDOMImplementation();

      Document fibonacci 
       = impl.createDocument(null, "Fibonacci_Numbers", null);

      BigInteger low  = BigInteger.ONE;
      BigInteger high = BigInteger.ONE;

      Element root = fibonacci.getDocumentElement();

      for (int i = 1; i <= 25; i++) {
        Element number = fibonacci.createElement("fibonacci");
        number.setAttribute("index", Integer.toString(i));
        Text text = fibonacci.createTextNode(low.toString());
        number.appendChild(text);
        root.appendChild(number);

        BigInteger temp = high;
        high = high.add(low);
        low = temp;
      }

      // Now the document has been created and exists in memory
    }
    catch (DOMException e) {
      e.printStackTrace();
    }

  }

}

A JAXP/DOM program that writes Fibonacci numbers into an XML document

import java.math.BigInteger;
import java.io.*;
import org.w3c.dom.*;
import javax.xml.parsers.*;


public class FibonacciJAXP {

  public static void main(String[] args) {

    try {       
      DocumentBuilderFactory factory 
       = DocumentBuilderFactory.newInstance();
      DocumentBuilder builder = factory.newDocumentBuilder();
      DOMImplementation impl = builder.getDOMImplementation();

      Document fibonacci 
       = impl.createDocument(null, "Fibonacci_Numbers", null);

      BigInteger low  = BigInteger.ONE;
      BigInteger high = BigInteger.ONE;

      Element root = fibonacci.getDocumentElement();

      for (int i = 1; i <= 25; i++) {
        Element number = fibonacci.createElement("fibonacci");
        number.setAttribute("index", Integer.toString(i));
        Text text = fibonacci.createTextNode(low.toString());
        number.appendChild(text);
        root.appendChild(number);

        BigInteger temp = high;
        high = high.add(low);
        low = temp;
      }

      // Now the document has been created and exists in memory
    }
    catch (DOMException e) {
      e.printStackTrace();
    }
    catch (ParserConfigurationException e) {
      System.err.println("You need to install a JAXP aware DOM implementation.");
    }
    
  }

}

Serialization

A DOM program that writes Fibonacci numbers onto System.out

import java.math.BigInteger;
import java.io.*;
import org.w3c.dom.*;
import org.apache.xerces.dom.*;
import org.apache.xml.serialize.*; 


public class FibonacciDOMSerializer {

  public static void main(String[] args) {
   
    try {
      
      DOMImplementation impl 
       = DOMImplementationImpl.getDOMImplementation();

      Document fibonacci 
       = impl.createDocument(null, "Fibonacci_Numbers", null);
      
      BigInteger low  = BigInteger.ONE;
      BigInteger high = BigInteger.ONE;      
      
      Element root = fibonacci.getDocumentElement(); 

      for (int i = 1; i <= 25; i++) {
        Element number = fibonacci.createElement("fibonacci");
        number.setAttribute("index", Integer.toString(i));
        Text text = fibonacci.createTextNode(low.toString());
        number.appendChild(text);
        root.appendChild(number);
        
        BigInteger temp = high;
        high = high.add(low);
        low = temp;
      }
      
      try {
        // Now that the document is created we need to *serialize* it
        OutputFormat format = new OutputFormat(fibonacci);
        XMLSerializer serializer 
         = new XMLSerializer(System.out, format);
        serializer.serialize(fibonacci);
      }
      catch (IOException e) {
        System.err.println(e); 
      }
    }
    catch (DOMException e) {
      e.printStackTrace();
    }

  }

}

fibonacci.xml

<?xml version="1.0" encoding="UTF-8"?>
<Fibonacci_Numbers><fibonacci index="0">0</fibonacci><fibonacci index="1">1</fibonacci><fibonacci index="2">1</fibonacci><fibonacci index="3">2</fibonacci><fibonacci index="4">3</fibonacci><fibonacci index="5">5</fibonacci><fibonacci index="6">8</fibonacci><fibonacci index="7">13</fibonacci><fibonacci index="8">21</fibonacci><fibonacci index="9">34</fibonacci><fibonacci index="10">55</fibonacci><fibonacci index="11">89</fibonacci><fibonacci index="12">144</fibonacci><fibonacci index="13">233</fibonacci><fibonacci index="14">377</fibonacci><fibonacci index="15">610</fibonacci><fibonacci index="16">987</fibonacci><fibonacci index="17">1597</fibonacci><fibonacci index="18">2584</fibonacci><fibonacci index="19">4181</fibonacci><fibonacci index="20">6765</fibonacci><fibonacci index="21">10946</fibonacci><fibonacci index="22">17711</fibonacci><fibonacci index="23">28657</fibonacci><fibonacci index="24">46368</fibonacci><fibonacci index="25">75025</fibonacci></Fibonacci_Numbers>

OutputFormat

package org.apache.xml.serialize;

public class OutputFormat extends Object {

  public OutputFormat()
  public OutputFormat(String method, 
   String encoding, boolean indenting)
  public OutputFormat(Document doc)
  public OutputFormat(Document doc, 
   String encoding, boolean indenting)
  
  public String   getMethod()
  public void     setMethod(String method)
  public String   getVersion()
  public void     setVersion(String version)
  public int      getIndent()
  public boolean  getIndenting()
  public void     setIndent(int indent)
  public void     setIndenting(boolean on)
  public String   getEncoding()
  public void     setEncoding(String encoding)
  public String   getMediaType()
  public void     setMediaType(String mediaType)
  public void     setDoctype(String publicID, String systemID)
  public String   getDoctypePublic()
  public String   getDoctypeSystem()
  public boolean  getOmitXMLDeclaration()
  public void     setOmitXMLDeclaration(boolean omit)
  public boolean  getStandalone()
  public void     setStandalone(boolean standalone)
  public String[] getCDataElements()
  public boolean  isCDataElement(String tagName)
  public void     setCDataElements(String[] cdataElements)
  public String[] getNonEscapingElements()
  public boolean  isNonEscapingElement(String tagName)
  public void     setNonEscapingElements(String[] nonEscapingElements)
  public String   getLineSeparator()
  public void     setLineSeparator(String lineSeparator)
  public boolean  getPreserveSpace()
  public void     setPreserveSpace(boolean preserve)
  public int      getLineWidth()
  public void     setLineWidth(int lineWidth)
  public char     getLastPrintable()
  
  public static String whichMethod(Document doc)
  public static String whichDoctypePublic(Document doc)
  public static String whichDoctypeSystem(Document doc)
  public static String whichMediaType(String method)
  
}

Better formatted output

 try {
  // Now that the document is created we need to *serialize* it
  OutputFormat format = new OutputFormat(fibonacci, "8859_1", true);
  format.setLineSeparator("\r\n");
  format.setLineWidth(72);
  format.setDoctype(null, "fibonacci.dtd");
  XMLSerializer serializer = new XMLSerializer(System.out, format);
  serializer.serialize(root);
}
catch (IOException ex) {
  System.err.println(ex); 
}

formatted_fibonacci.xml

<?xml version="1.0" encoding="8859_1"?>
<!DOCTYPE Fibonacci_Numbers SYSTEM "fibonacci.dtd">
<Fibonacci_Numbers>
    <fibonacci index="0">0</fibonacci>
    <fibonacci index="1">1</fibonacci>
    <fibonacci index="2">1</fibonacci>
    <fibonacci index="3">2</fibonacci>
    <fibonacci index="4">3</fibonacci>
    <fibonacci index="5">5</fibonacci>
    <fibonacci index="6">8</fibonacci>
    <fibonacci index="7">13</fibonacci>
    <fibonacci index="8">21</fibonacci>
    <fibonacci index="9">34</fibonacci>
    <fibonacci index="10">55</fibonacci>
    <fibonacci index="11">89</fibonacci>
    <fibonacci index="12">144</fibonacci>
    <fibonacci index="13">233</fibonacci>
    <fibonacci index="14">377</fibonacci>
    <fibonacci index="15">610</fibonacci>
    <fibonacci index="16">987</fibonacci>
    <fibonacci index="17">1597</fibonacci>
    <fibonacci index="18">2584</fibonacci>
    <fibonacci index="19">4181</fibonacci>
    <fibonacci index="20">6765</fibonacci>
    <fibonacci index="21">10946</fibonacci>
    <fibonacci index="22">17711</fibonacci>
    <fibonacci index="23">28657</fibonacci>
    <fibonacci index="24">46368</fibonacci>
    <fibonacci index="25">75025</fibonacci>
</Fibonacci_Numbers>

DOM based XMLPrettyPrinter

import org.apache.xerces.parsers.*;
import org.w3c.dom.*;
import org.xml.sax.*;
import java.io.*;
import org.apache.xerces.dom.*;
import org.apache.xml.serialize.*; 


public class DOMPrettyPrinter {

  public static void main(String[] args) { 
     
    DOMParser parser = new DOMParser();
    
    for (int i = 0; i < args.length; i++) {
      try {
        // Read the entire document into memory
        parser.parse(args[i]); 
       
        Document document = parser.getDocument();
        
        OutputFormat format 
         = new OutputFormat(document, "UTF-8", true);
        format.setLineSeparator("\r\n");
        format.setIndenting(true);
        format.setIndent(2);
        format.setLineWidth(72);
        format.setPreserveSpace(false);
        XMLSerializer serializer 
         = new XMLSerializer(System.out, format);
        serializer.serialize(document);     
      }
      catch (SAXException e) {
        System.err.println(e); 
      }
      catch (IOException e) {
        System.err.println(e); 
      }
      
    }
  
  } // end main

}

Output from a DOM based XMLPrettyPrinter

<?xml version="1.0" encoding="UTF-8"?>
<!-- <!DOCTYPE foo SYSTEM "http://msdn.microsoft.com/xml/general/htmlentities.dtd"> -->
<weblogs>
  <log>
    <name>MozillaZine</name>
    <url>http://www.mozillazine.org</url>
    <changesUrl>http://www.mozillazine.org/contents.rdf</changesUrl>
    <ownerName>Jason Kersey</ownerName>
    <ownerEmail>kerz@en.com</ownerEmail>
    <description>THE source for news on the Mozilla Organization.
      DevChats, Reviews, Chats, Builds, Demos, Screenshots, and more.</description>
    <imageUrl/>
    <adImageUrl>http://static.userland.com/weblogMonitor/ads/kerz@en.com.gif</adImageUrl>
  </log>
  <log>
    <name>SalonHerringWiredFool</name>
    <url>http://www.salonherringwiredfool.com/</url>
    <ownerName>Some Random Herring</ownerName>
    <ownerEmail>salonfool@wiredherring.com</ownerEmail>
    <description/>
  </log>
  <log>
    <name>Scripting News</name>
    <url>http://www.scripting.com/</url>
    <ownerName>Dave Winer</ownerName>
    <ownerEmail>dave@userland.com</ownerEmail>
    <description>News and commentary from the cross-platform scripting community.</description>
    <imageUrl>http://www.scripting.com/gifs/tinyScriptingNews.gif</imageUrl>
    <adImageUrl>http://static.userland.com/weblogMonitor/ads/dave@userland.com.gif</adImageUrl>
  </log>
  <log>
    <name>SlashDot.Org</name>
    <url>http://www.slashdot.org/</url>
    <ownerName>Simply a friend</ownerName>
    <ownerEmail>afriendofweblogs@weblogs.com</ownerEmail>
    <description>News for Nerds, Stuff that Matters.</description>
  </log>
</weblogs>

The point is this:

To Learn More


To Learn More

Index | Cafe con Leche
Copyright 2002-2005 Elliotte Rusty Harold
elharo@metalab.unc.edu
Last Modified December 10, 2005