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Chapter 19. Simple API for XML (SAX)

Contents:

The ContentHandler Interface
SAX Features and Properties
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The Simple API for XML (SAX) is a straightforward, event-based API for reading XML documents. Many different XML parsers, including Xerces, Crimson, MSXML, the Oracle XML Parser for Java, and Ælfred, implement the SAX API. SAX was originally defined as a Java API and is primarily intended for parsers written in Java. Therefore, this chapter focuses on the Java version of the API. However, SAX has been ported to most other major object-oriented languages, including C++, Python, Perl, and Eiffel. The translation from Java is usually fairly obvious.

The SAX API is unusual among XML APIs because it's an event-based push model rather than a tree-based pull model. As the XML parser reads an XML document, it sends your program information from the document in real time. Each time the parser sees a start-tag, an end-tag, character data, or a processing instruction, it tells your program. The document is presented to your program one piece at a time from beginning to end. You can either save the pieces you're interested in until the entire document has been read or process the information as soon as you receive it. You do not have to wait for the entire document to be read before acting on the data at the beginning of the document. Most importantly, the entire document does not have to reside in memory. This feature makes SAX the API of choice for very large documents that do not fit into available memory.

TIP: This chapter covers SAX2 exclusively. In 2002 all major parsers that support SAX support SAX2. The major change in SAX2 from SAX1 is the addition of namespace support. This addition necessitated changing the names and signatures of almost every method and class in SAX. The old SAX1 methods and classes are still available, but they're now deprecated, and you shouldn't use them.

SAX is primarily a collection of interfaces in the org.xml.sax package. One such interface is XMLReader . This interface represents the XML parser. It declares methods to parse a document and configure the parsing process, for instance, by turning validation on or off. To parse a document with SAX, first create an instance of XMLReader with the XMLReaderFactory class in the org.xml.sax.helpers package. This class has a static createXMLReader( ) factory method that produces the parser-specific implementation of the XMLReader interface. The Java system property org.xml.sax.driver specifies the concrete class to instantiate: 

try {
  XMLReader parser = XMLReaderFactory.createXMLReader( );
  // parse the document...
}
catch (SAXException e) {
  // couldn't create the XMLReader
}

The call to XMLReaderFactory.createXMLReader( ) is wrapped in a try-catch block that catches SAXException . This is the generic checked exception superclass for almost anything that can go wrong while parsing an XML document. In this case, it means either that the org.xml.sax.driver system property wasn't set or that it was set to the name of a class that Java couldn't find in the class path.

You can choose which concrete class to instantiate by passing its name as a string to the createXMLReader( ) method. This code fragment instantiates the Xerces parser by name: 

try {
  XMLReader parser = XMLReaderFactory.createXMLReader(
   "org.apache.xerces.parsers.SAXParser");
  // parse the document...
}
catch (SAXException e) {
  // couldn't create the XMLReader
}

Now that you've created a parser, you're ready to parse some documents with it. Pass the system ID of the document you want to parse to the parse( ) method. The system ID is either an absolute or a relative URL encoded in a string. For example, this code fragment parses the document at http://www.slashdot.org/slashdot.xml: 

try {
  XMLReader parser = XMLReaderFactory.createXMLReader( );
  parser.parse("http://www.slashdot.org/slashdot.xml");
}
catch (SAXParseException e) {
  // Well-formedness error
}
catch (SAXException e) {
  // Could not find an XMLReader implementation class
}
catch (IOException e) {
  // Some sort of I/O error prevented the document from being completely
  // downloaded from the server
}

The parse( ) method throws a SAXParseException if the document is malformed, an IOException if an I/O error such as a broken socket occurs while the document is being read, and a SAXException if anything else goes wrong. Otherwise, it returns void. To receive information from the parser as it reads the document, you must configure it with a ContentHandler.

19.1. The ContentHandler Interface

ContentHandler, shown in stripped-down form in Example 19-1, is an interface in the org.xml.sax package. You implement this interface in a class of your own devising. Next, you configure an XMLReader with an instance of your implementation. As the XMLReader reads the document, it invokes the methods in your object to tell your program what's in the XML document. You can respond to these method invocations in any way you see fit.

TIP: This class has no relation to the moribund java.net.ContentHandler class. However, you may encounter a name conflict if you import both java.net.* and org.xml.sax.* in the same class. It's better to import just the java.net classes you actually need, rather than the entire package.

Example 19-1. The org.xml.sax.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;

}

Every time the XMLReader reads a piece of the document, it calls a method in its ContentHandler. Suppose a parser reads the simple document shown in Example 19-2.

Example 19-2. A simple XML document 

<?xml version="1.0" encoding="ISO-8859-1"?>
<?xml-stylesheet type='text/css' href='person.css'?>
<!DOCTYPE person SYSTEM "person.dtd">
<person xmlns="http://xml.oreilly.com/person">
  <name:name xmlns:name="http://xml.oreilly.com/name">
    <name:first>Sydney</name:first>
    <name:last>Lee</name:last>
  </name:name>
  <assignment project_id="p2"/>
</person>

The parser will call these methods in its ContentHandler with these arguments in this order. The values of the arguments passed to each method are given after each method name: 

  1. setDocumentLocator(Locator locator)
locator: org.apache.xerces.readers.DefaultEntityHandler@1f953d
  2. startDocument( )
  3. processingInstruction(String target, String data)
target: "xml-stylesheet"
data: "type='text/css' href='person.css'"
  4. startPrefixMapping(String prefix, String namespaceURI)
prefix: ""
namespaceURI: "http://xml.oreilly.com/person"
  5. startElement(String namespaceURI, String localName, 
String qualifiedName, Attributes atts)
namespaceURI: "http://xml.oreilly.com/person"
localName: "person"
qualifiedName: "person"
atts: {} (no attributes, an empty list)
  6. ignorableWhitespace(char[] text, int start, int length)
text: <?xml version="1.0" encoding="ISO-8859-1"?>
<?xml-stylesheet type='text/css' href='person.css'?>
<!DOCTYPE person SYSTEM "person.dtd">
<person xmlns="http://xml.oreilly.com/person">
  <name:name xmlns:name="http://xml.oreilly.com/name">
    <name:first>Sydney</name:first>
    <name:last>Lee</name:last>
  </name:name>
  <assignment project_id="p2"/>
</person>
start: 181
length: 3
  7. startPrefixMapping(String prefix, String uri)
prefix: "name"
uri: "http://xml.oreilly.com/name")
  8. startElement(String namespaceURI, String localName, 
String qualifiedName, Attributes atts)
namespaceURI: "http://xml.oreilly.com/name"
localName: "name"
qualifiedName: "name:name"
atts: {} (no attributes, an empty list)
  9. ignorableWhitespace(char[] text, int start, int length)
text: <?xml version="1.0" encoding="ISO-8859-1"?>
<?xml-stylesheet type='text/css' href='person.css'?>
<!DOCTYPE person SYSTEM "person.dtd">
<person xmlns="http://xml.oreilly.com/person">
  <name:name xmlns:name="http://xml.oreilly.com/name">
    <name:first>Sydney</name:first>
    <name:last>Lee</name:last>
  </name:name>
  <assignment project_id="p2"/>
</person>
start: 236
length: 5
  10. startElement(String namespaceURI, String localName, 
String qualifiedName, Attributes atts)
namespaceURI: "http://xml.oreilly.com/name"
localName: "first"
qualifiedName: "name:first"
atts: {} (no attributes, an empty list)
  11. characters(char[] text, int start, int length)
text: <?xml version="1.0" encoding="ISO-8859-1"?>
<?xml-stylesheet type='text/css' href='person.css'?>
<!DOCTYPE person SYSTEM "person.dtd">
<person xmlns="http://xml.oreilly.com/person">
  <name:name xmlns:name="http://xml.oreilly.com/name">
    <name:first>Sydney</name:first>
    <name:last>Lee</name:last>
  </name:name>
  <assignment project_id="p2"/>
</person>
start: 253
length: 6
  12. endElement(String namespaceURI, String localName, String qualifiedName)
namespaceURI: "http://xml.oreilly.com/name"
localName: "first"
qualifiedName: "name:first"
  13. ignorableWhitespace(char[] text, int start, int length)
text: <?xml version="1.0" encoding="ISO-8859-1"?>
<?xml-stylesheet type='text/css' href='person.css'?>
<!DOCTYPE person SYSTEM "person.dtd">
<person xmlns="http://xml.oreilly.com/person">
  <name:name xmlns:name="http://xml.oreilly.com/name">
    <name:first>Sydney</name:first>
    <name:last>Lee</name:last>
  </name:name>
  <assignment project_id="p2"/>
</person>
start: 272
length: 5
  14. startElement(String namespaceURI, String localName, String qualifiedName, Attributes atts)
namespaceURI: "http://xml.oreilly.com/name"
localName: "last"
qualifiedName: "name:last"
atts: {} (no attributes, an empty list)
  15. characters(char[] text, int start, int length)
text: <?xml version="1.0" encoding="ISO-8859-1"?>
<?xml-stylesheet type='text/css' href='person.css'?>
<!DOCTYPE person SYSTEM "person.dtd">
<person xmlns="http://xml.oreilly.com/person">
  <name:name xmlns:name="http://xml.oreilly.com/name">
    <name:first>Sydney</name:first>
    <name:last>Lee</name:last>
  </name:name>
  <assignment project_id="p2"/>
</person>
start: 288
length: 3
  16. endElement(String namespaceURI, String localName, String qualifiedName)
namespaceURI: "http://xml.oreilly.com/name"
localName: "last"
qualifiedName: "name:last"
  17. ignorableWhitespace(char[] text, int start, int length)
text: <?xml version="1.0" encoding="ISO-8859-1"?>
<?xml-stylesheet type='text/css' href='person.css'?>
<!DOCTYPE person SYSTEM "person.dtd">
<person xmlns="http://xml.oreilly.com/person">
  <name:name xmlns:name="http://xml.oreilly.com/name">
    <name:first>Sydney</name:first>
    <name:last>Lee</name:last>
  </name:name>
  <assignment project_id="p2"/>
</person>
start: 303
length: 3
  18. endElement(String namespaceURI, String localName, String qualifiedName)
namespaceURI: "http://xml.oreilly.com/name"
localName: "name"
qualifiedName: "name:name"
  19. endPrefixMapping(String prefix)
prefix: "name"
  20. ignorableWhitespace(char[] text, int start, int length)
text: <?xml version="1.0" encoding="ISO-8859-1"?>
<?xml-stylesheet type='text/css' href='person.css'?>
<!DOCTYPE person SYSTEM "person.dtd">
<person xmlns="http://xml.oreilly.com/person">
  <name:name xmlns:name="http://xml.oreilly.com/name">
    <name:first>Sydney</name:first>
    <name:last>Lee</name:last>
  </name:name>
  <assignment project_id="p2"/>
</person>
start: 318
length: 3
  21. startElement(String namespaceURI, String localName, String qualifiedName, Attributes atts)
namespaceURI: "http://xml.oreilly.com/person"
localName: "assignment"
qualifiedName: "assignment
atts: {project_id="p2"}
  22. endElement(String namespaceURI, String localName, String qualifiedName)
namespaceURI: "http://xml.oreilly.com/person"
localName: "assignment"
qualifiedName: "assignment"
  23. ignorableWhitespace(char[] text, int start, int length)
text: <?xml version="1.0" encoding="ISO-8859-1"?>
<?xml-stylesheet type='text/css' href='person.css'?>
<!DOCTYPE person SYSTEM "person.dtd">
<person xmlns="http://xml.oreilly.com/person">
  <name:name xmlns:name="http://xml.oreilly.com/name">
    <name:first>Sydney</name:first>
    <name:last>Lee</name:last>
  </name:name>
  <assignment project_id="p2"/>
</person>
start: 350
length: 1
  24. endElement(String namespaceURI, String localName, String qualifiedName)
namespaceURI: "http://xml.oreilly.com/person"
localName: "person"
qualifiedName: "person"
  25. endPrefixMapping(String prefix)
prefix: ""
  26. endDocument( )

Some pieces of this are not deterministic. Note that the char array passed to each call to characters( ) and ignorableWhitespace( ) actually contains the entire document! The specific text block that the parser really returns is indicated by the second two arguments. This is an optimization that Xerces-J performs. Other parsers are free to pass different char arrays as long as they set the start and length arguments to match. Indeed, the parser is also free to split a long run of plain text across multiple calls to characters( ) or ignorableWhitespace( ), so you cannot assume that these methods necessarily return the longest possible contiguous run of plain text. Other details that may change from parser to parser include attribute order within a tag and whether a Locator object is provided by calling setDocumentLocator( ).

Suppose you want to count the number of elements, attributes, processing instructions, and characters of plain text that exist in a given XML document. To do so, first write a class that implements the ContentHandler interface. The current count of each of the four items of interest is stored in a field. The field values are initialized to zero in the startDocument( ) method, which is called exactly once for each document parsed. Each callback method in the class increments the relevant field. The endDocument( ) method reports the total for that document. Example 19-3 is such a class.

Example 19-3. The XMLCounter ContentHandler 

import org.xml.sax.*;

public class XMLCounter implements ContentHandler {

  private int numberOfElements;
  private int numberOfAttributes;
  private int numberOfProcessingInstructions;
  private int numberOfCharacters;

  public void startDocument( ) throws SAXException {
    numberOfElements = 0;
    numberOfAttributes = 0;
    numberOfProcessingInstructions = 0;
    numberOfCharacters = 0;
  }

  // We should count either the start-tag of the element or the end-tag,
  // but not both. Empty elements are reported by each of these methods.
  public void startElement(String namespaceURI, String localName,
   String qualifiedName, Attributes atts) throws SAXException {
    numberOfElements++;
    numberOfAttributes += atts.getLength( );
  }

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

  public void characters(char[] text, int start, int length)
   throws SAXException {
    numberOfCharacters += length;
  }

  public void ignorableWhitespace(char[] text, int start, int length)
   throws SAXException {
    numberOfCharacters += length;
  }

  public void processingInstruction(String target, String data)
   throws SAXException {
    numberOfProcessingInstructions++;
  }

  // Now that the document is done, we can print out the final results
  public void endDocument( ) throws SAXException {
    System.out.println("Number of elements: " + numberOfElements);
    System.out.println("Number of attributes: " + numberOfAttributes);
    System.out.println("Number of processing instructions: "
     + numberOfProcessingInstructions);
    System.out.println("Number of characters of plain text: "
     + numberOfCharacters);
  }

  // Do-nothing methods we have to implement only to fulfill
  // the interface requirements:
  public void setDocumentLocator(Locator locator) {}
  public void startPrefixMapping(String prefix, String uri)
   throws SAXException {}
  public void endPrefixMapping(String prefix) throws SAXException {}
  public void skippedEntity(String name) throws SAXException {}

}
TIP: This class needs to override most methods in the ContentHandler interface. However, if you only really want to provide one or two ContentHandler methods, you may want to subclass the DefaultHandler class instead. This adapter class implements all methods in the ContentHandler interface with do-nothing methods, so you only have to override methods in which you're genuinely interested.

Next, build an XMLReader, and configure it with an instance of this class. Finally, parse the documents you want to count, as in Example 19-4.

Example 19-4. The DocumentStatistics driver class 

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

public class DocumentStatistics {

  public static void main(String[] args) {

    XMLReader parser;
    try {
     parser = XMLReaderFactory.createXMLReader( );
    }
    catch (SAXException e) {
      // fall back on Xerces parser by name
      try {
        parser = XMLReaderFactory.createXMLReader(
         "org.apache.xerces.parsers.SAXParser");
      }
      catch (SAXException ee) {
        System.err.println("Couldn't locate a SAX parser");
        return;
      }
    }

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

    // Install the Content Handler
    parser.setContentHandler(new XMLCounter( ));

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

      // command line should offer URIs or file names
      try {
        parser.parse(args[i]);
      }
      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 report on " + args[i]
         + " because of the IOException " + e);
      }

    }

  }

}

Running the program in Example 19-4 across the document in Example 19-2 results in the following output: 

D:\books\xian\examples\18>java DocumentStatistics 18-2.xml
Number of elements: 5
Number of attributes: 1
Number of processing instructions: 1
Number of characters of plain text: 29

This generic program of Example 19-4 works on any well-formed XML document. Most SAX programs are more specific and only work with certain XML applications. They look for particular elements or attributes in particular places and respond to them accordingly. They may rely on patterns that are enforced by a validating parser. Still, this behavior comprises the fundamentals of SAX.

The complicated part of most SAX programs is the data structure you must build to store information returned by the parser until you're ready to use it. Sometimes this information can be as complicated as the XML document itself, in which case you may be better off using DOM, which at least provides a ready-made data structure for an XML document. You usually want only some information, though, and the data structure you construct should be less complex than the document itself.

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