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NAME

XML::SAX::Intro - An Introduction to SAX Parsing with Perl
 

Introduction

Replacing XML::Parser

  sub handle_start {
    my ($e, $el, %attrs) = @_;
    if ($el eq 'foo') {
      $e->{inside_foo}++; # BAD! $e is an XML::Parser::Expat object.
    }
  }

As you can see, we're using the $e object to hold our state information, which is a bad idea because we don't own that object - we didn't create it. It's an internal object of XML::Parser, that happens to be a hashref. We could all too easily overwrite XML::Parser internal state variables by using this, or Clark could change it to an array ref (not that he would, because it would break so much code, but he could).

The only way currently with XML::Parser to safely maintain state is to use a closure:

  my $state = MyState->new();
  $parser->setHandlers(Start => sub { handle_start($state, @_) });

This closure traps the $state variable, which now gets passed as the first parameter to your callback. Unfortunately very few people use this technique, as it is not documented in the XML::Parser POD files.

Another reason you might not want to use XML::Parser is because you need some feature that it doesn't provide (such as validation), or you might need to use a library that doesn't use expat, due to it not being installed on your system, or due to having a restrictive ISP. Using SAX allows you to work around these restrictions.  

Introducing SAX

  use XML::SAX;
  use MySAXHandler;

  my $parser = XML::SAX::ParserFactory->parser(
        Handler => MySAXHandler->new
  );

  $parser->parse_uri("foo.xml");

The important concept to grasp here is that SAX uses a factory class called XML::SAX::ParserFactory to create a new parser instance. The reason for this is so that you can support other underlying parser implementations for different feature sets. This is one thing that XML::Parser has always sorely lacked.

In the code above we see the parse_uri method used, but we could have equally well called parse_file, parse_string, or parse(). Please see XML::SAX::Base for what these methods take as parameters, but don't be fooled into believing parse_file takes a filename. No, it takes a file handle, a glob, or a subclass of IO::Handle. Beware.

SAX works very similarly to XML::Parser's default callback method, except it has one major difference: rather than setting individual callbacks, you create a new class in which to recieve the callbacks. Each callback is called as a method call on an instance of that handler class. An example will best demonstrate this:

  package MySAXHandler;
  use base qw(XML::SAX::Base);

  sub start_document {
    my ($self, $doc) = @_;
    # process document start event
  }

  sub start_element {
    my ($self, $el) = @_;
    # process element start event
  }

Now, when we instantiate this as above, and parse some XML with this as the handler, the methods start_document and start_element will be called as method calls, so this would be the equivalent of directly calling:

  $object->start_element($el);

Notice how this is different to XML::Parser's calling style, which calls:

  start_element($e, $name, %attribs);

It's the difference between function calling and method calling which allows you to subclass SAX handlers which contributes to SAX being a powerful solution.

As you can see, unlike XML::Parser, we have to define a new package in which to do our processing (there are hacks you can do to make this uneccessary, but I'll leave figuring those out to the experts). The biggest benefit of this is that you maintain your own state variable ($self in the above example) thus freeing you of the concerns listed above. It is also an improvement in maintainability - you can place the code in a separate file if you wish to, and your callback methods are always called the same thing, rather than having to choose a suitable name for them as you had to with XML::Parser. This is an obvious win.

SAX parsers are also very flexible in how you pass a handler to them. You can use a constructor parameter as we saw above, or we can pass the handler directly in the call to one of the parse methods:

  $parser->parse(Handler => $handler, 
                 Source => { SystemId => "foo.xml" });
  # or...
  $parser->parse_file($fh, Handler => $handler);

This flexibility allows for one parser to be used in many different scenarios throughout your script (though one shouldn't feel pressure to use this method, as parser construction is generally not a time consuming process).  

Callback Parameters

start_element

LocalName

The name of the element minus any namespace prefix it may have come with in the document.

NamespaceURI

The URI of the namespace associated with this element, or the empty string for none.

Attributes

A set of attributes as described below.

Name

The name of the element as it was seen in the document (i.e. including any prefix associated with it)

Prefix

The prefix used to qualify this element's namespace, or the empty string if none.

The Attributes are a hash reference, keyed by what we have called ``James Clark'' notation. This means that the attribute name has been expanded to include any associated namespace URI, and put together as {ns}name, where ``ns'' is the expanded namespace URI of the attribute if and only if the attribute had a prefix, and ``name'' is the LocalName of the attribute.

The value of each entry in the attributes hash is another hash structure consisting of:

LocalName

The name of the attribute minus any namespace prefix it may have come with in the document.

NamespaceURI

The URI of the namespace associated with this attribute. If the attribute had no prefix, then this consists of just the empty string.

Name

The attribute's name as it appeared in the document, including any namespace prefix.

Prefix

The prefix used to qualify this attribute's namepace, or the empty string if none.

Value

The value of the attribute.

So a full example, as output by Data::Dumper might be:

  ....

end_element

The end_element handler receives exactly the same structure as start_element, minus the Attributes entry. One must note though that it should not be a reference to the same data as start_element receives, so you may change the values in start_element but this will not affect the values later seen by end_element.  

characters

The characters handler is called with a very simple structure - a hash reference consisting of just one entry:

Data

The text data that was received.

comment

Data

The text of the comment.

processing_instruction

The handler is passed a structure containing just two entries:

Target

The target of the processing instrcution

Data

The text data in the processing instruction. Can be an empty string for a processing instruction that has no data element. For example <?wiggle?> is a perfectly valid processing instruction.

Tip of the iceberg

People who hate Object Oriented code for the sake of it may be thinking here that creating a new package just to parse something is a waste when they've been parsing things just fine up to now using procedural code. But there's reason to all this madness. And that reason is SAX Filters.

As you saw right at the very start, to let the parser know about our class, we pass it an instance of our class as the Handler to the parser. But now imagine what would happen if our class could also take a Handler option, and simply do some processing and pass on our data further down the line? That in a nutshell is how SAX filters work. It's Unix pipes for the 21st century!

There are two downsides to this. Number 1 - writing SAX filters can be tricky. If you look into the future and read the advanced tutorial I'm writing, you'll see that Handler can come in several shapes and sizes. So making sure your filter does the right thing can be tricky. Secondly, constructing complex filter chains can be difficult, and simple thinking tells us that we only get one pass at our document, when often we'll need more than that.

Luckily though, those downsides have been fixed by the release of two very cool modules. What's even better is that I didn't write either of them!

The first module is XML::SAX::Base. This is a VITAL SAX module that acts as a base class for all SAX parsers and filters. It provides an abstraction away from calling the handler methods, that makes sure your filter or parser does the right thing, and it does it FAST. So, if you ever need to write a SAX filter, which if you're processing XML -> XML, or XML -> HTML, then you probably do, then you need to be writing it as a subclass of XML::SAX::Base. Really - this is advice not to ignore lightly. I will not go into the details of writing a SAX filter here. Kip Hampton, the author of XML::SAX::Base has covered this nicely in his article on XML.com here <URI>.

To construct SAX pipelines, Barrie Slaymaker, a long time Perl hacker who's modules you will probably have heard of or used, wrote a very clever module called XML::SAX::Machines. This combines some really clever SAX filter-type modules, with a construction toolkit for filters that makes building pipelines easy. But before we see how it makes things easy, first lets see how tricky it looks to build complex SAX filter pipelines.

  use XML::SAX::ParserFactory;
  use XML::Filter::Filter1;
  use XML::Filter::Filter2;
  use XML::SAX::Writer;

  my $output_string;
  my $writer = XML::SAX::Writer->new(Output => \$output_string);
  my $filter2 = XML::SAX::Filter2->new(Handler => $writer);
  my $filter1 = XML::SAX::Filter1->new(Handler => $filter2);
  my $parser = XML::SAX::ParserFactory->parser(Handler => $filter1);

  $parser->parse_uri("foo.xml");

This is a lot easier with XML::SAX::Machines:

  use XML::SAX::Machines qw(Pipeline);

  my $output_string;
  my $parser = Pipeline(
        XML::SAX::Filter1 => XML::SAX::Filter2 => \$output_string
        );

  $parser->parse_uri("foo.xml");

One of the main benefits of XML::SAX::Machines is that the pipelines are constructed in natural order, rather than the reverse order we saw with manual pipeline construction. XML::SAX::Machines takes care of all the internals of pipe construction, providing you at the end with just a parser you can use (and you can re-use the same parser as many times as you need to).

Just a final tip. If you ever get stuck and are confused about what is being passed from one SAX filter or parser to the next, then Devel::TraceSAX will come to your rescue. This perl debugger plugin will allow you to dump the SAX stream of events as it goes by. Usage is really very simple just call your perl script that uses SAX as follows:

  $ perl -d:TraceSAX <scriptname>

And preferably pipe the output to a pager of some sort, such as more or less. The output is extremely verbose, but should help clear some issues up.  

AUTHOR

$Id: Intro.pod,v 1.3 2002/04/30 07:16:00 matt Exp $

Index

NAME

Introduction

Replacing XML::Parser

Introducing SAX

Callback Parameters

start_element

end_element

characters

comment

processing_instruction

Tip of the iceberg

AUTHOR

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