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NAME
XML::SAX::Intro - An Introduction to SAX Parsing with Perl
Introduction
XML::SAX is a new way to work with XML Parsers in Perl. In this article we'll discuss why
you should be using SAX, why you should be using XML::SAX, and we'll see some of the finer
implementation details. The text below assumes some familiarity with callback, or push
based parsing, but if you are unfamiliar with these techniques then a good place to start
is Kip Hampton's excellent series of articles on XML.com.
Replacing XML::Parser
The de-facto way of parsing XML under perl is to use Larry Wall and Clark Cooper's
XML::Parser. This module is a Perl and XS wrapper around the expat XML parser library by
James Clark. It has been a hugely successful project, but suffers from a couple of rather
major flaws. Firstly it is a proprietary API, designed before the SAX API was conceived,
which means that it is not easily replaceable by other streaming parsers. Secondly it's
callbacks are subrefs. This doesn't sound like much of an issue, but unfortunately leads
to code like:
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
SAX stands for the Simple API for XML. And simple it really is. Constructing a SAX parser
and passing events to handlers is done as simply as:
use XML::SAX;
use MySAXHandler;
my $parser = XML::SAX::ParserFactory->parser(
Handler => MySAXHandler->new
);
$parser->parse_uri("foo.xml");
The important concept to grasp here is that SAX uses a factory class called
XML::SAX::ParserFactory to create a new parser instance. The reason for this is so that
you can support other underlying parser implementations for different feature sets. This
is one thing that XML::Parser has always sorely lacked.
In the code above we see the parse_uri method used, but we could have equally well called
parse_file, parse_string, or parse(). Please see XML::SAX::Base for what these methods
take as parameters, but don't be fooled into believing parse_file takes a filename. No, it
takes a file handle, a glob, or a subclass of IO::Handle. Beware.
SAX works very similarly to XML::Parser's default callback method, except it has one major
difference: rather than setting individual callbacks, you create a new class in which to
receive the callbacks. Each callback is called as a method call on an instance of that
handler class. An example will best demonstrate this:
package MySAXHandler;
use base qw(XML::SAX::Base);
sub start_document {
my ($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 => $handler);
This flexibility allows for one parser to be used in many different scenarios throughout
your script (though one shouldn't feel pressure to use this method, as parser construction
is generally not a time consuming process).
Callback Parameters
The only other thing you need to know to understand basic SAX is the structure of the
parameters passed to each of the callbacks. In XML::Parser, all parameters are passed as
multiple options to the callbacks, so for example the Start callback would be called as
my_start($e, $name, %attributes), and the PI callback would be called as
my_processing_instruction($e, $target, $data). In SAX, every callback is passed a hash
reference, containing entries that define our "node". The key callbacks and the structures
they receive are:
start_element
The start_element handler is called whenever a parser sees an opening tag. It is passed an
element structure consisting of:
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 is called either when a parser sees a closing tag, or after
start_element has been called for an empty element (do note however that a parser may if
it is so inclined call characters with an empty string when it sees an empty element.
There is no simple way in SAX to determine if the parser in fact saw an empty element, a
start and end element with no content..
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 callback may be called in several circumstances. The most obvious one is
when seeing ordinary character data in the markup. But it is also called for text in a
CDATA section, and is also called in other situations. A SAX parser has to make no
guarantees whatsoever about how many times it may call characters for a stretch of text in
an XML document - it may call once, or it may call once for every character in the text.
In order to work around this it is often important for the SAX developer to use a bundling
technique, where text is gathered up and processed in one of the other callbacks. This is
not always necessary, but it is a worthwhile technique to learn, which we will cover in
XML::SAX::Advanced (when I get around to writing it).
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
The comment callback is called for comment text. Unlike with "characters()", the comment
callback *must* be invoked just once for an entire comment string. It receives a single
simple structure - a hash reference containing just one entry:
Data
The text of the comment.
processing_instruction
The processing instruction handler is called for all processing instructions in the
document. Note that these processing instructions may appear before the document root
element, or after it, or anywhere where text and elements would normally appear within the
document, according to the XML specification.
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
What we have discussed above is really the tip of the SAX iceberg. And so far it looks
like there's not much of interest to SAX beyond what we have seen with XML::Parser. But it
does go much further than that, I promise.
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 whose 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
Matt Sergeant, matt@sergeant.org
$Id$