PC Professionell 2004 December/PCpro_2004_12.ISO/files/webserver/tsw/TSW_3.4.0.exe/Apache2/perl/FAQ.pod

Text File | 2002-10-12 | 18.7 KB | 629 lines

package XML::Simple::FAQ;
1;

__END__

=head1 Frequently Asked Questions about XML::Simple


=head1 Basics


=head2 What is XML::Simple designed to be used for?

XML::Simple is a Perl module that was originally developed as a tool for
reading and writing configuration data in XML format.  You can use it for
many other purposes that involve storing and retrieving structured data in
XML.

You might also find XML::Simple a good starting point for playing with XML
from Perl.  It doesn't have a steep learning curve and if you outgrow its
capabilities there are plenty of other Perl/XML modules to 'step up' to.


=head2 Why store configuration data in XML anyway?

The many advantages of using XML format for configuration data include:

=over 4

=item *

Using existing XML parsing tools requires less development time, is easier
and more robust than developing your own config file parsing code

=item *

XML can represent relationships between pieces of data, such as nesting of
sections to arbitrary levels (not easily done with .INI files for example)

=item *

XML is basically just text, so you can easily edit a config file (easier than
editing a Win32 registry)

=item *

XML provides standard solutions for handling character sets and encoding
beyond basic ASCII (important for internationalization)

=item *

If it becomes necessary to change your configuration file format, there are
many tools available for performing transformations on XML files

=item *

XML is an open standard (the world does not need more proprietary binary
file formats)

=item *

Taking the extra step of developing a DTD allows the format of configuration
files to be validated before your program reads them (not directly supported
by XML::Simple)

=item *

Combining a DTD with a good XML editor can give you a GUI config editor for
minimal coding effort

=back


=head2 What isn't XML::Simple good for?

The main limitation of XML::Simple is that it does not work with 'mixed
content' (see the next question).  If you consider your XML files contain
marked up text rather than structured data, you should probably use another
module.

If you are working with very large XML files, XML::Simple's approach of
representing the whole file in memory as a 'tree' data structure may not be
suitable.


=head2 What is mixed content?

Consider this example XML:

  <document>
    <para>This is <em>mixed</em> content.</para>
  </document>

This is said to be mixed content, because the E<lt>paraE<gt> element contains
both character data (text content) and nested elements.

Here's some more XML:

  <person>
    <first_name>Joe</first_name>
    <last_name>Bloggs</last_name>
    <dob>25-April-1969</dob>
  </person>

This second example is not generally considered to be mixed content.  The
E<lt>first_nameE<gt>, E<lt>last_nameE<gt> and E<lt>dobE<gt> elements contain
only character data and the  E<lt>personE<gt> element contains only nested
elements.  (Note: Strictly speaking, the whitespace between the nested
elements is character data, but it is ignored by XML::Simple).


=head2 Why doesn't XML::Simple handle mixed content?

Because if it did, it would no longer be simple :-)

Seriously though, there are plenty of excellent modules that allow you to
work with mixed content in a variety of ways.  Handling mixed content
correctly is not easy and by ignoring these issues, XML::Simple is able to
present an API without a steep learning curve.


=head2 Which Perl modules do handle mixed content?

Every one of them except XML::Simple :-)

If you're looking for a recommendation, I'd suggest you look at the Perl-XML
FAQ at:

  http://www.perlxml.net/perl-xml-faq.dkb


=head1 Installation


=head2 How do I install XML::Simple?

If you're running ActiveState Perl, you've probably already got XML::Simple
(although you may want to upgrade to version 1.09 or better for SAX support).

If you do need to install XML::Simple, you'll need to install an XML parser
module first.  Install either XML::Parser (which you may have already) or
XML::SAX.  If you install both, XML::SAX will be used by default.

Once you have a parser installed ...

On Unix systems, try:

  perl -MCPAN -e 'install XML::Simple'

If that doesn't work, download the latest distribution from
http://web.co.nz/~grantm/cpan/ , unpack it and run these commands:

  perl Makefile.PL
  make
  make test
  make install

On Win32, if you have a recent build of ActiveState Perl (618 or better) try
this command:

  ppm install XML::Simple

If that doesn't work, you really only need the Simple.pm file, so grab it
from the above site and save it in the \site\lib\XML directory under your
Perl installation (typically C:\Perl).


=head2 I'm trying to install XML::Simple and 'make test' fails

Is the directory where you've unpacked XML::Simple mounted from a file server
using NFS, SMB or some other network file sharing?  If so, that may cause
errors in the the following test scripts:

  3_Storable.t
  4_MemShare.t
  5_MemCopy.t

The test suite is designed to exercise the boundary conditions of all
XML::Simple's functionality and these three scripts exercise the caching
functions.  If XML::Simple is asked to parse a file for which it has a cached
copy of a previous parse, then it compares the timestamp on the XML file with
the timestamp on the cached copy.  If the cached copy is *newer* then it will
be used.  If the cached copy is older or the same age then the file is
re-parsed.  The test scripts will get confused by networked filesystems if
the workstation and server system clocks are not synchronised (to the
second).

If you get an error in one of these three test scripts but you don't plan to
use the caching options (they're not enabled by default), then go right ahead
and run 'make install'.  If you do plan to use caching, then try unpacking
the distribution on local disk and doing the build/test there.

It's probably not a good idea to use the caching options with networked
filesystems in production.  If the file server's clock is ahead of the local
clock, XML::Simple will re-parse files when it could have used the cached
copy.  However if the local clock is ahead of the file server clock and a
file is changed immediately after it is cached, the old cached copy will be
used.

Is one of the three test scripts (above) failing but you're not running on
a network filesystem?  Are you running Win32?  If so, you may be seeing a bug
in Win32 where writes to a file do not affect its modfication timestamp.

If none of these scenarios match your situation, please confirm you're
running the latest version of XML::Simple and then email the output of
'make test' to me at [e-mail]

=head2 Why is XML::Simple so slow?

If you find that XML::Simple is very slow reading XML, the most likely reason
is that you have XML::SAX installed but no additional SAX parser module.  The
XML::SAX distribution includes an XML parser written entirely in Perl.  This is
very portable but not very fast.  For better performance install either
XML::SAX::Expat or XML::LibXML.


=head1 Usage

=head2 How do I use XML::Simple?

If you had an XML document called /etc/appconfig/foo.xml you could 'slurp' it
into a simple data structure (typically a hashref) with these lines of code:

  use XML::Simple;

  my $config = XMLin('/etc/appconfig/foo.xml');

The XMLin() function accepts options after the filename.


=head2 There are so many options, which ones do I really need to know about?

Although you can get by without using any options, you shouldn't even
consider using XML::Simple in production until you know what these two
options do:

=over 4

=item *

forcearray

=item *

keyattr

=back

The reason you really need to read about them is because the default values
for these options will trip you up if you don't.  Although everyone agrees
that these defaults are not ideal, there is not wide agreement on what they
should be changed to.  The answer therefore is to read about them (see below)
and select values which are right for you.


=head2 What is the forcearray option all about?

Consider this XML in a file called ./person.xml:

  <person>
    <first_name>Joe</first_name>
    <last_name>Bloggs</last_name>
    <hobbie>bungy jumping</hobbie>
    <hobbie>sky diving</hobbie>
    <hobbie>knitting</hobbie>
  </person>

You could read it in with this line:

  my $person = XMLin('./person.xml');

Which would give you a data structure like this:

  $person = {
    'first_name' => 'Joe',
    'last_name'  => 'Bloggs',
    'hobbie'     => [ 'bungy jumping', 'sky diving', 'knitting' ]
  };

The E<lt>first_nameE<gt> and E<lt>last_nameE<gt> elements are represented as
simple scalar values which you could refer to like this:

  print "$person->{first_name} $person->{last_name}\n";

The E<lt>hobbieE<gt> elements are represented as an array - since there is
more than one.  You could refer to the first one like this:

  print $person->{hobbie}->[0], "\n";

Or the whole lot like this:

  print join(', ', @{$person->{hobbie}} ), "\n";

The catch is, that these last two lines of code will only work for people
who have more than one hobbie.  If there is only one E<lt>hobbieE<gt>
element, it will be represented as a simple scalar (just like
E<lt>first_nameE<gt> and E<lt>last_nameE<gt>).  Which might lead you to write
code like this:

  if(ref($person->{hobbie})) {
    print join(', ', @{$person->{hobbie}} ), "\n";
  }
  else {
    print $person->{hobbie}, "\n";
  }

Don't do that.

One alternative approach is to set the forcearray option to a true value:

  my $person = XMLin('./person.xml', forcearray => 1);

Which will give you a data structure like this:

  $person = {
    'first_name' => [ 'Joe' ],
    'last_name'  => [ 'Bloggs' ],
    'hobbie'     => [ 'bungy jumping', 'sky diving', 'knitting' ]
  };

Then you can use this line to refer to all the list of hobbies even if there
was only one:

  print join(', ', @{$person->{hobbie}} ), "\n";

The downside of this approach is that the E<lt>first_nameE<gt> and
E<lt>last_nameE<gt> elements will also always be represented as arrays even
though there will never be more than one:

  print "$person->{first_name}->[0] $person->{last_name}->[0]\n";

This might be OK if you change the XML to use attributes for things that
will always be singular and nested elements for things that may be plural:

  <person first_name="Jane" last_name="Bloggs">
    <hobbie>motorcycle maintenance</hobbie>
  </person>

On the other hand, if you prefer not to use attributes, then you could
specify that any E<lt>hobbieE<gt> elements should always be represented as
arrays and all other nested elements should be simple scalar values unless
there is more than one:

  my $person = XMLin('./person.xml', forcearray => [ 'hobbie' ]);

The forcearray option accepts a list of element names which should always
be forced to an array representation:

  forcearray => [ qw(hobbie qualification childs_name) ]

See the XML::Simple manual page for more information.


=head2 What is the keyattr option all about?

Consider this sample XML:

  <catalog>
    <part partnum="1842334" desc="High pressure flange" price="24.50" />
    <part partnum="9344675" desc="Threaded gasket"      price="9.25" />
    <part partnum="5634896" desc="Low voltage washer"   price="12.00" />
  </catalog>

You could slurp it in with this code:

  my $catalog = XMLin('./catalog.xml');

Which would return a data structure like this:

  $catalog = {
      'part' => [
          {
            'partnum' => '1842334',
            'desc'    => 'High pressure flange',
            'price'   => '24.50'
          },
          {
            'partnum' => '9344675',
            'desc'    => 'Threaded gasket',
            'price'   => '9.25'
          },
          {
            'partnum' => '5634896',
            'desc'    => 'Low voltage washer',
            'price'   => '12.00'
          }
      ]
  };

Then you could access the description of the first part in the catalog
with this code:

  print $catalog->{part}->[0]->{desc}, "\n";

However, if you wanted to access the description of the part with the
part number of "9344675" then you'd have to code a loop like this:

  foreach my $part (@{$catalog->{part}}) {
    if($part->{partnum} eq '9344675') {
      print $part->{desc}, "\n";
      last;
    }
  }

The knowledge that each E<lt>partE<gt> element has a unique partnum attribute
allows you to eliminate this search.  You can pass this knowledge on to
XML::Simple like this:

  my $catalog = XMLin($xml, keyattr => ['partnum']);

Which will return a data structure like this:

  $catalog = {
    'part' => {
      '5634896' => { 'desc' => 'Low voltage washer',   'price' => '12.00' },
      '1842334' => { 'desc' => 'High pressure flange', 'price' => '24.50' },
      '9344675' => { 'desc' => 'Threaded gasket',      'price' => '9.25'  }
    }
  };

XML::Simple has been able to transform $catalog->{part} from an arrayref to
a hashref (keyed on partnum).  This transformation is called 'array folding'.

Through the use of array folding, you can now index directly to the
description of the part you want:

  print $catalog->{part}->{9344675}->{desc}, "\n";

The 'keyattr' option also enables array folding when the unique key is in a
nested element rather than an attribute.  eg:

  <catalog>
    <part>
      <partnum>1842334</partnum>
      <desc>High pressure flange</desc>
      <price>24.50</price>
    </part>
    <part>
      <partnum>9344675</partnum>
      <desc>Threaded gasket</desc>
      <price>9.25</price>
    </part>
    <part>
      <partnum>5634896</partnum>
      <desc>Low voltage washer</desc>
      <price>12.00</price>
    </part>
  </catalog>

See the XML::Simple manual page for more information.


=head2 So what's the catch with 'keyattr'?

One thing to watch out for is that you might get array folding even if you
don't supply the keyattr option.  The default value for this option is:

  [ 'name', 'key', 'id']

Which means if your XML elements have a 'name', 'key' or 'id' attribute (or
nested element) then they may get folded on those values.  This means that
you can take advantage of array folding simply through careful choice of
attribute names.  On the hand, if you really don't want array folding at all,
you'll need to set 'key attr to an empty list:

  my $ref = XMLin($xml, keyattr => []);

A second 'gotcha' is that array folding only works on arrays.  That might
seem obvious, but if there's only one record in your XML and you didn't set
the 'forcearray' option then it won't be represented as an array and
consequently won't get folded into a hash.  The moral is that if you're
using array folding, you should always turn on the forcearray option.

You probably want to be as specific as you can be too.  For instance, the
safest way to parse the E<lt>catalogE<gt> example above would be:

  my $catalog = XMLin($xml, keyattr => { part => 'partnum'}
                            forcearray => ['part']);

By using the hashref for keyattr, you can specify that only E<lt>partE<gt>
elements should be folded on the 'partnum' attribute (and that the
E<lt>partE<gt> elements should not be folded on any other attribute).

By supplying a list of element names for forcearray, you're ensuring that
folding will work even if there's only one E<lt>partE<gt>.  You're also
ensuring that if the 'partnum' unique key is supplied in a nested element
then that element won't get forced to an array too.


=head2 How do I know what my data structure should look like?

The rules are fairly straightforward:

=over 4

=item *

each element gets represented as a hash

=item *

unless it contains only text, in which case it'll be a simple scalar value

=item *

or unless there's more than one element with the same name, in which case
they'll be represented as an array

=item *

unless you've got array folding enabled, in which case they'll be folded into
a hash

=item *

empty elements (no text contents B<and> no attributes) will either be
represented as an empty hash, an empty string or undef - depending on the value
of the 'suppressempty' option.

=back

If you're in any doubt, use Data::Dumper, eg:

  use XML::Simple;
  use Data::Dumper;

  my $ref = XMLin($xml);

  print Dumper($ref);


=head2 I'm getting 'Use of uninitialized value' warnings

You're probably trying to index into a non-existant hash key - try
Data::Dumper.


=head2 I'm getting a 'Not an ARRAY reference' error

Something that you expect to be an array is not.  The two most likely causes
are that you forgot to use 'forcearray' or that the array got folded into a
hash - try Data::Dumper.


=head2 I'm getting a 'No such array field' error

Something that you expect to be a hash is actually an array.  Perhaps array
folding failed because one element was missing the key attribute - try
Data::Dumper.


=head2 I'm getting an 'Out of memory' error

Something in the data structure is not as you expect and Perl may be trying
unsuccessfully to autovivify things - try Data::Dumper.

If you're already using Data::Dumper, try calling Dumper() immediately after
XMLin() - ie: before you attempt to access anything in the data structure.


=head2 My element order is getting jumbled up

If you read an XML file with XMLin() and then write it back out with
XMLout(), the order of the elements will likely be different.  (However, if
you read the file back in with XMLin() you'll get the same Perl data
structure).

The reordering happens because XML::Simple uses hashrefs to store your data
and Perl hashes do not really have any order.

It is possible that a future version of XML::Simple will use Tie::IxHash
to store the data in hashrefs which do retain the order.  However this will
not fix all cases of element order being lost.

If your application really is sensitive to element order, don't use
XML::Simple (and don't put order-sensitive values in attributes).


=head2 XML::Simple turns nested elements into attributes

If you read an XML file with XMLin() and then write it back out with
XMLout(), some data which was originally stored in nested elements may end up
in attributes.  (However, if you read the file back in with XMLin() you'll
get the same Perl data structure).

There are a number of ways you might handle this:

=over 4

=item *

use the 'forcearray' option with XMLin()

=item *

use the 'noattr' option with XMLout()

=item *

live with it

=item *

don't use XML::Simple

=back


=head2 Why does XMLout() insert E<lt>nameE<gt> elements (or attributes)?

Try setting keyattr => [].

When you call XMLin() to read XML, the 'keyattr' option controls whether arrays
get 'folded' into hashes.  Similarly, when you call XMLout(), the 'keyattr'
option controls whether hashes get 'unfolded' into arrays.  As described above,
'keyattr' is enabled by default.

=head2 Why are empty elements represented as empty hashes?

An element is always represented as a hash unless it contains only text, in
which case it is represented as a scalar string.

If you would prefer empty elements to be represented as empty strings or the
undefined value, set the 'suppressempty' option to '' or undef respectively.

=cut