ch14_05.htm

用perl编写CGI的好书。本书从解释CGI和底层HTTP协议如何工作开始

<a name="INDEX-2865" />arrays whose
elements are the URL itself, the title of the URL, plus another array
pointing to second-level related URLs. The subarray of second-level
related URLs contains only two elements: the URL and the title. <a href="ch14_05.htm#ch14-15165">Figure 14-4</a> illustrates this data structure.</p>


<a name="ch14-15165" /><div class="figure"><img width="427" src="figs/cgi2.1404.gif" height="206" alt="Figure 14-4" /></div><h4 class="objtitle">Figure 14-4. Perl data structure that contains the related URLs and subsequent related URLs</h4>

<p>If there are no related items found at the top level submitted URL, a
message is printed to notify the user.</p>


<p>Later, we want to print out <a name="INDEX-2867" />
<a name="INDEX-2868" /> <a name="INDEX-2,869" />self-referencing hypertext links back
to this script. In preparation for this action, we create a variable
called <tt class="literal">$scriptname</tt> that will hold the current
scriptname for referencing in &lt;A HREF&gt; tags. CGI.pm's
<tt class="function">script_name</tt> method provides a convenient way of
getting this data.</p>


<p>Of course, we could have simply chosen a static name for this script.
However, it is generally considered good practice to code for
flexibility where possible. In this case, we can name the script
anything we want and the code here will not have to change.</p>


<p>
<a name="INDEX-2870" /> <a name="INDEX-2,871" />
<a name="INDEX-2872" />For each related URL, we print out
"[*]" embedded in an &lt;A&gt; tag that will contain a
reference to the script itself plus the current URL being passed to
it as a search parameter. If one element of
<tt class="literal">@related</tt> contains
<tt class="literal">["http://www.eff.org/",</tt> <tt class="literal">"The</tt>
<tt class="literal">Electronic</tt> <tt class="literal">Frontier</tt>
<tt class="literal">Foundation"]</tt> the resulting HTML would look like
this:</p>


<blockquote><pre class="code">
&lt;A HREF="whatsrelated.cgi?url=http://www.eff.org/" &gt;[*]&lt;/A&gt;
&lt;A HREF="http://www.eff.org/"&gt;The Electronic Frontier Foundation&lt;/A&gt;</pre></blockquote>


<p>This will let the user pursue the "What's Related"
trail another step by running this script on the chosen URL.
Immediately afterwards, the title (<tt class="literal">$_-&gt;[1]</tt>) is
printed with a hypertext reference to the URL that the title
represents (<tt class="literal">$_-&gt;[0]</tt>).</p>


<p><tt class="literal">@subrelated</tt>
<a name="INDEX-2873" /> contains the URLs
that are related to the URL we just printed for the user
(<tt class="literal">$_-&gt;[2]</tt>). If there are second-level related
URLs, we can proceed to print them. The second-level related URL
array follows the same format as the related URL array except that
there is no third element containing further references to more
related URLs. <tt class="literal">$_-&gt;[0]</tt> is the URL and
<tt class="literal">$_-&gt;[1]</tt> is the title of the URL itself. If
<tt class="literal">@subrelated</tt> is empty, the user is told that there
are no related items to the URL that is currently being displayed.</p>


<p>Finally, we output the footer for the What's Related query
results page. In addition, the user is presented with another text
field in which they can enter in a new URL to search on.</p>


<p>The
<tt class="function">get_whats_related_to_whats_related</tt>
<a name="INDEX-2874" />
subroutine contains logic to take a URL and construct a data
structure that contains not only URLs that are related to the passed
URL, but also the second-level related URLs.
<tt class="literal">@related</tt> contains the list of what's related
to the first URL.</p>


<p>Then, each record is examined in <tt class="literal">@related</tt> to see
if there is anything related to that URL. If there is, the third
element (<tt class="literal">$record-&gt;[2]</tt>) of the record is set to
a reference to the second-level related URLs we are currently
examining. Finally, the entire <tt class="literal">@related</tt> data
structure is returned.</p>


<p>The <tt class="function">get_whats_related</tt> subroutine returns an
array of references to an array with two elements: a related URL and
the title of that URL. The key to getting this information is to
parse it from an XML document. <tt class="literal">$parser</tt> is the
XML::Parser object that will be used to perform this task.</p>


<p>
<a name="INDEX-2875" />
<a name="INDEX-2876" />XML parsers do not simply parse
data in a linear fashion. After all, XML itself is hierarchical in
nature. There are two different ways that XML parsers can look at XML
data.</p>


<p>One way is to have the XML parser take the entire document and simply
return a tree of objects that represents the XML document hierarchy.
Perl supports this concept via the <a name="INDEX-2877" />
<a name="INDEX-2878" />XML::Grove
module by Ken MacLeod. The second way to parse XML documents is using
a <a name="INDEX-2879" />SAX (Simple API for XML) style of parser.
This type of parser is <a name="INDEX-2880" /> <a name="INDEX-2,881" />event-based and is the one that XML::Parser
is based on.</p>


<p>The event based parser is popular because it starts returning data to
the calling program as it parses the document. There is no need to
wait until the whole document is parsed before getting a picture of
how the XML elements are placed in the document. XML::Parser accepts
a file handle or the text of an XML document and then goes through
its structure looking for certain events. When a particular event is
encountered, the parser calls the appropriate Perl subroutine to
handle it on the fly.</p>


<p>For this program, we define a handler that looks for the
<a name="INDEX-2882" />
<a name="INDEX-2883" />start of any XML
tag. This handler is declared as a reference to a
<a name="INDEX-2884" />subroutine
called <tt class="function">handle_start</tt>. The
<tt class="function">handle_start</tt> subroutine is declared further
below within the local context of the subroutine we are discussing.</p>


<p>
<a name="INDEX-2885" />XML::Parser can handle more than
just start tags. XML::Parser also supports the capability of writing
handlers for other types of parsing events such as end tags, or even
for specific tag names. However, in this program, we only need to
declare a handler that will be triggered any time an XML start tag is
encountered.</p>


<p><tt class="literal">$data</tt> contains the raw XML code to be parsed. The
<tt class="function">get</tt>
<a name="INDEX-2886" />
<a name="INDEX-2887" /> <a name="INDEX-2,888" />
subroutine was previously imported by pulling the LWP::Simple module
into the Perl script. When we pass
<tt class="literal">WHATS_RELATED_URL</tt> along with the URL we are
looking for to the <tt class="function">get</tt> subroutine,
<tt class="function">get</tt> will go out on the Internet and retrieve the
output from the "What's Related" web server.</p>


<p>You will notice that as soon as <tt class="literal">$data</tt> is
collected, there is some additional manipulation done to it.
<a name="INDEX-2889" />
<a name="INDEX-2890" />XML::Parser will
parse only well-formed XML documents. Unfortunately, the Netscape XML
server sometimes returns data that is not entirely well-formed, so a
generic XML parser has a little difficulty with it.</p>


<p>To get around this problem, we filter out potentially bad data inside
of the tags. The <a name="INDEX-2891" />
<a name="INDEX-2892" />regular
expressions in the above code respectively transform ampersands,
double-quotes, HTML tags, and stray &lt; and &gt; characters into
well-formed counterparts. The last regular expression deals with
filtering out non-ASCII characters.</p>


<p>Before parsing the data, we set the baseline global variables
<tt class="literal">@RECORDS</tt> to the empty set and
<tt class="literal">$RELATED_RECORDS</tt> to true (1).</p>


<p>Simply calling the <em class="filename">parse</em> method on the
<tt class="literal">$parser</tt> object starts the parsing process. The
<tt class="literal">$data</tt> variable that is passed to
<tt class="function">parse</tt> is the XML subject to be read. The
<tt class="function">parse</tt> method also accepts other types of data
including file handles to XML files.</p>


<p>Recall that the <tt class="function">handle_start</tt> subroutine was
passed to the <tt class="literal">$parser</tt> object upon its creation.
The <tt class="function">handle_start</tt> subroutine that is declared
within the <tt class="function">get_whats_related</tt> subroutine is
called by XML::Parser every time a start tag is encountered.</p>


<p><tt class="literal">$expat</tt>
<a name="INDEX-2893" /> is a reference to the
XML::Parser object itself.
<tt class="literal">$element</tt>
<a name="INDEX-2894" />
<a name="INDEX-2895" /> <a name="INDEX-2,896" />
is the start element name and <tt class="literal">%attributes</tt> is a
hash table of attributes that were declared inside the XML element.</p>


<p>For this example, we are concerned only with tags that begin with the
name "child" and contain the
<tt class="literal">href</tt> attribute. In addition, the
<tt class="literal">$href</tt> tag is filtered so any non-URL information
is stripped out of the URL.</p>


<p>If there is no name attribute, or if the name attribute contains the
phrase "Smart Browsing", or if there were no related
records found previously for this URL, we do not want to add anything
to the <tt class="literal">@RECORDS</tt> array. In addition, if the name
attribute contains the phrase "no related", the
<tt class="literal">$RELATED_RECORDS</tt> flag is set to false (0).</p>


<p>Otherwise, if these conditions are not met, we will add the URL to
the <tt class="literal">@RECORDS</tt> array. This is done by making a
reference to an array with two elements: the URL and the title of the
URL. At the end of the subroutine, the compiled
<tt class="literal">@RECORDS</tt> array is returned.</p>


<p>This program was a simple example of using a CGI program to pull data
automatically from an XML-based server. While the What's
Related server is just one XML server, it is conceivable that as XML
grows, there will be more database engines on the Internet that
deliver even more types of data. Since XML is the standard language
for delivering data markup on the Web, extensions to this CGI script
can be used to access those new <a name="INDEX-2897" />data repositories.</p>


<p>More information about <a name="INDEX-2898" /> <a name="INDEX-2,899" /> <a name="INDEX-2,900" /> <a name="INDEX-2,901" />XML, DTD, RDF, and even the Perl
XML::Parser library can be found at <a href="http://www.xml.com/">http://www.xml.com/</a>. Of course, XML::Parser
can also be found on <a name="INDEX-2902" /> <a name="INDEX-2,903" /> <a name="INDEX-2,904" />CPAN.</p>


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