ch05.htm

Web_Programming_with_Perl5,一个不错的Perl语言教程。

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>502</TT></TD>

		<TD WIDTH="104" ALIGN="LEFT" VALIGN="TOP"><TT>Bad Gateway</TT></TD>

		<TD ALIGN="LEFT">The server received an invalid response from the gateway or an upstream server.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT" VALIGN="TOP"><TT>503</TT></TD>

		<TD WIDTH="104" ALIGN="LEFT" VALIGN="TOP"><TT>Service Unavailable</TT></TD>

		<TD ALIGN="LEFT">The server cannot process the request due to a high load or maintenance of the server.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT" VALIGN="TOP"><FONT COLOR="#000000">504</FONT></TD>

		<TD WIDTH="104" ALIGN="LEFT" VALIGN="TOP"><FONT COLOR="#000000">Gateway Timeout</FONT></TD>

		<TD ALIGN="LEFT"><FONT COLOR="#000000">The response did not return within a time that the gateway

			was prepared to wait. Similar to Internal error 500 but has more diagnostic value.</FONT></TD>

	</TR>

</TABLE>

In the next section, we will look at how the <TT>header()</TT> method in CGI.pm can

be used in your script to set the <TT>Status</TT> response header. <B><TT>Content-Type

Response Header</TT></B> The most common response header is <TT>Content-Type</TT>,

which contains the MIME type of the object being returned (usually &quot;text/html&quot;),

and <TT>Content-Length</TT>, which indicates the size of the object. <B><TT>Location

Response Header</TT></B> If you did not want to return a file but instead wanted

your script to send the browser to another URI, the <TT>Location</TT> response header

can be used.</P>

<P>The headers are terminated by an empty line and followed by the &quot;body&quot;

of the message. The &quot;body&quot; refers to the text or HTML or other MIME compliant

message in the object.</P>

<P>So, bringing all this together, here is the syntax of a basic HTTP response header:</P>

<PRE><FONT COLOR="#0066FF">&quot;HTTP/1.0&quot; result-code message CrLf

Header: Value CrLf

Header: Value CrLf

CrLf

BODY

Connection closed by foreign host.

</FONT></PRE>

<P>Here is a real-world example of the complete response generated by a Netscape

Communications Server. This response was generated after a simple <TT>GET</TT> request

was sent for an HTML file. A similar response could be generated by your CGI script

using the <TT>header()</TT> method in CGI.pm.</P>

<PRE><FONT COLOR="#0066FF">HTTP/1.0 200 OK

Server: Netscape-Communications/1.12

Date: Monday, 21-Oct-96 01:42:01 GMT

Last-modified: Thursday, 10-Oct-96 17:17:20 GMT

Content-length: 1048

Content-type: text/html

&lt;HTML&gt;

&lt;HEAD&gt;

&lt;TITLE&gt;Welcome to My Web Page&lt;/TITLE&gt;

&lt;/HEAD&gt;

&lt;BODY&gt;

Here is my web page.  &lt;b&gt;Do you like it?&lt;/b&gt;

&lt;/BODY&gt;

&lt;/HTML&gt;

Connection closed by foreign host.

</FONT></PRE>

<P>Notice that after the response headers are sent, a blank line (CrLf) is sent,

which is followed by the body of the response. The body is the Web page, or the data

your script outputs. After the data has been sent (the body), the server drops the

connection.</P>

<P>When generating your own response headers for NPH scripts (highly recommended),

the contents of these response headers are easily modified using CGI.pm, or the HTTP::Response

class modules. In the next section, I will show you how CGI.pm or the HTTP::Response

class can be used to easily generate response headers.

<CENTER>

<H3><A NAME="Heading14"></A><FONT COLOR="#000077">Perl5 and the WWW Libraries in

Action</FONT></H3>

</CENTER>

<P>Perl, with the help of CGI.pm or the WWW libraries, allows complete control over

every aspect of the client/server HTTP transaction. Using the CGI::Request WWW library

in your scripts to manage and process variables passed between the client and the

server during a transaction makes your code more secure, easier to maintain, and

easier to write.

<CENTER>

<H4><A NAME="Heading15"></A><FONT COLOR="#000077">CGI.pm versus Individual libwww

Modules</FONT></H4>

</CENTER>

<P>CGI.pm and the modular WWW libraries are very similar in function but very different

in purpose. CGI.pm includes all of the functionality of the individual modules, and

in most cases, more. In fact, Lincoln Stein, the author of CGI.pm and the WWW modules,

developed the CGI.pm module in parallel with the CGI, URL, and HTML modules. The

individual modules are designed to provide a very optimized solution to about nine

different aspects of programming with Perl for the Web. Modules exist for the Web

functions listed in Table 5.4. <TT>Table 5.4. Function of libwww modules. </TT>

<TABLE BORDER="0">

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT"><B>Module</B></TD>

		<TD ALIGN="LEFT"><B>Function</B></TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT">CGI::</TD>

		<TD ALIGN="LEFT">Allows control over HTTP request and response headers. Mini HTTP server, CGI-based

			imagemapping, &quot;smart&quot; forms.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT">File::</TD>

		<TD ALIGN="LEFT">Parsing of directory listings and a persistent counter.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT">Font::</TD>

		<TD ALIGN="LEFT">Interface to Adobe Font Metrics Files.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT">HTML::</TD>

		<TD ALIGN="LEFT">Creation and parsing of HTML elements. Format HTML as text or PostScript.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT">HTTP::</TD>

		<TD ALIGN="LEFT">Generation of HTTP requests, responses, and headers. Content negotiation and status

			code processing.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT">LWP::</TD>

		<TD ALIGN="LEFT">Low-level I/O, socket interface, file type recognition.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT">MIME::</TD>

		<TD ALIGN="LEFT">Encoding and decoding Base64 and QP.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT">URI::</TD>

		<TD ALIGN="LEFT">Filtering, processing of URIs, elimination of unsafe characters in strings.</TD>

	</TR>

	<TR ALIGN="LEFT" rowspan="1">

		<TD ALIGN="LEFT">WWW::</TD>

		<TD ALIGN="LEFT">Parse robot.txt files.</TD>

	</TR>

</TABLE>

Think of each of the individual modules as a very focused and optimized solution

to a specific task. Most of the time, you will be writing scripts that use features

from more than one of the preceding modules. If you have to load any more than three

modules, it is actually more efficient to just load the entire CGI.pm. Loading each

of the modules separately actually is more resource intensive due to the overhead

associated with loading multiple files. The CGI.pm module also has a more robust

feature set than the individual modules.</P>

<P>Anything your CGI prints to STDOUT gets sent back to the Web server and out to

the browser, unless you are using nph-CGIs, in which case, STDOUT goes directly to

the browser without being parsed by the server. Traditionally, before Perl5 and CGI.pm,

HTML forms were created by CGI scripts by printing the HTML form elements to STDOUT.

Traditionally, the name/value pairs would have to be URI-decoded from either the

URI (<TT>GET</TT> method) or STDIN (<TT>POST</TT> method). Back in the days of Perl

4, include scripts such as cgi-lib.pl were written to deal with these nuisances.

However, there were still problems. Using cgi-lib.pl, the state of a form (the <BR>

contents of the variables) was lost from invocation to invocation because neither

the browser nor the server retained data from a previous transaction, debugging was

difficult, namespace was trampled, and the code was messy and would emit errors before

the HTTP response headers were printed if Perl was run in strict mode.



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<CENTER>

<H3><A NAME="Heading16"></A><FONT COLOR="#000077">URL:</FONT></H3>

</CENTER>





<BLOCKQUOTE>

	<P>If you still aren't convinced, a complete rundown on all of the benefits of using

	CGI.pm and Perl5 over the old methods such as cgi-lib.pl can be found at the following

	URL:



</BLOCKQUOTE>



<PRE><FONT COLOR="#0066FF">http://perl.com/perl/info/www/!cgi-lib.html

</FONT></PRE>

<P>CGI.pm was created to address these problems and build a wide and extensible bridge

between your CGI Perl script and the CGI interface. With CGI.pm, HTML forms can be

generated easily by making a few Perl function calls. When you use CGI.pm to generate

HTML forms, the values of the input fields from previous queries (if there were any)

are automatically used to initialize the form, so the state of the form is preserved

from invocation to invocation. URI encoding and <BR>

decoding of special characters is handled transparently by CGI.pm. <TT>GET</TT> and

<TT>POST</TT> methods are handled appropriately. If your script is run from the UNIX

shell, the option of inputting variables from the command line is given. In short,

CGI.pm takes care of almost all of the gory details of CGI and lets you focus on

writing your program.</P>

<P>The easiest way to learn to use CGI.pm or any of the modules is to simply start

using them in your scripts.

<CENTER>

<H3><A NAME="Heading17"></A><FONT COLOR="#000077">Example 1: Creating Forms with

CGI.pm</FONT></H3>

</CENTER>

<P>The example in Listing 5.1 introduces the usage of CGI.pm in a typical script

that generates and displays an HTML form. When run, the script parses the input from

<TT>GET</TT> and <TT>POST</TT> methods, then prints the HTML output for a Web page

with a form on it. If the script finds any data in the input stream (a previously

filled-out form), it will be printed at the bottom of the page. The output is shown

in Figure 5.3.



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<CENTER>

<H3>

<HR WIDTH="83%">

<BR>

<FONT COLOR="#000077">NOTE:</FONT></H3>

</CENTER>





<BLOCKQUOTE>

	<P>The line numbers in Listing 5.1 are for reference only and are not to be typed

	in.<BR>

	

<HR>





</BLOCKQUOTE>



<CENTER>

<H3><A NAME="Heading19"></A><FONT COLOR="#000077">Listing 5.1. Using CGI.pm with

forms</FONT></H3>

</CENTER>

<PRE><FONT COLOR="#0066FF">1   #!/usr/local/bin/perl

2   use CGI;

3   $query = new CGI;

4

5   print $query-&gt;header;

6

7   print $query-&gt;start_html(&quot;Using CGI.pm with Forms&quot;);

8   print &quot;&lt;H2&gt; Example 1: Using CGI.pm with Forms&lt;/H2&gt;

9         &lt;HR&gt;\n&quot;;

10

11  &amp;print_prompt($query);

12  &amp;do_work($query);

13  print $query-&gt;end_html;

14

15  # Subroutines ###

16

17  sub print_prompt {

18  my($query) = @_;

19  print $query-&gt;start_multipart_form;

20

21  print &quot;

22  &lt;table border=0 cellpadding=0 cellspacing=0 width=550&gt;

23  &lt;tr&gt;&lt;td&gt;&quot;,

24  $query-&gt;textfield(-name=&gt;`first_name',

25                    -default=&gt;`John',

26                    -size=&gt;25),

27  &quot;&lt;BR&gt;First Name&lt;/td&gt;&lt;td&gt;&quot;,

28  $query-&gt;textfield(-name=&gt;`mi', -size=&gt;2, -default=&gt;`A'),

29  &quot;&lt;BR&gt;M.I.&lt;/td&gt;&lt;td&gt;&quot;,

30  $query-&gt;textfield(-name=&gt;`last_name',

31                    -default=&gt;`Doe',

32                    -size=&gt;25),

33  &quot;&lt;BR&gt;Last Name&lt;/tr&gt;&lt;/td&gt;

34  &lt;tr&gt;&lt;td colspan=3&gt;&quot;,

35  $query-&gt;textfield(-name=&gt;`address', -size=&gt;55),

36  &quot;&lt;BR&gt;Street Address&lt;/td&gt;&lt;/tr&gt;

37  &lt;tr&gt;&lt;td&gt;&quot;,

38  $query-&gt;textfield(-name=&gt;`city',

39                    -size=&gt;25),

40  &quot;&lt;BR&gt;City&lt;/td&gt;&lt;td&gt;&quot;,

41  $query-&gt;textfield(-name=&gt;`state', -size=&gt;2),

42  &quot;&lt;BR&gt;State&lt;/td&gt;&lt;td&gt;&quot;,

43  $query-&gt;textfield(-name=&gt;`zip',

44                    -size=&gt;10),

45  &quot;&lt;BR&gt;Zip Code&lt;/tr&gt;&lt;/td&gt;

46   &lt;/table&gt;&quot;;

47

48  print &quot;&lt;HR&gt;&lt;table border=0 cellspacing=0

49              cellpadding=0 width=550&gt;

50        &lt;tr&gt;&lt;td&gt;&lt;EM&gt;What Opera