lwpcook.pod

ARM上的如果你对底层感兴趣

=head1 NAME

lwpcook - libwww-perl cookbook

=head1 DESCRIPTION

This document contain some examples that show typical usage of the
libwww-perl library.  You should consult the documentation for the
individual modules for more detail.

All examples should be runnable programs. You can, in most cases, test
the code sections by piping the program text directly to perl.



=head1 GET

It is very easy to use this library to just fetch documents from the
net.  The LWP::Simple module provides the get() function that return
the document specified by its URL argument:

  use LWP::Simple;
  $doc = get 'http://www.sn.no/libwww-perl/';

or, as a perl one-liner using the getprint() function:

  perl -MLWP::Simple -e 'getprint "http://www.sn.no/libwww-perl/"'

or, how about fetching the latest perl by running this command:

  perl -MLWP::Simple -e '
    getstore "ftp://ftp.sunet.se/pub/lang/perl/CPAN/src/latest.tar.gz",
             "perl.tar.gz"'

You will probably first want to find a CPAN site closer to you by
running something like the following command:

  perl -MLWP::Simple -e 'getprint "http://www.perl.com/perl/CPAN/CPAN.html"'

Enough of this simple stuff!  The LWP object oriented interface gives
you more control over the request sent to the server.  Using this
interface you have full control over headers sent and how you want to
handle the response returned.

  use LWP::UserAgent;
  $ua = new LWP::UserAgent;
  $ua->agent("$0/0.1 " . $ua->agent);
  # $ua->agent("Mozilla/8.0") # pretend we are very capable browser

  $req = new HTTP::Request 'GET' => 'http://www.sn.no/libwww-perl';
  $req->header('Accept' => 'text/html');

  # send request
  $res = $ua->request($req);

  # check the outcome
  if ($res->is_success) {
     print $res->content;
  } else {
     print "Error: " . $res->status_line . "\n";
  }

The lwp-request program (alias GET) that is distributed with the
library can also be used to fetch documents from WWW servers.
  


=head1 HEAD

If you just want to check if a document is present (i.e. the URL is
valid) try to run code that looks like this:

  use LWP::Simple;

  if (head($url)) {
     # ok document exists
  }

The head() function really returns a list of meta-information about
the document.  The first three values of the list returned are the
document type, the size of the document, and the age of the document.

More control over the request or access to all header values returned
require that you use the object oriented interface described for GET
above.  Just s/GET/HEAD/g.


=head1 POST

There is no simple procedural interface for posting data to a WWW server.  You
must use the object oriented interface for this. The most common POST
operation is to access a WWW form application:

  use LWP::UserAgent;
  $ua = new LWP::UserAgent;

  my $req = new HTTP::Request 'POST','http://www.perl.com/cgi-bin/BugGlimpse';
  $req->content_type('application/x-www-form-urlencoded');
  $req->content('match=www&errors=0');

  my $res = $ua->request($req);
  print $res->as_string;

Lazy people use the HTTP::Request::Common module to set up a suitable
POST request message (it handles all the escaping issues) and has a
suitable default for the content_type:

  use HTTP::Request::Common qw(POST);
  use LWP::UserAgent;
  $ua = new LWP::UserAgent;

  my $req = POST 'http://www.perl.com/cgi-bin/BugGlimpse',
                [ search => 'www', errors => 0 ];

  print $ua->request($req)->as_string;

The lwp-request program (alias POST) that is distributed with the
library can also be used for posting data.



=head1 PROXIES

Some sites use proxies to go through fire wall machines, or just as
cache in order to improve performance.  Proxies can also be used for
accessing resources through protocols not supported directly (or
supported badly :-) by the libwww-perl library.

You should initialize your proxy setting before you start sending
requests:

  use LWP::UserAgent;
  $ua = new LWP::UserAgent;
  $ua->env_proxy; # initialize from environment variables
  # or
  $ua->proxy(ftp  => 'http://proxy.myorg.com');
  $ua->proxy(wais => 'http://proxy.myorg.com');
  $ua->no_proxy(qw(no se fi));

  my $req = new HTTP::Request 'wais://xxx.com/';
  print $ua->request($req)->as_string;

The LWP::Simple interface will call env_proxy() for you automatically.
Applications that use the $ua->env_proxy() method will normally not
use the $ua->proxy() and $ua->no_proxy() methods.

Some proxies also require that you send it a username/password in
order to let requests through.  You should be able to add the
required header, with something like this:

 use LWP::UserAgent;

 $ua = new LWP::UserAgent;
 $ua->proxy(['http', 'ftp'] => 'http://proxy.myorg.com');

 $req = new HTTP::Request 'GET',"http://www.perl.com";
 $req->proxy_authorization_basic("proxy_user", "proxy_password");

 $res = $ua->request($req);
 print $res->content if $res->is_success;

Replace C<proxy.myorg.com>, C<proxy_user> and
C<proxy_password> with something suitable for your site.


=head1 ACCESS TO PROTECTED DOCUMENTS

Documents protected by basic authorization can easily be accessed
like this:

  use LWP::UserAgent;
  $ua = new LWP::UserAgent;
  $req = new HTTP::Request GET => 'http://www.sn.no/secret/';
  $req->authorization_basic('aas', 'mypassword');
  print $ua->request($req)->as_string;

The other alternative is to provide a subclass of I<LWP::UserAgent> that
overrides the get_basic_credentials() method. Study the I<lwp-request>
program for an example of this.


=head1 MIRRORING

If you want to mirror documents from a WWW server, then try to run
code similar to this at regular intervals:

  use LWP::Simple;

  %mirrors = (
     'http://www.sn.no/'             => 'sn.html',
     'http://www.perl.com/'          => 'perl.html',
     'http://www.sn.no/libwww-perl/' => 'lwp.html',
     'gopher://gopher.sn.no/'        => 'gopher.html',
  );

  while (($url, $localfile) = each(%mirrors)) {
     mirror($url, $localfile);
  }

Or, as a perl one-liner:

  perl -MLWP::Simple -e 'mirror("http://www.perl.com/", "perl.html")';

The document will not be transfered unless it has been updated.



=head1 LARGE DOCUMENTS

If the document you want to fetch is too large to be kept in memory,
then you have two alternatives.  You can instruct the library to write
the document content to a file (second $ua->request() argument is a file
name):

  use LWP::UserAgent;
  $ua = new LWP::UserAgent;

  my $req = new HTTP::Request 'GET',
                'http://www.sn.no/~aas/perl/www/libwww-perl-5.00.tar.gz';
  $res = $ua->request($req, "libwww-perl.tar.gz");
  if ($res->is_success) {
     print "ok\n";
  }

Or you can process the document as it arrives (second $ua->request()
argument is a code reference):

  use LWP::UserAgent;
  $ua = new LWP::UserAgent;
  $URL = 'ftp://ftp.unit.no/pub/rfc/rfc-index.txt';

  my $expected_length;
  my $bytes_received = 0;
  $ua->request(HTTP::Request->new('GET', $URL),
               sub {
                   my($chunk, $res) = @_;
                   $bytes_received += length($chunk);
	           unless (defined $expected_length) {
	              $expected_length = $res->content_length || 0;
                   }
		   if ($expected_length) {
		        printf STDERR "%d%% - ",
	                          100 * $bytes_received / $expected_length;
                   }
	           print STDERR "$bytes_received bytes received\n";

                   # XXX Should really do something with the chunk itself
	           # print $chunk;
               });



=head1 HTML FORMATTING

It is easy to convert HTML code to "readable" text.

  use LWP::Simple;
  use HTML::Parse;
  print parse_html(get 'http://www.sn.no/libwww-perl/')->format;



=head1 PARSE URLS

To access individual elements of a URL, try this:

  use URI::URL;
  $host = url("http://www.sn.no/")->host;

or

  use URI::URL;
  $u = url("ftp://ftp.sn.no/test/aas;type=i");
  print "Protocol scheme is ", $u->scheme, "\n";
  print "Host is ", $u->host, " at port ", $u->port, "\n";

or even

  use URI::URL;
  my($host,$port) = (url("ftp://ftp.sn.no/test/aas;type=i")->crack)[3,4];


=head1 EXPAND RELATIVE URLS

This code reads URLs and print expanded version.

  use URI::URL;
  $BASE = "http://www.sn.no/some/place?query";
  while (<>) {
     print url($_, $BASE)->abs->as_string, "\n";
  }

We can expand URLs in an HTML document by using the parser to build a
tree that we then traverse:

  %link_elements =
  (
   'a'    => 'href',
   'img'  => 'src',
   'form' => 'action',
   'link' => 'href',
  );

  use HTML::Parse;
  use URI::URL;

  $BASE = "http://somewhere/root/";
  $h = parse_htmlfile("xxx.html");
  $h->traverse(\&expand_urls, 1);

  print $h->as_HTML;

  sub expand_urls
  {
     my($e, $start) = @_;
     return 1 unless $start;
     my $attr = $link_elements{$e->tag};
     return 1 unless defined $attr;
     my $url = $e->attr($attr);
     return 1 unless defined $url;
     $e->attr($attr, url($url, $BASE)->abs->as_string);
  }



=head1 BASE URL

If you want to resolve relative links in a page you will have to
determine which base URL to use.  The HTTP::Response objects now has a
base() method.

  $BASE = $res->base;



=head1 COPYRIGHT

Copyright 1996-1997, Gisle Aas

This library is free software; you can redistribute it and/or
modify it under the same terms as Perl itself.