HTTP::Proxy::BodyFilter - A base class for HTTP messages body filters

    package MyFilter;

    use base qw( HTTP::Proxy::BodyFilter );

    # a simple modification, that may break things
    sub filter {
        my ( $self, $dataref, $message, $protocol, $buffer ) = @_;
        $$dataref =~ s/PERL/Perl/g;
    }

    1;

The HTTP::Proxy::BodyFilter class is used to create filters for HTTP request/response body data.

A BodyFilter is just a derived class that implements some methods called by the proxy. Of all the methods presented below, only CWfilter() must be defined in the derived class.

filter()

The signature of the filter() method is the following:

    sub filter {
        my ( $self, $dataref, $message, $protocol, $buffer ) = @_;
        ...
    }

where CW$self is the filter object, CW$dataref is a reference to the chunk of body data received, CW$headers is a reference to HTTP::Headers object, CW$message is a reference to either a HTTP::Request or a HTTP::Response object, and CW$protocol is a reference to the LWP::Protocol protocol object. Note that this subroutine signature looks a lot like that of the call- backs of LWP::UserAgent (except that CW$message is either a HTTP::Request or a HTTP::Response object). $buffer is a reference to a buffer where some of the unprocessed data can be stored for the next time the filter will be called (see Using a buffer to store data for a later use for details). Thanks to the built-in HTTP::Proxy::BodyFilter::* filters, this is rarely needed. The CW$headers HTTP::Headers object is the one that was sent to the client (if the filter is on the response stack) or origin server (if the filter is on the request stack). Modifying it in the filter() method is useless, since the headers have already been sent. Since CW$dataref is a reference to the data string, the referent can be modified and the changes will be transmitted through the filters that follows, until the data reaches its recipient. A HTTP::Proxy::BodyFilter object is a blessed hash, and the base class reserves only hash keys that start with CW_hpbf.

new()

The constructor is defined for all subclasses. Initialisation tasks (if any) for subclasses should be done in the CWinit() method (see below).

init()

This method is called by the CWnew() constructeur to perform all initisalisation tasks. It's called once in the filter lifetime. It receives all the parameters passed to CWnew().

begin()

Some filters might require initialisation before they are able to handle the data. If a CWbegin() method is defined in your subclass, the proxy will call it before sending data to the CWfilter() method. It's called once per HTTP message handled by the filter, before data processing begins. The method signature is as follows:

    sub begin {
        my ( $self, $message ) = @_
        ...
    }

end()

Some filters might require finalisation after they are finished handling the data. If a CWend() method is defined in your subclass, the proxy will call it after it has finished sending data to the CWfilter() method. It's called once per HTTP message handled by the filter, after all data processing is done. This method does not expect any parameters.

Some filters cannot handle arbitrary data: for example a filter that basically lowercases tag name will apply a simple regex such as CWs/<\s*(\w+)([^>]*)>/<\L$1\E$2>/g. But the filter will fail is the chunk of data contains a tag that is cut before the final CW>.

It would be extremely complicated and error-prone to let each filter (and its author) do its own buffering, so the HTTP::Proxy architecture handles this too. The proxy passes to each filter, each time it is called, a reference to an empty string ($buffer in the above signature) that the filter can use to store some data for next run.

When the reference is CWundef, it means that the filter cannot store any data, because this is the very last run, needed to gather all the data left in all buffers.

It is recommended to store as little data as possible in the buffer, so as to avoid (badly) reproducing what HTTP::Proxy::BodyFilter::complete does.

In particular, you have to remember that all the data that remains in the buffer after the last piece of data is received from the origin server will be sent back to your filter in one big piece.

HTTP::Proxy implements a store and forward mechanism, for those filters which need to have the whole message body to work. It's enabled simply by pushing the HTTP::Proxy::BodyFilter::complete filter on the filter stack.

The data is stored in memory by the complete filter, which passes it on to the following filter once the full message body has been received.

Standard HTTP::Proxy::BodyFilter classes are lowercase.

The following BodyFilters are included in the HTTP::Proxy distribution:

lines

This filter makes sure that the next filter in the filter chain will only receive complete lines. The chunks of data received by the following filters with either end with CW\n or will be the last piece of data for the current HTTP message body.

htmltext

This class lets you create a filter that runs a given code reference against text included in a HTML document (outside CW<script> and CW<style> tags). HTML entities are not included in the text.

htmlparser

Creates a filter from a HTML::Parser object.

simple

This class lets you create a simple body filter from a code reference.

save

Store the message body to a file.

complete

This filter stores the whole message body in memory, thus allowing some actions to be taken only when the full page has been received by the proxy.

tags

The HTTP::Proxy::BodyFilter::tags filter makes sure that the next filter in the filter chain will only receive complete tags. The current implementation is not 100% perfect, though.

Please read each filter's documentation for more details about their use.

Some methods are available to filters, so that they can eventually use the little knowledge they might have of HTTP::Proxy's internals. They mostly are accessors.

proxy()

Gets a reference to the HTTP::Proxy objects that owns the filter. This gives access to some of the proxy methods.

Philippe BooK Bruhat, <book@cpan.org>.

HTTP::Proxy, HTTP::Proxy::HeaderFilter.

Copyright 2003-2005, Philippe Bruhat.

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