HTML::Mason::Subclassing(3) Documentation on Subclassing Internal Mason classes


This is the deep voodoo guide, for folks who want to create their own custom subclasses for parts of Mason, such as the Request or Interp objects.


A number of modules in Mason are subclasses of "Class::Container". This module was originally part of the Mason core as "HTML::Mason::Container", but Ken Williams decided to release it separately on CPAN.

It was created to encapsulate some common behaviors for Mason objects such as parameter validation and the creation of ``contained'' objects.

Basically, any Mason object which takes parameters to its constructor must inherit from this module. Of course, since all of the classes that you might consider subclassing already inherit from "Class::Container", you won't need to inherit from it directly. However, you may need to use some of its methods.

So before you go further we highly recommend familiarizing yourself with "Class::Container" and its methods. Also feel free to look at some of the Mason core modules to see how "Class::Container" is used within Mason itself.


The following classes have been designed with subclassing in mind:
  • HTML::Mason::Request

    This object is your old friend $m. The request contains information about the current request context, and provides methods for calling other components.

  • HTML::Mason::Resolver

    The resolver's job is to translate a component paths into an actual component. Mason comes with a single Resolver subclass, "HTML::Mason::Resolver::File", which is used to translate component paths into filesystem paths.

  • HTML::Mason::ComponentSource

    An object of this class represents a component's source. These objects are instantiated by the resolver when it finds a component matching a given path.

  • HTML::Mason::Lexer

    The lexer is responsible for parsing a component. Creating a new lexer would allow you to change Mason's component syntax.

  • HTML::Mason::Compiler

    The compiler takes the parsed chunks from the lexer and gives them meaning. The default compiler, "HTML::Mason::Compiler::ToObject", turns a Mason component into a Mason ``object file'', which contains actual Perl code.

  • HTML::Mason::ApacheHandler

    The ApacheHandler class is the bridge between the mod_perl world and Mason, primarily Mason's Interp class.

    It also provides its own "HTML::Mason::Request" and "HTML::Resolver::File" subclasses which implement some mod_perl specific behaviors and features.

  • HTML::Mason::Interp

    The Interp is the core of Mason, and is primarily responsible for making all the other objects do their jobs.


If you choose to override the constructor, which is always "new" with Mason objects, that you make sure to call the superclass's constructor and that you use the object returned by it. A good boilerplate for an overridden constructor looks something like this:

  sub new
      my $class = shift;
      my $self = $class->SUPER::new(@_);
      return $self;


What to Subclass?

One important thing to know about this class is that it is actually several classes. The first, "HTML::Mason::Request", is used when ApacheHandler is not loaded. The other, "HTML::Mason::Request::ApacheHandler", is loaded by ApacheHandler and used to provide some mod_perl specific features. Similar, the CGIHandler class provides its own request subclass, "HTML::Mason::Request::CGIHandler".

It is impossible to know which one of these to subclass at compile time, since it is possible that your subclass will be loaded before either ApacheHandler or CGIHandler.

To handle this, simply call the "alter_superclass()" method in your constructor, like this:

  sub new
      my $class = shift;
      $class->alter_superclass( $HTML::Mason::ApacheHandler::VERSION ?
                                'HTML::Mason::Request::ApacheHandler' :
                                $HTML::Mason::CGIHandler::VERSION ?
                                'HTML::Mason::Request::CGI' :
                                'HTML::Mason::Request' );
      my $self = $class->SUPER::new(@_);
      return $self;

It is quite important that you do this as these handler-specific subclasses provide important functionality. The "alter_superclass()" method is implemented in the "HTML::Mason::Request" base class, and will do the right thing even in cases of multiple inheritance. It also cooperates with "Class::Container" to make sure that it sees changes to the inheritance hierarchy.

The exec() method

The "exec" method is called in order to execute a request, and is the method that you are most likely to want to override.

However, if you do override it we suggest that you make sure to call the parent class's "exec" method to implement the actual component execution and there is no need for you to re-implement them.

Since the "exec()" method is scalar/list context-sensitive, your "exec" method will need to preserve that. Here is a boilerplate:

  sub exec
      my $self = shift;
      ... # do something cool
      my @r;
      if (wantarray)
          @r = $self->SUPER::exec(@_);
          $r[0] = $self->SUPER::exec(@_);
      ... # maybe do some cleanup
      return wantarray ? @r : $r[0];


Your custom request class will also be used to implement subrequests, which are implemented by calling "exec" just like any other method. If you only want to do certain things in "exec" for the first request, you can simply check the value of "$self->is_subrequest".


See the "MasonX::Request::WithApacheSession" module on CPAN.

Resolver and ComponentSource

The resolver takes a component path and figures out what component that path corresponds to.

All resolver classes must implement two methods, "get_info" and "glob_path". The first takes a component path and returns a new "HTML::Mason::ComponentSource" object. This object contains information about the component, such as its last modified time and its source. See the "HTML::Mason::ComponentSource" documentation for more details.

You may choose to provide your own ComponentSource subclass as well, if your resolver implementation can take advantage of it.

The "glob_path" method is responsible for translating a component path like /foo/*/bar into a list of component paths that match that glob pattern.


The rationale for providing your own lexer would be to extend or replace Mason's syntax.

The lexer is called by the compiler via its "lex" method. The arguments it receives are the component name, source, and the compiler object. See the Compiler class documentation for details on what methods the lexer can call.


See the Compiler class documentation for details on what methods a subclass of this class needs to provide.

If you simply want to tweak Mason's existing behavior, you will probably want to subclass "HTML::Mason::Compiler::ToObject", which is the default Compiler class. For example, if you wanted to do something like make attributes dynamic, you could override the "_flags_or_attr()" method in ToObject.

If you want to drastically change the behavior, you can subclass "HTML::Mason::Compiler" instead. An example of this would be creating a compiler that generates "EmbPerl" or "Apache::ASP" as output.


The methods that you are most likely to want to subclass are documented in the "ApacheHandler class" documentation.

Providing an ApacheHandler subclass gives you a chance to do your own client parameter parsing, as well as the capability of providing a different way of handling requests.


Like the ApacheHandler, you could subclass this module in order to provide your own argument processing or to step in and provide a different way to handle requests.


When using your custom subclasses, we recommend that you take advantage of Mason's ability to construct subclassed object on the fly.

For example, if you're subclassed the Interp object, you can still let the ApacheHandler object create the Interp object for you, as long as you give it the appropriate interp_class parameter. This is important because Mason may internally set up certain defaults for contained objects. For example, the ApacheHandler, by default, will tell the Interp object to use the "HTML::Mason::Request::ApacheHandler" Request subclass. If you create an Interp object manually and you want to use that Interp object with ApacheHandler, you'll have to specify the same Request class.

For example:

  my $interp =
          ( request_class  => 'HTML::Mason::Request::ApacheHandler',
            my_new_interp_param => 42,
  my $ah = HTML::Mason::ApacheHandler->new( interp => $interp );

It is far easier to simply do this:

  my $ah =
          ( interp_class => 'My::Interp',
            my_new_interp_param => 42,

Your new parameter, "my_new_interp_param", will still be passed to the "My::Interp" constructor, but this also gives ApacheHandler a chance to set various parameters for the Interp object. Of course, you can still override these defaults explicitly:

  my $ah =
          ( interp_class => 'My::Interp',
            resolver_class => 'My::Resolver'.
            my_new_interp_param => 42,

If you need access to the interp object's methods directly, it will be always be available via "$ah->interp".