SelfLoader

Section: Perl Programmers Reference Guide (3pm)
Updated: 2007-12-18
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NAME

SelfLoader - load functions only on demand  

SYNOPSIS

    package FOOBAR;
    use SelfLoader;

    ... (initializing code)

    __DATA__
    sub {....
 

DESCRIPTION

This module tells its users that functions in the FOOBAR package are to be autoloaded from after the "__DATA__" token. See also ``Autoloading'' in perlsub.  

The __DATA__ token

The "__DATA__" token tells the perl compiler that the perl code for compilation is finished. Everything after the "__DATA__" token is available for reading via the filehandle FOOBAR::DATA, where FOOBAR is the name of the current package when the "__DATA__" token is reached. This works just the same as "__END__" does in package 'main', but for other modules data after "__END__" is not automatically retrievable, whereas data after "__DATA__" is. The "__DATA__" token is not recognized in versions of perl prior to 5.001m.

Note that it is possible to have "__DATA__" tokens in the same package in multiple files, and that the last "__DATA__" token in a given package that is encountered by the compiler is the one accessible by the filehandle. This also applies to "__END__" and main, i.e. if the 'main' program has an "__END__", but a module 'require'd (_not_ 'use'd) by that program has a 'package main;' declaration followed by an '"__DATA__"', then the "DATA" filehandle is set to access the data after the "__DATA__" in the module, _not_ the data after the "__END__" token in the 'main' program, since the compiler encounters the 'require'd file later.  

SelfLoader autoloading

The SelfLoader works by the user placing the "__DATA__" token after perl code which needs to be compiled and run at 'require' time, but before subroutine declarations that can be loaded in later - usually because they may never be called.

The SelfLoader will read from the FOOBAR::DATA filehandle to load in the data after "__DATA__", and load in any subroutine when it is called. The costs are the one-time parsing of the data after "__DATA__", and a load delay for the _first_ call of any autoloaded function. The benefits (hopefully) are a speeded up compilation phase, with no need to load functions which are never used.

The SelfLoader will stop reading from "__DATA__" if it encounters the "__END__" token - just as you would expect. If the "__END__" token is present, and is followed by the token DATA, then the SelfLoader leaves the FOOBAR::DATA filehandle open on the line after that token.

The SelfLoader exports the "AUTOLOAD" subroutine to the package using the SelfLoader, and this loads the called subroutine when it is first called.

There is no advantage to putting subroutines which will _always_ be called after the "__DATA__" token.  

Autoloading and package lexicals

A 'my $pack_lexical' statement makes the variable $pack_lexical local _only_ to the file up to the "__DATA__" token. Subroutines declared elsewhere _cannot_ see these types of variables, just as if you declared subroutines in the package but in another file, they cannot see these variables.

So specifically, autoloaded functions cannot see package lexicals (this applies to both the SelfLoader and the Autoloader). The "vars" pragma provides an alternative to defining package-level globals that will be visible to autoloaded routines. See the documentation on vars in the pragma section of perlmod.  

SelfLoader and AutoLoader

The SelfLoader can replace the AutoLoader - just change 'use AutoLoader' to 'use SelfLoader' (though note that the SelfLoader exports the AUTOLOAD function - but if you have your own AUTOLOAD and are using the AutoLoader too, you probably know what you're doing), and the "__END__" token to "__DATA__". You will need perl version 5.001m or later to use this (version 5.001 with all patches up to patch m).

There is no need to inherit from the SelfLoader.

The SelfLoader works similarly to the AutoLoader, but picks up the subs from after the "__DATA__" instead of in the 'lib/auto' directory. There is a maintenance gain in not needing to run AutoSplit on the module at installation, and a runtime gain in not needing to keep opening and closing files to load subs. There is a runtime loss in needing to parse the code after the "__DATA__". Details of the AutoLoader and another view of these distinctions can be found in that module's documentation.  

__DATA__, __END__, and the FOOBAR::DATA filehandle.

This section is only relevant if you want to use the "FOOBAR::DATA" together with the SelfLoader.

Data after the "__DATA__" token in a module is read using the FOOBAR::DATA filehandle. "__END__" can still be used to denote the end of the "__DATA__" section if followed by the token DATA - this is supported by the SelfLoader. The "FOOBAR::DATA" filehandle is left open if an "__END__" followed by a DATA is found, with the filehandle positioned at the start of the line after the "__END__" token. If no "__END__" token is present, or an "__END__" token with no DATA token on the same line, then the filehandle is closed.

The SelfLoader reads from wherever the current position of the "FOOBAR::DATA" filehandle is, until the EOF or "__END__". This means that if you want to use that filehandle (and ONLY if you want to), you should either

1. Put all your subroutine declarations immediately after the "__DATA__" token and put your own data after those declarations, using the "__END__" token to mark the end of subroutine declarations. You must also ensure that the SelfLoader reads first by calling 'SelfLoader->load_stubs();', or by using a function which is selfloaded;

or

2. You should read the "FOOBAR::DATA" filehandle first, leaving the handle open and positioned at the first line of subroutine declarations.

You could conceivably do both.  

Classes and inherited methods.

For modules which are not classes, this section is not relevant. This section is only relevant if you have methods which could be inherited.

A subroutine stub (or forward declaration) looks like 

  sub stub;

i.e. it is a subroutine declaration without the body of the subroutine. For modules which are not classes, there is no real need for stubs as far as autoloading is concerned.

For modules which ARE classes, and need to handle inherited methods, stubs are needed to ensure that the method inheritance mechanism works properly. You can load the stubs into the module at 'require' time, by adding the statement 'SelfLoader->load_stubs();' to the module to do this.

The alternative is to put the stubs in before the "__DATA__" token BEFORE releasing the module, and for this purpose the "Devel::SelfStubber" module is available. However this does require the extra step of ensuring that the stubs are in the module. If this is done I strongly recommend that this is done BEFORE releasing the module - it should NOT be done at install time in general.  

Multiple packages and fully qualified subroutine names

Subroutines in multiple packages within the same file are supported - but you should note that this requires exporting the "SelfLoader::AUTOLOAD" to every package which requires it. This is done automatically by the SelfLoader when it first loads the subs into the cache, but you should really specify it in the initialization before the "__DATA__" by putting a 'use SelfLoader' statement in each package.

Fully qualified subroutine names are also supported. For example, 

   __DATA__
   sub foo::bar {23}
   package baz;
   sub dob {32}

will all be loaded correctly by the SelfLoader, and the SelfLoader will ensure that the packages 'foo' and 'baz' correctly have the SelfLoader "AUTOLOAD" method when the data after "__DATA__" is first parsed.