SelfLoader
SelfLoader(3) Perl Programmers Reference Guide SelfLoader(3)
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 recog-
nized 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 pack-
age 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 mod-
ule 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 docu-
mentation.
__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 FOO-
BAR::DATA filehandle. "__END__" can still be used to denote the end of
the "__DATA__" section if followed by the token DATA - this is sup-
ported by the SelfLoader. The "FOOBAR::DATA" filehandle is left open if
an "__END__" followed by a DATA is found, with the filehandle posi-
tioned 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 "FOO-
BAR::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 'Self-
Loader->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::SelfStub-
ber" 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 "Self-
Loader::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.
perl v5.8.6 2001-09-21 SelfLoader(3)
