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SelfLoader - load functions only on demand |
SelfLoader - load functions only on demand
package FOOBAR;
use SelfLoader;
... (initializing code)
__DATA__
sub {....
This module tells its users that functions in the FOOBAR package are to be
autoloaded from after the __DATA__ token. See also
Autoloading in the perlsub manpage.
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.
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.
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 the perlmod manpage.
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.
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.
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.
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.
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SelfLoader - load functions only on demand |