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Programming Perl

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The Standard Perl Library
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7.2.55 SelfLoader - Load Functions Only on Demand

package GoodStuff;
use SelfLoader;

[initializing code]
__DATA__
sub {...};

This module is used for delayed loading of Perl functions that (unlike AutoLoader functions) are packaged within your script file. This gives the appearance of faster loading.

In the example above, SelfLoader tells its user (GoodStuff) that functions in the GoodStuff package are to be autoloaded from after the __DATA__ token.

The __DATA__ token tells Perl that the code for compilation is finished. Everything after the __DATA__ token is available for reading via the filehandle GoodStuff::DATA, where GoodStuff is the name of the current package when the __DATA__ token is reached. This token works just the same as __END__ does in package main, except that data after __END__ is retrievable only in package main, whereas data after __DATA__ is retrievable in whatever the current package is.

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. That is, whenever the __DATA__ token is parsed, any DATA filehandle previously open in the current package (opened in a different file, presumably) is closed so that the new one can be opened. (This also applies to __END__ and the main::DATA filehandle: main::DATA is reopened whenever __END__ is encountered, so any former association is lost.)

7.2.55.1 SelfLoader autoloading

The SelfLoader will read from the GoodStuff::DATA filehandle to get definitions for functions placed after __DATA__, and then eval the requested subroutine the first time it's 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 are a speeded up compilation phase, with no need to load functions that are never used.

You can use __END__ after __DATA__. The SelfLoader will stop reading from DATA if it encounters the __END__ token, just as you might expect. If the __END__ token is present, and is followed by the token DATA, then the SelfLoader leaves the GoodStuff::DATA filehandle open on the line after that token.

The SelfLoader exports the AUTOLOAD subroutine to the package using the SelfLoader, and this triggers the automatic loading of an undefined subroutine out of its DATA portion the first time that subroutine is called.

There is no advantage to putting subroutines that will always be called after the __DATA__ token.

7.2.55.2 Autoloading and file-scoped lexicals

A my $pack_lexical statement makes the variable $pack_lexical visible only up to the __DATA__ token. That means that subroutines declared elsewhere cannot see lexical variables. Specifically, autoloaded functions cannot see such lexicals (this applies to both the SelfLoader and the Autoloader). The use vars pragma (see later in this chapter) provides a way to declare package-level globals that will be visible to autoloaded routines.

7.2.55.3 SelfLoader and AutoLoader

The SelfLoader can replace the AutoLoader - just change use AutoLoader to use SelfLoader[13] and the __END__ token to __DATA__.

[13] Be aware, however, that the SelfLoader exports an AUTOLOAD function into your package. But if you have your own AUTOLOAD and are using the AutoLoader too, you probably know what you're doing.

There is no need to inherit from the SelfLoader.

The SelfLoader works similarly to the AutoLoader, but picks up the subroutine definitions from after the __DATA__ instead of in the lib/auto/ directory. SelfLoader needs less maintenance at the time the module is installed, since there's no need to run AutoSplit. And it can run faster at load time because it doesn't need to keep opening and closing files to load subroutines. On the other hand, it can run slower because it needs 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.

7.2.55.4 How to read DATA from your Perl program

(This section is only relevant if you want to use the GoodStuff::DATA together with the SelfLoader.)

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

You could even conceivably do both.

7.2.55.5 Classes and inherited methods

This section is only relevant if your module is a class, and has methods that could be inherited.

A subroutine stub (or forward declaration) looks like:

sub stub;

That is, it is a subroutine declaration without the body of the subroutine. For modules that aren't classes, there is no real need for stubs as far as autoloading is concerned.

For modules that 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 you do this, we strongly recommended that you do it before releasing the module and not at install time.

7.2.55.6 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 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.

See the discussion of autoloading in Chapter 5. Also see the AutoLoader module, a utility that handles modules that have been into a collection of files for autoloading.


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