Speeding Module Loading with SelfLoader (Perl Cookbook, 2nd Edition)

## 12.10. Speeding Module Loading with SelfLoader

### 12.10.1. Problem

You'd like to load a very large module quickly.

### 12.10.2. Solution

Use the SelfLoader module:

>     require Exporter;
>     require SelfLoader;
>     @ISA = qw(Exporter SelfLoader);
>     #
>     # other initialization or declarations here
>     #
>     _ _DATA_ _
>     sub abc { .... }
>     sub def { .... }

### 12.10.3. Discussion

When you load a module using `require` or `use`, the entire module file must be read and compiled (into internal parse trees, not into byte code or native machine code) right then. For very large modules, this annoying delay is unnecessary if you need only a few functions from a particular file.

To address this problem, the SelfLoader module delays compilation of each subroutine until that subroutine is actually called. SelfLoader is easy to use: just place your module's subroutines underneath the `_ _DATA_ _` marker so the compiler will ignore them, use a `require` to pull in the SelfLoader, and include SelfLoader in the module's `@ISA` array. That's all there is to it. When your module is loaded, the SelfLoader creates stub functions for all routines below `_ _DATA_ _`. The first time a function gets called, the stub replaces itself by first compiling the real function and then calling it.

There is one significant restriction on modules that employ the SelfLoader (or the AutoLoader for that matter, described in Recipe 12.11). SelfLoaded or AutoLoaded subroutines have no access to lexical variables in the file whose `_ _DATA_ _` block they are in because they are compiled via `eval` in an imported AUTOLOAD block. Such dynamically generated subroutines are therefore compiled in the scope of SelfLoader's or AutoLoader's AUTOLOAD.

Whether the SelfLoader helps or hinders performance depends on how many subroutines the module has, how large they are, and whether they are all called over the lifetime of the program or not.

You should initially develop and test your module without SelfLoader. Commenting out the `_ _DATA_ _` line will take care of that, making those functions visible to the compiler.

### 12.10.4. See Also

The documentation for the standard module SelfLoader; Recipe 12.11

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