Zend Framework 2 autoloading

Question

I am just starting to look at Zend Framework 2 (and am new to ZF in general), and in the user guide, they are using autoloading when adding a new module. However, I find the explanation to be quite challenging for a rookie. They are adding a Module.php file within the module directory, which among others contains the following code:

public function getAutoloaderConfig()
    {
        return array(
            'Zend\Loader\ClassMapAutoloader' => array(
                __DIR__ . '/autoload_classmap.php',
            ),
            'Zend\Loader\StandardAutoloader' => array(
                'namespaces' => array(
                    __NAMESPACE__ => __DIR__ . '/src/' . __NAMESPACE__,
                ),
            ),
        );
    }

Now I did some digging around to try and figure out what this autoloading is all about. As far as I understand, the autoloading uses spl_autoload_register() and is a way to avoid having require_once() everywhere in the code. So, when trying to use a class that is not defined, the autoload() method that was registered will be run, which simply does an array lookup and includes the file like below if it was added.

// Zend/Loader/ClassMapAutoloader.php
public function autoload($class)
{
    if (isset($this->map[$class])) {
        require_once $this->map[$class];
    }
}

This seems clever due to performance. I hope what I just wrote is correct. Based on this, I am trying to figure out what is going on in getAutoloaderConfig() from the first code snippet, but I am quite confused. It seems as if the array that is returned by this method is used for AutoloaderFactory::factory(), but I am not sure for what purpose. Instantiating autoloaders with options it seems, but exactly what that does, I am not sure. I guess the second entry of the array specifies where to find the source files for the module's namespace - at least that would be my guess. The first entry I am, however, not sure about. In the user guide, it says the following:

As we are in development, we don’t need to load files via the classmap, so we provide an empty array for the classmap autoloader.

The file just returns an empty array. I am not sure what the purpose of this ClassMapAutoloader.

Sorry if my point is unclear; basically I am trying to figure out what is happening in getAutoloaderConfig() and what mymodule/autoload_classmap.php is used for. If someone could shed some light on this, that would be much appreciated!

Answer 1

The classmap is there to show PHP the most direct way to a class. It's essentially saying "You you're looking for A\Class\Youre\Looking\For, look no further than this file: xyz.php. This would be expressed like this:

return array(
    'A\Class\Youre\Looking\For' => ___DIR__.'/xyz.php'
)

Without it PHP has to run through the whole autoloader chain, which can be pretty expensive. Why is it saying something about "as we're in development"? Because classmap files are typically generated on the production server by some script. Basically, just don't worry about it too much right now. It's micro-optimization...

The getAutoloaderConfig() method is just there to give you some flexibility in really advanced applications. Most of the time you can just use the SkeletonApplication's and SkeletonModule's boilerplate code and leave it alone. Really, you can even kill the 'Zend\Loader\ClassMapAutoloader' => array(__DIR__ . '/autoload_classmap.php',) part for now.

It's just a hook for future improvements and nothing to worry about too much if you're just starting out with ZF2 (like me ;).