Every application consists of a large number of classes in many different locations. The framework provides classes for core functionality. Your application will have a number of libraries, models, and other entities to make it work. You might have third-party classes that your project is using. Keeping track of where every single file is, and hard-coding that location into your files in a series of requires() is a massive headache and very error-prone. That’s where autoloaders come in.
CodeIgniter provides a very flexible autoloader that can be used with very little configuration. It can locate individual non-namespaced classes, namespaced classes that adhere to PSR4 autoloading directory structures, and will even attempt to locate classes in common directories (like Controllers, Models, etc).
For performance improvement, the core CodeIgniter components have been added to the classmap.
The autoloader works great by itself, but can also work with other autoloaders, like Composer, or even your own custom autoloaders, if needed. Because they’re all registered through spl_autoload_register, they work in sequence and don’t get in each other’s way.
The autoloader is always active, being registered with spl_autoload_register() at the beginning of the framework’s execution.
Initial configuration is done in /application/Config/Autoload.php. This file contains two primary arrays: one for the classmap, and one for PSR4-compatible namespaces.
The recommended method for organizing your classes is to create one or more namespaces for your application’s files. This is most important for any business-logic related classes, entity classes, etc. The psr4 array in the configuration file allows you to map the namespace to the directory those classes can be found in:
$psr4 = [ 'App' => APPPATH, 'CodeIgniter' => BASEPATH, ];
The key of each row is the namespace itself. This does not need a trailing slash. If you use double-quotes to define the array, be sure to escape the backwards slash. That means that it would be My\\App, not My\App. The value is the location to the directory the classes can be found in. They should have a trailing slash.
By default, the application folder is namespace to the App namespace. While you are not forced to namespace the controllers, libraries, or models in the application directory, if you do, they will be found under the App namespace. You may change this namespace by editing the /application/Config/Constants.php file and setting the new namespace value under the APP_NAMESPACE setting:
You will need to modify any existing files that are referencing the current namespace.
Config files are namespaced in the Config namespace, not in App\Config as you might expect. This allows the core system files to always be able to locate them, even when the application namespace has changed.
The classmap is used extensively by CodeIgniter to eke the last ounces of performance out of the system by not hitting the file-system with extra file_exists() calls. You can use the classmap to link to third-party libraries that are not namespaced:
$classmap = [ 'Markdown' => APPPATH .'third_party/markdown.php' ];
The key of each row is the name of the class that you want to locate. The value is the path to locate it at.
If neither of the above methods find the class, and the class is not namespaced, the autoloader will look in the /application/Libraries and /application/Models directories to attempt to locate the files. This provides a measure to help ease the transition from previous versions.
There are no configuration options for legacy support.