Why can Container::getInstance() return an application class

Question

I am I'm wondering why the Container::getInstance() can return a application class.

For example:

I want to make a hash str, I want to know how they work：

app('hash')->make('password');

and I found the source code in laravel ：

vendor/laravel/framework/src/Illuminate/Foundation/helpers.php

if (! function_exists('app')) {
    /**
     * Get the available container instance.
     *
     * @param  string  $make
     * @param  array   $parameters
     * @return mixed|\Illuminate\Foundation\Application
     */
    function app($make = null, $parameters = [])
    {
        if (is_null($make)) {
            return Container::getInstance();
        }

        return Container::getInstance()->make($make, $parameters);

    }
}

I dont know what the Container::getInstance() will return, then I dd（Container::getInstance()） and I know it will can return an application class, but I dont know how they work.

Answer 1

Maybe I'm a little bit late with my answer, but anyway.

Description is current as of Laravel framework version 5.3.24.

Why calling app(), that then calls Container::getInstance() returns object, instance of Application? For example, why

Route::get('/', function () {
    var_dump(app());
});

outputs:

object(Illuminate\Foundation\Application)
...

Because... And here we have to go step by step though it to understand everything.

1. User initiates a web request. The request is processed by /public/index.php

2. /public/index.php contains the following:

   $app = require_once __DIR__.'/../bootstrap/app.php';
   

3. /bootstrap/app.php has the following lines:

   $app = new Illuminate\Foundation\Application(
       realpath(__DIR__.'/../')
   );
   

4. When an $app object is instantiated, the Illuminate\Foundation\Application class constructor is called.

/vendor/laravel/framework/src/Illuminate/Foundation/Application.php

    class Application extends Container implements ...
    {
        // ...
        public function __construct($basePath = null)
        {
            // 5. constructor triggers the following method:
            $this->registerBaseBindings();
            // ...
        }
        // ...
        protected function registerBaseBindings()
        {
            // 6. which then triggers the following:
            static::setInstance($this);
            // 7. Important! $this points to class Application here
            // and is passed to Container
            // ...
        }
        // ...
    }

8. static::setInstance($this); refers us to class Container, because class Application extends Container

/vendor/laravel/framework/src/Illuminate/Container/Container.php

    class Container implements ...
    {
        // ...
        // 11. $instance now contains an object,
        // which is an instance of Application class
        protected static $instance;
        // ...
        public static function setInstance(ContainerContract $container = null)
        {
            // 9. $container = Application here, because it has been passed
            // from class Application while calling static::setInstance($this);
            // 10. Thus, static::$instance is set to Application here
            return static::$instance = $container;
        }
        // ...
    }

---

12. Now, suppose, we have written the following lines in our routes file. /routes/web.php

    Route::get('/', function () {
        dd(app()); // 13. We a calling an app() helper function
    });
    

14 Calling app() leads us to /vendor/laravel/framework/src/Illuminate/Foundation/helpers.php

    // ...
    /** @return mixed|\Illuminate\Foundation\Application */
    function app($make = null, $parameters = [])
    {
        // 15. $make is null, so this is the case
        if (is_null($make)) {
            // 16. The following static method is called:
            return Container::getInstance();
        }
        // ...
    }
    // ...

17. Now we are back in our Container class /vendor/laravel/framework/src/Illuminate/Container/Container.php

    public static function getInstance()
    {
        // 18. Important!
        // To this point static::$instance is NOT null,
        // because it has already been set (up to "step 11").
        if (is_null(static::$instance)) {
            static::$instance = new static; // Thus, we skip this.
        }
        // 19. static::$instance is returned
        // that contains an object,
        // which is an instance of Application class
        return static::$instance;
    }
    

Some important notes for steps 16-19.

Important note 1!

Static Keyword

Declaring class properties or methods as static makes them accessible without needing an instantiation of the class.

Important note 2!

static::$instance = new static; is NOT related to calling our app() function in step 13. And was somewhat misleading for me at first...

But just to note, it makes use of Late Static Bindings.