PSR4无法正常工作？

Class not found, apparently. I've tried various things but nothing works.

Composer:

"autoload": {
    "psr-4": {
        "App\\": "application/"
    }
}

File structure: https://i.imgur.com/h9wOEqI.png

<?php

namespace App\Library\Classes;

defined('START') or exit('We couldn\'t process your request right now.');

class Application
{
    private static $libraries = array();

    public static function get($library) {
        if (isset(self::$libraries[$library]) && isset(self::$classes[$library])) {
            return self::$libraries[$library];
        }

        $fixedLibrary = str_replace('.', '/', $library);
        $file = ROOT . '/application/library/classes/' . strtolower($fixedLibrary) . '.php';

        self::$libraries[$library] = $library;

        $declared = get_declared_classes();
        $workingClass = end($declared);

        self::$libraries[$library] = new $workingClass();

        return self::$libraries[$library];
    }
}

?>

Error is on this line:

Application::get('test')->test();

Yet, if I change it to this, it works:

include ROOT . '/application/Library/Application.php';
App\Library\Classes\Application::get('test')->test();

The PSR4 is not built-in part or PHP, you need an implementation of autoloader to use this standard such as provided by the Composer.

When you install or update depedencies, composer generates the relevant code of autoloading, but you can directly update it by the command dump-autoload, as @jibsteroos said. Next you should explicitly include the file vendor/autoload.php in the entry point of your application.

Also, error message says about class Application, but you should add the use statement at first:

use App\Library\Classes\Application;

Application::get('test')->test();

Or use the fully qualified class name (class name with namespace prefix):

\App\Library\Classes\Application::get('test')->test();