code: public static function autoload($className)
      {
          if (isset(static::$classMap[$className])) {
              $classFile = static::$classMap[$className];
              if ($classFile[0] === '@') {
                  $classFile = static::getAlias($classFile);
              }
          } elseif (strpos($className, '\\') !== false) {
              $classFile = static::getAlias('@' . str_replace('\\', '/', $className) . '.php', false);
              if ($classFile === false || !is_file($classFile)) {
                  return;
              }
          } else {
              return;
          }
  
          include $classFile;
  
          if (YII_DEBUG && !class_exists($className, false) && !interface_exists($className, false) && !trait_exists($className, false)) {
              throw new UnknownClassException("Unable to find '$className' in file: $classFile. Namespace missing?");
          }
      }
docs: Class autoload loader.
  
  This method is invoked automatically when PHP sees an unknown class.
  The method will attempt to include the class file according to the following procedure:
  
  1. Search in [[classMap]];
  2. If the class is namespaced (e.g. `yii\base\Component`), it will attempt
  to include the file associated with the corresponding path alias
  (e.g. `@yii/base/Component.php`);
  
  This autoloader allows loading classes that follow the [PSR-4 standard](http://www.php-fig.org/psr/psr-4/)
  and have its top-level namespace or sub-namespaces defined as path aliases.
  
  Example: When aliases `@yii` and `@yii/bootstrap` are defined, classes in the `yii\bootstrap` namespace
  will be loaded using the `@yii/bootstrap` alias which points to the directory where bootstrap extension
  files are installed and all classes from other `yii` namespaces will be loaded from the yii framework directory.
  
  Also the [guide section on autoloading](guide:concept-autoloading).
  
  @param string $className the fully qualified class name without a leading backslash "\"
  @throws UnknownClassException if the class does not exist in the class file