yii\base\Application::bootstrap PHP Method

    protected function bootstrap()
    {
        if ($this->extensions === null) {
            $file = Yii::getAlias('@vendor/yiisoft/extensions.php');
            $this->extensions = is_file($file) ? include $file : [];
        }
        foreach ($this->extensions as $extension) {
            if (!empty($extension['alias'])) {
                foreach ($extension['alias'] as $name => $path) {
                    Yii::setAlias($name, $path);
                }
            }
            if (isset($extension['bootstrap'])) {
                $component = Yii::createObject($extension['bootstrap']);
                if ($component instanceof BootstrapInterface) {
                    Yii::trace('Bootstrap with ' . get_class($component) . '::bootstrap()', __METHOD__);
                    $component->bootstrap($this);
                } else {
                    Yii::trace('Bootstrap with ' . get_class($component), __METHOD__);
                }
            }
        }
        foreach ($this->bootstrap as $class) {
            $component = null;
            if (is_string($class)) {
                if ($this->has($class)) {
                    $component = $this->get($class);
                } elseif ($this->hasModule($class)) {
                    $component = $this->getModule($class);
                } elseif (strpos($class, '\\') === false) {
                    throw new InvalidConfigException("Unknown bootstrapping component ID: {$class}");
                }
            }
            if (!isset($component)) {
                $component = Yii::createObject($class);
            }
            if ($component instanceof BootstrapInterface) {
                Yii::trace('Bootstrap with ' . get_class($component) . '::bootstrap()', __METHOD__);
                $component->bootstrap($this);
            } else {
                Yii::trace('Bootstrap with ' . get_class($component), __METHOD__);
            }
        }
    }

 /**
  * @inheritdoc
  */
 protected function bootstrap()
 {
     $request = $this->getRequest();
     Yii::setAlias('@webroot', dirname($request->getScriptFile()));
     Yii::setAlias('@web', $request->getBaseUrl());
     parent::bootstrap();
 }