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Your LimeSurvey version:5.35
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Survey theme/template:
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When I created a new controller file in the directory, I encountered a 404 error accessing it through routing. 


File directory：application\controllers\admin\Hjj.php

Code such as：
<?php
class Hjj extends Survey_Common_Action{
    protected function _init()
    {
            parent::_init();
    }
   public function actionIndex()
   {
echo "Helloworld";
   }
}

Url such as:
https://localhost/index.php/admin/Hjj

Later, I directly copied the existing normal controller, but changed the name and also encountered a 404 error.

How to add a new controller？

Controller are not in admin directory

Put in in controller directory.

And here : it's not a controller , but a PHP file
Example : gitlab.com/SondagesPro/coreAndTools/ResetPasswordController

If you want a new page for admin : use pluginhelper

$url = App()->getController()->createUrl(
'admin/pluginhelper',
array('plugin' => $this->getName(), 'sa' => 'fullpagewrapper','method' => 'actionList' )
);

Sample : gitlab.com/SondagesPro/Authentification/.../AuthRemoteToken.php