You need to wait for 3.10.X to become available; as per the documentation, "This guide assumes you are starting on Joomla 3.10.x. If you are on a earlier version make sure you upgrade to Joomla 3.10 first before moving to Joomla 4."
Joomla 3.10 is currently in development, at 86% completion, most notably, with no completion date for release. V3.10-...
This should still work in 4.0 but will break in 5.0. You are advised to use fully qualified class names. Remove JLoader::register() call and instead add a use statement:
Thanks to @Zollie-- the plugin is enabled by default and does seem to work. You'd need to check the libraries/autoload_psr4.php to verify that your component's namespace (set in the new namespace parameter in the manifest xml) is being generated correctly.
The default namespace format for custom components is DeveloperName\Component\ComponentName.
According to the documentation, JApplicationWeb seems to be alive and well in Joomla4.
note As of 4.0 this class will be abstract
Relevant references and examples:
Just an FYI that Joomla 3.10-alpha has now been released so the upgrade process can now easily done.
Go to the Joomla Update Component Options and switch the Update Channel to Testing and the Minimum Stability option to Alpha.
Update to 3.10 alpha1 using the Joomla Update Component.
After you have updated to 3.10 alpha please switch the Update
You will firstly need to create a helper class that extends ContentHelper, like so:
class MyComponentHelper extends ContentHelper
protected $config = null;
public function __construct($config)
$this->config = new ...
The view file that your component is looking for MycomponentViewViewname which was used in the Joomla 3, so make sure you are extending the right controller class in your Controller(i.e Joomla\CMS\MVC\Controller\BaseController or any other class extending it such as AdminController). If it is from correct namespace track the setView() for any such changes ...