In fact, Joomla 4 installs and enables by default a plugin 'Behaviour - Backward Compatibility'. Its description says "This plugin should be enabled before updating to Joomla 5."To my knowledge, no such backward compatibility plugin for J4 exists.
Under the hood, the Joomla 4 plugin looks simple because it does not even have class maps, the scripts that the Joomla 5 plugin loads and maps earlier class names like JRegistry, JModelAdmin and JUser, so that the calls are directed to Registry, AdminModel and User.
As a quick test, just installed Joomla 4.4.3 on localhost with Blog Sample Data, disabled the plugin 'Behaviour - Backward Compatibility'. Then installed Gantry 5 and Helium and changed the default template to Helium. There were no problems whatsoever, neither in the Joomla 4 frontend nor the backend.
Then changed the Update Channel to 'Joomla Next' and updated the site to 5.0.3, while the plugin 'Behaviour - Backward Compatibility' was still disabled.
Went to the backend and tried to access Gantry 5 Themes - Default Theme. The following error message was displayed: "An error has occurred.
0 An exception has been thrown during the rendering of a template ("Unable to load application: site")."
Then went to the front end, where the usual red page popped up with the message "Sorry, there was a problem we could not recover from. The server returned a '500 - Whoops, looks like something went wrong.' "
Turned on the plugin 'Behaviour - Backward Compatibility' and both error messages were gone.
So, even though the compatibility plugin does not do much in Joomla 4, there is a good reason why it should be enabled before updating the Joomla 4 site to Joomla 5, especially if the site already has third party extensions that have not been updated to be fully compatible with Joomla 5.
Statistics: Posted by toivo — Sat Mar 09, 2024 3:25 am