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Version: 4.4

Extension Class for Components

There are a number of occasions where other Joomla code would want to interact with our component, for example

  • the router might want to use our component's custom router to parse and build SEF routes
  • if our component supports categories, then com_categories will want to display on the categories view a summary by category containing the number of items with that category, split by published status
  • if our component supports custom fields then com_fields will want to call getContexts() to get the types of items which can have custom fields associated with them
  • if our component supports multilingual associations then com_associations will want to know the types of items which can have associations.
  • and of course Joomla will want to run our component to capture the output for the web page.

The reasons for introducing the Extension class becomes clearer if we review how other Joomla code interacted with our extension in Joomla 3.

Joomla 3 component access

In Joomla 3 all these other bits of code dipped into our component's code base in a rather haphazard fashion – calling functions in various helper files.

In Joomla 4 this is streamlined:

Joomla 4 component access

From Joomla 4 components get a handle on our com_example component by calling:

$extension = $app->bootComponent("com_example");

They can then call their required function on this Extension instance.

Immediately after the component Extension class is instantiated the Joomla library code will call your Extension's boot function, passing your child Dependency Injection Container instance:


This is just an opportunity to let you do, well, really, whatever you like. Sometimes it's used to set up particular classes for using with HtmlHelper::_() calls. Or you can use it to save a reference to your child DIC (which can be hard to get otherwise).

After the first instantiation of your component Joomla caches the instance, and if there is another call to

$extension = $app->bootComponent("com_example");

it just returns your Extension instance, rather than performing the class instantiation and calling of boot() again. You can even call bootComponent passing your own component, if you need to get a reference to your own Extension object.