Service interface implemented by extensions. An extension is a service provider declared in META-INF/services.

Service providers may have observer methods, which may observe any event, including any container lifecycle event, and obtain an injected BeanManager.

The container instantiates a single instance of each extension at the beginning of the application initialization process and maintains a reference to it until the application shuts down. The container delivers event notifications to this instance by calling its observer methods.

Service providers are made available for injection as beans with the qualifier @Default.