Rijkswatch: Servlet listeners

Scope	Lifecycle listeners	Attribute change listeners	Register
Application	ServletContextListener	ServletContextAttributeListener	@WebListener
Servlet	HttpServlet		@PostConstruct @PreDestroy
Session	HttpSessionListener	HttpSessionAttributeListener	@WebListener
Request	ServletRequestListener	ServletRequestAttributeListener	@WebListener
Asynchronous Request	AsyncListener		AsyncContext#addListener

Additional Session Listeners:
- HttpSessionBindingListener: Notifies an attribute that it is bound/unbound from the session attribute map
- HttpSessionActivationListener: Notifies an attribute that it is passivated to/activated from persistent storage (session passivation)
WebListeners can also be registered using
- web.xml
```
<web-app>
  <listener>
    <listener-class>org.edu.MyListener</listener-class>
  </listener>
…
</web-app> 
```
- programmatically using ServletContext#addListener from a ServletContextListener

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