What is the ContextLoaderListener and what does it do?

The ContextLoaderListener is a listener that helps to bootstrap Spring MVC. As the name suggests, it loads and creates the ApplicationContext, so you don't have to write explicit code to do create it.

The application context is where Spring bean leaves. For a web application, there is is a subclass called WebAppliationContext.

The ContextLoaderListener also ties the lifecycle of the ApplicationContext to the lifecycle of the ServletContext. You can get the ServletContext from the WebApplicationContext using the getServletContext() method.