There are different types of factory patterns like Simple Factory (Or Class Factory), Factory Method, Abstract Factory Method and Creator Method (Or Static Factory Method). Out of these, Factory Method and Abstract Factory Method are part of the GoF Design Patterns. All factory design patterns help you create objects without using the new keyword directly within your code. Concept is similar to any other real world factories: You use cars, but you don’t develop cars by your own.

Design patterns gained popularity in computer science after the book Design Patterns: Elements of Reusable Object-Oriented Software was published in 1994 by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides; and together they are called the "Gang of Four" or "GoF".

The GOF design patterns can be broadly classified into the following three categories:

1. Creational patterns

   • These are patterns related to object creation. 

All web applications in a Java EE server have their own unique context roots. A web application's context is reprecented by an implementation of the ServletContext interface. Each web application has only one servletContext and one ServletContext belongs to only one application. Servlets need to make use of the container interface to communicate with other components outside of their application or even within their application. 

A servlet can use a ServletContext object for various purposes, including:

Listeners are classes that will wait for certain life cycle events and the container will invoke certain methods of these classes when that life cycle event happens. There will be listener interfaces, with methods that need to be invoked when a life cycle event occur. The listener implementation class will need to implement this interface.

You can specify a listener in either of the ways:

• @WebListener annocation over the listener implementation class

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