Visitor Design Pattern

Definition
A behavioral pattern which lets you define new methods to a class hierarchy without changing the class hierarchy on which it operates

Applicability
1. When the primary class hierarchy is fixed or it is coming from a different vendor and you cannot make any changes to that hierarchy
2. Several different operations have to be performed on all or some of the classes in the hierarchy

Visitor and Composite Pattern normally go hand in hand together. First, we will try to create an example of composite pattern. A composite pattern is a structural pattern which helps to create tree like hierarchial structures. Consider a linux filesystem where every element is considered as a file. We are trying to achieve the same tree structure using Directory, File and Link elements. All of these classes implement the interface FileSystemElement

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A Servlet Refresher

A Servlet is a dynamic web component developed in Java technology, loaded and run by Java technology enabled web server. The Servlet interface is the core of the Java Servlet API. The two classes which implement the Servlet interface are the GenericServlet and the HttpServlet. The Servlet interface defines the init(), service() and the destroy() methods which are implemented by GenericServlet class. The HttpServlet is an abstract class extending GenericServlet and basically helps in processing HTTP requests.

Servlet Life Cycle
The Servlet life cycle is expressed in the API by the init, service and destroy methods of the Servlet interface

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Configuring and retrieving a DataSource object

In the JDBC API, a DataSource object is a means to access a database. A DataSource object is identified by set of properties which is used to communicate with the server. These properties include information such as the location of the database server, the name of the database, the protocol used to communicate with the server, the username and password of the privileged database user.
In the application server, a data source is known as JDBC Resource

The rest of the post demonstrates how to configure a JDBC Resource in GlassFish application server and retrieve the same data source using JNDI through a simple web servlet

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Prototype Design Pattern

Definition
A creational pattern which allows creation of new instances by copying existing instance or prototype.

Applicability
1. When the classes to instantiate are specified at the run time
2. When creating a new instance is more expensive due to high computation or a call over the network

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