Life Cycle of a Servlet

 

A Simple Servlet Example

This is a simple servlet example using tomcat and eclipse. It answers the questions like, what is a servlet? How a servlet works in the Tomcat container? The life cycle of a servlet etc.

Servlet Example using Tomcat and Eclipse

What is a Servlet?

A Servlet is a server side java component that can process the request sent by a browser and send back response to it. A Servlet is a Simple java class implementing the Servlet interface in the javax.servlet interface. The response sent by the Servlet back to the browser is purely html. Servlets can execute inside a servlet container like tomcat,weblogic etc. The javax.servlet package contains all the classes and interfaces that will be required to build a servlet.

Interfaces inside the javax.servlet package

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Servlet ServletConfig ServletContext ServletRequest ServletResponse SingleThreadModel

How to Create a Servlet?

To create a servlet you have to implement the Servlet interface and provide implementation of all the methods in that interface.. The important methods are init ,service, destroy.So while creating a servlet we are actually defining all the required methods like init,service,destroy. When the request for a servlet reaches Tomcat, it will call the methods following a particular order which is called the life cycle of a Servlet.

How Tomcat Works?

When a request for a servlet reaches Tomcat from a browser, the first thing Tomcat does is it finds out weather an object of that servlet is loaded or not. If not loaded tomcat loads a new object for that Servlet. If loaded tomcat compares the time stamp of the loaded Servlet object with the Servlet class file. If the loaded servlet object is older then the servlet class file, tomcat loads a new servlet object that will process the request.

Life Cycle of a Servlet

Tomcat will first call the init method, then the service method. destroy method will be called just prior to the destruction of the servlet instance the init method gets called only once for an object The things that may be done once for an application can be put in the init method What ever is the functionality of the servlet should be placed in side the service method.

Creating a Example in Eclipse and Tomcat

First we should install jdk, eclipse and tomcat. In this servlet tutorial we have used jdk 1.8, Eclipse for EE developers and Tomcat 9. Below are the step by step instructions to create a Servlet.

Create a Dynamic web project

In Eclipse click on File -> New -> Dynamic Web Project. We have named the project as MyWebApp

Generate the web.xml

In a dynamic web project we can have the web.xml in the WEB-INF directory. This file is also called the deployment descriptior of the project. The web.xml should contain configuration information regarding the project. When we create a servlet we can enter information regarding the servlet in the web.xml file. We can click the check box to generate the web.xml automatically.

Create a new Servlet

Right click on the java src folder and click on new -> servlet.

Url pattern for the Servlet

Provide url pattern for the servlet

Simple Servlet Code

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import java.io.IOException; import javax.servlet.Servlet; import javax.servlet.ServletConfig; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; import javax.servlet.annotation.WebServlet; @WebServlet("/Testing") public class Testing implements Servlet { public void destroy() { System.out.println("From Destroy Method"); }   public ServletConfig getServletConfig() { return null; }   public String getServletInfo() { return null; }   public void init(ServletConfig config) throws ServletException { System.out.println("From Init Method"); }   public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException { System.out.println("From Service Method"); } }

Output of the Servlet

When accessed from various browsers the output will be as follows

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From Init Method From Service Method From Service Method From Service Method