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Introduction to Servlet technology

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A Servlet lifecycle

A Servlet has certain life cycle which includes process from creation till destruction.

The Servlet container manages Servlet life cycle. The life cycle contains below steps:

Servlet Class should be Loaded.
Create Instance of Servlet.
The init() method is used for servlet initialization.
The service () method is used process clients request.
The destroy () method is used terminate the servlet.

Finally, Servlet is garbage collected by garbage collector of JVM.

Load Servlet Class

Before a servlet can be invoked servlet container must first load its class definition.

Create Instance of Servlet

When servlet class is loaded, servlet container creates an instance of servlet.

The init() method

The web container calls init method only once after creating servlet instance. The init method is used to initialize servlet. The servlet is created when a user invokes a URL corresponding to servlet. The init() method creates or loads data that are used throughout life of servlet.

public void init() throws ServletException {
  // code Initialization.
}

The service () method

The service() method is used to perform actual task. The servlet container calls service() method to handle requests coming from client( browsers) and to write formatted response back to client.

Whenever servlet receives request from server it creates a new thread and call service. The service() method check for HTTP request type (POST, GET, DELETE, PUT, etc.) and call doPut, doGet, doPost, doDelete, etc. methods as appropriate.

public void service(ServletRequest request, ServletResponse response)
      throws ServletException, IOException{
}

The destroy () method

The servlet is unloaded by servlet container, destroy() method is called. This step executed once, since servlet is unloaded only once. After destroy() method is called, servlet object is marked for garbage collection.

  public void destroy() {
    // Code Finalization.
  }

A Hello World Servlet

The servlets can be created as follows:

Servlet interface Implementation
GenericServlet class Implementation (or)
HttpServlet class Implementation

Here is an example of a HttpServlet

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public class HelloWorld extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
    throws IOException, ServletException
    {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<html>");
        out.println("<head>");
        out.println("<title>Hello World!</title>");
        out.println("</head>");
        out.println("<body>");
        out.println("<h1>Hello World!</h1>");
        out.println("</body>");
        out.println("</html>");
    }
}

Compiling a Servlet

Save the file as HelloWorld.Java compile the file using

Javac HelloWorld.Java

This will create a file HelloWorld.Class.

Servlet Deployment

Copy HelloWorld.class into <CATALINA_HOME>/webapps/ROOT/WEB-INF/classes and create following entries in web.xml file located in <CATALINA_HOME >/webapps/ROOT/WEB-INF/

<servlet>
   <servlet-name>HelloWorld</servlet-name>
   <servlet-class>HelloWorld</servlet-class>
 </servlet>
<servlet-mapping>
    <servlet-name>HelloWorld</servlet-name>
    <url-pattern>/HelloWorld</url-pattern>
 </servlet-mapping>

Restart apache server and access below link to get output.

http://localhost:8080/HelloWorld

Alright, so that was introduction to a servlet life cycle, in next part we will explore handling client requests.

Description

This guide introduces Servlet technology and we will cover below topics

What is a Servlet?
Initial Setup
Life Cycle
Various Examples
Client Request
Server Response
Http Status Codes
Writing Filters
Exception Handling
Cookies Handling
Session Tracking
Database Access
Packaging
Internationalization

This is to the point introduction to the topic to get you started

Prerequisites

Working knowledge of Core Java is essential

Audience

Beginner to Java Servlet technology or students looking to brush up their skills quickly

Learning Objectives

Learn Java Servlets

Author: Subject Coach
Added on: 2nd May 2015

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