Programming for beginners: void init (ServletConfig config) throws ServletException

public void init(ServletConfig config) throws ServletException

The Servlet container calls the init method, once the Instance for the servlet is created. The servlet won't process any service request until the init method finish.

The servlet container cannot place the servlet into service if the init method

a. Throws a ServletException

b. Does not return within a time period defined by the Web server

A servlet that cannot complete its initialization process should throw UnavailableException.

A servlet configuration object used by a servlet container to pass information to a servlet during initialization.

<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <title>Servlet Config</title>
    </head>
    <body>
        <form method="get" action="/servlet/ServletInit">
            <input type="submit" value="Click Me">
        </form>
    </body>
</html>

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.1" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd">
    <servlet>
        <servlet-name>ServletInit</servlet-name>
        <servlet-class>ServletInit</servlet-class>
        <init-param>
            <param-name>Author</param-name>
            <param-value>Krishna</param-value>
        </init-param>
        <init-param>
            <param-name>Book</param-name>
            <param-value>Self Learning Java</param-value>
        </init-param>
        <init-param>
            <param-name>URL</param-name>
            <param-value>self-learning-java-tutorial.blogspot.com</param-value>
        </init-param>    
    </servlet>
    
    <servlet-mapping>
        <servlet-name>ServletInit</servlet-name>
        <url-pattern>/ServletInit</url-pattern>
    </servlet-mapping>
</web-app>

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

public class ServletInit extends HttpServlet {
    @Override
    public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException{
       PrintWriter out = res.getWriter();
       res.setContentType("text/html");
       
       /* Get the Servlet Config Object */
       ServletConfig conf = getServletConfig();
       String author = conf.getInitParameter("Author");
       String book = conf.getInitParameter("Book");
       String url = conf.getInitParameter("URL");
       String str = "<html>" +
                    "<head>" +
                    "<title> Hello World </title>" +
                    "</head>" +
                    "<body>" +
                    "<h1> <br />" +
                    "Author :" + author +"<br />" +
                    "Book : "+ book +"<br />" +
                    "URL : " + url +
                    "</h1> </body>" +
                    "</html>";
       out.println(str);
   } 
   
}

After a servlet properly initialized, the servlet container use it to handle client requests. In the case of an HTTP request, the objects provided by the container are of types HttpServletRequest and HttpServletResponse.

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Programming for beginners

Tuesday, 15 July 2014

void init (ServletConfig config) throws ServletException

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