Programming for beginners: @WebListener annotation

This annotation is used to declare a WebListener. Any class annotated with WebListener must implement one or more of the ServletContextListener, ServletContextAttributeListener, ServletRequestListener, ServletRequestAttributeListener, HttpSessionListener, or HttpSessionAttributeListener, or HttpSessionIdListener interfaces.

main.jsp

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

Admin.java

public class Admin {
    String name;
    String description;
    
    Admin(String name, String description){
        this.name = name;
        this.description = description;
    }
    
    String getName(){
        return name;
    }
    
    String getDescription(){
        return description;
    }
}

InitializeListener.java

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.ServletContext;
import javax.servlet.annotation.WebListener;

@WebListener(value="Initailize Adimins")
public class InitializeListener implements ServletContextListener {

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        ServletContext context = sce.getServletContext();
        String adm1 = context.getInitParameter("admin1");
        String adm2 = context.getInitParameter("admin2");
        String desc1 = context.getInitParameter("admin1Des");
        String desc2 = context.getInitParameter("admin2Des");
        
        Admin admin1 = new Admin(adm1, desc1);
        Admin admin2 = new Admin(adm2, desc2);
        
        context.setAttribute("admin1", admin1);
        context.setAttribute("admin2", admin2);
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
      System.out.println("Context Destroyed");
    }
}

ContextListenerEx.java

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

public class ContextListenerEx extends HttpServlet {
    @Override
    public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException{
        
        try(PrintWriter out = res.getWriter()){
            String header = "<html>" +
                    "<head>" +
                    "<title> Hello World </title>" +
                    "</head>" +
                    "<body>" +
                    "<h1> <br />";                     
            String footer = "</h1> </body>" +
                    "</html>";
            ServletContext context = req.getServletContext();
            Admin adm1 = (Admin)context.getAttribute("admin1");
            Admin adm2 = (Admin)context.getAttribute("admin2");
            
            StringBuilder str = new StringBuilder();
            str.append("<br />").append(adm1.getName()).append(":").append(adm1.getDescription());
            str.append("<br />").append(adm2.getName()).append(":").append(adm2.getDescription());
            
            out.println(header);
            out.println(str);
            out.println(footer);
        }
    }
}

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">
    <context-param>
        <param-name>admin1</param-name>
        <param-value>Krishna</param-value>
    </context-param>
    <context-param>
        <param-name>admin2</param-name>
        <param-value>Hari</param-value>
    </context-param>
    <context-param>
        <param-name>admin1Des</param-name>
        <param-value>Primary Admin</param-value>
    </context-param>
    <context-param>
        <param-name>admin2Des</param-name>
        <param-value>Secondary Admin</param-value>
    </context-param>
    <servlet>
        <servlet-name>ContextListenerEx</servlet-name>
        <servlet-class>ContextListenerEx</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>ContextListenerEx</servlet-name>
        <url-pattern>/ContextListenerEx</url-pattern>
    </servlet-mapping>
</web-app>

Output

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Programming for beginners

Friday, 15 August 2014

@WebListener annotation

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