Front Controller Pattern

A Front Controller Pattern says that if you want to provide the centralized request handling mechanism so that all the requests will be handled by a single handler". This handler can do the authentication or authorization or logging or tracking of request and then pass the requests to corresponding handlers.

Usage:

When you want to control the page flow and navigation.
When you want to access and manage the data model.
When you want to handle the business processing.

Benefits:

It reduces the duplication of code in JSP pages, especially in those cases where several resources require the same processing.
It maintains and controls a web application more effectively.
A web application of two-tier architecture, the recommended approach is front controller to deal with user requests.

UML for Front Controller Pattern:

Implementation of Front Controller Pattern:

Step 1

Create a Login.html web page.

<html>
<head>
<title>
Login page
</title>
</head>
<body style="color:green;">
<h1 style="font-family:Comic Sans Ms;text-align="center";font-size:20pt;
color:#00FF00;>
Login Page
</h1>
<form method="POST" action="FrontControllerServlet" onsubmit="return checkForm(this);">
<p>Username: <input type="text" name="username"></p>
<p>Password: <input type="password" name="pwd1"></p>
<p>Confirm Password: <input type="password" name="pwd2"></p>
<p><input type="submit" value="Login"></p>
</form>
<script type="text/javascript">

  function checkForm(form)
  {
    if(form.username.value == "") {
      alert("Error: Username cannot be blank!");
      form.username.focus();
      return false;
    }
    re = /^\w+$/;
    if(!re.test(form.username.value)) {
      alert("Error: Username must contain only letters, numbers and underscores!");
      form.username.focus();
      return false;
    }

    if(form.pwd1.value != "" && form.pwd1.value == form.pwd2.value) {
      if(form.pwd1.value.length < 6) {
        alert("Error: Password must contain at least six characters!");
        form.pwd1.focus();
        return false;
      }
      if(form.pwd1.value == form.username.value) {
        alert("Error: Password must be different from Username!");
        form.pwd1.focus();
        return false;
      }
	re = /[0-9]/;
      if(!re.test(form.pwd1.value)) {
        alert("Error: password must contain at least one number (0-9)!");
        form.pwd1.focus();
        return false;
      }
      re = /[a-z]/;
      if(!re.test(form.pwd1.value)) {
        alert("Error: password must contain at least one lowercase letter (a-z)!");
        form.pwd1.focus();
        return false;
      }
      re = /[A-Z]/;
      if(!re.test(form.pwd1.value)) {
        alert("Error: password must contain at least one uppercase letter (A-Z)!");
        form.pwd1.focus();
        return false;
      }
    } else {
      alert("Error: Please check that you've entered and confirmed your password!");
      form.pwd1.focus();
      return false;
    }
    return true;
  }

</script>
</body>
</html>

Step 2

Create a FrontControllerServlet.java class which is a servlet and it may be a JSP page also.

package controller;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class FrontControllerServlet
 */
@WebServlet("/FrontControllerServlet")
public class FrontControllerServlet extends HttpServlet {
	
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
                 
		response.setContentType("text/html");  
	    PrintWriter out = response.getWriter();  
		
		String username=request.getParameter("username");
		String password=request.getParameter("pwd2");
		
		if (password.equals("Ashwani1987")) {
			
			RequestDispatcher rd=request.getRequestDispatcher("/Success.jsp");
			rd.forward(request, response);
		} else {

			RequestDispatcher rd=request.getRequestDispatcher("/Error.jsp");
			rd.forward(request, response);
		}

	}

}

Step 3

Create a Success.jsp page.

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Welcome Page</title>
</head>
<body style="background-color: gray;">

   <%  String name=request.getParameter("username"); %>

    <b>Welcome,</b> <% out.print(name); %>

</body>
</html>

Step 4

Create a Error.jsp page.

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
</head>
<body style="background-color: olive;">

    <b>You are have entered wrong username or password!!</b><br>

    <a href="Login.html">Back To Home Page</a>
</body>
</html>

Step 5

Create a web.xml file.

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" version="3.0">
  <display-name>Front Controller Example</display-name>
  <welcome-file-list>
    <welcome-file>Login.html</welcome-file>
  </welcome-file-list>
</web-app>

Output: