Intercepting Filter Pattern

An Intercepting Filter Pattern says that "if you want to intercept and manipulate a request and response before and after the request is processed".

Usage:

When you want centralization, common processing across requests, such as logging information about each request, compressing an outgoing response or checking the data encoding scheme of each request.
When you want pre and post processing the components which are loosely coupled with core request-handling services to facilitate which are not suitable for addition and removal.

Benefits:

It provides central control with loosely coupled handlers.
It improves reusability.

UML for Intercepting Filter Pattern:

Implementation of Intercepting Filter Pattern:

Step 1

Create a Login.html web page.

<!DOCTYPE html>
<html>
<head>
<meta charset="US-ASCII">
<title>Login Page</title>
</head>
<body>
 
<form action="LoginServlet" method="post">
 
Username: <input type="text" name="username">
<br><br>
Password: <input type="password" name="password">
<br><br>
<input type="submit" value="Login">

</form>
</body>

Step 2

Create a LoginServlet class.

package sessions;

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.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
**
 * Servlet implementation class LoginServlet
 */
@WebServlet("/LoginServlet")
public class LoginServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
	private final String user = "admin";
	private final String password = "admin@1234";

public LoginServlet() {
        super();
    }
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		
		// get request parameters for username and passwd
        String username = request.getParameter("username");
        String passwd = request.getParameter("password");
         
        if(user.equals(username) && password.equals(passwd)){
            HttpSession session = request.getSession();
            session.setAttribute("user", "ashwani");
            
            //setting session to expire in 1 hour
            session.setMaxInactiveInterval(60*60);
            
            Cookie userName = new Cookie("user", user);
            userName.setMaxAge(60*60);
            response.addCookie(userName);
            response.sendRedirect("LoginSuccess.jsp");
        }else{
            RequestDispatcher rd = getServletContext().getRequestDispatcher("/Login.html");
            PrintWriter out= response.getWriter();
            out.println("<font color=red>Either user name or password is wrong.</font>");
            rd.include(request, response);
        }
	}
}//End of the LoginServlet class.

Step 3

Create a LoginSuccess.jsp page.

<%@ page language="java" contentType="text/html; charset=US-ASCII"
    pageEncoding="US-ASCII"%>
<!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=US-ASCII">
<title>Login Success Page</title>
</head>
<body>
<%
//allow access only if session exists
String user = (String) session.getAttribute("user");
String userName = null;
String sessionID = null;
Cookie[] cookies = request.getCookies();
if(cookies !=null){
for(Cookie cookie : cookies){
    if(cookie.getName().equals("user")) userName = cookie.getValue();
    if(cookie.getName().equals("JSESSIONID")) sessionID = cookie.getValue();
}
}
%>
<h3>Hi <%=userName %>, Login successful.Your Session ID=<%=sessionID %></h3>
<br>
User=<%=user %>
<br>
<a href="CheckoutPage.jsp">Checkout Page</a><br>
<form action="LogoutServlet" method="post">
<input type="submit" value="Logout" >
</form>
</body>
</html>

Step 4

Create an AdminPage.jsp page.

<%@ page language="java" contentType="text/html; charset=US-ASCII"
    pageEncoding="US-ASCII"%>
<!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=US-ASCII">
<title>Login Success Page</title>
</head>
<body>
<%
String userName = null;
String sessionID = null;
Cookie[] cookies = request.getCookies();
if(cookies !=null){
for(Cookie cookie : cookies){
    if(cookie.getName().equals("user")) userName = cookie.getValue();
}
}
%>
<h3>Hi <%=userName %>, These services are only for you to take action.</h3>
<br>
<form action="LogoutServlet" method="post">
<input type="submit" value="Logout" >
</form>
</body>
</html>

Step 5

Create an LogoutServlet class.

package sessions;

import java.io.IOException;

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

/**
 * Servlet implementation class LogoutServlet
 */
@WebServlet("/LogoutServlet")
public class LogoutServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
    
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html");
        Cookie[] cookies = request.getCookies();
        if(cookies != null){
        for(Cookie cookie : cookies){
            if(cookie.getName().equals("JSESSIONID")){
                System.out.println("JSESSIONID="+cookie.getValue());
                break;
            }
        }
        }
        //invalidate the session if exists
        HttpSession session = request.getSession(false);
        System.out.println("User="+session.getAttribute("user"));
        if(session != null){
            session.invalidate();
        }
        response.sendRedirect("Login.html");
    }

}//End of the LogoutServlet class

Step 6

Create an AuthenticationFilter class.

package filters;

import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 * Servlet Filter implementation class AuthenticationFilter
 */
@WebFilter("/AuthenticationFilter")
public class AuthenticationFilter implements Filter {

	private ServletContext context;
    
    public void init(FilterConfig fConfig) throws ServletException {
        this.context = fConfig.getServletContext();
        this.context.log("AuthenticationFilter initialized");
    }
     
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
 
        HttpServletRequest req = (HttpServletRequest) request;
        HttpServletResponse res = (HttpServletResponse) response;
         
        String uri = req.getRequestURI();
        this.context.log("Requested Resource::"+uri);
         
        HttpSession session = req.getSession(false);
         
        if(session == null && !(uri.endsWith("html") || uri.endsWith("LoginServlet"))){
            this.context.log("Unauthorized access request");
            res.sendRedirect("Login.html");
        }else{
            // pass the request along the filter chain
            chain.doFilter(request, response);
        }
    }
    public void destroy() {
        //close any resources here
    }
}//End of the AuthenticationFilter class

Step 7

Create an RequestLoggingFilter class.

package filters;
import java.io.IOException;
import java.util.Enumeration;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServletRequest;
@WebFilter("/RequestLoggingFilter")
public class RequestLoggingFilter implements Filter {
private ServletContext context;
 public void init(FilterConfig fConfig) throws ServletException {
        this.context = fConfig.getServletContext();
        this.context.log("RequestLoggingFilter initialized");
    }
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest req = (HttpServletRequest) request;
        Enumeration<String> params = req.getParameterNames();
while(params.hasMoreElements()){
            String name = params.nextElement();
            String value = request.getParameter(name);
            this.context.log(req.getRemoteAddr() + "::Request Params::{"+name+"="+value+"}");
        }
         
        Cookie[] cookies = req.getCookies();
        if(cookies != null){
            for(Cookie cookie : cookies){
                this.context.log(req.getRemoteAddr() + "::Cookie::{"+cookie.getName()+","+cookie.getValue()+"}");
            }
        }
        // pass the request along the filter chain
        chain.doFilter(request, response);
    }
 
    public void destroy() {
        //we can close resources here
    }
}// End of the RequestLoggingFilter class

Step 8

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>ServletFilterExample</display-name>
  <welcome-file-list>
    <welcome-file>Login.html</welcome-file>
  </welcome-file-list>
  <filter>
    <filter-name>RequestLoggingFilter</filter-name>
    <filter-class>filters.RequestLoggingFilter</filter-class>
  </filter>
  <filter>
    <filter-name>AuthenticationFilter</filter-name>
    <filter-class>filters.AuthenticationFilter</filter-class>
  </filter>
  <filter-mapping>
    <filter-name>RequestLoggingFilter</filter-name>
    <url-pattern>/*</url-pattern>
    <dispatcher>REQUEST</dispatcher>
  </filter-mapping>
  <filter-mapping>
    <filter-name>AuthenticationFilter</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>
</web-app>

Output: