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Servlet filter in java

Servlet filter:

Servlet filters are the objects which are used to perform some filtering task. A filter can be applied to a servlet, jsp or html.

Servlet filters are mainly used for following tasks:

  1. 1. Pre-processing: Servlet filter is used for pre-processing of request before it accesses any resource at server side.
  2. 2. Post-processing: Servlet filter is used for post-processing of response before it sent back to client.

How to create a filter?

Implement javax.servlet.Filter interface to create a filter.

Filter interface:

To create a filter you have to implement filter interface. Filter interface is in javax.servlet package javax.servlet.Filter. It provides life cycle methods of a filter.

Methods of filter interface:

1. init(FilterConfig config): This method is used to initialize the filter. It is called only once by web container.

Syntax: public void init(FilterConfig config)

2. doFilter(HttpServletRequest request,HttpServletResponse response, FilterChain chain): This method is used for performing pre-processing and post-processing tasks. It is called every time for a request/response comes for a resource to which filter is mapped.

Syntax: public void doFilter(HttpServletRequest request,HttpServletResponse response, FilterChain chain)

3. destroy(): This method is called only once by the web container when filter is taken out of the service.

Syntax: public void destroy()

FilterChain interface:

FilterChain object is used to call the next filter or a resource if it is the last filter in filter chaining.

Method of FilterChain interface:

1. doFilter(HttpServletRequest request, HttpServletResponse response): This method is used to call the next filter in filter chaining.

Syntax: public void doFilter(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException

How to define a filter in web.xml?

<filter> attribute is used to define a filter in web.xml.



//Other attributes.


<filter-name>filterName </filter-name>







//Other attributes.




import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
* This class is used as a filter.
* @author javawithease
public class LoginFilter implements Filter {
	private static final long serialVersionUID = 1L;
	public void init(FilterConfig filterConfig) 
		throws ServletException {
      public void doFilter(ServletRequest request, 
			ServletResponse response, FilterChain chain) 
	                throws IOException, ServletException {  
    	PrintWriter out = response.getWriter();
    	//get parameters from request object.
    	String userName = request.getParameter("userName").trim();
    	String password = request.getParameter("password").trim();
    	//check for null and empty values.
    	if(userName == null || userName.equals("") || 
    			password == null || password.equals("")){
    		out.print("Please enter both username " +
    				"and password. <br/><br/>");
    		RequestDispatcher requestDispatcher = 
    		requestDispatcher.include(request, response);
    	}//Check for valid username and password.
    	else if(userName.equals("jai") && password.equals("1234")){
    		 chain.doFilter(request, response);
    		out.print("Wrong username or password. <br/><br/>");
    		RequestDispatcher requestDispatcher = 
    		requestDispatcher.include(request, response);
    public void destroy() {


import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 * This class is used to print login message 
 * if user logged in successfully.
public class WelcomeServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
   //no-argument constructor
    public WelcomeServlet() {
    protected void doPost(HttpServletRequest request, 
		HttpServletResponse response)
	            throws ServletException, IOException {
        PrintWriter out = response.getWriter();  
        out.print("<h1>You are logged in successfully.</h1>");  


<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 
Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
	<form action="WelcomeServlet" method="POST">
		Username:<input type="text" name="userName"/>
		Password:<input type="password" name="password"/>
		<input type="submit" value="login"/> 


<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
id="WebApp_ID" version="2.5">


servlet example 14 login
Enter username: jai and password: 1234
servlet example 14 value
Click on login button.
servlet example 14 welcome
Download this example.
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