Servlets Filter

Introduction

  • The Filter is an object that is used to perform some filtering task.
  • It works on either the preprocessing request or post processing response.
  • It is used to perform filtering tasks such as conversion, compression, encryption and decryption etc.
  • The doFilter method is used to perform filtering and it is configured in deployment descriptor of a web application.
The main tasks performed by Filter are:
i. It blocks the request and response pair.               
ii. Interacts with external resources.
iii. It performs encryption and decryption, conversion, logout etc.
iv. It logs the IP address of computers from which the request originates.

Servlet Filter Methods

MethodDescription
public void init(FilterConfig, filterConfig)Invoked by the web container to specify a filter that it is placed into service.
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)Called by the web container each time. The FilterChain is used to invoke the next filter in the chain.
public void destroy( ) It is called by the web container to specify that the servlet class is taken out of the service.

Example : Illustrating the uses of Servlet Filter

//welcome.html

<form method="post" action="test">
    Username:<input type="text" name="user" /><br/>
    Password:<input type="text" name="pass" /><br/>
    <input type="submit" value="submit" />
</form>

//web.xml

<web-app>
    <filter>
        <filter-name>TestFilter</filter-name>
        <filter-class>TestFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>TestFilter</filter-name>
        <servlet-name>test</servlet-name>
    </filter-mapping>
    <servlet>
        <servlet-name>test</servlet-name>
        <servlet-class>test</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>test</servlet-name>
        <url-pattern>/test</url-pattern>
    </servlet-mapping>
    <welcome-file-list>
        <welcome-file>welcome.html</welcome-file>
    </welcome-file-list>
</web-app>

//TestFilter.java

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class TestFilter implements Filter
{
     public void init(FilterConfig fc) throws ServletException {}
     public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException
     {
         PrintWriter out = response.getWriter();
         String pass = request.getParameter("pass");
         if(pass.equals("123456"))
         {
             chain.doFilter(request, response);   
         }
         else
         {
              out.println("Password does not match");
              RequestDispatcher rs = request.getRequestDispatcher("index.html");
              rs.include(request, response);
         }
     }
     public void destroy()
     {
     
     }
}

//test.java

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class test extends HttpServlet
{
     protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException
     {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        String user = request.getParameter("user");
        out.println("Welcome "+user);
     }
}

Servlet Authentication

  • The Servlet Authentication Filter should check the pre and post-processing for authentication functions.
  • It is the extension of the filter object that allows filter to replace or extends container-based authentication.
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