ViralPatel.net

[ad name=”AD_INBETWEEN_POST”]Servlet Filters are the latest components that are added in Servlet 2.3 specifications. These filters are used basically for intercepting and modifying requests and response from server.  Consider a scenario where you want to check session from the every users request and if it is valid then only you want to let the user access the page. You can acheive this by checking sessions on all the servlet pages (or JSP pages) which users queries or you can do this by using Filter. Let us create a Servlet Filter that just prints the clients ip address and date time. This is just to log users who are accessing the application. We we use Eclipse for developing our application and Apache Tomcat for deploying and running our application.

Step 1: Create dynamic web project in Eclipse.

Starts eclipse and create a new dynamic web project with name ServletFilterProject. Select Target runtime environment. I have selected Apache Tomcat v6.0, you can select any Tomcat version that you have installed. Click on Finish.

Step 2: Create package & Servlet Filter class.

Create a package for Servlet Filters in your source folder of Project. I have created a package net.viralpatel.servlet.filters. Inside the package, create a Java class file called LogFilter.java.

Step 3: Writting Servlet Filter class code.

Modify the LogFilter class and add following code into it.

package net.viralpatel.servlet.filters;

import java.io.IOException;
import java.util.Date;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;

public class LogFilter implements Filter {

 public void doFilter(ServletRequest req, ServletResponse res,
   FilterChain chain) throws IOException, ServletException {

  HttpServletRequest request = (HttpServletRequest) req;
  
  //Get the IP address of client machine.
  String ipAddress = request.getRemoteAddr();
  
  //Log the IP address and current timestamp.
  System.out.println("IP "+ipAddress + ", Time " 
       + new Date().toString());
  
  chain.doFilter(req, res);
 }
 public void init(FilterConfig config) throws ServletException {
  
  //Get init parameter
  String testParam = config.getInitParameter("test-param");
  
  //Print the init parameter
  System.out.println("Test Param: " + testParam);
 }
 public void destroy() {
  //add code to release any resource
 }
}
Code language: Java (java)

In this filter example, we have implemented an interface javax.servlet.Filter and override its methods init, doFilter and destroy. The init() method is used to initialize any code that is used by Filter. Also note that, init() method will get an object of FilterConfig which contains different Filter level information as well as init parameters which is passed from Web.xml (Deployment descriptor). The doFilter() method will do the actual logging of information. You can modify this method and add your code which can modify request/session/response, add any attribute in request etc. The destroy() method is called by the container when it wants to garbage collect the filter. This is usually done when filter is not used for long time and server wants to allocate memory for other applications.

Step 4: Create Servlet Filter Mapping in Web.xml

Open web.xml file from WEB-INF directory of your Project and add following entry for filter tag.

<filter>
 <filter-name>LogFilter</filter-name>
 <filter-class>
  net.viralpatel.servlet.filters.LogFilter
 </filter-class>
 <init-param>
  <param-name>test-param</param-name>
  <param-value>This parameter is for testing.</param-value>
 </init-param>
</filter>
<filter-mapping>
 <filter-name>LogFilter</filter-name>
 <url-pattern>/*</url-pattern>
</filter-mapping>
Code language: HTML, XML (xml)

In this entry, we have added LogFilter class in Web xml and mapped it with URL /*. Hence any request from client will generated a call to this filter. Also we have passed a parameter test-param. This is just to show how to pass and retrieve a parameter in servlet filter.

Step 5: Execute the web application

We are done with the coding part of Servlet Filter example. Now execute the project by Run -> Run As -> Run on server (shortcut Alt+Shift+X, R). Check the console you will see the output that we print using System.out. Change the code in the way you want your filter to work. Let me know your comments and suggestions. :)