Introduction to Servlet filters

This section contains the introduction to Servlet filters

Introduction to Servlet filters

A filter is an object that can transform a request or alter a response. Filters not create a response like servlet. It process request before it reaches to a servlet and can process response before it leaves to a servlet.

For implementing servlet we need to import  javax.servlet.Filter . This class defines three methods :

• void init(FilterConfig config) throws ServletException : 

         Sets the filter's configuration object.

• void destroy() :

          For destroying the filter's configuration instance.

• void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException:

          Perform the actual filter work.

Example :

This filter records the duration of all request.

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class TimerFilter implements Filter {

private FilterConfig config = null;

public void init(FilterConfig config) throws ServletException {
this.config = config;
}

public void destroy() {
config = null;
}

public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
long before = System.currentTimeMillis();
chain.doFilter(request, response);
long after = System.currentTimeMillis();

String name = "";
if (request instanceof HttpServletRequest) {
name = ((HttpServletRequest)request).getRequestURI();
}
config.getServletContext().log(name + ": " + (after - before) + "ms");
}
}

Output :

Under the "logs" folder of tomcat , if you check the log file, you will find the following log :

Dec 1, 2010 3:27:39 PM org.apache.catalina.core.ApplicationContext log
INFO: /ankit/servlet/RewriteServletURL: 0 millisecond

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