JAVA EE: doFilter

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Thursday, 1 May 2014

Servlets : Single Filter Demo

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What is a Servlet Filter?

In Java web applications, a Filter is a pluggable Java component that intercepts incoming client requests before they hit a servlet, and inspects outgoing responses before they are returned to the browser.

Filters are ideal for cross-cutting tasks such as logging request metadata, verifying user credentials, modifying headers, or compressing data—without modifying the business logic inside the target servlet itself.

How a Single Filter Works in Practice

To implement a servlet filter, your class must implement the javax.servlet.Filter interface and override its core methods:

init(FilterConfig config): Called once by the web container when initializing the filter.
doFilter(ServletRequest request, ServletResponse response, FilterChain chain): Executes every time a matching request arrives.
destroy(): Called once when the filter is taken out of service.

// Step 1: Pre-processing before servlet invocation

    System.out.println("Filter: Pre-processing client request...");


    // Step 2: Pass control to the destination servlet or next filter

    chain.doFilter(request, response);


    // Step 3: Post-processing after servlet generates response

    System.out.println("Filter: Post-processing server response...");

Key Takeaways from the Demo

web.xml Configuration: The filter is mapped to specific servlet names or URL patterns using <filter> and <filter-mapping> tags.
chain.doFilter() is Crucial: Failing to invoke chain.doFilter() blocks the request and prevents the servlet from executing.
Two-Way Interception: Code written before chain.doFilter() executes during the request phase, while code written after runs during the response phase.

Recommended Related Tutorials

Explore more in-depth filter concepts with these tutorials from Ram N Java:

Click here to watch in Youtube : https://www.youtube.com/watch?v=xHl_0yuDN6c

Click the below Image to Enlarge

SingleFilterDemo Project Dir Structure

LogFilter.java

import java.io.IOException;
import java.io.PrintWriter;
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 init(FilterConfig config) throws ServletException
    {
        System.out
                .println("------------------------------------------------------");
        System.out.println(" init method is called in "
                + this.getClass().getName());
        System.out
                .println("------------------------------------------------------");
    }

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

        System.out.println(" doFilter method is called in "
                + this.getClass().getName());

        PrintWriter out = res.getWriter();
        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());

        out.print("LogFilter is invoked before<br>");

        chain.doFilter(req, res);

        out.print("LogFilter is invoked after <br>");

    }

    public void destroy()
    {
        // add code to release any resource
        System.out
                .println("------------------------------------------------------");
        System.out.println(" destroy method is called in "
                + this.getClass().getName());
        System.out
                .println("------------------------------------------------------");
    }
}

HelloWorldServlet.java

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;

public class HelloWorldServlet extends HttpServlet
{

    private static final long serialVersionUID = 1L;

    public void init() throws ServletException
    {
        System.out
                .println("------------------------------------------------------");
        System.out.println(" Init method is called in "
                + this.getClass().getName());
        System.out
                .println("------------------------------------------------------");
    }

    // Method to handle GET method request.
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException
    {

        System.out
                .println("------------------------------------------------------");
        System.out.println(" doGet is called in "
                + this.getClass().getName());
        System.out
                .println("------------------------------------------------------");

        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.print("Hello World <br>");

    }

    public void destroy()
    {
        System.out
                .println("------------------------------------------------------");
        System.out.println(" destroy method is called in "
                + this.getClass().getName());
        System.out
                .println("------------------------------------------------------");
    }
}

web.xml

<?xml version="1.0" encoding="ISO-8859-1"?>
<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"
    metadata-complete="true" version="3.0">
    <display-name>SingleFilterDemo</display-name>
    <description>
        This is a simple web application with a source code organization
        based on the recommendations of the Application Developer's Guide.
    </description>
    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>HelloWorldServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
    <filter>
        <filter-name>LogFilter</filter-name>
        <filter-class>LogFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>LogFilter</filter-name>
        <url-pattern>/hello</url-pattern>
    </filter-mapping>
</web-app>

index.html

<!DOCTYPE HTML>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Single Filter Demo</title>
</head>
<body>
    <li><a href="hello">Single Filter Demo</a></li>
</body>
</html>

Environment Used

JDK version :1.6.0_30
Tomcat version : 7.0.50

To Download SingleFilterDemoApp Project Click the below link

https://sites.google.com/site/javaee4321/servlets/SingleFilterDemoApp.zip?attredirects=0&d=1

See also:

Servlets Tutorial

All Design Patterns Links