As a continuation of my previous post about Log4j MDC (Mapped Diagnostic Context), here's a fully working code sample explaining the concept. This is a simple example where we have a one servlet and one filter. The filter intercepts each and every request and put the user name in the MDC.

The Servlet Class:

package com.veerasundar.code.log4jmdc;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.log4j.Logger;

/**
 * A good-for-nothing servlet which is just writing few messages to the logger
 * object. Since we've configured in the log4j.properties file to include the
 * userName, taken from MDC, every message will be appended with the user name
 * that is set from the AuthenticationFilter
 *
 */
public class Log4jMdcDemo extends HttpServlet {

    private static Logger logger = Logger.getLogger(Log4jMdcDemo.class);

    public Log4jMdcDemo() {
    }

    protected void doGet(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
        doService(request, response);
    }

    protected void doPost(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
        doService(request, response);
    }

    protected void doService(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {

        logger.info("This is  demo for the Log4j MDC concept");
        logger.info("From Veerasundar.com/blog");
        logger.debug("Just some sample messages");
    }
}

A Filter to put the user name in MDC for every request call

package com.veerasundar.code.log4jmdc;

import java.io.IOException;

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 org.apache.log4j.MDC;

/**
 * An example authentication filter which is used to intercept all the requests
 * for fetching the user name from it and put the user name to the Log4j Mapped
 * Diagnostic Context (MDC), so that the user name could be used for
 * differentiating log messages.
 *
 * @author veerasundar.com/blog
 *
 */
public class AuthenticationFilter implements Filter {

    @Override
    public void doFilter(ServletRequest request, ServletResponse response,
            FilterChain chain) throws IOException, ServletException {

        try {
            /*
            * This code puts the value "userName" to the Mapped Diagnostic
            * context. Since MDc is a static class, we can directly access it
            * with out creating a new object from it. Here, instead of hard
            * coding the user name, the value can be retrieved from a HTTP
            * Request object.
            */
            MDC.put("userName", "veera");

            chain.doFilter(request, response);

        } finally {
            MDC.remove("userName");
        }

    }

}

Web.xml file to glue together Servlet and Filter

<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" version="2.4"   xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"   xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
    <servlet>
        <description/>
        <display-name>Log4jMdcDemo</display-name>
        <servlet-name>Log4jMdcDemo</servlet-name>
        <servlet-class>com.veerasundar.code.log4jmdc.Log4jMdcDemo</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>Log4jMdcDemo</servlet-name>
        <url-pattern>/Log4jMdcDemo</url-pattern>
    </servlet-mapping>
    <filter>
        <filter-name>AuthFilter</filter-name>
        <filter-class>com.veerasundar.code.log4jmdc.AuthenticationFilter</filter-class>
    </filter>
    <filter-mapping>
        <filter-name>AuthFilter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>
</web-app>

Log4j.properties file which uses the data present in MDC

# sample log4j.properties file, explaining log4j MDC concept
# author: veerasundar.com/blog

log4j.appender.consoleAppender = org.apache.log4j.ConsoleAppender
log4j.appender.consoleAppender.layout = org.apache.log4j.PatternLayout

#note the %X{userName} - this is how you fetch data from Mapped Diagnostic Context (MDC)
log4j.appender.consoleAppender.layout.ConversionPattern = %-4r [%t] %5p %c %x - %m - %X{userName}%n

log4j.rootLogger = DEBUG, consoleAppender

I hope that code explains the concept of MDC. Do let me know if you have any questions/suggestions in the comments.

Output from the log file:

0    [http-8084-2]  INFO Log4jMDCDemo  - This is  demo for the Log4j MDC concept - veera
0    [http-8084-2]  INFO Log4jMDCDemo  - From Veerasundar.com/blog - veera
0    [http-8084-2] DEBUG Log4jMDCDemo  - Just some sample messages - veera