Have a look at the following link:

Log4j Tutorials

Log4j webapplication - servlet,log4j.xml

   // Program by Nishanth Thomas â?? Insolutions Global Bangalore

  // example  class file   

 package com.logs;


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

  import javax.servlet.http.HttpServletRequest;
  import javax.servlet.http.HttpSession;

  import org.apache.log4j.BasicConfigurator;
  import org.apache.log4j.FileAppender;

  import org.apache.log4j.Logger;
  import org.apache.log4j.PatternLayout;

  import org.apache.log4j.PropertyConfigurator;
  import org.apache.log4j.xml.DOMConfigurator;


public class Logscollected {


   static Logger log = Logger.getLogger(Logscollected.class);

   public static void logscollection(Exception e,       HttpServletRequest request) throws IOException{


      HttpSession session = request.getSession();

              String  exceptiondetail = â??â?? ;

//create a folder in your application config and place  log4j.xml file there  and       

  // get path by the following;

      String strPath =  request.getRealPath("config");

      DOMConfigurator.configure(strPath+"/"+"log4jexp.xml");

        //create log4j.xml file as give name like  shown above log4jexp;

  try{

      ////  /* Get the Stack Trace messages //////

    e = ((e==null)?((Exception)request.getAttribute("Exception")):e);

        StackTraceElement stem[]=e.getStackTrace();

        for(int i=0;i<stem.length;i++){


      if (!exceptiondetail.equals(""))

          exceptiondetail += "\n";

          exceptiondetail += "\t";

               exceptiondetail=exceptiondetail+stem[i].toString();
        }

          log.debug(exceptiondetail);
                  //log.info(e);
                    // log.warn("Here is some WARN");
                    //  log.info(object);
          log.error(e);

          //  log.fatal("Here is some FATAL");

    }catch(Exception e1){
        e1.printStackTrace();
    }
  }

}

In the catch of any servlet call that function like this

       try{

    //     Some code����

      }

    catch(Exception e)
        {

                    // Logs collection log 4j------------------------

            Logscollected.logscollection(e,request);

        //---------------------------

}

Finally log4j config file

Lo4j.xml here we renamed to log4jexp.xml to avoid conflicts with server log4j.xml file palce log4jexp.xml in config folder ( here I used)

<--configure path according to your requrmts to save log file -->

<!--code  between <log4j:configuration>  </log4j:configuration>  -->  

<appender name="daoAppender" 
   class="org.apache.log4j.RollingFileAppender">   
    <param name="maxFileSize" value="100KB" />   
    <param name="maxBackupIndex" value="2" />   
    <param name="File" value="${jboss.server.home.dir}/deploy/myexampleprj.war/serverlogs/serverlogs.txt" />   
    <layout class="org.apache.log4j.PatternLayout">   
        <param name="ConversionPattern"  
               value="%d{dd-MMM-yyyy HH:mm};%5p %c %m%n" />   
    </layout>   
</appender>   

  <--configure path according for diff packge to your requrmts to save log file -->   

<!--appender name="appAppender"  
          class="org.apache.log4j.RollingFileAppender">   
    <param name="maxFileSize" value="100KB" />   
    <param name="maxBackupIndex" value="2" />   
    <param name="File" value="${jboss.server.home.dir}/deploy/myexampleprj.war/serverlogs/serverlogs.txt" />   
    <layout class="org.apache.log4j.PatternLayout">   
        <param name="ConversionPattern"  
               value="%d{ABSOLUTE} %5p %c{1}: %m%n " />   
    </layout>    
</appender-->   

<!â??package name given in category here com is the package give package name  -->    

<!--category name="com.ex.actions">     
    <priority value="DEBUG"/>     
    <appender-ref ref="appAppender"/>       
</category-->

<category name="com">     
    <priority value="DEBUG"/>     
    <appender-ref ref="daoAppender"/>       
</category>    

<root>   
    <!--     
    <appender-ref ref="appAppender" />   
    <appender-ref ref="daoAppender" />   
    -->    
</root>