ServletRequestAttributeListener

[Mahtab Alam]

This is my Servlet That I am hitting from browser

package com.emind;
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 TargetServlet extends HttpServlet
{
          private static final long serialVersionUID = 1L;
          @Override
          protected void doGet(HttpServletRequest req, HttpServletResponse resp)
                             throws ServletException, IOException
          {
                   PrintWriter  out = resp.getWriter();
                   
                   req.setAttribute("name", "Mahtab");
                 
                   req.setAttribute("name", "Mahtab Alam");
                  
                   req.removeAttribute("name");
            }
}

And This is the ServletRequestAttributeListener I have implemented

package com.emind;
import javax.servlet.ServletRequestAttributeEvent;
import javax.servlet.ServletRequestAttributeListener;

public class ServletRequestAttributeListenerDemo implements ServletRequestAttributeListener
{
          public void attributeAdded(ServletRequestAttributeEvent e)
          {
          System.out.println(e.getName()+" : Attribute added . .Value "+e.getValue());
          }

          public void attributeRemoved(ServletRequestAttributeEvent e)
          {
           System.out.println(e.getName()+" : Attribute removed . " );
          }

          public void attributeReplaced(ServletRequestAttributeEvent e)
          {
           System.out.println(e.getName()+" : Attribute replaced . .Value "+e.getValue());
          }
}

My output from apache tomcat log is this

org.apache.catalina.ASYNC_SUPPORTED : Attribute replaced . .Value true
name : Attribute added . .Value Mahtab
name : Attribute replaced . .Value Mahtab
name : Attribute removed .

I am not understanding this output.
I first called the setAttribute() method in the servlet , from where that first line in output comes.
Then I again call setAttribute on the same attribute to change its value but its showing Mahtab again it should have displayed Mahtab Alam.

setAttribute() calls attributeAdded() , does calling setAttribute() on the same attribute again calls attributeReplaced().

[Ulf Dittmer]

I first called the setAttribute() method in the servlet , from where that first line in output comes.

From the name of it, org.apache.catalina.ASYNC_SUPPORTED looks like something Tomcat uses internally.

Then I again call setAttribute on the same attribute to change its value but its showing Mahtab again it should have displayed Mahtab Alam.

No, that's correct. See the javadocs of ServletContextAttributeEvent.getValue.

setAttribute() calls attributeAdded() , does calling setAttribute() on the same attribute again calls attributeReplaced().

Yes, that's the difference between "adding" and "replacing".