Java Servlets attributeRemoved（）不触发

Question

I'm new to servlets, I'm following this tutorial everything works fine, but when I remove an attribute from request it doesn't trigger the proper event here is my code.
I couldn't find similar case on SO.

Servlet

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
    {
            ServletContext ctx = request.getServletContext();
    ctx.setAttribute("User", "Pankaj");
            String user = (String) ctx.getAttribute("User");
            System.out.println("removing attr");
            ctx.removeAttribute("User");   
            HttpSession session = request.getSession();
            session.invalidate();
            PrintWriter out = response.getWriter();
            out.write("Hi "+user);
    }

Listener

@WebListener
public class AppContextAttributeListener implements
        ServletRequestAttributeListener
{

    public void attributeAdded(ServletRequestAttributeEvent arg0) {
        System.out.println("ServletContext attribute added::{"
                + arg0.getName() + ","
                + arg0.getValue() + "}");

    }
    public void attributeRemoved(ServletRequestAttributeEvent arg0) {
        System.out.println("here");
        System.out.println("ServletContext attribute removed::{"
                + arg0.getName() + ","
                + arg0.getValue() + "}");
        }
    public void attributeReplaced(ServletRequestAttributeEvent arg0) {
        System.out.println("ServletContext attribute replaced::{"
                + arg0.getName() + ","
                + arg0.getValue() + "}");
    }
}

expected output

ServletRequest initialized. Remote IP=0:0:0:0:0:0:0:1%0
ServletContext attribute added::{User,Pankaj}
removing attr
here
ServletContext attribute removed::{User,Pankaj}
Session Created:: ID=8805E7AE4CCCF98AFD60142A6B300CD6
Session Destroyed:: ID=8805E7AE4CCCF98AFD60142A6B300CD6
ServletRequest destroyed. Remote IP=0:0:0:0:0:0:0:1%0

my output

ServletRequest initialized. Remote IP=0:0:0:0:0:0:0:1%0
ServletContext attribute added::{User,Pankaj}
removing attr
Session Created:: ID=8805E7AE4CCCF98AFD60142A6B300CD6
Session Destroyed:: ID=8805E7AE4CCCF98AFD60142A6B300CD6
ServletRequest destroyed. Remote IP=0:0:0:0:0:0:0:1%0

I can't get to trigger attributeRemoved() though attributeAdded() is printing which means listener is well defined!
What's wrong here? I'm using tomcat 7 and servlets 3

Answer 1

Methods from ServletRequestAttributeListener are called when you add, remove or replace attribute in request eg:

request.removeAttribute("User");

They are not called when you remove attribue from servletContext:

ctx.removeAttribute("User");

So you should use ServletContextAttributeListener , because methods of this filter are called when attribute has been added/removed/replaced to the ServletContext.

Thera are three types of filters to listen attributes change:

1. ServletContextAttributeListener - receives notification that an attribute has been added/removed/replaced to the ServletContext.
2. ServletRequestAttributeListener - receives notification that an attribute has been added/removed/replaced to the ServletRequest.
3. HttpSessionAttributeListener - receives notification that an attribute has been added/removed/replaced from a session.

Thera are another three filter to listen lifecycle changes ServletContext, ServletRequest and HttpSession:

1. ServletContextListener - receiving notification events about ServletContext when it is initializated or destroyed.
2. ServletRequestListener - receiving notification events about requests coming into and going out
3. HttpSessionListener - receiving notification events about HttpSession when it is created or destroyed.