Servlets :: Update HashMap And ServletContextListener With Timer

I have two java classes, the first is a Servlet that register users access to the same servlet, the second class implements a ServletContextListener and update the HashMap removing the expired access.I need this Application to obtain a list of online users on an android app, this is the code:

import java.util.HashMap;
import java.util.Timer;
import java.util.TimerTask;
import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

[code]....

at the moment the application doesn't work fine, because the update doens't happen and after few minutes a ConcurrentModificationException occurs...