public class UserCounterListener implements ServletContextListener,
HttpSessionAttributeListener {
public static final String COUNT_KEY = "userCounter";
public static final String USERS_KEY = "userNames";
private Log log = LogFactory.getLog(UserCounterListener.class);
private ServletContext servletContext;
private int counter;
private Set users;

public void contextInitialized(ServletContextEvent sce) {
servletContext = sce.getServletContext();
servletContext.setAttribute((COUNT_KEY), Integer.toString(counter));
}

public void contextDestroyed(ServletContextEvent sce) {
users = null;
counter = 0;
}

synchronized void incrementUserCounter() {
counter =
Integer.parseInt((String) servletContext.getAttribute(COUNT_KEY));
counter++;
servletContext.setAttribute(COUNT_KEY, Integer.toString(counter));

if (log.isDebugEnabled()) {
log.debug("User Count: " + counter);
}
}

synchronized void decrementUserCounter() {
int counter =
Integer.parseInt((String) servletContext.getAttribute(COUNT_KEY));
counter--;

if (counter < 0) {
counter = 0;
}

servletContext.setAttribute(COUNT_KEY, Integer.toString(counter));

if (log.isDebugEnabled()) {
log.debug("User Count: " + counter);
}
}

synchronized void addUsername(Object user) {
users = (Set) servletContext.getAttribute(USERS_KEY);

if (users == null) {
users = new HashSet();
}

if (log.isDebugEnabled()) {
if (users.contains(user)) {
log.debug("User already logged in, adding anyway...");
}
}

users.add(user);
servletContext.setAttribute(USERS_KEY, users);
incrementUserCounter();
}

synchronized void removeUsername(Object user) {
users = (Set) servletContext.getAttribute(USERS_KEY);

if (users != null) {
users.remove(user);
}

servletContext.setAttribute(USERS_KEY, users);
decrementUserCounter();
}

/**
* This method is designed to catch when user's login and record their name
* @see javax.servlet.http.HttpSessionAttributeListener#attributeAdded(javax.servlet.http.HttpSessionBindingEvent)
*/
public void attributeAdded(HttpSessionBindingEvent event) {
if (event.getName().equals(Constants.USER_KEY)) {
addUsername(event.getValue());
}
}

/**
* When user's logout, remove their name from the hashMap
* @see javax.servlet.http.HttpSessionAttributeListener#attributeRemoved(javax.servlet.http.HttpSessionBindingEvent)
*/
public void attributeRemoved(HttpSessionBindingEvent event) {
if (event.getName().equals(Constants.USER_KEY)) {
removeUsername(event.getValue());
}
}

/**
* @see javax.servlet.http.HttpSessionAttributeListener#attributeReplaced(javax.servlet.http.HttpSessionBindingEvent)
*/
public void attributeReplaced(HttpSessionBindingEvent event) {
// I don't really care if the user changes their information
}
}