Servlet 3.0 sample code found at http://www.javaworld.com/javaworld/jw-02-2009/jw-02-servlet3.htm

BidPushService.java

@WebServlet(name="myServlet", urlPatterns={"/auctionservice"}, asyncSupported=true)
public class MyServlet extends HttpServlet {
   
   // track bid prices
   public void doGet(HttpServletRequest request, HttpServletResponse response) {
      AsyncContext aCtx = request.startAsync(request, response); 
      // This could be a cluser-wide cache.
      ServletContext appScope = request.getServletContext();    
      Map<String, List<AsyncContext>> aucWatchers = (Map<String, List<AsyncContext>>)appScope.getAttribute("aucWatchers");
      List<AsyncContext> watchers = (List<AsyncContext>)aucWatchers.get(request.getParameter("auctionId"));
      watchers.add(aCtx); // register a watcher
   }

   // place a bid
   public void doPost(HttpServletRequest request, HttpServletResponse response) {
      // run in a transactional context 
      // save a new bid
      AsyncContext aCtx = request.startAsync(request, response); 
      ServletContext appScope = request.getServletContext(); 
      Queue<Bid> aucBids = (Queue<Bid>)appScope.getAttribute("aucBids");
      aucBids.add((Bid)request.getAttribute("bid"));  // a new bid event is placed queued.  
   }
}

@WebServletContextListener
public class BidPushService implements ServletContextListener{

   public void contextInitialized(ServletContextEvent sce) {   
      Map<String, List<AsyncContext>> aucWatchers = new HashMap<String, List<AsyncContext>>();
      sce.getServletContext().setAttribute("aucWatchers", aucWatchers);
      // store new bids not published yet
      Queue<Bid> aucBids = new ConcurrentLinkedQueue<Bid>();
      sce.getServletContext().setAttribute("aucBids", aucBids);
        
      Executor bidExecutor = Executors.newCachedThreadPool(); 
      final Executor watcherExecutor = Executors.newCachedThreadPool();
      while(true)
      {        
         if(!aucBids.isEmpty()) // There are unpublished new bid events.
         {
            final Bid bid = aucBids.poll();
            bidExecutor.execute(new Runnable(){
               public void run() {
                  List<AsyncContext> watchers = aucWatchers.get(bid.getAuctionId()); 
                  for(final AsyncContext aCtx : watchers)
                  {
                     watcherExecutor.execute(new Runnable(){
                        public void run() {
                           // publish a new bid event to a watcher
                           aCtx.getResponse().getWriter().print("A new bid on the item was placed. The current price ..., next bid price is ...");
                        };
                     });
                  }                           
               }
            });
         }
      }
   }
    
   public void contextDestroyed(ServletContextEvent sce) {
   }
}

SlowWebService.java

@WebServlet(name="myServlet", urlPatterns={"/slowprocess"}, asyncSupported=true)
public class MyServlet extends HttpServlet {
   
    public void doGet(HttpServletRequest request, HttpServletResponse response) {
        AsyncContext aCtx = request.startAsync(request, response); 
        ServletContext appScope = request.getServletContext();
        ((Queue<AsyncContext>)appScope.getAttribute("slowWebServiceJobQueue")).add(aCtx);
    }
}

@WebServletContextListener
public class SlowWebService implements ServletContextListener {

    public void contextInitialized(ServletContextEvent sce) {
        Queue<AsyncContext> jobQueue = new ConcurrentLinkedQueue<AsyncContext>();
        sce.getServletContext().setAttribute("slowWebServiceJobQueue", jobQueue);
        // pool size matching Web services capacity
        Executor executor = Executors.newFixedThreadPool(10);
        while(true)
        {
            if(!jobQueue.isEmpty())
            {
                final AsyncContext aCtx = jobQueue.poll();
                executor.execute(new Runnable(){
                    public void run() {
                        ServletRequest request = aCtx.getRequest();
                        // get parameteres
                        // invoke a Web service endpoint
                        // set results
                        aCtx.forward("/result.jsp");
                    }                    
                });             
            }
        }
    }
    
    public void contextDestroyed(ServletContextEvent sce) {
    }
}

Just curious, what package does implement the annotation
@WebServletContextListener? it is supposed to be @weblistener instead. LINK.