Servlet 3 Async task on Tomcat 7

Question

I'm trying to implement Simple chat using Servlet 3.0 and Comet pattern based on its async support.

I'm inspired by this article: http://www.javaworld.com/javaworld/jw-02-2009/jw-02-servlet3.html?page=3

My servlet looks like this.

@WebServlet(name="chatServlet", urlPatterns={"/ChatServlet"}, asyncSupported=true)
public class ChatServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
         AsyncContext aCtx = request.startAsync(request, response); 
         ServletContext appScope = request.getServletContext();    
         List<AsyncContext> watchers = (List<AsyncContext>) appScope.getAttribute("watchers");
         watchers.add(aCtx); //register the watcher
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
          AsyncContext aCtx = request.startAsync(request, response); 
          ServletContext appScope = request.getServletContext(); 
          Queue<String> messages = (Queue<String>)appScope.getAttribute("messages");
          messages.add(someMessage);
    }
} 

now my Listener looks like this:

@WebListener
public class ChatPushService implements ServletContextListener {

        @Override
        public void contextInitialized(ServletContextEvent sce) {
              final List<AsyncContext> watchers = new  ArrayList<AsyncContext>();
             sce.getServletContext().setAttribute("watchers", watchers);
              // store new messages not published yet
             Queue<String> messages = new ConcurrentLinkedQueue<String>();
             sce.getServletContext().setAttribute("messages", messages);
             Executor messageExecutor = Executors.newCachedThreadPool(); 
             final Executor watcherExecutor = Executors.newCachedThreadPool();
             while(true)
              {      

                 if(!messages.isEmpty()) 
                 {
                     System.out.println("notEmpty");
                    String message =  messages.poll(); 
                    messageExecutor.execute(new Runnable(){

                        @Override
                        public void run() {
                             for(final AsyncContext aCtx : watchers){
                                 watcherExecutor.execute(new Runnable(){

                                     @Override
                                        public void run() {
                                           try {
                                            aCtx.getResponse().getWriter().print("brrrrr");
                                        } catch (IOException e) {
                                            // TODO Auto-generated catch block
                                            e.printStackTrace();
                                        }
                                     }
                                 });
                             }
                        }
                 });
              }
        }

    }
    }

When I'm starting my it's freezing during container initiation.

Nov 1, 2011 1:12:09 AM org.apache.catalina.core.AprLifecycleListener init
INFO: The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: /usr/lib/jvm/java-6-openjdk/jre/lib/amd64/server:/usr/lib/jvm/java-6-openjdk/jre/lib/amd64:/usr/lib/jvm/java-6-openjdk/jre/../lib/amd64:/usr/java/packages/lib/amd64:/usr/lib/jni:/lib:/usr/lib
Nov 1, 2011 1:12:09 AM org.apache.tomcat.util.digester.SetPropertiesRule begin
WARNING: [SetPropertiesRule]{Server/Service/Engine/Host/Context} Setting property 'source' to 'org.eclipse.jst.jee.server:Servlet3Comet' did not find a matching property.
Nov 1, 2011 1:12:09 AM org.apache.coyote.AbstractProtocol init
INFO: Initializing ProtocolHandler ["http-bio-8080"]
Nov 1, 2011 1:12:09 AM org.apache.coyote.AbstractProtocol init
INFO: Initializing ProtocolHandler ["ajp-bio-8009"]
Nov 1, 2011 1:12:09 AM org.apache.catalina.startup.Catalina load
INFO: Initialization processed in 624 ms
Nov 1, 2011 1:12:09 AM org.apache.catalina.core.StandardService startInternal
INFO: Starting service Catalina
Nov 1, 2011 1:12:09 AM org.apache.catalina.core.StandardEngine startInternal
INFO: Starting Servlet Engine: Apache Tomcat/7.0.22

It looks like public void contextInitialized function is not running asynchronously on background and is blocking further container initialization.

Why?

can anybody help me on this issue?

Answer 1

You are running while loop inside contextInitialized() method which is wrong. contextInitialized() is invoked by Servlet Container as part of the application startup, having while loop there will block your app start.

Modified the code such the ContextListener will start one daemon thread which publishes the messages to the watchers

@WebListener
public class ChatPushService implements ServletContextListener {

    @Override
    public void contextInitialized(ServletContextEvent sce) {
          final List<AsyncContext> watchers = new  ArrayList<AsyncContext>();
         sce.getServletContext().setAttribute("watchers", watchers);
          // store new messages not published yet
         Queue<String> messages = new ConcurrentLinkedQueue<String>();
         sce.getServletContext().setAttribute("messages", messages);
         new chatManager(sce.getServletContext()).start(); //START DAEMON

      }
}
public class ChatManager implements Runnable
{
ServletContext servletCtx;
public ChatManager(ServletContext ctx)
{
     this.servletCtx = ctx;
}
public void run()
{
         List<AsyncContext> watchers = (List<AsyncContext>) servletCtx.getAttribute("watchers");
     Queue<String> messages = (Queue<String>)appScope.getAttribute("messages");
     Executor messageExecutor = Executors.newCachedThreadPool(); 
         final Executor watcherExecutor = Executors.newCachedThreadPool();
         while(true)
          {      

             if(!messages.isEmpty()) 
             {
                 System.out.println("notEmpty");
                String message =  messages.poll(); 
                messageExecutor.execute(new Runnable(){

                    @Override
                    public void run() {
                         for(final AsyncContext aCtx : watchers){
                             watcherExecutor.execute(new Runnable(){

                                 @Override
                                    public void run() {
                                       try {
                                        aCtx.getResponse().getWriter().print("brrrrr");
                                    } catch (IOException e) {
                                        // TODO Auto-generated catch block
                                        e.printStackTrace();
                                    }
                                 }
                             });
                         }
                    }
             });
          }
    }

}

}