Standard Java class communicating with a servlet

Question

Hey,
I am trying to access a standard java classes member via a servlet class however the data is never received even though the member is visible on compilation. is there something Im doing wrong ?
public class Start {
//lots of stuff
     static List<Plug> plugList = Collections.synchronizedList(new LinkedList<Plug>());
//lots more stuff
public class wah extends HttpServlet{
        public void doGet( HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException{
                doPost(request, response);
    public void doPost(HttpServletRequest request,
                                HttpServletResponse res)throws ServletException, IOException{
                res.setContentType("text/html");
                PrintWriter out = res.getWriter();
                out.println("wahwahwah");
                synchronized(Start.plugList){
                        out.println("pluglist");
                        Iterator i1 = Start.plugList.iterator();
                        while (i1.hasNext()) {
                                Plug oldPlug = (Plug)i1.next();
                                out.println("PLug : " + oldPlug.getaddress64());
        }//doPost
}

Answer 1

Don't post all your code. Instead, make a simplified version of the code that demonstrates what your problem is and that we could compile and see what is going on. Make it as simple as possible but still demonstrating the problem.
Who knows, when you make the SSCCP (small, self contained, compileable program) you will find your mistake as well.
But without seeing any code all I can say is that the date you are accessing in the list has not been made available when you try to get it from the servlet. Perhaps the servlet call is blocking the other threads from accessing the list to put data into it, perhaps the other threads haven't begun working yet, perhaps the list you give to the servlet isn't the same list (but instead a copy of the list) that the data is being entered into, or many other reasons.
One thing I would assume you need to do, since the Servlet is the data consumer, you need to make sure the data has been entered before the servlet tries to read it. The best way to do that is to put a wait() in the servlet code so that it knows it can't do anything until the producers are finished. Then the producers would add a notify() to tell the servlet to go ahead and display the data.
So some modification might be:
public class Start {
//lots of stuff
     static List<Plug> plugList = Collections.synchronizedList(new LinkedList<Plug>());
        //marker to let servlet know that it can consume data
        private static boolean done = false;
        //producers call this method when list is ready for use
        static void setReadyToConsume() { done = true; }
        //servlet calls this to check if the list is ready
        public static boolean isReadyToConsume() { return done; }
//lots more stuff
    public void doPost(HttpServletRequest request,
                                HttpServletResponse res)throws ServletException, IOException{
                res.setContentType("text/html");
                PrintWriter out = res.getWriter();
                out.println("wahwahwah");
                synchronized(Start.plugList){
                       //Hold off trying to use list until list is ready
                        while (!Start.isReadyToConsume()) {
                              try {
                                      Start.plugList.wait();
                              } catch (InterruptedException ie) {
                                      log("Waiting for Plugs to be ready interrupted.  Continuing to wait.");
                                      log(ie.getMessage(), ie);
                        out.println("pluglist");
                        Iterator i1 = Start.plugList.iterator();
                        while (i1.hasNext()) {
                                Plug oldPlug = (Plug)i1.next();
                                out.println("PLug : " + oldPlug.getaddress64());
        }//doPostAnd don't forget to add a Start.plugList.notify() or Start.plugList().notifyAll() in the producer code after it calls the setReadyToConsume() method.