Why notify/notifyAll method doesn't resume the waiting thread quickly?????

Question

I am currently working on a socket based IM server, I get messages from multiple clients simultaneously and put them into queues, for each queue in my server I have a separate listener running in a thread, as soon as a packet comes in the queue (from any client) it��s related thread is notified to get that packet from thread and process it.
Here I am using wait/notify blocking mechanism to control loop in my listener thread. My logic works like this
1)     When Listener thread starts it calls the synchronized getNextPacket method on that queue if the queue has any packet (i.e. it��s length is greater then zero) then it will remove that packet from the underlying vector (NOTE: I am using Vector to contain packets) and return that packet to listener thread, else if queue doesn��t have any packet (i.e. its size is zero) then I will call wait() method which causes the listener thread to wait.
2)     Now when any client adds some packet in that queue using synchronized addPacket( ) method I first add that packet in the underlying vector and then call notify()/notifyAll() to awake that listener thread, which again calls the getNextPacket() method and this cycle continuous like this.
So multiple threads (clients) are adding data in the queue using synchronized method, only one listener thread is getting data from that queue using a synchronized method again �� .
This approach works fine but sometimes I have noticed that the listener thread doesn��t resume just after notiy() / notifyAll() has been called , sometimes it resumes after a long time sometimes it even don��t resume (after waiting a long time I assumed this).
Solutions I tried
1)     I did set the listener Thread��s priority to Maximum but facing same problem again.
For better understanding I am also sending you the code for my queue class and it��s listener thread.
CODE OF QUEUE CLASS
import java.util.Vector;
import org.apache.log4j.Logger;
import com.tcm.unicorn.server.UnicornCustomeObject;
* @author sajjad.paracha
public class UIMPCommandsQueue {
     private static final Logger logger=Logger.getLogger(UIMPCommandsQueue.class);
      * Contains all the packets from clients
     private Vector <UnicornCustomeObject> unicornCustomeObjectQueue = new Vector<UnicornCustomeObject>();
     private UIMPCommandsQueue(){
     private static UIMPCommandsQueue uIMPCommandsQueue = null;
     public static UIMPCommandsQueue getInstance(){
          synchronized(UIMPCommandsQueue.class){
               if(uIMPCommandsQueue!=null){
                    return uIMPCommandsQueue;
               }else
                    return uIMPCommandsQueue = new UIMPCommandsQueue();
      * Adds a new command
      * @param unicornCustomeObject
     public synchronized void addCommandPakcet(UnicornCustomeObject unicornCustomeObject){
          logger.debug("[[[[[[[[[[[[[[[[[[[[[[[[[[   Going to add a packet in queue  no "+unicornCustomeObject.getClientSession().getRequestQueueNo());
          unicornCustomeObjectQueue.add(unicornCustomeObject);
          //** Notify the Listener (RequestProcessor) Thread that a new packet has been arrived in the queue
          //** So it now can again start it's processing
          notifyAll();
      * Removes an object from queue whose processing has been started or completed
      * @param unicornCustomeObject
      * @return
     private boolean removeCommandPacket(UnicornCustomeObject unicornCustomeObject){
          return unicornCustomeObjectQueue.remove(unicornCustomeObject);
      * <p> If no packet is available in queue it retuns null value
      *     otherwise returns an object from that queue
      * <p>
      * @return unicornCustomeObject
     public synchronized UnicornCustomeObject getNextCommandPacket(){
          if(unicornCustomeObjectQueue.size()>0){
               UnicornCustomeObject unicornCustomeObject = unicornCustomeObjectQueue.get(0);
               logger.debug("[[[[[[[[[[[[[[[[[[[[[[[[[[   Got a packet from queue no  "+unicornCustomeObject.getClientSession().getRequestQueueNo());
               logger.debug("[[[[[[[[[[[[[[[[[[[[[[[[[[   Going to remove a packet from queue no  "+unicornCustomeObject.getClientSession().getRequestQueueNo());
               removeCommandPacket(unicornCustomeObject);
               return unicornCustomeObject;
          }else{
               try {
                    //** Force the Listener (RequestProcessor) Thread to wait for notification
                    //** This Thread will be only notified if a new command packet has been arrived(added) in the
                    //** Queue i.e in addCommandPacket Method
                    wait();
               } catch (InterruptedException e) {
                    logger.error("",e);
               return null;
CODE OF LISTENER CLASS
import org.apache.log4j.Logger;
import com.tcm.unicorn.server.UnicornCustomeObject;
public class RequestProcessor implements Runnable {
      * will listen on Request queue for any new massages
     public void run() {
               //** get an instance of RequestQueue before the loop  
               UIMPCommandsQueue requestQueue= UIMPCommandsQueue.getInstance();
               while(true){
                    try{
                         //**call the blocking method getNextCommandPacket()     
                         UnicornCustomeObject unicornCustomeObject= requestQueue.getNextCommandPacket();
                         if(unicornCustomeObject!=null){
                              System.out.println("Got a pcket will process it now.......");                    
                    }catch(Exception exp){
                         exp.printStackTrace();
Can anybody please tell me where I am doing something wrong and whats the best way to get rid of this situation .
Thanks in advance
Message was edited by:
meetsaju

Answer 1

Another question !
in my previous programe i have seen a starange behavior , my processor thread sometimes processes the later message before the message came before that in queue here is an output of my debug statements
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 10
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 11
INFO :03 May 2007 11:21:17,>>>>>>>>>>>>>>>>>>>>>>>>requestProcessorThread is in block state
INFO :03 May 2007 11:21:17,>>>>>>>>>>>>>>>>>>>>>>>>requestProcessorThread is in block state
INFO :03 May 2007 11:21:17,>>>>>>>>>>>>>>>>>>>>>>>>requestProcessorThread is in block state
INFO :03 May 2007 11:21:17,>>>>>>>>>>>>>>>>>>>>>>>>requestProcessorThread is in block state
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 30
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 13
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 0
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 12
INFO :03 May 2007 11:21:17,>>>>>>>>>>>>>>>>>>>>>>>>requestProcessorThread is in block state
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 20
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 40
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 31
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 14
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 15
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 32
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 33
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 16
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 34
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 17
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 35
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 18
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 36
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 19
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 37
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 41
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 38
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 39
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 42
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 43
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 21
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 44
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 22
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 45
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 23
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 46
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 24
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 47
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 25
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 26
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 49
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 48
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 27
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 28
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 1
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 2 look at the lines
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 49
INFO :03 May 2007 11:21:17,[[[[[[[[[[[[[[[[[[[[[[[process the packet no 48
as for as i know it shouldnt be the case as we are using a FIFO queue here....just querious how it is possible that a later message is taken from a FIFO queue.