NullPointerExceotion on Timer.cancel

Question

Hi everyone!
I'm a bit messed up with a rebellious timer in my program.
It is supposed to measure a timeout between a messege sent by a server and a response from a client. Everything runs in UDP.
So, the server sends a keepalive message periodically by executing this method (called from another timerTask):
public void sendKeepalive() {
          Message message = new Message("KPA","",this, this);         // Message is a class used for wrapping messages in the server
          tx(message);                                                                       // Sends a message over a UDP socket
          waitForACK = new Timer();                                           
          waitForACK.schedule(new WaitForACKTask(this), keepaliveTimeout);
}AFAIK, this code works fine, because the timer task works fine, and it does diisconnect the client once the time is up. This is the code to WaitForACKTask:
public class waitForACKTask extends TimerTask {
     private Client client;                            // Client is a class that deals with one client (it is part of the server; ie: the server has one Client class per connected client)
     public WaitForACKTask(Client client){
          this.client = client;
     public void run(){
          client.disconnect();    //deletes the user from the server list and notifies it that it has been disconnected
}Again, this seems to work fine.
Now, the problem comes when the client answers the petition. It sends an "ACK" message, wich is processed as follows:
     public void process(Message message) {
          if (message.returnType().equals("ACK")){              
               System.out.println("ACK received");                  // I have tested that it does recognize the type of the message correctly
               waitForACK.cancel();                                       // This is the line where NullPointerException is launched
          } else if (message.returnType().equals("DIS")){
               disconnect();
          else {
               control.queue(message);                 
     }So, here is where the code fails; on waitForACK.cancel():
java.lang.NullPointerException
     at chatServer.Client.process(Client.java:72)
     at chatServer.ListenClient.run(ListenClient.java:29)After that, te timer simply keeps on counting (as if no ACK has been received) and disconnects the client after timeout.
I've looked up in the java documentation and Timer.cancel() is not supposed to launch any exceptions.
Thank's in advance!

Answer 1

Just figured it out by myself! I had tested everything with a fake client (that is, me writing the protocol messages on a dumb client made just by a window and a socket), and I saw it didn't fail. So it turns out that the ACK was received before the timer had had time to be created. It didn't fail when I was the client, because I was much slower. So everything is fixed by changing the order in the code of sendKeepAlive:
public void sendKeepalive() {
                waitForACK = new Timer();                                           
          waitForACK.schedule(new WaitForACKTask(this), keepaliveTimeout);
          Message message = new Message("KPA","",this, this);       
          tx(message);                                                                   
}Edited by: fenom_ on May 16, 2009 8:50 AM