Question

I’m using Timer and TimerTask to long poll for new messages for a chat application.
I would like to examine two “slightly” different possibilities:

1 : Timer declared as local variable

public List<MessageBean> getLastMessages(...) {
    [...]
    Timer timer = new Timer(true); //**Timer declared as local variable**
    while (someCondiction) {
        MessagesTimerTask returnMessagesTask = new MessagesTimerTask([...]);
        timer.schedule(returnMessagesTask, 6000);
        synchronized (listMessageBean) {
            listMessageBean.wait();
            //notify is called in the MessagesTimerTask which extends TimerTask
        }
    }
}

*Problem: Every time i call the method, i can see that a new thread is created, [Timer-1], [Timer-2], etc.. And in Eclipse Debug window they all seem to be running even after the getLastMessages(..) finishes running and returns a value to the client.
This may cause a huge problem if the timers are actually using threads, and after few transactions the server will eventually consume all the machine resources.

2 : Timer declared as local field

private final Timer timer = new Timer(true); //**Timer declared as local field**

public List<MessageBean> getLastMessages(...) {
    [...]
    while (someCondiction) {
        MessagesTimerTask returnMessagesTask = new MessagesTimerTask([...]);
        timer.schedule(returnMessagesTask, 6000);
        synchronized (listMessageBean) {
            listMessageBean.wait();
            //notify is called in the MessagesTimerTask which extends TimerTask
        }
    }
}

*Problem: every time i call the method, the same thread is used [Thread-1], but i’m not sure if i make two consecutive calls, the latter will cancel/override the former (the class is @Autowired by spring) ?

Any suggestions ?
Thank you.

Answer 1

Here is the source code of the schedule method:

190       public void schedule(TimerTask task, long delay) {
191           if (delay < 0)
192               throw new IllegalArgumentException("Negative delay.");
193           sched(task, System.currentTimeMillis()+delay, 0);
194       }

and the sched method:

386       private void sched(TimerTask task, long time, long period) {
387           if (time < 0)
388               throw new IllegalArgumentException("Illegal execution time.");
389   
390           // Constrain value of period sufficiently to prevent numeric
391           // overflow while still being effectively infinitely large.
392           if (Math.abs(period) > (Long.MAX_VALUE >> 1))
393               period >>= 1;
394   
395           synchronized(queue) {
396               if (!thread.newTasksMayBeScheduled)
397                   throw new IllegalStateException("Timer already cancelled.");
398   
399               synchronized(task.lock) {
400                   if (task.state != TimerTask.VIRGIN)
401                       throw new IllegalStateException(
402                           "Task already scheduled or cancelled");
403                   task.nextExecutionTime = time;
404                   task.period = period;
405                   task.state = TimerTask.SCHEDULED;
406               }
407   
408               queue.add(task);
409               if (queue.getMin() == task)
410                   queue.notify();
411           }
412       }

From here you can clearly see that a queue is being used to store tasks internally, meaning that later tasks will not overwrite the earlier ones. If you also check the mainLoop method in the file, you can see that it takes tasks from the queue one-by-one ordered by their schedule time and executes them.

So it should not be a problem to schedule multiple tasks on the same Timer object.

As a side-note, consider replacing Timer with ScheduledThreadPoolExecutor available since Java 1.5.