Question

I’m a newbie in Java and I haven’t work with threads in past. Now I have a really tricky issue for my level of knowledge.

I have developed two functionalities the “Send mail” and the “Check mail”. Each user register his preferences and the system creates two catalogues with many instructions for periodic work like

A) send message every 1500 msec

B) send message every 1800 msec

C) send message every 3000 msec

And
A) check message every 2000 msec

B) check message every 6000 msec

C) check message every 8000 msec

I tried many ways with threads but I fail to do it work.
My question is which is the best way to synchronize them? Below is code from my last effort using threads.

public class MailCreatorThread extends Thread {
    @Override
    public void run() {
        CreateMail(_date); //creates a mail with _date as subject
    }
}

public class GPSThread extends Thread {
    @Override
    public void run() {
        // TODO Auto-generated method stub
        while (!_isTimeToStop) {
            try {
                while (_servicesToUpdate.size() == 0) {
                    Thread.sleep(500);
                }
                _currentService = (MyService) _servicesToUpdate.get(0)
                        .clone();
                _servicesToUpdate.remove(0);

                        MailCreatorThread mailCreatorThread = new MailCreatorThread();
                        mailCreatorThread.start();


            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }
}

public class CheckServicesThread extends Thread {
    @Override
    public void run() {
        // TODO Auto-generated method stub
        while (!_isTimeToStop) {
            try {
                Thread.sleep(1000);
                    for (int j = 0; j < _servicesList.length; j++) {
                        MyService currentService = ((MyService) _servicesList[j]);
                        if (myService.getTimeToNextUpdate() - 1000 <= 0) {
                            _servicesToUpdate
                                    .add((MyService) currentService
                                            .clone());
                            currentService
                                    .setTimeToNextUpdate(currentService
                                            .getUpdatePeriod());
                        } else {
                            currentService
                                    .setTimeToNextUpdate(currentService
                                            .getTimeToNextUpdate() - 1000);
                        }
                }

            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
        stopSelf();
    }
}

Answer 1

You can use a ScheduledExecutorService to run thos periodic tasks. The syntax is fairly straightforward:

Runnable check = new Runnable() {
    public void run() {
        checkMessage();
    }
}
Runnable send = new Runnable() {
    public void run() {
        sendMessage();
    }
}

//since what you are doing is mostly I/O you probably want to have 
//more than one thread available so that if one operation blocks,
//the other ones can be launched in parallel

ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(10);
scheduler.scheduleAtFixedRate(check, 0, 1500, MILLISECONDS);
scheduler.scheduleAtFixedRate(send, 0, 6000, MILLISECONDS);

Note: Timer mentioned in Ozzy’s answer should not be used any more as it has been improved in Java 1.5 by ScheduledThreadPoolExecutor, as explained in Timer’s javadoc :

Java 5.0 introduced the java.util.concurrent package and one of the concurrency utilities therein is the ScheduledThreadPoolExecutor which is a thread pool for repeatedly executing tasks at a given rate or delay. It is effectively a more versatile replacement for the Timer/TimerTask combination, as it allows multiple service threads, accepts various time units, and doesn’t require subclassing TimerTask (just implement Runnable). Configuring ScheduledThreadPoolExecutor with one thread makes it equivalent to Timer.