Waiting list of threads in java

Question

I'm writing a swing application with HttpClient and I need a way to make a download list because I need to wait 1 minute (for example) before starting a new download.

So I would like to create a waiting list of threads (downloads). I would have a class that takes a time parameter and contains a list of threads and when I add a thread in the list it starts if there is no running thread. Otherwise it waits for its turn.

Is there any tool to do that ?

Thanks a lot for your help.

Answer 1

Yes. ScheduledExecutorService. You can create a fixed length service via Executors.newScheduledThreadPool(corePoolSize). When you are ready to submit the task to wait the amount of time just submit it to ScheduledExecutorService.schedule

ScheduledExecutorService e = Executors.newScheduledThreadPool(10)
private final long defaultWaitTimeInMinutes = 1;
public void submitTaskToWait(Runnable r){
    e.schedule(r, defaultWaitTimeInMinutes, TimeUnit.MINUTES);
}

Here the task will launch in 1 minute from the time of being submitted. And to address your last point. If there are currently tasks being downloaded (this configuration means 10 tasks being downloaded) after the 1 minute is up the runnable submitted will have to wait until one of the other downloads are complete.

Keep in mind this deviates a bit from the way you are designing it. For each new task you wouldnt create a new thread, rather you would submit to a service that already has thread(s) waiting. For instance, if you only want one task to download at a time you change from Executors.newScheduledThreadPool(10) to Executors.newScheduledThreadPool(1)

Edit: I'll leave my previous answer but update it with a solution to submit a task to start exactly 1 minute after the previous task completes. You would use two ExecutorServices. One to submit to the scheuled Executor and the other to do the timed executions. Finally the first Executor will wait on the completion and continue with the other tasks queued up.

ExecutorService e = Executors.newSingleThreadExecutor();
ScheduledExecutorService scheduledService = Executors.newScheduledThreadPool(1)
public void submitTask(final Runnable r){
    e.submit(new Runnable(){
       public void run(){
           ScheduledFuture<?> future= scheduledService.schedule(r, defaultWaitTimeInMinutes, TimeUnit.MINUTES);
           future.get(); 

       }
    });
}

Now when the future.get(); completes the next Runnable submitted through submitTask will be run and then scheduled for a minute. Finally this will work only if you require the task to wait the 1 minute even if there is no other tasks submitted.