Java - BlockingQueue freezes multithread application

Question

I'm making an application that contains of two threads: one of them writes a value to LinkedBlockingQueue, another is reading. I'm using ScheduledExecutorService for running this operations in some period in seconds. The problem is that my application is freezing on the method take of BlockingQueue and i cant understand why.

This is a common resourse:

class Res{
    AtomicInteger atomicInteger = new AtomicInteger(0);
    BlockingQueue<String> q = new LinkedBlockingQueue<>();
}

This is reader

Semaphore semaphore = new Semaphore(1); /this is for reader does not take two places in thread pool
Runnable reader = ()->{
    try {
        semaphore.acquire();
        System.out.println(res.q.take()+" "+res.atomicInteger.incrementAndGet());
        semaphore.release();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
};

Writer:

Runnable writer = ()->{
    res.q.add("hi");
};

Full code:

class Res{
    AtomicInteger atomicInteger = new AtomicInteger(0);
    BlockingQueue<String> q = new LinkedBlockingQueue<>();
}
public class Main {

    public static void main(String[] args) throws InterruptedException {

        ScheduledExecutorService executorService = Executors.newScheduledThreadPool(2);
        Res res = new Res();
        Semaphore semaphore = new Semaphore(1); //this is for reader does not take two places in thread pool
        Runnable reader = ()->{
            try {
                semaphore.acquire();
                System.out.println(res.q.take()+" "+res.atomicInteger.incrementAndGet());
                semaphore.release();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        };
        Runnable writer = ()->{
            res.q.add("hi");
        };
        Random rnd = new Random();

        for (int i = 0; i < 20; i++) {
            int time = rnd.nextInt(5)+ 2;
            executorService.schedule(writer,time, TimeUnit.SECONDS);
        }
        for (int i = 0; i < 20; i++) {
            int time = rnd.nextInt(5)+ 2;
            executorService.schedule(reader,time, TimeUnit.SECONDS);
        }

        executorService.shutdown();
}

It should print twenty lines "hi [number]", but freezes on some line. For example, my current print:

hi 1
hi 2
hi 3
hi 4
hi 5

I found out If i increase count of threads newScheduledThreadPool(20) it starts work, but how can I make it with two threads? Thanks!

Your executor starts off with two threads. What do you think will happen when both of those threads are blocked in calls to your `reader` object, and the queue is empty? — Solomon Slow, Apr 07 '20 at 20:49

score 2 · Accepted Answer · answered Apr 07 '20 at 20:48

2

It's a bit hard to follow your code, though it is obvious at the same time what is going on. You can run two threads at a time, at most, because of Executors.newScheduledThreadPool(2);. Both of these threads are reader threads.

So Thread-1 entered the try block and acquired the semaphore permit via semaphore.acquire();, but the queue was empty - as such it blocks on res.q.take(). The next thread - Thread-2 is a reader thread too, but it can not acquire a permit, since it is already taken by Thread-1 and is blocked on semaphore.acquire();. Since you have no room for other Threads (you pool is blocked working with these two Threads), there are no writers that would put something in your queue and as such unblock Thread-1 (so that res.q.take() would work).

Adding more worker Threads just delays the problem - you could end up in the same position as you were before.

answered Apr 07 '20 at 20:48

Eugene

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Thanks! it's seems to be true. I think i can user two separate executer service, such as `ScheduledExecutorService executorServiceWriter = Executors.newScheduledThreadPool(1); ScheduledExecutorService executorServiceReader = Executors.newScheduledThreadPool(1);` Is it a good solution? How do you think? – Alexander Tukanov Apr 07 '20 at 21:40
@AlexanderTukanov yes, if you have separate pools for readers and writers, you should be fine – Eugene Apr 07 '20 at 21:41
And do I understand correctly, there is no another solution in my case? – Alexander Tukanov Apr 07 '20 at 21:43
@AlexanderTukanov there could be multiple solutions, but I don't _fully_ understand what you are trying to do - if you simply want to read and write to a queue and you don't want threads to block, your threads need to somehow coordinate their actions and there are multiple ways for that. You might wan to ask a separate, clear, question. – Eugene Apr 07 '20 at 21:45
Yes, I just want to read and write to a queue without blocking. Do you mean I should create new question? – Alexander Tukanov Apr 07 '20 at 21:48
@AlexanderTukanov right and clearly explain what you want to do, so that people could help. – Eugene Apr 07 '20 at 22:03

Java - BlockingQueue freezes multithread application

1 Answers1