I am trying to understand the working of semaphores in the context of producers and consumers. Have a simple bounded queue containing push & pop APIs. Also, managing multiple producers and consumers using Semaphores but looks like there's still some race condition.
Queue class
class BoundedQueue { private Queue<Integer> q; int maxSize; Semaphore produce; Semaphore consume; BoundedQueue(int size) throws InterruptedException { maxSize = size; q = new LinkedList<>(); produce = new Semaphore(maxSize); consume = new Semaphore(-1); } void push(int val) { try { produce.acquire(); q.offer(val); System.out.println(Thread.currentThread().getName() +" produced " + val); if (q.size() > maxSize) { System.out.println("Max size breached"); } consume.release(); } catch (InterruptedException e) { System.out.println("Interrupted exception " + e); produce.release(); } } int pop() { int val = -1; try { consume.acquire(); if (q.isEmpty()) { System.out.println(Thread.currentThread().getName() +" is trying to consume empty queue"); return val; } val = q.remove(); System.out.println(Thread.currentThread().getName() +" consumed " + val); if (q.size() > maxSize) { System.out.println("Max size breached"); } produce.release(); } catch (InterruptedException e) { System.out.println("Interrupted exception " + e); consume.release(); } return val; }}Multiple producers and consumers that are expected to work in tandem. Producers produce till the queue is not full. Consumers consume when at least one item is available.
public class ProducerConsumerSemaphores extends Thread { private BoundedQueue que; private String workerType; ProducerConsumerSemaphores(BoundedQueue que, String workerType) { this.que = que; this.workerType = workerType; } public void run() { System.out.println("Starting worker " + workerType +" " + Thread.currentThread().getName()); if (workerType.equals("producer")) { Random rand = new Random(); int val = rand.nextInt(1, 100); for (int i=0; i<10; ++i) { que.push(val+i); try { Thread.sleep(10); } catch (Exception e) { System.out.println("Caught " + e); } } } else { for (int i=0; i<10; ++i) { que.pop(); try { Thread.sleep(100); } catch (Exception e) { System.out.println("Caught " + e); } } } System.out.println("Ending woker " + workerType +" " + Thread.currentThread().getName()); } public static void main(String args[]) { System.out.println(); try { BoundedQueue que = new BoundedQueue(5); ProducerConsumerSemaphores p1 = new ProducerConsumerSemaphores(que, "producer"); p1.setName("p1"); ProducerConsumerSemaphores p2 = new ProducerConsumerSemaphores(que, "producer"); p2.setName("p2"); ProducerConsumerSemaphores p3 = new ProducerConsumerSemaphores(que, "producer"); p3.setName("p3"); p1.start(); p2.start(); ProducerConsumerSemaphores c1 = new ProducerConsumerSemaphores(que, "consumer"); c1.setName("c1"); c1.start(); ProducerConsumerSemaphores c2 = new ProducerConsumerSemaphores(que, "consumer"); c2.setName("c2"); c2.start(); p3.start(); ProducerConsumerSemaphores c3 = new ProducerConsumerSemaphores(que, "consumer"); c3.setName("c3"); c3.start(); } catch (Exception e) { System.out.println("Exception occurred"); } }}sample output
Starting worker producer p2Starting worker producer p1Starting worker consumer c1Starting worker consumer c3Starting worker producer p3Starting worker consumer c2p2 produced 87p1 produced 61p3 produced 90c1 consumed 87Exception in thread "c2" java.util.NoSuchElementException at java.base/java.util.LinkedList.removeFirst(LinkedList.java:274) at java.base/java.util.LinkedList.remove(LinkedList.java:689) at BoundedQueue.pop(ProducerConsumerSemaphores.java:42) at ProducerConsumerSemaphores.run(ProducerConsumerSemaphores.java:85)p2 produced 88c3 consumed 88p2 produced 89What is going wrong, any idea?
