Waitgroups in Prolog

Our example demonstrates how to wait for multiple threads to finish using Java’s CountDownLatch. This is similar to the concept of WaitGroups in other languages.

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;

public class WaitGroups {

    // This is the method we'll run in every thread.
    private static void worker(int id, CountDownLatch latch) {
        try {
            System.out.printf("Worker %d starting%n", id);

            // Sleep to simulate an expensive task.
            TimeUnit.SECONDS.sleep(1);

            System.out.printf("Worker %d done%n", id);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        } finally {
            // Signal that this worker is done.
            latch.countDown();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        // This CountDownLatch is used to wait for all the
        // threads launched here to finish.
        CountDownLatch latch = new CountDownLatch(5);

        // Launch several threads and decrement the CountDownLatch
        // counter for each.
        for (int i = 1; i <= 5; i++) {
            final int workerId = i;
            new Thread(() -> worker(workerId, latch)).start();
        }

        // Block until the CountDownLatch counter goes back to 0;
        // all the workers notified they're done.
        latch.await();

        // Note that this approach has no straightforward way
        // to propagate exceptions from workers. For more
        // advanced use cases, consider using ExecutorService
        // with Future objects.
    }
}

To run the program:

$ javac WaitGroups.java
$ java WaitGroups
Worker 1 starting
Worker 2 starting
Worker 3 starting
Worker 4 starting
Worker 5 starting
Worker 3 done
Worker 1 done
Worker 2 done
Worker 5 done
Worker 4 done

The order of workers starting up and finishing is likely to be different for each invocation.

In this Java version:

We use CountDownLatch instead of WaitGroup. It serves a similar purpose of allowing one thread to wait for multiple other threads.
The worker method takes a CountDownLatch as a parameter and calls countDown() when it’s done, similar to wg.Done() in the original example.
Instead of goroutines, we create and start regular Java threads.
We use latch.await() to wait for all threads to complete, which is equivalent to wg.Wait().
Java’s TimeUnit.SECONDS.sleep(1) is used to simulate the delay, replacing time.Sleep(time.Second).
We use System.out.printf for formatted printing, which is similar to fmt.Printf.

This example demonstrates how to manage concurrency in Java, waiting for multiple threads to complete before proceeding with the main thread’s execution.

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Waitgroups in Prolog