Asynchronous equivalent of BlockingQueue in Java?

Question

I'm looking for a queue that would be the asynchronous (non-blocking) equivalent of java.util.concurrent.BlockingQueue. Its interface would include:

public interface AsynchronousBlockingQueue<E> {
    // - if the queue is empty, return a new CompletableFuture,
    //   that will be completed next time `add` is called
    // - if the queue is not empty, return a completed CompletableFuture,
         containing the first element of the list
    public CompletableFuture<E> poll();

    // if polling is in progress, complete the ongoing polling CompletableFuture.
    // otherwise, add the element to the queue
    public synchronized void add(E element);
}

If that matters, there should be just one poller thread, and polling should be done sequentially (poll will not be called when polling is already in progress).

I expected it to already exist in the JVM, but I couldn't find it, and of course I'd rather use something from the JVM than write it myself.

Another constraint, I'm stuck with Java 8 (even though I'm definitely interested in knowing what exists in more recent versions).

Answer 1

So finally I wrote my own class... Interested in comments :)

import java.util.LinkedList;
import java.util.Queue;
import java.util.concurrent.CompletableFuture;

public class AsynchronousBlockingQueue<E> {
    CompletableFuture<E> incompletePolling = null;
    Queue<E> elementsQueue = new LinkedList<>();

    // if the queue is empty, return a new CompletableFuture, that will be completed next time `add` is called
    // if the queue is not empty, return a completed CompletableFuture containing the first element of the list
    public synchronized CompletableFuture<E> poll() {
        // polling must be done sequentially, so this shouldn't be called if there is a poll ongoing.
        if (incompletePolling != null)
            throw new IllegalStateException("Polling is already ongoing");
        if (elementsQueue.isEmpty()) {
            incompletePolling = new CompletableFuture<>();
            return incompletePolling;
        }
        CompletableFuture<E> result = new CompletableFuture<>();
        result.complete(elementsQueue.poll());
        return result;
    }

    // if polling is in progress, complete the ongoing polling CompletableFuture.
    // otherwise, add the element to the queue
    public synchronized void add(E element) {
        if (incompletePolling != null) {
            CompletableFuture<E> result = incompletePolling;
            // removing must be done first because the completion could trigger code that needs the queue state to be valid
            incompletePolling = null;
            result.complete(element);
            return;
        }
        elementsQueue.add(element);
    }


}