How Can Callbacks Enable Non-Blocking Task Completion Notifications in Java Executors?

Java Executors: Non-Blocking Notifications for Task Completion

When faced with processing numerous tasks through an executor service, blocking can become a significant obstacle due to stack space constraints. To circumvent this issue, consider employing a callback approach, where tasks submit non-blocking notifications upon completion.

Creating a Callback Interface

Define a callback interface encapsulating the desired parameters for completion notification. In the provided code example:

public interface Callback {
  void complete();
}

Wrapping Tasks with a Callback

Utilize this callback interface to create a wrapper class for Runnable tasks:

class CallbackTask implements Runnable {
  private final Runnable task;
  private final Callback callback;

  CallbackTask(Runnable task, Callback callback) {
    this.task = task;
    this.callback = callback;
  }

  @Override
  public void run() {
    task.run();
    callback.complete();
  }
}

Submitting Tasks with Callbacks

With the callback wrapper in place, submit tasks to the executor service as follows:

ExecutorService executor = Executors.newSingleThreadExecutor();
executor.submit(new CallbackTask(task, callback));

Notification Callback

Implement the callback to handle completion notifications and execute the next task from the queue:

public class CallbackImpl implements Callback {
  private Queue<Task> queue;

  @Override
  public void complete() {
    if (!queue.isEmpty()) {
      executor.submit(new CallbackTask(queue.poll(), this));
    }
  }
}

Java 8's CompletableFuture

Since Java 8, the CompletableFuture class offers a more comprehensive approach for creating and managing asynchronous processes:

CompletableFuture<String> f = CompletableFuture.supplyAsync(svc::work);
f.thenAccept(listener::notify);

This code creates a CompletableFuture representing the asynchronous task, and defines a callback to be triggered upon its completion.

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