Java executors: how to be notified, without blocking, when a task completes?

Define a callback interface to receive whatever parameters you want to pass along in the completion notification. Then invoke it at the end of the task.

You could even write a general wrapper for Runnable tasks, and submit these to ExecutorService. Or, see below for a mechanism built into Java 8.

class CallbackTask implements Runnable {

  private final Runnable task;

  private final Callback callback;

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

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

}

With CompletableFuture, Java 8 included a more elaborate means to compose pipelines where processes can be completed asynchronously and conditionally. Here's a contrived but complete example of notification.

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.TimeUnit;

public class GetTaskNotificationWithoutBlocking {

  public static void main(String... argv) throws Exception {
    ExampleService svc = new ExampleService();
    GetTaskNotificationWithoutBlocking listener = new GetTaskNotificationWithoutBlocking();
    CompletableFuture<String> f = CompletableFuture.supplyAsync(svc::work);
    f.thenAccept(listener::notify);
    System.out.println("Exiting main()");
  }

  void notify(String msg) {
    System.out.println("Received message: " + msg);
  }

}

class ExampleService {

  String work() {
    sleep(7000, TimeUnit.MILLISECONDS); /* Pretend to be busy... */
    char[] str = new char[5];
    ThreadLocalRandom current = ThreadLocalRandom.current();
    for (int idx = 0; idx < str.length; ++idx)
      str[idx] = (char) ('A' + current.nextInt(26));
    String msg = new String(str);
    System.out.println("Generated message: " + msg);
    return msg;
  }

  public static void sleep(long average, TimeUnit unit) {
    String name = Thread.currentThread().getName();
    long timeout = Math.min(exponential(average), Math.multiplyExact(10, average));
    System.out.printf("%s sleeping %d %s...%n", name, timeout, unit);
    try {
      unit.sleep(timeout);
      System.out.println(name + " awoke.");
    } catch (InterruptedException abort) {
      Thread.currentThread().interrupt();
      System.out.println(name + " interrupted.");
    }
  }

  public static long exponential(long avg) {
    return (long) (avg * -Math.log(1 - ThreadLocalRandom.current().nextDouble()));
  }

}

ThreadPoolExecutor also has beforeExecute and afterExecute hook methods that you can override and make use of. Here is the description from ThreadPoolExecutor's Javadocs.

Hook methods

This class provides protected overridable beforeExecute(java.lang.Thread, java.lang.Runnable) and afterExecute(java.lang.Runnable, java.lang.Throwable) methods that are called before and after execution of each task. These can be used to manipulate the execution environment; for example, reinitializing ThreadLocals, gathering statistics, or adding log entries. Additionally, method terminated() can be overridden to perform any special processing that needs to be done once the Executor has fully terminated. If hook or callback methods throw exceptions, internal worker threads may in turn fail and abruptly terminate.

Use a CountDownLatch.

It's from java.util.concurrent and it's exactly the way to wait for several threads to complete execution before continuing.

In order to achieve the callback effect you're looking after, that does require a little additional extra work. Namely, handling this by yourself in a separate thread which uses the CountDownLatch and does wait on it, then goes on about notifying whatever it is you need to notify. There is no native support for callback, or anything similar to that effect.

EDIT: now that I further understand your question, I think you are reaching too far, unnecessarily. If you take a regular SingleThreadExecutor, give it all the tasks, and it will do the queueing natively.

If you want to make sure that no tasks will run at the same time then use a SingleThreadedExecutor. The tasks will be processed in the order the are submitted. You don't even need to hold the tasks, just submit them to the exec.

You can implement future tasks in java executors that returns the callback on the completion of the tasks.

• Use callable for tasks instead of runnable
• Can perform non related tasks when execution of threads are in progress asynchronously
• Multiple tasks can be executed and can get result of each one

Follwoing is the class that returns random integer values from a task and printed on main thread

public class ExecutorCallable {

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

        //fix number of threads
        //blocking queue-thread safe
        ExecutorService service = Executors.newFixedThreadPool(100);

        //submit the tasks for execution
        Future<Integer> result = service.submit(new Task());

        //*** perform some unrelated operations

        System.out.println("Result of submitted task is : " + result.get());//blocks until the future is ready after completion

        System.out.println("Thread name " + Thread.currentThread().getName());
    }

    static class Task implements Callable<Integer> {

        @Override
        public Integer call() throws Exception {
            return new Random().nextInt();
        }
    }
 }

The Output is:

Result of submitted task is : 16645418
Thread name main