Guava: Using ListenableFuture

Google Guava has many interesting classes which we can use on our application. The ones from collection package have been already used by many developers and this blog has tutorial on how to use the computing map.

I want to move to the other package. This one is, specifically I want to introduce ListenableFuture. The documentation of the class is as follow:

This interface defines a future that has listeners attached to it, which is useful for asynchronous workflows. Each listener has an associated executor, and is invoked using this executor once the Future’s computation is Future#isDone() complete}. The listener will be executed even if it is added after the computation is complete.

Consider following example. If we have task T1, T2, and T3. T2 can only be done when T1 is finished and T3 can only be done once T2 is ended. The diagram below shows the dependency.

The easiest solution without any concurrency is of course to just run each task one after another. But consider that we have 5 set of these operation. Without thread we can end up with serial solution depicted by the following picture.

ListenableFuture made it easy to create the concurrent version of the solution.

This is a code example of this solution which will print “1”, then pause for a second before printing “2”, and another stop for 1 second before finally printing “3”. Note that ListenableFutureTask extends ListenableFuture.

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;


public class TestListenableFuture {

	static class SimpleTask extends
			ListenableFutureTask<Void> {
		SimpleTask(final String message) {
			super(new Callable<Void>() {

				public Void call() throws Exception {
					return null;

	public static void main(String[] args) {
		ListenableFutureTask<Void> task1 =
			new SimpleTask("1");
		ListenableFutureTask<Void> task2 =
			new SimpleTask("2");
		ListenableFutureTask<Void> task3 =
			new SimpleTask("3");

		ExecutorService exec =
		task1.addListener(task2, exec);
		task2.addListener(task3, exec);

		try {
		} catch (InterruptedException e) {

Pretty easy, isn’t it? And we can probably extend this solution to create a full workflow framework solution.

UPDATE: Remember that the API is still in Beta version. I’ll try to update it once the release version is released

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