An implementation of ScheduledExecutorService for LibGDX which executes tasks on the main game thread.

GdxScheduledExecutorService.java

import com.badlogic.gdx.Gdx;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicReference;
import java.util.stream.Collectors;

/**
 * A {@link ScheduledExecutorService} implementation that executes operations on the game loop
 * thread using {@link com.badlogic.gdx.Application#postRunnable(Runnable)} via {@link Gdx#app}
 * and a delegated scheduled executor service.
 */
public class GdxScheduledExecutorService implements ScheduledExecutorService {
  private final ScheduledExecutorService delegate;

  public GdxScheduledExecutorService(ScheduledExecutorService delegate) {
    this.delegate = delegate;
  }

  /**
   * Creates a new {@link GdxScheduledExecutorService} with a thread pool as the delegate.
   * @param corePoolSize the number of threads to keep in the pool
   * @return the new executor service
   */
  public static GdxScheduledExecutorService create(int corePoolSize) {
    return new GdxScheduledExecutorService(Executors.newScheduledThreadPool(corePoolSize));
  }

  @Override
  public ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit) {
    return delegate.schedule(() -> runTask(command), delay, unit);
  }

  @Override
  public <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit) {
    return delegate.schedule(() -> runCallable(callable), delay, unit);
  }

  @Override
  public ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period, TimeUnit unit) {
    return delegate.scheduleAtFixedRate(() -> runTask(command), initialDelay, period, unit);
  }

  @Override
  public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit) {
    return delegate.scheduleWithFixedDelay(() -> runTask(command), initialDelay, delay, unit);
  }

  @Override
  public void shutdown() {
    delegate.shutdown();
  }

  @Override
  public List<Runnable> shutdownNow() {
    return delegate.shutdownNow();
  }

  @Override
  public boolean isShutdown() {
    return delegate.isShutdown();
  }

  @Override
  public boolean isTerminated() {
    return delegate.isTerminated();
  }

  @Override
  public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException {
    return delegate.awaitTermination(timeout, unit);
  }

  @Override
  public <T> Future<T> submit(Callable<T> task) {
    return delegate.submit(() -> runCallable(task));
  }

  @Override
  public <T> Future<T> submit(Runnable task, T result) {
    return delegate.submit(() -> runTask(task), result);
  }

  @Override
  public Future<?> submit(Runnable task) {
    return delegate.submit(() -> runTask(task));
  }

  @Override
  public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
      throws InterruptedException {
    return delegate.invokeAll(gdxTasks(tasks));
  }

  @Override
  public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks, long timeout,
      TimeUnit unit) throws InterruptedException {
    return delegate.invokeAll(gdxTasks(tasks), timeout, unit);
  }

  @Override
  public <T> T invokeAny(Collection<? extends Callable<T>> tasks)
      throws InterruptedException, ExecutionException {
    return delegate.invokeAny(gdxTasks(tasks));
  }

  @Override
  public <T> T invokeAny(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit)
      throws InterruptedException, ExecutionException, TimeoutException {
    return delegate.invokeAny(gdxTasks(tasks), timeout, unit);
  }

  @Override
  public void execute(Runnable command) {
    delegate.execute(() -> runTask(command));
  }

  private void runTask(Runnable command) {
    CountDownLatch lock = new CountDownLatch(1);
    Gdx.app.postRunnable(() -> {
      command.run();
      lock.countDown();
    });

    try {
      lock.await();
    }
    catch (InterruptedException e) {
      // ignored
    }
  }

  private <V> V runCallable(Callable<V> callable) throws Exception {
    AtomicReference<V> result = new AtomicReference<>();
    AtomicReference<Exception> exception = new AtomicReference<>();
    CountDownLatch lock = new CountDownLatch(1);
    Gdx.app.postRunnable(() -> {
      try {
        result.set(callable.call());
      }
      catch (Exception e) {
        exception.set(e);
      }
      lock.countDown();
    });

    lock.await();

    if (exception.get() != null) {
      throw exception.get();
    }
    return result.get();
  }

  private <T> List<? extends Callable<T>> gdxTasks(Collection<? extends Callable<T>> tasks) {
    return tasks.stream()
        .map(task -> (Callable<T>) () -> runCallable(task))
        .collect(Collectors.toList());
  }
}