Special use-case ExecutorService which collects tasks for execution and invokes them all on #flush() method in current thread. Useful for setup tests, so one can gain control of code execution scheduled by classes under test.

OnDemandExecutorService.java

import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.List;
import java.util.Queue;
import java.util.concurrent.AbstractExecutorService;
import java.util.concurrent.TimeUnit;

/**
 *
 * @author witoldsz
 */
public class OnDemandExecutorService extends AbstractExecutorService {

    private final Queue<Runnable> commands = new ArrayDeque<>();

    @Override
    public void shutdown() {
        //noop
    }

    @Override
    public List<Runnable> shutdownNow() {
        return new ArrayList<>(commands);
    }

    @Override
    public boolean isShutdown() {
        return false;
    }

    @Override
    public boolean isTerminated() {
        return false;
    }

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

    @Override
    public void execute(Runnable command) {
        commands.add(command);
    }

    public void flush() {
        Runnable command;
        while ((command = commands.poll()) != null) {
            command.run();
        }
    }

}