import java.util.Random;
import java.util.concurrent.Semaphore;

public class SemApp {
    public static void main(String[] args) {

        Runnable limitedCall = new Runnable() {
            final Random rand = new Random();
            final Semaphore available = new Semaphore(3);
            int count = 0;

            @Override
            public void run() {
                int time = rand.nextInt(15);
                int num = count++;

                try {
                    available.acquire();
                    System.out.println("Executing long-running action for " + time + " second... #" + num);

                    Thread.sleep(time * 1000);

                    System.out.println("Done with #" + num + "!");

                    available.release();
                } catch (InterruptedException intEx) {
                    intEx.printStackTrace();
                }
            }
        };

        for (int i = 0; i < 10; i++) {
            new Thread(limitedCall).start();
        }
    }
}