final Phaser phaser = new Phaser(1);
List<Runnable> runnables = getHelloWorldTasks(15);

for (final Runnable runnable : runnables) {
  phaser.register();
  new Thread() {
    public void run() {
      runnable.run();

      if(phaser.getArrivedParties()==1) {
        int phaserNum = phaser.arrive();
        //Some Logic
        try {
          Thread.sleep(5000);
          phaser.awaitAdvance(phaserNum);
          System.out.println("After Sleep");
        } catch (InterruptedException e) {
           e.printStackTrace();
        }
      } else {
        phaser.arriveAndAwaitAdvance();
      }
    }
  }.start();
}

phaser.arriveAndAwaitAdvance();
System.out.println("END");