Those sound useful. I haven't had to do much cross-thread synchronization thankfully, but I have had to use a BlockingDeque to check that some events came and in the right order. The CountDownLatch and Phaser may have been better.

I had never heard of Phaser, but it looks pretty cool. I just read Baeldung's Guide to Phaser and correct me if I'm wrong, but doesn't it kind of seem like a race condition (it could just be how they use it in the examples)?

class LongRunningAction implements Runnable {
    private String threadName;
    private Phaser ph;

    LongRunningAction(String threadName, Phaser ph) {
        this.threadName = threadName;
        this.ph = ph;
        ph.register();
    }

    @Override
    public void run() {
        ph.arriveAndAwaitAdvance();
        try {
            Thread.sleep(20);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        ph.arriveAndDeregister();
    }
}

then

executorService.submit(new LongRunningAction("thread-1", ph));
executorService.submit(new LongRunningAction("thread-2", ph));
executorService.submit(new LongRunningAction("thread-3", ph));

if ph.arriveAndAwaitAdvance(); is called before all of the LongRunningActions are initialized, won't it proceed before it is supposed to?