CountDownLatch waits for a set of tasks

A latch counts down as work completes and releases every waiter at zero. It is single-use - create a new one for the next round.

Code

var latch = new CountDownLatch(3);
var done = Collections.synchronizedList(new ArrayList<String>());

for (int i = 1; i <= 3; i++) {
    int id = i;
    Thread.ofVirtual().start(() -> {
        try { Thread.sleep(Duration.ofMillis(10L * id)); } catch (InterruptedException e) { }
        done.add("worker" + id);
        latch.countDown();
    });
}

latch.await();
System.out.println("all workers finished, count = " + done.size());
System.out.println("latch count now = " + latch.getCount());

Output

all workers finished, count = 3
latch count now = 0

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Published 2026-07-30