🔸 What it is (Definition)

▪️ CountDownLatch is a concurrency utility that starts with a count and lets threads await until the count reaches zero. Each completed task calls countDown(), and waiting threads resume when the latch hits 0. 🚦

🔸 How it works (Tiny example)

int workers = 3;

CountDownLatch latch = new CountDownLatch(workers);

for (int i = 0; i 〈 workers; i++) {

new Thread(() -〉 {

try { /* do work */ }

finally { latch.countDown(); } // ✅ always decrement

}).start();

}

latch.await(); // waits until 3 countDown() calls happened

// proceed when all workers are done

🔸 Common Use Cases

▪️ Start gate: Delay a set of threads until you say “go” (use a second latch or a barrier). 🏁

▪️ Finish gate: Main thread waits for N worker tasks to complete before continuing. ✅

▪️ Parallel init/bootstrap: Load configs/resources in parallel, then continue when all ready. 🚀

▪️ Testing: Wait for async callbacks/events to finish in unit/integration tests. 🧪

🔸 Gotchas & Tips

▪️ One-shot only: CountDownLatch cannot be reset; use CyclicBarrier or Phaser if you need reuse. 🔁

▪️ Always decrement in finally to avoid deadlocks on exceptions. 🛡️

▪️ Timeouts: Prefer await(timeout, unit) to avoid waiting forever. ⏳

▪️ When tasks produce results: Consider CompletableFuture.allOf(...) or an ExecutorService. 📦

▪️ For “N start simultaneously”: Pair with a second latch (start signal) or use CyclicBarrier. 🧩

🔸 TL;DR

▪️ CountDownLatch lets one or more threads wait until other threads complete N signals.

▪️ Perfect for a one-time rendezvous (finish gate).

▪️ Use finally { latch.countDown(); } and consider timeouts.

🔸 Takeaways

▪️ Use CountDownLatch for one-shot coordination; not reusable.

▪️ Prefer CyclicBarrier/Phaser for repeated phases; CompletableFuture for result aggregation.

▪️ Keep it safe with timeouts and finally blocks.

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