Java.Multithreading.What is the difference between CyclicBarrier and CountDownLatch?

Excellent question, Stanley! 🚧 Both CyclicBarrier and CountDownLatch are powerful synchronization tools in Java’s java.util.concurrent package — but they serve different purposes and behave differently in key ways.

Let’s break them down and compare side by side.

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🧩 Quick Summary:

Feature	CountDownLatch	CyclicBarrier
Purpose	Wait for other threads to finish	Wait until a group of threads is ready
Reusable?	❌ One-time use	✅ Reusable (cyclic)
Threads block on	await() (main thread waits)	await() (all threads wait for each other)
When it proceeds	When count reaches zero	When all threads reach the barrier
Can reset?	❌ No	✅ Yes (reset() method)
Optional barrier action	❌ No	✅ Yes (Runnable runs once all arrive)

📦 CountDownLatch — Countdown & Go

Used when one or more threads must wait until other threads complete some work.

Example:

CountDownLatch latch = new CountDownLatch(3);

Runnable worker = () -> {
    System.out.println(Thread.currentThread().getName() + " working...");
    latch.countDown();
};

new Thread(worker).start();
new Thread(worker).start();
new Thread(worker).start();

latch.await(); // Main thread waits here until count = 0
System.out.println("All workers done. Proceeding...");

🧠 Key Points:

• The latch starts with a count.
• Each countDown() decrements the count.
• await() blocks until count hits 0.
• Not reusable — once it hits 0, it’s done forever.

🧱 CyclicBarrier — Meet at the Checkpoint

Used when multiple threads must wait for each other to reach a common point.

Example:

CyclicBarrier barrier = new CyclicBarrier(3, () -> {
    System.out.println("All threads reached the barrier!");
});

Runnable worker = () -> {
    System.out.println(Thread.currentThread().getName() + " waiting at barrier");
    barrier.await(); // Wait for 2 other threads
    System.out.println(Thread.currentThread().getName() + " passed the barrier");
};

new Thread(worker).start();
new Thread(worker).start();
new Thread(worker).start();

🧠 Key Points:

• All threads call await() and wait for others.
• Once all parties have arrived, the barrier releases them all.
• The optional Runnable barrier action runs just once when the last thread arrives.
• Reusable — can be used again after all threads pass the barrier.

🔁 Real-World Analogies

Situation	Tool	Analogy
Wait for workers to finish	CountDownLatch	Wait for 3 friends to finish chores before going to the movies
Synchronize tasks in phases	CyclicBarrier	3 friends wait at a checkpoint before continuing a hike together

✅ When to Use What?

Scenario	Use
Main thread waits for tasks to complete	CountDownLatch
Threads wait for each other at a common point	CyclicBarrier
Need to reuse the barrier for multiple rounds	CyclicBarrier
One-shot coordination (e.g., service startup)	CountDownLatch