CountDownLatch

This Article is part of Series of Articles on Java 8 Concurrency Tutorial.
In this article, we’ll focus on a the concept of CountDownLatch in the Java language.

CountDownLatch

CountDownLatch enables a java thread to wait until other set of threads completes their tasks.

A CountDownLatch is initialized with a given count.
The await methods block until the current count reaches zero due to invocations of the countDown() method, after which all waiting threads are released and any subsequent invocations of await return immediately.
This is a one-shot phenomenon – the count cannot be reset. If you need a version that resets the count, consider using a CyclicBarrier.

e.g. Assume we have divided one task in 5 small independent task.
Now main thread should wait, till other 5 Threads finish there work.
In these scenarios CountDownLatch can be used.

CountDownLatch Example

class CountDownLatchWorker implements Runnable
{
	private CountDownLatch countDownLatch;
	private int workerId;

	public CountDownLatchWorker(CountDownLatch countDownLatch ,int workerId) {
		this.countDownLatch=countDownLatch;
		this.workerId=workerId;
	}
	@Override
	public void run() {
		System.out.println("Worker " + workerId + " Started" );
		try {
			Thread.sleep(workerId*1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println("Worker " + workerId + " Completed it's work, Reducing  count of countDownLatch " );
		countDownLatch.countDown();		
	}	
}
public class CountDownLatchExample {
	public static void main(String[] args) throws InterruptedException {
		CountDownLatch countDownLatch = new CountDownLatch(5);
		ExecutorService newCachedThreadPool = Executors.newCachedThreadPool();
		IntStream.range(1, 6)
				  .forEach(cnt -> {
					  newCachedThreadPool.submit(new CountDownLatchWorker(countDownLatch, cnt));
				  });		
		System.out.println("Main Thread is wating for workers to finish!!!!!!");
		countDownLatch.await();
		System.out.println("Work of All Worker is Completed");
		newCachedThreadPool.shutdown();		
	}
}

Key Points

await Method
Causes the current thread to wait until the latch has counted down to zero, unless the thread is interrupted.
If the current count is zero then this method returns immediately.
If the current count is greater than zero then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happen:

countDown Method
Decrements the count of the latch, releasing all waiting threads if the count reaches zero.
If the current count is greater than zero then it is decremented. If the new count is zero then all waiting threads are re-enabled for thread scheduling purposes.
If the current count equals zero then nothing happens.