CountDownLatch Example in Java

Here is a simple example of how and when to use CountDownLatch in Java.

	package concurrency;
	import java.util.concurrent.CountDownLatch;
	import java.util.concurrent.ExecutorService;
	import java.util.concurrent.Executors;
	public class CountdownLatchExample {
	//In this example, our requirement is that main thread should wait till 3 child threads are done with their tasks
	public static void main(String[] args) {
	//Define a latch initialized with 3.
	CountDownLatch latch = new CountDownLatch(3);
	//Create a new thread pool with 3 threads.
	ExecutorService executorService = Executors.newFixedThreadPool(3);
	for (int i = 0; i < 3; i++) {
	//Execute the runnable code using thread.
	executorService.submit(new LatchTask(latch, "Thread_" + i));
	//Introduce some delay between each thread call.
	try {
	Thread.sleep(1000);
	} catch (InterruptedException e) {
	e.printStackTrace();
	}
	}
	System.out.println("Waiting for threads to finish");
	//Main thread gets blocked here till all the latches are opened
	try {
	latch.await();
	} catch (InterruptedException e) {
	e.printStackTrace();
	}
	executorService.shutdown();
	System.out.println("All threads are done");
	}
	}
	class LatchTask extends Thread {
	CountDownLatch latch;
	String threadName;
	public LatchTask(CountDownLatch latch, String threadName) {
	this.latch = latch;
	this.threadName = threadName;
	}
	public void run() {
	//Time consuming code.
	System.out.println(threadName + " executing long running process");
	try {
	Thread.sleep(5000);
	} catch (InterruptedException e) {
	e.printStackTrace();
	}
	System.out.println(threadName + " opening latch");
	//Once done, thread opens the latch.
	latch.countDown();
	System.out.println("Latches still on: " + latch.getCount());
	}
	}
	/*
	Output:
	Thread_0 executing long running process
	Thread_1 executing long running process
	Thread_2 executing long running process
	Waiting for threads to finish
	Thread_0 opening latch
	Latches still on: 2
	Thread_1 opening latch
	Latches still on: 1
	Thread_2 opening latch
	Latches still on: 0
	All threads are done
	*/

view raw CountdownLatchExample.java hosted with ❤ by GitHub