CountDownLatch in java

CountDownLatch is a java synchronizer that comes under java.util.concurrent package and is available in java from jdk 1.5.

It is used in the scenario where multiple threads are working together and one or more threads need to wait until a set of task is finished by other threads.

Important methods used
await() ; The current thread which called this method will go into waiting state and will wait until the count becomes 0.

CountDown() ;This method when called will decrements the count and when count becomes 0, the current thread will notify the thread which called await() on the same CountDownLatch instance.

getCount() : This method will return the current count.

How it works:

When we create an object of CountDownLatch, we pass an int value to its constructor which specify the number of threads it should wait for, all the thread are required to decrement this count by calling countDown() . As soon as count becomes zero, the waiting thread on which await method is called completes its waiting time and resumes further processing.

Example application

import java.util.concurrent.CountDownLatch;

public class CountDownLatchExample{
	public static void main(String args[]) throws InterruptedException
	{
		CountDownLatch latch = new CountDownLatch(3);
		System.out.println("Initial count is :"+latch.getCount());
		// Let us create three worker threads (subTask-1,subtask-2 and subTask-3)and start them.
		for(int i=0;i<=2;i++){
			new Thread(new MyRunnableDemo(latch),"subTask-"+i).start();
		}

		// The main task wait for three threads
		latch.await();
		System.out.println("count after completing all subtasks is :"+latch.getCount());
		// Main thread has started
		System.out.println("All sub tasks are finished and the main thread is starting now ...");
		Thread.sleep(1000);
		System.out.println(Thread.currentThread().getName() +" thread has finished");
	}
}

import java.util.concurrent.CountDownLatch;

public class CountDownLatchExample{

public static void main(String args[]) throws InterruptedException

{

CountDownLatch latch = new CountDownLatch(3);

System.out.println("Initial count is :"+latch.getCount());

// Let us create three worker threads (subTask-1,subtask-2 and subTask-3)and start them.

for(int i=0;i<=2;i++){

new Thread(new MyRunnableDemo(latch),"subTask-"+i).start();

}

// The main task wait for three threads

latch.await();

System.out.println("count after completing all subtasks is :"+latch.getCount());

// Main thread has started

System.out.println("All sub tasks are finished and the main thread is starting now ...");

Thread.sleep(1000);

System.out.println(Thread.currentThread().getName() +" thread has finished");

}

import java.util.concurrent.CountDownLatch;

class MyRunnableDemo implements Runnable
{
	private CountDownLatch latch;
	public MyRunnableDemo(CountDownLatch latch)
	{
		this.latch = latch;
	}
	@Override
	public void run() { 
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println(Thread.currentThread().getName()+" finished the task");
		latch.countDown();
	}
}

import java.util.concurrent.CountDownLatch;

class MyRunnableDemo implements Runnable

{

private CountDownLatch latch;

public MyRunnableDemo(CountDownLatch latch)

{

this.latch = latch;

}

@Override

public void run() {

try {

Thread.sleep(1000);

} catch (InterruptedException e) {

e.printStackTrace();

}

System.out.println(Thread.currentThread().getName()+" finished the task");

latch.countDown();

}

OutPut :

Initial count is :3
subTask-0 finished the task
subTask-1 finished the task
subTask-2 finished the task
count after completing all subtasks is :0
All sub tasks are finished and the main thread is starting now …
main thread has finished

Real-time use cases

In an amusement park, there are certain rides in which it is mandate to have atleast 2 people (2 is count) to share seat in order to take that ride. So, ride keeper waits for 2 persons for each seat so that he can start the ride.

Suppose a big heavy rectangular box can be lifted by 4 people from four corners, so you will wait for all 4 to come then only you can lift that heavy box.

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