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Syncing with CountDownLatch

A CountDownLatch is a synchronisation mechanism which allows threads to wait until some events occur. By using CountDownLatch you can wait for events to happen in other threads and can proceed only if they have completed.


From docs

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.

Note: This is a one-shot phenomenon – the count cannot be reset. If you need a version that resets the count, consider using a CyclicBarrier.

There are 3 important methods in CountDownLatch class.

  • await()
  • await(long timeout, TimeUnit unit)
  • countDown()


void await() throws InterruptedException

await() causes current thread to wait until event count in latch reaches to zero (by calling countDown() method on Latch) or thread is interrupted. If Latch has already zero count, this method returns immediately.

If a thread has interrupted flag set upon entering this method, it will throw InterruptedException and will clear the flag.

await(long timeout, TimeUnit unit)

boolean await(long timeout, TimeUnit unit) throws InterruptedException

This method is same as await with one difference. It will stop waiting if given timeout is elapsed.It returns false if waiting timeout elapses and true, if event count in latch become zero.

If the time is less than or equal to zero, the method will not wait at all.


void countDown()

It decrements the latch count if count is greater than zero. If count reaches to zero, it awakes all awaiting threads.


To understand everything I have written, let’s create a scenario in which we can use CountDownLatch. Say, we have 3 threads, first two threads insert value in a shared list and third thread waits until each of them completes inserts value in the queue.

Once both thread have inserted values, we will calculate sum of those values in third thread.

Now, create a general NumberThread class which inserts values in the list and invokes countDown on latch.

class NumberThread extends Thread {

    List<Integer> list;
    CountDownLatch latch;

    NumberThread(String name, List<Integer> list, CountDownLatch latch){
        this.list = list;
        this.latch = latch;
    } // NumberThread

    public void run() {;
        String threadName = getName();

        System.out.printf("%s - starts waiting\n", threadName);
        try {
            Thread.sleep(new Random().nextInt(1000));
        } catch (InterruptedException e) {

        int value = new Random().nextInt(100);
        System.out.printf("%s - inserting %d in the list\n", threadName, value);

        System.out.printf("%s - invoking countDown on latch\n", threadName);

    } // run

} // NumberThread

Create ThreadOne class which extends from NumberThread

class ThreadOne extends NumberThread {

    ThreadOne(List<Integer> list, CountDownLatch latch) {
        super(ThreadOne.class.getSimpleName(), list, latch);
    } // ThreadOne

} // ThreadOne

Create ThreadTwo class which extends from NumberThread

class ThreadTwo extends NumberThread {

    ThreadTwo(List<Integer> list, CountDownLatch latch) {
        super(ThreadTwo.class.getSimpleName(), list, latch);
    } // ThreadTwo

} // ThreadTwo

Create ThreadThree class which extends from NumberThread. This class alos overrides the run method and waits for ThreadOne & ThreadTwo to insert value in list by invoking await on latch.

class ThreadThree extends NumberThread {

    ThreadOne threadOne;
    ThreadTwo threadTwo;

    ThreadThree(List<Integer> list, CountDownLatch latch, ThreadOne threadOne, ThreadTwo threadTwo) {
        super(ThreadThree.class.getSimpleName(), list, latch);
        this.threadOne = threadOne;
        this.threadTwo = threadTwo;

    public void run() {
        try {

            System.out.printf("%s - starts waiting for other threads to complete\n", getName());
            latch.await(); // wait for other threads to insert values

            int sum = list.get(0) + list.get(1);

            System.out.printf("%s - sum = %d\n", getName(), sum);

            System.out.printf("\nThread States\n");
            System.out.printf("%s - %s [%s], %s [%s]", getName(), threadOne.getName(), threadOne.getState(), threadTwo.getName(), threadTwo.getState());
        } catch (InterruptedException e) {
    } // run

} // ThreadThree

Finally, here is the App class.

public class App {

    public static void main(String[] args) throws InterruptedException {
        List<Integer> list = new ArrayList<>();
        CountDownLatch latch = new CountDownLatch(2);

        ThreadOne one = new ThreadOne(list, latch);
        ThreadTwo two = new ThreadTwo(list, latch);
        NumberThread three = new ThreadThree(list, latch, one, two);



    } // main

} // App

Output - (it can vary)

ThreadThree - starts waiting for other threads to complete
ThreadTwo - starts waiting
ThreadOne - starts waiting
ThreadTwo - inserting 74 in the list
ThreadTwo - invoking countDown on latch
ThreadOne - inserting 95 in the list
ThreadOne - invoking countDown on latch
ThreadThree - sum = 169

Thread States
ThreadThree - ThreadOne [TERMINATED], ThreadTwo [TERMINATED]