How is CountDownLatch used in Java Multithreading?

Question

Can someone help me to understand what Java CountDownLatch is and when to use it?

I don't have a very clear idea of how this program works. As I understand all three threads start at once and each Thread will call CountDownLatch after 3000ms. So count down will decrement one by one. After latch becomes zero the program prints "Completed". Maybe the way I understood is incorrect.

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

class Processor implements Runnable {
    private CountDownLatch latch;

    public Processor(CountDownLatch latch) {
        this.latch = latch;
    }

    public void run() {
        System.out.println("Started.");

        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        latch.countDown();
    }
}

// -----------------------------------------------------

public class App {

    public static void main(String[] args) {

        CountDownLatch latch = new CountDownLatch(3); // coundown from 3 to 0

        ExecutorService executor = Executors.newFixedThreadPool(3); // 3 Threads in pool

        for(int i=0; i < 3; i++) {
            executor.submit(new Processor(latch)); // ref to latch. each time call new Processes latch will count down by 1
        }

        try {
            latch.await();  // wait until latch counted down to 0
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Completed.");
    }

}

Answer 1

CoundDownLatch enables you to make a thread wait till all other threads are done with their execution.

Pseudo code can be:

// Main thread starts
// Create CountDownLatch for N threads
// Create and start N threads
// Main thread waits on latch
// N threads completes there tasks are returns
// Main thread resume execution

Answer 2

package practice;

import java.util.concurrent.CountDownLatch;

public class CountDownLatchExample {

    public static void main(String[] args) throws InterruptedException {
        CountDownLatch c= new CountDownLatch(3);  // need to decrements the count (3) to zero by calling countDown() method so that main thread will wake up after calling await() method 
        Task t = new Task(c);
        Task t1 = new Task(c);
        Task t2 = new Task(c);
        t.start();
        t1.start();
        t2.start();
        c.await(); // when count becomes zero main thread will wake up 
        System.out.println("This will print after count down latch count become zero");
    }
}

class Task extends Thread{
    CountDownLatch c;

    public Task(CountDownLatch c) {
        this.c = c;
    }

    @Override
    public void run() {
        try {
            System.out.println(Thread.currentThread().getName());
            Thread.sleep(1000);
            c.countDown();   // each thread decrement the count by one 
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

Answer 3

Yes, you understood correctly. CountDownLatch works in latch principle, the main thread will wait until the gate is open. One thread waits for n threads, specified while creating the CountDownLatch.

Any thread, usually the main thread of the application, which calls CountDownLatch.await() will wait until count reaches zero or it's interrupted by another thread. All other threads are required to count down by calling CountDownLatch.countDown() once they are completed or ready.

As soon as count reaches zero, the waiting thread continues. One of the disadvantages/advantages of CountDownLatch is that it's not reusable: once count reaches zero you cannot use CountDownLatch any more.

Edit:

Use CountDownLatch when one thread (like the main thread) requires to wait for one or more threads to complete, before it can continue processing.

A classical example of using CountDownLatch in Java is a server side core Java application which uses services architecture, where multiple services are provided by multiple threads and the application cannot start processing until all services have started successfully.

P.S. OP's question has a pretty straightforward example so I didn't include one.

Answer 4

It is used when we want to wait for more than one thread to complete its task. It is similar to join in threads.

Where we can use CountDownLatch

Consider a scenario where we have requirement where we have three threads "A", "B" and "C" and we want to start thread "C" only when "A" and "B" threads completes or partially completes their task.

It can be applied to real world IT scenario

Consider a scenario where manager divided modules between development teams (A and B) and he wants to assign it to QA team for testing only when both the teams completes their task.

public class Manager {
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(2);
        MyDevTeam teamDevA = new MyDevTeam(countDownLatch, "devA");
        MyDevTeam teamDevB = new MyDevTeam(countDownLatch, "devB");
        teamDevA.start();
        teamDevB.start();
        countDownLatch.await();
        MyQATeam qa = new MyQATeam();
        qa.start();
    }   
}

class MyDevTeam extends Thread {   
    CountDownLatch countDownLatch;
    public MyDevTeam (CountDownLatch countDownLatch, String name) {
        super(name);
        this.countDownLatch = countDownLatch;       
    }   
    @Override
    public void run() {
        System.out.println("Task assigned to development team " + Thread.currentThread().getName());
        try {
                Thread.sleep(2000);
        } catch (InterruptedException ex) {
                ex.printStackTrace();
        }
    System.out.println("Task finished by development team Thread.currentThread().getName());
            this.countDownLatch.countDown();
    }
}

class MyQATeam extends Thread {   
    @Override
    public void run() {
        System.out.println("Task assigned to QA team");
        try {
                Thread.sleep(2000);
        } catch (InterruptedException ex) {
            ex.printStackTrace();
        }
        System.out.println("Task finished by QA team");
    }
}

Output of above code will be:

Task assigned to development team devB

Task assigned to development team devA

Task finished by development team devB

Task finished by development team devA

Task assigned to QA team

Task finished by QA team

Here await() method waits for countdownlatch flag to become 0, and countDown() method decrements countdownlatch flag by 1.

Limitation of JOIN: Above example can also be achieved with JOIN, but JOIN can not be used in two scenarios:

1. When we use ExecutorService instead of Thread class to create threads.
2. Modify above example where Manager wants to handover code to QA team as soon as Development completes their 80% task. It means that CountDownLatch allow us to modify implementation which can be used to wait for another thread for their partial execution.

Answer 5

This example from Java Doc helped me understand the concepts clearly:

class Driver { // ...
  void main() throws InterruptedException {
    CountDownLatch startSignal = new CountDownLatch(1);
    CountDownLatch doneSignal = new CountDownLatch(N);

    for (int i = 0; i < N; ++i) // create and start threads
      new Thread(new Worker(startSignal, doneSignal)).start();

    doSomethingElse();            // don't let run yet
    startSignal.countDown();      // let all threads proceed
    doSomethingElse();
    doneSignal.await();           // wait for all to finish
  }
}

class Worker implements Runnable {
  private final CountDownLatch startSignal;
  private final CountDownLatch doneSignal;
  Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
     this.startSignal = startSignal;
     this.doneSignal = doneSignal;
  }
  public void run() {
     try {
       startSignal.await();
       doWork();
       doneSignal.countDown();
     } catch (InterruptedException ex) {} // return;
  }

  void doWork() { ... }
}

Visual interpretation:

Evidently, CountDownLatch allows one thread (here Driver) to wait until a bunch of running threads (here Worker) are done with their execution.

Answer 6

From oracle documentation about CountDownLatch:

A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.

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.

A CountDownLatch is a versatile synchronization tool and can be used for a number of purposes.

A CountDownLatch initialized with a count of one serves as a simple on/off latch, or gate: all threads invoking await wait at the gate until it is opened by a thread invoking countDown().

A CountDownLatch initialized to N can be used to make one thread wait until N threads have completed some action, or some action has been completed N times.

public void await()
           throws InterruptedException

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.

public void countDown()

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.

Explanation of your example.

1. You have set count as 3 for latch variable

   CountDownLatch latch = new CountDownLatch(3);
   

2. You have passed this shared latch to Worker thread : Processor

3. Three Runnable instances of Processor have been submitted to ExecutorService executor

4. Main thread ( App ) is waiting for count to become zero with below statement

    latch.await();  
   

5. Processor thread sleeps for 3 seconds and then it decrements count value with latch.countDown()

6. First Process instance will change latch count as 2 after it's completion due to latch.countDown().

7. Second Process instance will change latch count as 1 after it's completion due to latch.countDown().

8. Third Process instance will change latch count as 0 after it's completion due to latch.countDown().

9. Zero count on latch causes main thread App to come out from await

10. App program prints this output now : Completed