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:
- When we use ExecutorService instead of Thread class to create threads.
- 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.