What is CountDownLatch?

As per java docs, CountDownLatch is a synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes. CountDownLatch concept is very common interview question in java concurrency, so make sure you understand it well. In this post, I will cover following points related to CountDownLatch in java concurrency.

Sections in this post:

What is CountDownLatch?
How CountDownLatch works?
Possible usages in real time applications
Example application
Common interview questions

What is CountDownLatch?

CountDownLatch was introduced with JDK 1.5 along with other concurrent utilities like CyclicBarrier, Semaphore, ConcurrentHashMap and BlockingQueue in java.util.concurrent package. This class enables a java thread to wait until other set of threads completes their tasks. e.g. Application’s main thread want to wait, till other service threads which are responsible for starting framework services have completed started all services.

CountDownLatch works by having a counter initialized with number of threads, which is decremented each time a thread complete its execution. When count reaches to zero, it means all threads have completed their execution, and thread waiting on latch resume the execution.

CountDownLatch Concept

Pseudo code for CountDownLatch can be written like this:

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

How CountDownLatch works?

CountDownLatch.java class defines one constructor inside:

This count is essentially the number of threads, for which latch should wait. This value can be set only once, and CountDownLatch provides no other mechanism to reset this count.

The first interaction with CountDownLatch is with main thread which is goind to wait for other threads. This main thread must call, CountDownLatch.await() method immediately after starting other threads. The execution will stop on await() method till the time, other threads complete their execution.

Other N threads must have reference of latch object, because they will need to notify the CountDownLatch object that they have completed their task. This notification is done by method : CountDownLatch.countDown(); Each invocation of method decreases the initial count set in constructor, by 1. So, when all N threads have call this method, count reaches to zero, and main thread is allowed to resume its execution past await() method.

Possible usages in real time applications

Let’s try to identify some possible usage of CountDownLatch in real time java applications. I am listing, as much i can recall. If you have any other possible usage, please leave a comment. It will help others.

1. Achieving Maximum Parallelism : Sometimes we want to start a number of threads at the same time to achieve maximum parallelism. For example, we want to test a class for being singleton. This can be done easily if we create a CountDownLatch with initial count 1, and make wait all threads to wait of latch. A single call to countDown() method will resume execution for all waiting threads in same time.
2. Wait N threads to completes before start execution: For example an application start-up class want to ensure that all N external systems are Up and running before handling the user requests.
3. Deadlock detection: A very handy use case in which you can use N threads to access a shared resource with different number of threads in each test phase, and try to create a deadlock.

Example application using CountDownLatch

In this example, I have simulated an application startup class which starts N threads that will check for external systems and report back to latch, on which startup class is waiting. As soon as all services are verified and checked, startup proceeds.

public abstract class BaseHealthChecker implements Runnable {

	private CountDownLatch _latch;
	private String _serviceName;
	private boolean _serviceUp;

	// Get latch object in constructor so that after completing the task, thread
	// can countDown() the latch
	public BaseHealthChecker(String serviceName, CountDownLatch latch) {
		super();
		this._latch = latch;
		this._serviceName = serviceName;
		this._serviceUp = false;
	}

	@Override
	public void run() {
		try {
			verifyService();
			_serviceUp = true;
		} catch (Throwable t) {
			t.printStackTrace(System.err);
			_serviceUp = false;
		} finally {
			if (_latch != null) {
				_latch.countDown();
			}
		}
	}

	public String getServiceName() {
		return _serviceName;
	}

	public boolean isServiceUp() {
		return _serviceUp;
	}

	// This methos needs to be implemented by all specific service checker
	public abstract void verifyService();
}

public class ApplicationStartupUtil {
	// List of service checkers
	private static List<BaseHealthChecker> _services;

	// This latch will be used to wait on
	private static CountDownLatch _latch;

	private ApplicationStartupUtil() {
	}

	private final static ApplicationStartupUtil INSTANCE = new ApplicationStartupUtil();

	public static ApplicationStartupUtil getInstance() {
		return INSTANCE;
	}

	public static boolean checkExternalServices() throws Exception {
		// Initialize the latch with number of service checkers
		_latch = new CountDownLatch(3);

		// All add checker in lists
		_services = new ArrayList<BaseHealthChecker>();
		_services.add(new NetworkHealthChecker(_latch));
		_services.add(new CacheHealthChecker(_latch));
		_services.add(new DatabaseHealthChecker(_latch));

		// Start service checkers using executor framework
		Executor executor = Executors.newFixedThreadPool(_services.size());

		for (final BaseHealthChecker v : _services) {
			executor.execute(v);
		}

		// Now wait till all services are checked
		_latch.await();

		// Services are file and now proceed startup
		for (final BaseHealthChecker v : _services) {
			if (!v.isServiceUp()) {
				return false;
			}
		}
		return true;
	}
}

public class CacheHealthChecker extends BaseHealthChecker {
	public CacheHealthChecker(CountDownLatch latch) {
		super("CACHE Service", latch);
	}

	@Override
	public void verifyService() {
		System.out.println("Checking " + this.getServiceName());
		try {
			Thread.sleep(7000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println(this.getServiceName() + " is UP");
	}

}

public class Main {
	public static void main(String[] args) {
		boolean result = false;
		try {
			result = ApplicationStartupUtil.checkExternalServices();
		} catch (Exception e) {
			e.printStackTrace();
		}
		System.out
				.println("External services validation completed !! Result was :: "
						+ result);
	}
}