CountDownLatch

A CountDownLatch is a synchronization utility used to make one or more threads wait until a set of operations being performed by other threads completes. The latch count cannot be reset once it reaches zero.

API Mechanics

Key operations on a CountDownLatch:

• Instantiation: new CountDownLatch(N) sets the latch count to N.
• countDown(): Decrements the count by 1. Threads calling this do not block.
• await(): Blocks the calling thread until the latch count reaches zero. Once zero, all waiting threads are released.

Code Walkthrough

This program simulates a system startup gate where the main service waits for 3 database workers to initialize.

import java.util.concurrent.CountDownLatch;
public class StartupManager {
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch latch = new CountDownLatch(3);
Runnable service = () -> {
            try {
                System.out.println(Thread.currentThread().getName() + " is starting...");
                Thread.sleep(1000); // Simulate service load
                System.out.println(Thread.currentThread().getName() + " initialized.");
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            } finally {
                latch.countDown(); // Decrement count
            }
        };
new Thread(service, "CacheService").start();
        new Thread(service, "DBService").start();
        new Thread(service, "MessagingService").start();
System.out.println("Main server waiting for services to initialize...");
        latch.await(); // Blocks until latch count reaches 0
        System.out.println("All services initialized. Server started!");
    }
}

Interview-Relevant Information