Countdownlatch

Latching variables specify conditions that once set never change. This provides a way to start several threads and have them wait until a signal is received from a coordinating thread. [1]

The following program creates a set of threads, but doesn't let any thread start until all the threads are created.

import java.util.concurrent.*;
 
public class LatchTest
{
    private static final int COUNT = 10;
 
    private static class Worker implements Runnable
    {
        CountDownLatch startLatch;
 
        CountDownLatch stopLatch;
 
        String name;
 
        Worker(CountDownLatch startLatch, CountDownLatch stopLatch, String name)
        {
            this.startLatch = startLatch;
            this.stopLatch = stopLatch;
            this.name = name;
        }
 
        public void run()
        {
            try
            {
                startLatch.await(); // wait until the latch has counted down to
                                    // zero
            } catch (InterruptedException ex)
            {
                ex.printStackTrace();
            }
            System.out.println("Running: " + name);
            stopLatch.countDown();
        }
    }
 
    public static void main(String args[])
    {
        // CountDownLatch(int count)
        // Constructs a CountDownLatch initialized with the given count.
        CountDownLatch startSignal = new CountDownLatch(1);
        CountDownLatch stopSignal = new CountDownLatch(COUNT);
        for (int i = 0; i < COUNT; i++)
        {
            new Thread(new Worker(startSignal, stopSignal, Integer.toString(i)))
                    .start();
        }
        System.out.println("Go");
        startSignal.countDown();
        try
        {
            stopSignal.await();
        } catch (InterruptedException ex)
        {
            ex.printStackTrace();
        }
        System.out.println("Done");
    }
}

Bibliography

1. Getting to Know Synchronizers

: 2 Java API