Touring Additional Utility APIs - Beginning Java 7 - page 550

Note Afterwaitingthreadsarereleased,subsequentcallsto await() returnimme-
diately.Also,becausethecountcannotbereset,a CountDownLatch instancecanbe
usedonlyonce.Whenrepeateduseisarequirement,usethe CyclicBarrier class
instead.

We can use CountDownLatch to ensure that worker threads start working at ap-
proximately the same time. For example, check out Listing 6-2 .

Listing 6-2. Using a countdown latch to trigger a coordinated start

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

class CountDownLatchDemo
{

final static int NTHREADS = 3;
public static void main(String[] args)
{

final CountDownLatch startSignal = new Coun-

tDownLatch(1);
final CountDownLatch doneSignal = new Coun-

tDownLatch(NTHREADS);
Runnable r = new Runnable()
{

public void run()
{

try
{

report("entered run()");
startSignal.await(); // wait

until told to proceed

report("doing work");
Thread.sleep((int)(Math.random()*1000));
doneSignal.countDown(); // re-

duce count on which

//