import java.util.concurrent.CountDownLatch;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

public class CountDownLatchDemo{

final static int NTHREADS = 3;

public static void main(String[] args){

final CountDownLatch startSignal = new CountDownLatch(1);

final CountDownLatch doneSignal = new CountDownLatch(NTHREADS);

Runnable runnable = new Runnable(){

@Override

public void run(){

try{

report("entered run()");

startSignal.await(); // wait until count down to zero.

report("doing work");

Thread.sleep((int)(Math.random()*10000));

doneSignal.countDown(); // decrement the count if not zero

}catch(InterruptedException ie){

System.err.println(ie);

}

}

void report(String s){

System.out.println(System.currentTimeMillis() + ": " + Thread.currentThread() + ": " + s);

}

};

ExecutorService executor = Executors.newFixedThreadPool(NTHREADS);

for(int i=0; i<NTHREADS; i++){

executor.execute(runnable);

}

try{

System.out.println("main thread doing something");

Thread.sleep(1000);

startSignal.countDown();

System.out.println("main thread doing something else");

doneSignal.await(); // wait till count down to be zero. main thread is waiting for the end.

executor.shutdownNow();

System.out.println("main thread's wait finished. That's the end.");

}catch(InterruptedException ie){

System.err.println(ie);

}

}

}