What is CountDownLatch? Write CountDownLatch Example?

A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes. A CountDownLatch is initialized with a given count. The await methods block until the current count reaches zero due to invocations of the countDown() method, after which all waiting threads are released and any subsequent invocations of await return immediately. This is a one-shot phenomenon -- the count cannot be reset. If you need a version that resets the count, consider using a CyclicBarrier.

     A CountDownLatch is a versatile synchronization tool and can be used for a number of purposes. A CountDownLatch initialized with a count of one serves as a simple on/off latch, or gate: all threads invoking await wait at the gate until it is opened by a thread invoking countDown(). A CountDownLatch initialized to N can be used to make one thread wait until N threads have completed some action, or some action has been completed N times.

A useful property of a CountDownLatch is that it doesn't require that threads calling countDown wait for the count to reach zero before proceeding, it simply prevents any thread from proceeding past an await until all threads could pass.

Sample usage: Here is a pair of classes in which a group of worker threads use two countdown latches:

• The first is a start signal that prevents any worker from proceeding until the driver is ready for them to proceed;

• The second is a completion signal that allows the driver to wait until all workers have completed.

import java.util.concurrent.CountDownLatch;

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");

 }

}