Count Down Latch example code

Key point :

 

1) 1 task is composed of several portion.

 

2) every portion finished, the portion will call latch.countDown()

 

3) Threads depending on this task will call latch.await()

 

4) latch will be initialized using portion number, countDown() will decrese the number, when the number reaches 0, latch will release all those threads awaiting the latch open.

 

Sample code attached.

 

 

package concurrency;

 

//: concurrency/CountDownLatchDemo.java

import static java.lang.System.out;

import java.util.Random;

import java.util.concurrent.CountDownLatch;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

 

// Performs some portion of a task:

class TaskPortion implements Runnable

{

      private static int counter = 0;

      private final int id = counter++;

      private static Random rand = new Random(47);

      private final CountDownLatch latch;

 

      TaskPortion(CountDownLatch latch)

      {

            this.latch = latch;

      }

 

      public void run()

      {

            try

            {

                  doWork();

                  latch.countDown();

            }

            catch (InterruptedException ex)

            {

                  // Acceptable way to exit

            }

      }

 

      public void doWork() throws InterruptedException

      {

            TimeUnit.MILLISECONDS.sleep(rand.nextInt(2000));

            out.println(this + "completed");

      }

 

      public String toString()

      {

            return String.format("%1$-3d ", id);

      }

}

 

// Waits on the CountDownLatch:

class WaitingTask implements Runnable

{

      private static int counter = 0;

      private final int id = counter++;

      private final CountDownLatch latch;

 

      WaitingTask(CountDownLatch latch)

      {

            this.latch = latch;

      }

 

      public void run()

      {

            try

            {

                  latch.await();

                  out.println("Latch barrier passed for " + this);

            }

            catch (InterruptedException ex)

            {

                  out.println(this + " interrupted");

            }

      }

 

      public String toString()

      {

            return String.format("WaitingTask %1$-3d ", id);

      }

}

 

public class CountDownLatchDemo

{

      static final int SIZE = 100;

 

      public static void main(String[] args) throws Exception

      {

            ExecutorService exec = Executors.newCachedThreadPool();

            // All must share a single CountDownLatch object:

            CountDownLatch latch = new CountDownLatch(SIZE);

            for (int i = 0; i < 10; i++)

                  exec.execute(new WaitingTask(latch));

            for (int i = 0; i < SIZE; i++)

                  exec.execute(new TaskPortion(latch));

            out.println("Launched all tasks");

            exec.shutdown(); // Quit when all tasks complete

      }

} /* (Execute to see output) *///:~