Diferencia entre CountDownLatch y CyclicBarrier en Java

Aunque tanto CountDownLatch como CyclicBarrier se utilizan como una ayuda de sincronización que permite que al menos un subproceso espere, hay ciertas diferencias entre ellos. Conocer esos contrastes entre CountDownLatch y CyclicBarrier en Java lo ayudará a elegir cuándo una de estas utilidades le servirá mejor y, obviamente, también es una pregunta de consulta java decente.

CountDownLatch es un subproceso que espera que finalicen varios subprocesos o que llama a countDown(). Cuando todos los subprocesos han llamado a countDown(), el subproceso en espera continúa ejecutándose.

Ejemplo: 

// Java Program to demonstrate Usage of CountDownLatch
// Its used when a thread needs to wait for other
// threads before starting its work
 
// Importing required classes
import java.util.concurrent.CountDownLatch;
 
// Main class
public class CountDownLatchDemo {
 
    // Main driver method
    public static void main(String args[])
        throws InterruptedException
    {
        // Let us create task that is going to
        // wait for four threads before it starts
        CountDownLatch latch = new CountDownLatch(4);
 
        // Creating worker threads
        Worker first = new Worker(1000, latch, "WORKER-1");
        Worker second = new Worker(2000, latch, "WORKER-2");
        Worker third = new Worker(3000, latch, "WORKER-3");
        Worker fourth = new Worker(4000, latch, "WORKER-4");
 
        // Starting above 4 threads
        first.start();
        second.start();
        third.start();
        fourth.start();
 
        // The main task waits for four threads
        latch.await();
 
        // Main thread has started
        System.out.println(Thread.currentThread().getName()
                           + " has finished");
    }
}
 
// A class to represent threads for which
// the main thread waits.
class Worker extends Thread {
    private int delay;
    private CountDownLatch latch;
 
    public Worker(int delay, CountDownLatch latch,
                  String name)
    {
        super(name);
        this.delay = delay;
        this.latch = latch;
    }
 
    @Override public void run()
    {
        try {
            Thread.sleep(delay);
            latch.countDown();
            System.out.println(
                Thread.currentThread().getName()
                + " finished");
        }
        catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

WORKER-1 finished
WORKER-2 finished
WORKER-3 finished
main has finished
WORKER-4 finished

CyclicBarrier es cuando diferentes subprocesos se mantienen apretados entre sí (esperan el uno al otro) y cuando todos han terminado su ejecución, el resultado debe combinarse en el subproceso principal.

Ejemplo

// Java program to demonstrate Execution on Cyclic Barrier
 
// Importing required classes
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
 
// Class 1
// Class implementing Runnable interface
class Computation1 implements Runnable {
 
    public static int product = 0;
    public void run()
    {
        product = 2 * 3;
        try {
            // thread1 awaits for other threads
            Tester.newBarrier.await();
        }
        catch (InterruptedException
               | BrokenBarrierException e) {
            e.printStackTrace();
        }
    }
}
 
// Class 2
// Implementing Runnable interface
class Computation2 implements Runnable {
 
    public static int sum = 0;
    public void run()
    {
        // check if newBarrier is broken or not
        System.out.println("Is the barrier broken? - "
                           + Tester.newBarrier.isBroken());
        sum = 10 + 20;
        try {
            Tester.newBarrier.await(3000,
                                    TimeUnit.MILLISECONDS);
 
            // number of parties waiting at the barrier
            System.out.println(
                "Number of parties waiting at the barrier "
                + "at this point = "
                + Tester.newBarrier.getNumberWaiting());
        }
        catch (InterruptedException
               | BrokenBarrierException e) {
            e.printStackTrace();
        }
        catch (TimeoutException e) {
            e.printStackTrace();
        }
    }
}
 
public class Tester implements Runnable {
 
    // create a static CyclicBarrier instance
    public static CyclicBarrier newBarrier
        = new CyclicBarrier(3);
 
    public static void main(String[] args)
    {
        // parent thread
        Tester test = new Tester();
 
        Thread t1 = new Thread(test);
 
        // Starting the thread using start() method
        t1.start();
    }
 
    // Method
    public void run()
    {
        // Print statement
        System.out.println(
            "Number of parties required to trip the barrier = "
            + newBarrier.getParties());
        System.out.println(
            "Sum of product and sum = "
            + (Computation1.product + Computation2.sum));
 
        // Creating object of class 1 objects
        // on which the child thread has to run
        Computation1 comp1 = new Computation1();
        Computation2 comp2 = new Computation2();
 
        // creation of child thread
        Thread t1 = new Thread(comp1);
        Thread t2 = new Thread(comp2);
 
        // Moving child thread to runnable state
        t1.start();
        t2.start();
 
        try {
            // parent thread awaits
            Tester.newBarrier.await();
        }
 
        catch (InterruptedException
               | BrokenBarrierException e) {
 
            // Display exceptions along with line number
            // using printStackTrace() method
            e.printStackTrace();
        }
 
        // barrier breaks as the number of thread waiting
        // for the barrier at this point = 3
        System.out.println(
            "Sum of product and sum = "
            + (Computation1.product + Computation2.sum));
 
        // Resetting the newBarrier
        newBarrier.reset();
        System.out.println("Barrier reset successful");
    }
}

Number of parties required to trip the barrier = 3
Sum of product and sum = 0
Is the barrier broken? - false
Number of parties waiting at the barrier at this point = 0
Sum of product and sum = 36
Barrier reset successful

Diferencia entre CountDownLatch y CyclicBarrier