What is CyclicBarrier in Java? How and When to use CyclicBarrier with Example? Tutorial

What is CyclicBarrier in Java

CyclicBarrier in Java is a synchronizer introduced in JDK 5 on java.util.Concurrent package along with other concurrent utility like Counting Semaphore, BlockingQueue, ConcurrentHashMap, etc. CyclicBarrier is similar to CountDownLatch which we have seen in the last article  What is CountDownLatch in Java and allows multiple threads to wait for each other (barrier) before proceeding. The difference between CountDownLatch and CyclicBarrier is also a very popular multi-threading interview question in Java. CyclicBarrier is a natural requirement for a concurrent program because it can be used to perform the final part of the task once individual tasks are completed.

All threads which wait for each other to reach the barrier are called parties, CyclicBarrier is initialized with a number of parties to wait and threads wait for each other by calling CyclicBarrier.await() method which is a blocking method in Java and blocks until all Thread or parties call await(). 

In general calling await() is a shout out that Thread is waiting on the barrier. await() is a blocking call but can be timed out or Interrupted by other threads. 

In this Java concurrency tutorial, we will see What is CyclicBarrier in Java and an example of CyclicBarrier on which three Threads will wait for each other before proceeding further.

And, if you are serious about mastering Java multi-threading and concurrency then I also suggest you take a look at the Java Multithreading, Concurrency, and Performance Optimization course by Michael Pogrebinsky on Udemy. It's an advanced course to become an expert in Multithreading, concurrency, and Parallel programming in Java with a strong emphasis on high performance

How to use CyclicBarrier in Java – Example

Now we know what is CyclicBarrier in Java and it's time to see an example of CyclicBarrier in Java. Here is a simple example of CyclicBarrier in Java on which we initialize CyclicBarrier with 3 parties, means in order to cross the barrier, 3 thread needs to call await() method. 

Each thread calls await method in short duration but they don't proceed until all 3 threads reached the barrier, once all thread reaches the barrier, the barrier gets broker and each thread started their execution from that point. It's much clear with the output of the following example of CyclicBarrier in Java:

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 * Java program to demonstrate how to use CyclicBarrier in Java. CyclicBarrier is a

 * new Concurrency Utility added in Java 5 Concurrent package.

 *
 * @author Javin Paul
 */
public class CyclicBarrierExample {

    //Runnable task for each thread
    private static class Task implements Runnable {

        private CyclicBarrier barrier;

        public Task(CyclicBarrier barrier) {
            this.barrier = barrier;
        }

        @Override
        public void run() {
            try {
                System.out.println(Thread.currentThread()
    .getName() + " is waiting on barrier");
                barrier.await();
                System.out.println(Thread.currentThread()
    .getName() + " has crossed the barrier");
            } catch (InterruptedException ex) {
                Logger.getLogger(CyclicBarrierExample.class.getName())
      .log(Level.SEVERE, null, ex);
            } catch (BrokenBarrierException ex) {
                Logger.getLogger(CyclicBarrierExample.class.getName())
      .log(Level.SEVERE, null, ex);
            }
        }
    }

    public static void main(String args[]) {

        //creating CyclicBarrier with 3 parties i.e. 3 Threads needs to call await()
        final CyclicBarrier cb = new CyclicBarrier(3, new Runnable(){
            @Override
            public void run(){
                //This task will be executed once all thread reaches barrier
                System.out.println("All parties are arrived at barrier, lets play");
            }
        });

        //starting each of thread
        Thread t1 = new Thread(new Task(cb), "Thread 1");
        Thread t2 = new Thread(new Task(cb), "Thread 2");
        Thread t3 = new Thread(new Task(cb), "Thread 3");

        t1.start();
        t2.start();
        t3.start();
     
    }
}

Output:
Thread 1 is waiting on the barrier
Thread 3 is waiting on the barrier
Thread 2 is waiting on the barrier
All parties have arrived at the barrier, lets play
Thread 3 has crossed the barrier
Thread 1 has crossed the barrier
Thread 2 has crossed the barrier

When to use CyclicBarrier in Java? Example

Given the nature of CyclicBarrier, it can be very handy to implement map-reduce kind of task similar to the fork-join framework of Java 7, where a big task is broken down into smaller pieces, and to complete the task you need output from the individual small task.

For example, to count the population of India you can have 4 threads which count population from North, South, East, and West and once complete they can wait for each other When last thread completed their task, Main thread or any other thread can add result from each zone and print total population. 

You can use CyclicBarrier in Java :

1) To implement multiplayer game which can not begin until all player has joined.

2) Perform lengthy calculation by breaking it into smaller individual tasks, In general, to implement Map reduce technique.

If you want to learn more about CyclicBarrier in Java, then you can further check the Multithreading and Parallel Computing in Java course on Udemy, it's a good course to learn multithreading basics in Java and you can buy in just $10 on Udemy sales. 

Difference between CountDownLatch and CyclicBarrier in Java

In our last article, we have to see how CountDownLatch can be used to implement multiple threads waiting for each other. If you look at CyclicBarrier it also does the same thing but it is different you can not reuse CountDownLatch once the count reaches zero while you can reuse CyclicBarrier by calling the reset() method which resets Barrier to its initial State. 

What it implies that CountDownLatch is good for one-time events like application start-up time and CyclicBarrier can be used in case of the recurrent event like concurrently calculating a solution of the big problem etc. 

If you like to learn more about threading and concurrency in Java you can also check my post on When to use the Volatile variable in Java and How Synchronization works in Java.

Important points of CyclicBarrier in Java

1. CyclicBarrier can perform a completion task once all thread reaches to the barrier, This can be provided while creating CyclicBarrier.

2. If CyclicBarrier is initialized with 3 parties means 3 thread needs to call the await method to break the barrier.

3. The thread will block on await() until all parties reach the barrier, another thread interrupts or await timed out.

4. If another thread interrupts the thread which is waiting on the barrier it will throw BrokernBarrierException as shown below:

java.util.concurrent.BrokenBarrierException
        at java.util.concurrent.CyclicBarrier.dowait(CyclicBarrier.java:172)
        at java.util.concurrent.CyclicBarrier.await(CyclicBarrier.java:327)

5.CyclicBarrier.reset() put Barrier on its initial state, other thread which is waiting or not yet reached barrier will terminate with java.util.concurrent.BrokenBarrierException.

That's all on What is CyclicBarrier in Java When to use CyclicBarrier in Java and a Simple Example of How to use CyclicBarrier in Java. We have also seen the difference between CountDownLatch and CyclicBarrier in Java and got some idea where we can use CyclicBarrier in Java Concurrent code.

Other Java thread and concurrency tutorials from Javarevisited Blog

• Why wait and notify methods are declared in an Object class
• How to avoid deadlock in Java by carefully coding
• What is a race condition in a concurrent Java program?
• How to write a thread-safe class in Java
• Difference between Runnable and Thread in Java
• 5 Best Courses to learn Multithreading in Java
• 6 Books to learn Concurrency and Threads in Java
• What is happens-before in Java? 
• How to use Exchanger to share objects in Java?
• Difference between volatile, atomic, and synchronized in Java
• How Thread, Code, and Data works in Java Program

Thanks for reading this article of far. If you find this CyclicBarrier tutorial useful then please share it with your friends and colleagues. If you have any questions or feedback then please drop a note. 

Now, over to you, what is difference between Phaser and CyclicBarrier in Java? and What is your favorite Concurrency utility like CyclicBarrier, CountDownLatch, Semaphore, Phaser, CompletableFuture or anything else?