Saturday, July 28, 2012

What is CyclicBarrier Example in Java 5 – Concurrency 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 an also very popular multi-threading interview question in Java. CyclicBarrier is a natural requirement for a concurrent program because it can be used to perform final part of the task once individual tasks  are completed. All threads which wait for each other to reach 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 shout out that Thread is waiting on the barrier. await() is a blocking call but can be timed out or Interrupted by other thread. 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.

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 the does the same thing but there is different you can not reuse CountDownLatch once the count reaches zero while you can reuse CyclicBarrier by calling reset() method which resets Barrier to its initial State. What it implies that CountDownLatch is a good for one-time events like application start-up time and CyclicBarrier can be used to in case of the recurrent event e.g. 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 Volatile variable in Java  and How Synchronization works in Java.

CyclicBarrier in Java – Example

Java CyclicBarrier Example and Tutorial programNow 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 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 reach the barrier, barrier gets broker and each thread started their execution from that point. Its much clear with the output of 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;

        public void run() {
            try {
                System.out.println(Thread.currentThread().getName() + " is waiting on barrier");
                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(){
            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");


Thread 1 is waiting on barrier
Thread 3 is waiting on barrier
Thread 2 is waiting on barrier
All parties have arrived at 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
Given the nature of CyclicBarrier it can be very handy to implement map reduce kind of task similar to fork-join framework of Java 7, where a big task is broker down into smaller pieces and to complete the task you need output from individual small task e.g. to count 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 multi player 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.

Important point 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 await method to break the barrier.
3. The thread will block on await() until all parties reach to the barrier, another thread interrupt or await timed out.
4. If another thread interrupts the thread which is waiting on barrier it will throw BrokernBarrierException as shown below:

        at java.util.concurrent.CyclicBarrier.dowait(
        at java.util.concurrent.CyclicBarrier.await(

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.

Further Learning
Java Fundamentals: The Java Language
Java Concurrency in Practice
Applying Concurrency and Multi-threading to Common Java Patterns

Other Java thread and concurrency tutorials from Javarevisited Blog


Ran Biron said...

Using a barrier in a parallel calculation isn't the best example. You'd end up tying resources needlessly while they could be doing something useful - such as calculating more parts. Most implementations use an intermediate results repository and take threads from a pool according to need.

Javin @ transient vs volatile java said...

@Ran Biron, Thanks for your comments. This CyclicBarrier example was to demonstrate How main thread wait for supporting thread at a particular point. I think CyclicBarrier has real use case around multi-player games where Game should be start until all parties arrived and as you rightly pointed out initializing different parts of system etc.

Nerak usares said...

hi,i hope you could help me coz i have a final project in school and i don't know how to make final project is " to create a program using inheritance and polymorphism and that program is about a company program and i need that program this coming september 24,2012.....i hope you help me this...Thanks a lot... God Bless

Anonymous said...

Another key difference between CyclicBarrier vs CountDownLatch is that later is advanceble. By advanceble, I mean thread can still continue e.g. Threads waiting on CountDownLatch can still continue after calling countDown() method but In CyclicBarrier, all thread must wait at barrier for other parties to arrive. That's why CyclicBarrier is not advanceable.

Unknown said...

"..You can use CyclicBarrier in Java :
1) To implement multi player game which can not begin until all player has joined."

While I can see how one might be able to use a CyclicBarrier for the multi player example, a CountDown latch is actually more relevant here. Latches are precisely meant for this use - a one off event where some control thread wants to wait for N number of other threads before proceeding. As a matter of fact, that specific example (the multi-player example) is used in the Java concurrency in Practice book (5.5.1).

Rakesh Dewangan said...

There is one scenario where We have 3 Producer P1,P2,P3 and 3 Consumer C1,C2,C3,. The producer produce the X1,X2,X3 in random order and put in to Queue. Now I want to consume the things like C1 consume [X1,X2,X3] same like C2 [Y1,Y2,Y3] ….where the queue contains [x1,x2,y1,x3,y2,y3....]… How to implement this things

Anonymous said...

Great example, here is one more example of how you can use CyclicBarrier in Java to stop multiple threads at barrier point with easy to understand explanation.

Anonymous said...

What is difference between CyclicBarrier, CountDownLatch and Phaser in Java? When to use each of them or can we use them interchangebly?

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