Java.util.concurrent.CyclicBarrier Example

Anirudh BhatnagarDecember 15th, 2014Last Updated: December 15th, 2014

1 340 3 minutes read

In one of the previous posts, we talked about CountdownLatch and its use case. Today we are going to talk about a similar yet different synchronisation mechanism called as CyclicBarrier.

Java.util.Concurrent.CyclicBarrier is a synchronisation aid that allows a set of threads to wait for each other to reach a common barrier point. This means that all the threads reaching a specific point (called as barrier point) will have to wait for other threads to reach the same point. As soon as all the threads have reached the barrier point, all threads to are released to continue.

The barrier is called cyclic because it can be re-used after the waiting threads are released and that is where it is different than CountdownLatch. We can reuse CyclicBarrier by calling reset() method which resets the barrier to its initial state.

CountDownLatch is good for one time event like application/module start-up time and CyclicBarrier can be used to in case of recurrent event e.g. concurrently (re-)calculating each time when the input data changed.

Let’s try to understand CyclicBarrier by an example.

Suppose we have two services which should wait for each other to complete the execution. Moreover the service calling them should also wait for their competition. So in our example the 2 services and the main thread calling them will wait for each other to finish their execution.

JavaCyclicBarrierExample.java

package com.jcg.example;

import java.util.Date;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;

/**
 * 
 * @author anirudh
 *
 */
public class JavaCyclicBarrierExample {
	
	
	public static void main(String[] args) {
		
		//3 threads are part of the barrier, ServiceOne, ServiceTwo and this main thread calling them.
		final CyclicBarrier barrier = new CyclicBarrier(3);
		
		Thread serviceOneThread = new Thread(new ServiceOne(barrier));
		Thread serviceTwoThread = new Thread(new ServiceTwo(barrier));
		
		System.out.println("Starting both the services at"+new Date());
		
		serviceOneThread.start();
		serviceTwoThread.start();
		
		try {
			barrier.await();
		} catch (InterruptedException e) {
			System.out.println("Main Thread interrupted!");
			e.printStackTrace();
		} catch (BrokenBarrierException e) {
			System.out.println("Main Thread interrupted!");
			e.printStackTrace();
		}
		System.out.println("Ending both the services at"+new Date());
	}
	
	
}

ServiceOne.java

package com.jcg.example;

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;

/**
 * 
 * @author anirudh
 *
 */
public class ServiceOne implements Runnable {

	private final CyclicBarrier cyclicBarrier;

	public ServiceOne(CyclicBarrier cyclicBarrier) {
		this.cyclicBarrier = cyclicBarrier;
	}

	@Override
	public void run() {
		System.out.println("Starting service One...");
		try {
			Thread.sleep(3000);
		} catch (InterruptedException e1) {
			// TODO Auto-generated catch block
			e1.printStackTrace();
		}
		System.out
				.println("Service One has finished its work... waiting for others...");
		try {
			cyclicBarrier.await();
		} catch (InterruptedException e) {
			System.out.println("Service one interrupted!");
			e.printStackTrace();
		} catch (BrokenBarrierException e) {
			System.out.println("Service one interrupted!");
			e.printStackTrace();
		}
		System.out.println("The wait is over, lets complete Service One!");

	}

}

ServiceTwo.java

package com.jcg.example;

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;

/**
 * 
 * @author anirudh
 *
 */
public class ServiceTwo implements Runnable {
	
	private final CyclicBarrier cyclicBarrier;

	public ServiceTwo(CyclicBarrier cyclicBarrier) {
		this.cyclicBarrier = cyclicBarrier;
	}

	@Override
	public void run() {
		System.out.println("Starting service Two....");
		
		try {
			Thread.sleep(5000);
		} catch (InterruptedException e1) {
			// TODO Auto-generated catch block
			e1.printStackTrace();
		}
		System.out.println("Service Two has finished its work.. waiting for others...");
		try {
			cyclicBarrier.await();
		} catch (InterruptedException e) {
			System.out.println("Service one interrupted!");
			e.printStackTrace();
		} catch (BrokenBarrierException e) {
			System.out.println("Service one interrupted!");
			e.printStackTrace();
		}
		System.out.println("The wait is over, lets complete Service two!");

	}

}

Output :

Starting both the services atSun Dec 14 14:27:09 IST 2014
Starting service One...
Starting service Two....
Service One has finished its work... waiting for others...
Service Two has finished its work.. waiting for others...
The wait is over, lets complete Service two!
The wait is over, lets complete Service One!
Ending both the services atSun Dec 14 14:27:14 IST 2014

Here, we saw that the main thread was waiting for the two services to reach the barrier. Similarly, both the services were also waiting for each other to reach the barrier. Hence we saw that we can synchronise the 3 threads and make sure that they reach a barrier or in this case finish the execution, then only others will continue.