Java CyclicBarrier
last modified February 15, 2025
In this article we show how to synchronize Java threads using CyclicBarrier.
CyclicBarrier is a synchronization aid that allows a set of threads
to wait for each other to reach a common barrier point. Unlike
CountDownLatch, which is a one-time use, CyclicBarrier
can be reused after the waiting threads are released.
To use CyclicBarrier, we first create a CyclicBarrier
object with a specified number of threads and an optional Runnable
action that is executed once the barrier is tripped. Each thread calls the
await method on the barrier, which blocks until all threads have
reached the barrier. Once all threads have arrived, the barrier is tripped, and
the optional action is executed. The threads are then released, and the barrier
can be reused.
CyclicBarrier example
The following example shows the usage of CyclicBarrier.
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Random;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.stream.Stream;
class Worker implements Runnable {
private final List<String> messages;
private final CyclicBarrier cyclicBarrier;
public Worker(List<String> messages, CyclicBarrier cyclicBarrier) {
this.messages = messages;
this.cyclicBarrier = cyclicBarrier;
}
@Override
public void run() {
int r = new Random().nextInt(3000, 8000);
System.out.printf("%s starting, durations %dms %n",
Thread.currentThread().getName(), r);
try {
Thread.sleep(r);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
System.out.println(Thread.currentThread().getName() + " completed");
messages.add(Thread.currentThread().getName() + " completed");
try {
cyclicBarrier.await();
} catch (InterruptedException | BrokenBarrierException e) {
throw new RuntimeException(e);
}
}
}
void main() throws InterruptedException {
List<String> messages = Collections.synchronizedList(new ArrayList<>());
var cyclicBarrier = new CyclicBarrier(5, () ->
messages.add("Barrier tripped, all threads reached the barrier"));
List<Thread> workers = Stream.generate(() ->
new Thread(new Worker(messages, cyclicBarrier)))
.limit(5)
.toList();
workers.forEach(Thread::start);
for (Thread worker : workers) {
worker.join();
}
System.out.println(messages);
}
The main thread creates five new threads; these five threads are synchronized
with CyclicBarrier.
int r = new Random().nextInt(3000, 8000);
Each thread runs for a random number of milliseconds between 3000 and 8000.
messages.add(Thread.currentThread().getName() + " completed"); cyclicBarrier.await();
When the task completes, it writes a message into a synchronized list and
calls await on the barrier. The thread blocks until all threads
have reached the barrier.
List<String> messages = Collections.synchronizedList(new ArrayList<>());
var cyclicBarrier = new CyclicBarrier(5, () ->
messages.add("Barrier tripped, all threads reached the barrier"));
We create a synchronized list and a CyclicBarrier for five tasks.
The barrier also has an optional action that is executed once all threads reach
the barrier.
List<Thread> workers = Stream.generate(() ->
new Thread(new Worker(messages, cyclicBarrier)))
.limit(5)
.toList();
A list of five threads is created. Each thread receives a synchronized message
list and the cyclicBarrier.
workers.forEach(Thread::start);
We start all threads.
for (Thread worker : workers) {
worker.join();
}
The main thread waits for all worker threads to complete using join.
System.out.println(messages);
In the end, we print all the messages.
Reusable CyclicBarrier Example
The following example demonstrates the reusability of
CyclicBarrier. The barrier is used twice to synchronize threads at
two different points.
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Random;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.stream.Stream;
class Worker implements Runnable {
private final List<String> messages;
private final CyclicBarrier cyclicBarrier;
public Worker(List<String> messages, CyclicBarrier cyclicBarrier) {
this.messages = messages;
this.cyclicBarrier = cyclicBarrier;
}
@Override
public void run() {
int r = new Random().nextInt(3000, 8000);
System.out.printf("%s starting, durations %dms %n",
Thread.currentThread().getName(), r);
try {
Thread.sleep(r);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
System.out.println(Thread.currentThread().getName() + " completed phase 1");
messages.add(Thread.currentThread().getName() + " completed phase 1");
try {
cyclicBarrier.await(); // Wait for all threads to complete phase 1
} catch (InterruptedException | BrokenBarrierException e) {
throw new RuntimeException(e);
}
// Phase 2
r = new Random().nextInt(3000, 8000);
System.out.printf("%s starting phase 2, durations %dms %n",
Thread.currentThread().getName(), r);
try {
Thread.sleep(r);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
System.out.println(Thread.currentThread().getName() + " completed phase 2");
messages.add(Thread.currentThread().getName() + " completed phase 2");
try {
cyclicBarrier.await(); // Wait for all threads to complete phase 2
} catch (InterruptedException | BrokenBarrierException e) {
throw new RuntimeException(e);
}
}
}
void main() throws InterruptedException {
List<String> messages = Collections.synchronizedList(new ArrayList<>());
var cyclicBarrier = new CyclicBarrier(5, () ->
messages.add("Barrier tripped, all threads reached the barrier"));
List<Thread> workers = Stream.generate(() ->
new Thread(new Worker(messages, cyclicBarrier)))
.limit(5)
.toList();
workers.forEach(Thread::start);
for (Thread worker : workers) {
worker.join();
}
System.out.println(messages);
}
In this program, the CyclicBarrier is used twice to synchronize
threads at two different phases of execution. Each thread completes two phases
of work, and the barrier ensures that all threads finish each phase before
proceeding to the next.
cyclicBarrier.await(); // Wait for all threads to complete phase 1
The first call to await ensures all threads complete phase 1 before
moving to phase 2.
cyclicBarrier.await(); // Wait for all threads to complete phase 2
The second call to await ensures all threads complete phase 2
before the program terminates.
var cyclicBarrier = new CyclicBarrier(5, () ->
messages.add("Barrier tripped, all threads reached the barrier"));
The barrier action is executed each time the barrier is tripped, indicating that all threads have reached the synchronization point.
$ java Main.java Thread-0 starting, durations 4500ms Thread-1 starting, durations 3200ms Thread-2 starting, durations 7000ms Thread-3 starting, durations 5000ms Thread-4 starting, durations 6000ms Thread-1 completed phase 1 Thread-0 completed phase 1 Thread-3 completed phase 1 Thread-4 completed phase 1 Thread-2 completed phase 1 Barrier tripped, all threads reached the barrier Thread-0 starting phase 2, durations 4000ms Thread-1 starting phase 2, durations 5500ms Thread-2 starting phase 2, durations 3000ms Thread-3 starting phase 2, durations 2500ms Thread-4 starting phase 2, durations 6000ms Thread-3 completed phase 2 Thread-2 completed phase 2 Thread-0 completed phase 2 Thread-1 completed phase 2 Thread-4 completed phase 2 Barrier tripped, all threads reached the barrier [Thread-1 completed phase 1, Thread-0 completed phase 1, Thread-3 completed phase 1, Thread-4 completed phase 1, Thread-2 completed phase 1, Barrier tripped, all threads reached the barrier, Thread-3 completed phase 2, Thread-2 completed phase 2, Thread-0 completed phase 2, Thread-1 completed phase 2, Thread-4 completed phase 2, Barrier tripped, all threads reached the barrier]
Source
Java CyclicBarrier - reference
In this article we have shown how to synchronize Java threads using CyclicBarrier.
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