BrokenBarrierException in Java

What is BrokenBarrierException in Java?

BrokenBarrierException is a checked exception in Java found in the java.util.concurrent package. It is thrown when a thread is waiting at a CyclicBarrier and the barrier is broken before all threads arrive at the synchronization point.

This exception typically occurs in multithreaded programs that use CyclicBarrier to coordinate multiple threads, and one of the threads fails, times out, or gets interrupted while waiting at the barrier.

Class Declaration

package java.util.concurrent;

public class BrokenBarrierException extends Exception {
    public BrokenBarrierException() { }
    public BrokenBarrierException(String message) {
        super(message);
    }
}

As a subclass of Exception, this is a checked exception, so it must be handled explicitly.

What is a CyclicBarrier?

A CyclicBarrier is a synchronization aid that allows a set number of threads to wait for each other at a common point before proceeding. When all participating threads reach the barrier by calling await(), they are released simultaneously. If one thread gets interrupted or times out, the barrier is broken, and all other threads waiting at it will receive a BrokenBarrierException.

When Does BrokenBarrierException Occur?

This exception is thrown when:

• A thread is waiting at the barrier using await()
• The barrier gets broken because another thread was interrupted or timed out
• The barrier was reset before the wait completed

Example Program: Triggering BrokenBarrierException

Let’s look at a step-by-step example where we create a few threads and use a CyclicBarrier to synchronize them. We'll simulate an error where one thread doesn't wait at the barrier, causing the others to throw a BrokenBarrierException.

Step-by-Step Code

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

public class BrokenBarrierDemo {

    public static void main(String[] args) {
        // Create a CyclicBarrier for 3 threads
        CyclicBarrier barrier = new CyclicBarrier(3);

        // Start 3 threads
        for (int i = 1; i <= 3; i++) {
            int threadId = i;

            new Thread(() -> {
                try {
                    System.out.println("Thread " + threadId + " started");

                    if (threadId == 2) {
                        // Simulate an early exit - thread does not wait at the barrier
                        System.out.println("Thread " + threadId + " is exiting early.");
                        return;
                    }

                    Thread.sleep(1000); // Simulate work
                    System.out.println("Thread " + threadId + " waiting at barrier");

                    barrier.await(); // Wait at the barrier

                    System.out.println("Thread " + threadId + " passed the barrier");

                } catch (BrokenBarrierException e) {
                    System.out.println("Thread " + threadId + " caught BrokenBarrierException: " + e.getMessage());
                } catch (Exception e) {
                    System.out.println("Thread " + threadId + " encountered an error: " + e.getMessage());
                }
            }).start();
        }
    }
}

Expected Output

Thread 1 started
Thread 2 started
Thread 2 is exiting early.
Thread 3 started
Thread 1 waiting at barrier
Thread 3 waiting at barrier
Thread 1 caught BrokenBarrierException: null
Thread 3 caught BrokenBarrierException: null

Explanation of the Output

• We created a CyclicBarrier that expects 3 threads.
• Thread 2 simulates a failure and exits early — it never calls await().
• Thread 1 and Thread 3 reach the barrier and call await().
• Since Thread 2 never arrives, the barrier is broken.
• Thread 1 and Thread 3 throw and catch BrokenBarrierException.

How to Handle BrokenBarrierException

This exception should be handled to prevent your program from crashing unexpectedly in multi-threaded environments.

Basic Try-Catch Block

try {
    barrier.await();
} catch (BrokenBarrierException e) {
    System.err.println("Barrier was broken: " + e.getMessage());
} catch (InterruptedException e) {
    System.err.println("Thread was interrupted");
}

Using Barrier Reset

You can reset a barrier manually using barrier.reset() if it's broken and you want to reuse it:

if (barrier.isBroken()) {
    barrier.reset();
}

Real-World Use Cases

CyclicBarrier and BrokenBarrierException are useful in scenarios like:

• Parallel computations where tasks need to synchronize at certain points
• Game loops where multiple threads represent players or units synchronizing per tick
• Simulations where phases must complete before proceeding

Best Practices

• Always catch BrokenBarrierException when using await().
• Check if a barrier is broken using isBroken() before reusing it.
• Use await(timeout, TimeUnit) to avoid indefinite blocking.
• Document which threads are expected to reach the barrier — early exits can break synchronization.

Common Mistakes

• Not handling InterruptedException and BrokenBarrierException properly
• Expecting all threads to always reach the barrier without failure handling
• Reusing a broken barrier without resetting it

Summary

• BrokenBarrierException occurs when a CyclicBarrier breaks before all threads arrive.
• It is a checked exception and must be handled using try-catch.
• Use proper error handling and thread management to avoid broken barriers.
• Reset the barrier if you plan to reuse it after failure.

Final Thoughts

BrokenBarrierException highlights the importance of coordination in concurrent programming. When working with synchronization tools like CyclicBarrier, it's essential to anticipate failures — whether due to logic errors, early exits, or unexpected interruptions. Handling this exception properly ensures your multi-threaded applications are robust and reliable.