Java.util.concurrent.CyclicBarrier in Java

CyclicBarrier is used to make threads wait for each other. It is used when different threads process a part of computation and when all threads have completed the execution, the result needs to be combined in the parent thread. In other words, a CyclicBarrier is used when multiple thread carry out different sub tasks and the output of these sub tasks need to be combined to form the final output. After completing its execution, threads call await() method and wait for other threads to reach the barrier. Once all the threads have reached, the barriers then give the way for threads to proceed.

Working of CyclicBarrier

CyclicBarriers are defined in java.util.concurrent package. First a new instance of a CyclicBarriers is created specifying the number of threads that the barriers should wait upon.

CyclicBarrier newBarrier = new CyclicBarrier(numberOfThreads);

Each and every thread does some computation and after completing it’s execution, calls await() methods as shown:

public void run()
{
    // thread does the computation
    newBarrier.await();
}

Working of CyclicBarrier:

Once the number of threads that called await() equals numberOfThreads, the barrier then gives a way for the waiting threads. The CyclicBarrier can also be initialized with some action that is performed once all the threads have reached the barrier. This action can combine/utilize the result of computation of individual thread waiting in the barrier.

Runnable action = ... 
//action to be performed when all threads reach the barrier;
CyclicBarrier newBarrier = new CyclicBarrier(numberOfThreads, action);

Important Methods of CyclicBarrier:

1. getParties: Returns the number of parties required to trip this barrier.
   Syntax:

   public int getParties()

   Returns:
   the number of parties required to trip this barrier

2. reset: Resets the barrier to its initial state.
   Syntax:

   public void reset()

   Returns:
   void but resets the barrier to its initial state. If any parties are currently waiting at the barrier, they will return with a BrokenBarrierException.

3. isBroken: Queries if this barrier is in a broken state.
   Syntax:

   public boolean isBroken()

   Returns:
   true if one or more parties broke out of this barrier due to interruption or timeout since construction or the last reset, or a barrier action failed due to an exception; false otherwise.

4. getNumberWaiting: Returns the number of parties currently waiting at the barrier.
   Syntax:

   public int getNumberWaiting()

   Returns:
   the number of parties currently blocked in await()

5. await: Waits until all parties have invoked await on this barrier.
   Syntax:

   public int await() throws InterruptedException, BrokenBarrierException

   Returns:
   the arrival index of the current thread, where index getParties() – 1 indicates the first to arrive and zero indicates the last to arrive.

6. await: Waits until all parties have invoked await on this barrier, or the specified waiting time elapses.
   Syntax:

   public int await(long timeout, TimeUnit unit) 
   throws InterruptedException,
   BrokenBarrierException, TimeoutException

   Returns:
   the arrival index of the current thread, where index getParties() – 1 indicates the first to arrive and zero indicates the last to arrive

Output:

<Number of parties required to trip the barrier = 3
Sum of product and sum = 0
Is the barrier broken? - false
Number of parties waiting at the barrier at this point = 0
Sum of product and sum = 36
Barrier reset successful

Explanation: The value of (sum + product) = 0 is printed on the console because the child thread has’t yet ran to set the values of sum and product variable. Following this, (sum + product) = 36 is printed on the console because the child threads ran setting the values of sum and product. Furthermore, the number of waiting thread on the barrier reached 3, due to which the barrier then allowed all thread to pass and finally 36 was printed. The value of “Number of parties waiting at the barrier at this point” = 0 because all the three threads had already called await() method and hence, the barrier is no longer active. In the end, newBarrier is reset and can be used again.

BrokenBarrierException

A barrier breaks when any of the waiting thread leaves the barrier. This happens when one or more waiting thread is interrupted or when the waiting time is completed because the thread called the await() methods with a timeout as follows:

newBarrier.await(1000, TimeUnit.MILLISECONDS);
// thread calling this await() 
// methods waits for only 1000 milliseconds.

When the barrier breaks due to one of more participating threads, the await() methods of all the other threads throws a BrokenThreadException. Whereas, the threads that are already waiting in the barriers have their await() call terminated.

Difference between a CyclicBarrier and a CountDownLatch

• A CountDownLatch can be used only once in a program(until it’s count reaches 0).
• A CyclicBarrier can be used again and again once all the threads in a barriers is released.

Reference: Oracle

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