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 last article What is CountDownLatch in Java and allows multiple threads to wait for each other (barrier) before proceeding. Difference between CountDownLatch and CyclicBarrier is a also very popular multi-threading interview question in Java. CyclicBarrier is a natural requirement for concurrent program because it can be used to perform final part of task once individual tasks are completed. All threads which wait for each other to reach barrier are called parties, CyclicBarrier is initialized with number of parties to be 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 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 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 a different you can not reuse CountDownLatch once 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 good for one time event like application start-up time and CyclicBarrier can be used to in case of recurrent event e.g. concurrently calculating solution of 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
Now we know what is CyclicBarrier in Java and it's time to see 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 barrier, once all thread reach barrier, barrier gets broker and each thread started there execution from that point. Its much clear with the output of following example of CyclicBarrier in Java:
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 counts population from North, South, East and West and once complete they can wait for each other, When last thread completed there 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 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. Thread will block on await() until all parties reaches to barrier, another thread interrupt or await timed out.
4. If another thread interrupt the thread which is waiting on barrier it will throw BrokernBarrierException as shown below:
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 difference between CountDownLatch and CyclicBarrier in Java and got some idea where we can use CyclicBarrier in Java Concurrent code.
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