java11-concurrency-cyclicbarrier

preface

• a thread arriving at the barrier waits for other threads to arrive
• all threads waiting at the barrier point are released when the last arrives
• use-case: split task to subtasks and combine the results when all are ready
• tripping the barrier - situation where the required number of threads wait at the barrier

java

• barrier in java is represented by CyclicBarrier class
• can be reused (reset())
• could execute barrier action - Runnable task executed just before release
• constructors
  • CyclicBarrier​(int parties) - trip when the given number of parties (threads) are waiting upon it
  • CyclicBarrier​(int parties, Runnable barrierAction)
    • barrierAction - the command to execute when the barrier is tripped
• methods
  • int await()
    • waits until all parties have invoked await on this barrier
    • returns the arrival index of the current thread (countdown)
    • InterruptedException if the current thread was interrupted while waiting
    • BrokenBarrierException
      • if another thread was interrupted or timed out while the current thread was waiting
      • the barrier was reset
      • the barrier was broken when await was called
      • the barrier action (if present) failed due to an exception
• int await​(long timeout, TimeUnit unit)
• int getNumberWaiting()
• int getParties()
• boolean isBroken()
• void reset()
  • if any parties are currently waiting at the barrier, they will return with a BrokenBarrierException

project description

1. sub tasks

   class SubTask extends Thread {
       
       private final CyclicBarrier barrier;
       private final int id;
   
       SubTask(CyclicBarrier barrier, int id) {
           this.barrier = barrier;
           this.id = id;
       }
   
       @Override
       public void run() {
           try {
               System.out.println("SubTask " + id + " is performing work");
               TimeUnit.MILLISECONDS.sleep(new Random().nextInt(50) + 3);
               System.out.println("SubTask " + id + " ended the work");
               this.barrier.await();
               System.out.println("SubTask " + id + " passed the barrier");
           } catch (InterruptedException | BrokenBarrierException e) {
               // not used
           }
       }
   }
   

2. simulation

   Runnable barrierAction = () -> System.out.println("all subtasks arrive");
   CyclicBarrier barrier = new CyclicBarrier(5, barrierAction);
   
   var threads = new LinkedList<Thread>();
   
   for (int i = 0; i < 5; i++) {
       SubTask t = new SubTask(barrier, i);
       t.start();
       threads.add(t);
   }
   
   for (Thread thread : threads) {
       thread.join();
   }
   

   can produce output

   SubTask 4 is performing work
   SubTask 2 is performing work
   SubTask 0 is performing work
   SubTask 3 is performing work
   SubTask 1 is performing work
   
   SubTask 4 ended the work and is about to reach the barrier
   SubTask 0 ended the work and is about to reach the barrier
   SubTask 3 ended the work and is about to reach the barrier
   SubTask 2 ended the work and is about to reach the barrier
   SubTask 1 ended the work and is about to reach the barrier
   
   all subtasks arrive
   
   SubTask 0 passed the barrier
   SubTask 3 passed the barrier
   SubTask 2 passed the barrier
   SubTask 4 passed the barrier
   SubTask 1 passed the barrier